|> Kik Group||College of Optics and Photonics||UCF|
The following is an overview of recent publications, including the full text in PDF format where possible. Please note: These articles may be downloaded for personal use only. Any other use requires prior permission of the author and the publisher.
NEW Photonic Multitasking Interleaved Si Nanoantenna Phased Array Dianmin Lin, Aaron L. Holsteen, Elhanan Maguid, Gordon Wetzstein, Pieter G. Kik, Erez Hasman, and Mark L. Brongersma, Nano Letters 16, 7671 (2016) [pdf] [link]
Metasurfaces provide unprecedented control over light propagation by imparting local, space-variant phase changes on an incident electromagnetic wave. They can improve the performance of conventional optical elements and facilitate the creation of optical components with new functionalities and form factors. Here, we build on knowledge from shared aperture phased array antennas and Si-based gradient metasurfaces to realize various multifunctional metasurfaces capable of achieving multiple distinct functions within a single surface region. As a key point, we demonstrate that interleaving multiple optical elements can be accomplished without reducing the aperture of each subelement. Multifunctional optical elements constructed from Si-based gradient metasurface are realized, including axial and lateral multifocus geometric phase metasurface lenses. We further demonstrate multiwavelength color imaging with a high spatial resolution. Finally, optical imaging functionality with simultaneous color separation has been obtained by using multifunctional metasurfaces, which opens up new opportunities for the field of advanced imaging and display.
NEW Superabsorbing, artificial metal films constructed from semiconductor nanoantennas Soo Jin Kim, Junghyun Park, Majid Esfandyarpour, Emanuele Fancesco Pecora, Pieter G. Kik, and Mark L. Brongersma, Nano Lett. 16, 3801 (2016) [pdf] [link]
In 1934 Wilhelm Woltersdorff demonstrated that the absorption of light in an ultrathin, freestanding film is fundamentally limited to 50%. He concluded that reaching this limit would require a film with a real-valued sheet resistance that is exactly equal to R=eta/2=188.5 Ohm/sq, where eta = mu0 eps0 is the impedance of free space. This condition can be closely approximated over a wide frequency range in metals that feature a large imaginary relative permittivity eps'', i.e. a real-valued conductivity sigma = eps0 eps'' omega. A thin, continuous sheet of semiconductor material does not facilitate such strong absorption as its complex-valued permittivity with both large real and imaginary components preclude effective impedance matching. In this work, we show how a semiconductor metafilm constructed from optically resonant semiconductor nanostructures can be created whose optical response mimics that of a metallic sheet. For this reason the fundamental absorption limit mentioned above can also be reached with semiconductor materials, opening up new opportunities for the design of ultrathin optoelectronic and light harvesting devices.
NEW Omnidirectional excitation of sidewall gap-plasmons in a hybrid goldnanoparticle/aluminum-nanopore structure Chatdanai Lumdee and Pieter G. Kik, APL Photonics 1, 031301 (2016) [pdf] [link]
The gap-plasmon resonance of a gold nanoparticle in a nanopore in an aluminum film is investigated in polarization dependent single particle microscopy and spectroscopy. Scattering and transmission measurements reveal that gap-plasmons of this structure can be excited and observed under near-normal incidence excitation and collection, in contrast to the more common particle-on-a-mirror structure. Correlation of numerical simulations with optical spectroscopy suggests that a local electric field enhancement factor in excess of 50 is achieved under normal incidence excitation, with a hot-spot located near the top surface of the structure. It is shown that the strong field enhancement from this sidewall gap-plasmon mode can be efficiently excited over a broad angular range. The presented plasmonic structure lends itself to implementation in low-cost, chemically stable, easily addressable biochemical sensor arrays providing large optical field enhancement factors.
NEW Electrospray deposition of uniform thickness Ge23Sb7S70 and As40S60 chalcogenide glass films, Spencer Novak, Pao-Tai Lin, Cheng Li, Nikolay Borodinov, Zhaohong Han, Corentin Monmeyran, Neil Patel, Qingyang Du, Chatdanai Lumdee, Pieter G. Kik, Weiwei Deng, Juejun Hu, Anuradha Agarwal, Igor Luzinov, and Kathleen Richardson, JoVE 54379R3 (2016)
Solution-based electrospray film deposition, which is compatible with continuous, roll-to-roll processing, is applied to chalcogenide glasses. Two chalcogenide compositions are demonstrated: Ge23Sb7S70 and As40S60, which have both been studied extensively for planar mid-infrared (mid-IR) microphotonic devices. In this approach, uniform thickness films are fabricated through the use of computer numerical controlled (CNC) motion. Chalcogenide glass (ChG) is written over the substrate by a single nozzle along a serpentine path. Films were subjected to a series of heat treatments between 100 ˚C and 200 ˚C under vacuum to drive off residual solvent and densify the films. Based on transmission Fourier transform infrared (FTIR) spectroscopy and surface roughness measurements, both compositions were found to be suitable for the fabrication of planar devices operating in the mid-IR region. Residual solvent removal was found to be much quicker for the As40S60 film as compared to Ge23Sb7S70. Based on the advantages of electrospray, direct printing of a gradient refractive index
NEW Electrically Tunable Coherent Optical Absorption in Graphene with Ion Gel, Vrinda Thareja, Ju-Hyung Kang, Hongtao Yuan, Kaveh M. Milaninia, Harold Y. Hwang, Yi Cui, Pieter G. Kik, and Mark L. Brongersma, Nano Lett. 15, 1570 (2015) [pdf] [link]
We demonstrate electrical control over coherent optical absorption in a graphene-based Salisbury screen consisting of a single layer of graphene placed in close proximity to a gold back reflector. The screen was designed to enhance light absorption at a target wavelength of 3.2 µm by using a 600 nm-thick, nonabsorbing silica spacer layer. An ionic gel layer placed on top of the screen was used to electrically gate the charge density in the graphene layer. Spectroscopic reflectance measurements were performed in situ as a function of gate bias. The changes in the reflectance spectra were analyzed using a Fresnel based transfer matrix model in which graphene was treated as an infinitesimally thin sheet with a conductivity given by the Kubo formula. The analysis reveals that a careful choice of the ionic gel layer thickness can lead to optical absorption enhancements of up to 5.5 times for the Salisbury screen compared to a suspended sheet of graphene. In addition to these absorption enhancements, we demonstrate very large electrically induced changes in the optical absorption of graphene of ~3.3% per volt, the highest attained so far in a device that features an atomically thick active layer. This is attributable in part to the more effective gating achieved with the ion gel over the conventional dielectric back gates and partially by achieving a desirable coherent absorption effect linked to the presence of the thin ion gel that boosts the absorption by 40%.
NEW Heterogeneous plasmonic trimers for enhanced nonlinear optical absorption, Seyfollah Toroghi, Chatdanai Lumdee, and Pieter G. Kik, Appl. Phys. Lett. 106, 103102 (2015) [pdf] [link]
A dramatic enhancement of the thermally induced nonlinear optical response in compositionally heterogeneous plasmonic trimers is reported. It is demonstrated numerically that the nonlinear absorption performance of silver nanoparticle dimers under pulsed illumination can be enhanced by more than two orders of magnitude through the addition of only 0.1 vol.% of gold in the dimer gap. The nonlinear absorption performance of the resulting Ag-Au-Ag trimer exceeds the peak performance of isolated gold nanoparticles by a factor 40. This dramatic effect is enabled by cascaded plasmon resonance, resulting in extreme field concentration in the central nanoparticle of the trimer. The observed localized heat-generation, large optical response, and a predicted response time below 1 ns make these structures promising candidates for use in nonlinear optical limiting and optical switching.
Effect of surface roughness on substrate-tuned gold nanoparticle gap plasmon resonances, Chatdanai Lumdee, Binfeng Yun, and Pieter G. Kik, Nanoscale 7, 4250 (2015) [pdf] [link]
The effect of nanoscale surface roughness on the gap plasmon resonance of gold nanoparticles on thermally evaporated gold films is investigated experimentally and numerically. Single-particle scattering spectra obtained from 80 nm diameter gold particles on a gold film show significant particle-to-particle variation of the peak scattering wavelength of ±28 nm. The experimental results are compared with numerical simulations of gold nanoparticles positioned on representative rough gold surfaces, modeled based on atomic force microscopy measurements. The predicted spectral variation and average resonance wavelength show good agreement with the measured data. The study shows that nanometer scale surface roughness can significantly affect the performance of gap plasmon-based devices.
Gap-Plasmon Enhanced Gold Nanoparticle Photoluminescence, Chatdanai Lumdee, Binfeng Yun, and Pieter G. Kik, ACS Photonics 1, 1224 (2014) [pdf] [link] [cover] [supporting info]
Gap-plasmon enhanced gold nanoparticle photoluminescence is studied experimentally at the single particle level. The photoluminescence spectra of gold nanoparticles on an Al2O3-coated gold film under both 532 nm and 633 nm excitation show a clear peak near the measured gap plasmon resonance wavelength. Comparing the collected emission spectrum with that from a gold reference film under 633 nm excitation, a peak photoluminescence enhancement factor of 28000 is observed. The spectral shape and absolute magnitude of the enhancement factors for both excitation wavelengths are reproduced using numerical calculations without the use of any free parameters. The photoluminescence enhancement is explained in terms of a gap-mode enhanced e-h pair generation rate and a wavelength-dependent enhancement of the emission efficiency.
2013: Wide-band Spectral Control of Au Nanoparticle Plasmon Resonances pdf / link
2012: Voltage Controlled Resonance Tuning of Nanoscale Plasmonic Antennas pdf / link
2010: Metal-film-induced tuning of silver nanoparticle plasmon resonances pdf / link
Photothermal response enhancement in heterogeneous plasmon resonant nanoparticle trimers, Seyfollah Toroghi and Pieter G. Kik, Phys. Rev. B 90, 205414 (2014) [pdf] [link]
The optical response of heterogeneous plasmonic trimer structures composed of a silver nanoparticle dimer and a central gold nanoparticle is investigated analytically and numerically. The plasmon resonance of the silver dimer is controlled through near-field coupling, resulting in plasmon resonance frequency matching of the silver dimer and gold monomer. This coupling condition makes it possible to increase the energy dissipation per unit volume in the gold particle by over two orders of magnitude compared to a single-particle system. It is predicted that pulsed illumination of a trimer consisting of two 80 nm diameter silver particles and a 10 nm diameter central gold particle can raise the gold particle temperature by 100 K using a pump fluence as low as 20 nJ/mm2 at a wavelength of 530 nm. This finding may have practical applications in photothermal therapy, fast thermal nonlinear optical modulation, and could enable new fundamental thermal studies at picosecond time scales.
Catoptric electrodes: transparent metal electrodes using shaped surfaces, Pieter G. Kik, Opt. Lett. 39, 5114 (2014) [pdf] [link]
An optical electrode design is presented that theoretically allows 100% optical transmission through an interdigitated metallic electrode at 50% metal areal coverage. This is achieved by redirection of light incident on embedded metal electrode lines to an angle beyond that required for total internal reflection. Full-field electromagnetic simulations using realistic material parameters demonstrate 84% frequency-averaged transmission for unpolarized illumination across the entire visible spectral range using a silver interdigitated electrode at 50% areal coverage. The redirection is achieved through specular reflection, making it non-resonant and arbitrarily broadband, provided the electrode width exceeds the optical wavelength. These findings could significantly improve the performance of photovoltaic devices and optical detectors that require high-conductivity top contacts.
Numerical prediction of the effect of nanoscale surface roughness on film-coupled nanoparticle plasmon resonances, C. Lumdee and P. G. Kik, Proc. SPIE 9163, 91631I (2014) [pdf] [link] Abstract
Plasmon resonant metal nanoparticles on substrates have been considered for use in several nanophotonic applications due to the combination of large field enhancement factors, broadband frequency control, ease of fabrication, and structural robustness that they provide. Despite the existence of a large body of work on the dependence of the nanoparticle plasmon resonance on composition and particle-substrate separation, little is known about the role of substrate roughness in these systems. This is in fact an important aspect, since particle-substrate gap sizes for which large resonance shifts are observed are of the same order of typical surface roughness of deposited films. In the present study, the plasmon resonance response of 80 nm diameter gold nanoparticles on a thermally evaporated gold film are numerically calculated based on the measured surface morphology of the gold film. By combining the measured surface data with electromagnetic simulations, it is demonstrated that the plasmon resonance wavelength of single gold nanoparticles is blueshifted on a rough gold surface compared the response on a flat gold film. The anticipated degree of spectral variation of gold nanoparticles on the rough surface is also presented. This study demonstrates that nanoscale surface roughness can become an important source of spectral variation for substrate tuned resonances that use small gap sizes.
Plasmon-enhanced photothermal response in heterogeneous metallic trimers, S. Toroghi and P. G. Kik, Proc. SPIE 9163, 91631B (2014) [pdf] [link] Abstract
Heat generation in plasmonic nanostructures has attracted enormous attention due to the ability of these nanostructures to generate high temperatures in nanoscale volumes using far-field irradiation, enabling applications ranging from photothermal therapy to fast (sub-nanosecond) thermal optical switching. Here we investigate the optical and thermal response of a heterogeneous trimer structure composed of a gold nanoparticle surrounded by two larger silver nanoparticles analytically and numerically. We observe that this type of multi-scale multi-material plasmonic oligomer can produce temperature changes over two orders of magnitude higher than possible with isolated gold nanoparticles.
Wide-band Spectral Control of Au Nanoparticle Plasmon Resonances on a Thermally and Chemically Robust Sensing Platform, Chatdanai Lumdee, Binfeng Yun, and Pieter G. Kik, J. Phys. Chem. C 117, 19127 (2013) [pdf] [link]
Gold nanoparticles on Al2O3-coated gold films are presented as a chemically and thermally robust platform for molecular sensing. Single particle spectroscopy as a function of Al2O3 coating thickness shows reproducible gold nanoparticle scattering spectra in the range from 690 nm to 610 nm as the Al2O3 thickness increases from 0 to 3.4 nm. Numerical simulation of these structures indicates that surface enhanced Raman spectroscopy enhancement factors in excess of 10^6 can be achieved. The stability of the Al2O3 coated structures under high power laser irradiation was tested, revealing stable scattering spectra upon irradiation with 100 W/mm2 at the particle resonance wavelength. The presented structure solves challenges with thermal stability, wavelength tuning range, and Raman background signal associated with previously attempted approaches.
Cascaded plasmon resonances in multi-material nanoparticle trimers for extreme field enhancement, S. Toroghi, C. Lumdee, and P. G. Kik, Proc. SPIE 8809, 88091M (2013) [pdf] [link] Abstract
Optical field enhancement in coupled plasmonic nanostructures has attracted significant attention because of field enhancement factors that significantly exceed those observed in isolated nanostructures. While previous studies demonstrated the existence of such cascaded field enhancement in coupled nanospheres with identical composition, this effect has not yet been studied in systems containing multiple materials. Here, we investigate the polarization-dependent optical response of multi-material trimer nanostructures composed of Au nanoparticles surrounded by two Ag nanoparticles as a function of nanoparticle size and inter-particle spacing. We observe field enhancement factors that are ten times larger than observed in isolated Au nanoparticles. © (2013) COPYRIGHT Society of Photo-Optical Instrumentation Engineers (SPIE). Downloading of the abstract is permitted for personal use only.
Optical Characteristics and Numerical Study of Gold Nanoparticles on Al2O3 coated Gold Film for Tunable Plasmonic Sensing Platforms, C. Lumdee, B. Yun, and P. G. Kik, Proc. SPIE 8809, 88091S (2013) [pdf] [link] Abstract
Substrate-based tuning of plasmon resonances on gold nanoparticles (NP) is a versatile method of achieving plasmon resonances at a desired wavelength, and offers reliable nanogap sizes and large field enhancement factors. The reproducibility and relative simplicity of these structures makes them promising candidates for frequency-optimized sensing substrates. The underlying principle in resonance tuning of such a structure is the coupling between a metal nanoparticle and the substrate, which leads to a resonance shift and a polarization dependent scattering response. In this work, we experimentally investigate the optical scattering spectra of isolated 60 nm diameter gold nanoparticles on aluminum oxide (Al2O3) coated gold films with various oxide thicknesses. Dark-field scattering images and scattering spectra of gold particles reveal two distinct resonance modes. The experimental results are compared with numerical simulations, revealing the magnitude and phase relationships between the effective dipoles of the gold particle and the gold substrate. The numerical approach is described in detail, and enables the prediction of the resonance responses of a particle-on-film structure using methods that are available in many available electromagnetics simulation packages. The simulated scattering spectra match the experimentally observed data remarkably well, demonstrating the usefulness of the presented approach to researchers in the field.
Controlled Surface Plasmon Resonance on Stable Substrates as an Optimized Sensing Platform, C. Lumdee, B. Yun, and P. G. Kik, Paper FTh3C.8, Frontiers in Optics Conference (2013) [pdf] [link] Abstract
Precise control of localized plasmon resonance scattering spectra of gold nanoparticles on Al2O3 coated gold substrates were demonstrated. The scattering spectra remain stable after high power laser irradiation near the resonance wavelength.
Extreme plasmon resonant field enhancement in multi-material nanoparticle trimers, Seyfollah Toroghi, Chatdanai Lumdee, and Pieter G. Kik, Paper FTh3C.3, Frontiers in Optics Conference (2013) [pdf] [link] Abstract
Field enhancement of few-particle clusters consisting of spherical nanoparticles made from different materials is numerically investigated. Results demonstrate that multiplicative field enhancement can occur in multi-material trimer (Ag-Au-Ag) structures
Post-Fabrication Voltage Controlled Resonance Tuning of Nanoscale Plasmonic Antennas, Chatdanai Lumdee, Seyfollah Toroghi, and Pieter G. Kik, ACS Nano 6, 6301 (2012) [pdf] [link]
Voltage controlled wavelength tuning of the localized surface plasmon resonance of gold nanoparticles on an aluminum film is demonstrated in single particle microscopy and spectroscopy measurements. Anodization of the Al film after nanoparticle deposition forms an aluminum oxide spacer layer between the gold particles and the Al film, modifying the particle-substrate interaction. Darkfield microscopy reveals ring-shaped scattering images from individual Au nanoparticles, indicative of plasmon resonances with a dipole moment normal to the substrate. Single particle scattering spectra show narrow plasmon resonances that can be tuned from ~580 nm to ~550 nm as the anodization voltage increases to 12 V. All observed experimental trends could be reproduced in numerical simulations. The presented approach could be used as a general post-fabrication resonance optimization step of plasmonic nanoantennas and devices.
Cascaded field enhancement in plasmon resonant dimer nanoantennas compatible with two-dimensional nanofabrication methods, Seyfollah Toroghi and Pieter G. Kik, Appl. Phys. Lett. 101, 13116 (2012) [pdf] [link]
Cascaded field enhancement is demonstrated in asymmetric plasmon resonant dimer nanoantennas consisting of shape-tuned ellipsoidal nanoparticles. The nanoparticles that make up the dimer have identical thickness, suggesting that the presented approach can be used to design cascaded dimer antennas compatible with standard two-dimensional top-down nanofabrication tools such as electron beam lithography and nano-imprint lithography. Cascaded excitation is achieved by modification of the in-plane particle aspect ratios in a way that keeps the resonance frequency of the individual particles fixed, while significantly changing their polarizability. The achievable field enhancement is evaluated as a function of the particle volume ratio and spacing.
Cascaded plasmon resonant field enhancement in nanoparticle dimers in the point dipole limit, Seyfollah Toroghi and Pieter G. Kik, Appl. Phys. Lett. 100, 183105 (2012) [pdf] [link]
Cascaded field enhancement in silver dimer nanostructures is investigated using a dipole-dipole interaction model. Field enhancement spectra are evaluated as a function of the particle size difference and inter-particle spacing. We observe three distinct regimes of cascaded field enhancement: hindered cascading, multiplicative cascading, and the ultimate cascading limit, depending on the dimer interaction strength. Multiplicative cascading at small inter-particle spacing leads to analytic expressions for the ultimate internal and external field enhancement factors. For silver dimers in a host with index 1.5 we obtain a maximum internal field enhancement of 2.9E3, a factor of 75 larger than that of an isolated silver nanoparticle.
Cascaded plasmonic metamaterials for phase-controlled enhancement of nonlinear absorption and refraction, Seyfollah Toroghi and Pieter G. Kik, Phys. Rev. B 85, 045432 (2012) [pdf] [link]
The nonlinear optical properties of plasmon resonant metamaterials consisting of chains of metal nanoparticles are evaluated. Introducing particle size differences along the chains leads to the development of cascaded plasmon resonances exhibiting increased field enhancement and field confinement. The interplay between the different resonances on the structures induces a frequency dependent enhancement of the nonlinear refractive and absorptive response of the metamaterial, ultimately providing larger nonlinear susceptibility enhancement factors with engineered complex phase. It is shown that cascaded structures can provide a figure of merit for nonlinear absorption that is more than an order of magnitude larger than that obtained in non-cascaded structures. The presented approach could lead to new planar and integrated nonlinear optical modulation and switching media with improved performance compared to their non-cascaded counterparts.
Taking cascaded plasmonic field enhancement to the ultimate limit in silver nanoparticle dimers, Seyfollah Toroghi and Pieter G. Kik, Proc. SPIE 8457, 84570D (2012) [pdf] [link]
Cascaded optical field enhancement in coupled plasmonic nanostructures has attracted significant attention because of field enhancement factors that dramatically exceed those observed in isolated nanostructures. While previous studies demonstrated the existence of cascaded enhancement, little work has been done to identify the requirements for achieving maximum field enhancement. Here, we investigate cascaded field enhancement in silver nanosphere dimers as a function of volume ratio and center-to-center separation, and show the requirements for achieving the ultimate cascading limit in nanoparticle dimers. We observe field enhancements that are a factor 75 larger than observed in isolated silver nanoparticles.
Voltage controlled nanoparticle plasmon resonance tuning through anodization, Chatdanai Lumdee and Pieter G. Kik, Proc. SPIE 8457, 84570T (2012) [pdf] [link]
Frequency control of plasmon resonances is important for optical sensing applications such as Surface Enhanced Raman Spectroscopy. Prior studies that investigated substrate-based control of noble metal nanoparticle plasmon resonances mostly relied on metal substrates with organic or oxide spacer layers that provided a fixed resonance frequency after particle deposition. Here we present a new approach enabling continuous resonance tuning through controlled substrate anodization. Localized Surface Plasmon tuning of single gold nanoparticles on an Al film is observed in single-particle microscopy and spectroscopy experiments. Au nanoparticles (diameter 60 nm) are deposited on 100 nm thick Al films on silicon. Dark field microscopy reveals Au nanoparticles with a dipole moment perpendicular to the aluminum surface. Subsequently an Al2O3 film is formed with voltage controlled thickness through anodization of the particle coated sample. Spectroscopy on the same particles before and after various anodization steps reveal a consistent blue shift as the oxide thickness is increased. The observed trends in the scattering peak position are explained as a voltage controlled interaction between the nanoparticles and the substrate. The experimental findings are found to closely match numerical simulations. The effects of particle size variation and spacer layer dielectric functions are investigated numerically. The presented approach could provide a post-fabrication frequency tuning step in a wide range of plasmonic devices, could enable the investigation of the optical response of metal nanostructures in a precisely controlled local environment, and could form the basis of chemically stable frequency optimized sensors.
Design of cascaded plasmon resonances for ultrafast nonlinear optical switching, Seyfollah Toroghi and Pieter G. Kik, SPIE 8054, 80540E-1 (2011) [pdf] [link]
The optical properties of cascaded plasmon resonant metallic nanocomposites are investigated. Plasmon resonances and their related field distributions are numerically evaluated in two-dimensional arrays of spherical silver nanoparticles embedded in a dielectric host. The field distributions in structures with identical particle sizes indicate the presence of a largely dipolar particle response, with a small multipole resonance contribution at high frequency. However, in arrays consisting of particles with dissimilar sizes, an additional coupled mode appears in which the dipole moment in adjacent particles is found to be anti-parallel. For increasing size-dissimilarity a higher electric field enhancement is observed inside the small metal nanospheres, indicative of a cascaded field enhancement effect. This effect may be used to enhance the nonlinear optical response of an effective medium composed of particles with engineered size dispersion and particle placement.
Determination of optimum Si excess concentration in Er-doped Si-rich SiO2 for optical amplification at 1.54 um , Oleksandr Savchyn, Kevin R. Coffey, and Pieter G. Kik, Appl. Phys. Lett. 97, 201107 (2010) [pdf] [link]
The presence of indirect Er3+ excitation in Si-rich SiO2 is demonstrated for Si excess concentrations in the range 2.5 – 37 at.%. The Si excess concentration providing the highest density of sensitized Er3+ ions is demonstrated to be relatively insensitive to the presence of Si nanocrystals and is found to be ~ 14.5 at.% for samples without Si nanocrystals (annealed at 600°C) and ~ 11.5 at.% for samples with Si nanocrystals (annealed at 1100°C). The observed optimum is attributed to an increase in the density of Si-related sensitizers as the Si concentration is increased, with subsequent deactivation and removal of these sensitizers at high Si concentrations. The optimized Si excess concentration is predicted to generate maximum Er-related gain at 1.54 μm in devices based on Er-doped Si-rich SiO2.
Frequency dependent power efficiency of a nanostructured surface plasmon coupler, Amitabh Ghoshal and Pieter G. Kik, Phys. Stat. Sol. Rapid Research Letters 4, 280 (2010) [pdf] [link] [cover] [cover info]
Surface plasmon (SP) excitation on an extended thin metal film via a miniature nanoparticle-enhanced grating coupler is studied experimentally using leakage radiation spectroscopy. A universally applicable method for determining the efficiency of free-space SP excitation is developed, and the efficiency of the coupler is determined. Two distinct grating excitation modes are observed, as well as a particle-mediated excitation mode. The maximum observed coupling efficiency of the structure was approximately 3.5% at 615 nm and 670 nm, corresponding to the two grating modes of the structure.
Single particle spectroscopy study of metal-film-induced tuning of silver nanoparticle plasmon resonances, Min Hu, Amitabh Ghoshal, Manuel Marquez, and Pieter G. Kik, J. Phys. Chem. C 114, 7509 (2010) [pdf] [link]
We present an experimental study of the tunability of the silver nanoparticle localized plasmon resonance in close proximity to a gold film. Broadband tuning of the silver particle plasmon resonance from blue wavelengths into the near-IR region can be achieved due to strong electromagnetic coupling between the nanoparticle and the metal film. By altering the thickness of a thin silica spacer layer between the metal nanoparticle and the metal film the resonance frequency can be selected. Single particle spectroscopy of over 250 isolated silver nanoparticles revealed evidence for the excitation of both horizontal and vertical plasmon modes. Distinct resonance features observed in the scattering spectra were assigned to specific modes based on a dipole-dipole interaction model. The experimental results suggest that low-loss silver nanoparticles can be used in surface enhanced spectroscopy studies throughout the entire visible spectrum. The use of frequency tuned spherical metal nanoparticles on solid substrates could lead to thermally stable substrates for plasmon enhanced sensing applications, including surface enhanced Raman scattering and refractive index based biodetection methods.
High temperature optical properties of sensitized Er3+ in Si-rich SiO2 - implications for gain performance , Oleksandr Savchyn, Ravi M. Todi, Kevin R. Coffey, and Pieter G. Kik, Opt. Mat. 32, 1274 (2010) [pdf] [link]
The high-temperature photoluminescence of Er-doped Si-rich SiO2 with and without silicon nanocrystals is studied at sample temperatures in the range 20 – 200oC. The optical properties of Er-doped Si-rich SiO2 with and without silicon nanocrystals are shown to exhibit a similar temperature dependence. Based on the measured photoluminescence intensities and lifetimes it is predicted that an increase of the sample temperature from 20°C to 200oC results in a decrease of the maximum optical gain at 1535 nm by a factor of ~ 1.8 and ~ 1.6 for samples with and without nanocrystals respectively. Implementation of this material in silicon photonics requires stable operation at typical processor case temperatures up to 80 – 90oC. It is demonstrated that increasing the temperature from room temperature to 90°C leads to a predicted maximum optical gain reduction of ~ 1.26 for both materials. In addition, the predicted erbium related optical gain at significant inversion levels in samples processed at low temperature (600°C) is a factor ~ 9 higher than for samples processed at high temperature (1060°C). These findings demonstrate that relatively thermally stable gain performance of the Er-doped Si-rich SiO2 up to typical processor operating temperatures is possible and indicate that low-temperature-processed erbium-doped silicon-rich SiO2 is a technologically viable gain medium for use in silicon photonics.
Near-field enhancement of infrared intensities for f-f transitions in Er3+ ions close to the surface of silicon nanoparticles, Lesya Borowska, Stephan Fritzsche, Pieter G. Kik, and Artem E. Masunov, J. Mol. Model. 17, 423 (2010) [pdf] [link]
Erbium doped waveguide amplifiers can be used in optical integrated circuits to compensate for signal losses. Such amplifiers use stimulated emission from the first excited state (4I13/2) to the ground state (4I15/2) of Er3+ at 1.53 μm, the standard wavelength for optical communication. Since the intra-f transitions are parity forbidden for free Er3+ ions, the absorption and the emission cross sections are quite small for such doped amplifiers. To enhance the absorption, Si nanoclusters can be embedded in silica matrix. Here we investigate the effect of the Si nanocluster on the Er emission using ab initio theory for the first time. We combine multi-reference configuration interaction with one-electron spin-orbit Hamiltonian and relativistic effective core potentials. Our calculations show that the presence of a polarizable Be atom at 5A from the Er3+ ion in a crystalline environment can lead to an enhancement in the emission by a factor of three. The implications of this effect in designing more efficient optical gain materials are discussed.
Excitation wavelength-independent sensitized Er3+ concentration in as-deposited and low temperature annealed Si-rich SiO2 films, Oleksandr Savchyn, Ravi M. Todi, Kevin R. Coffey, Luis K. Ono, Beatriz Roldan Cuenya, and Pieter G. Kik, Appl. Phys. Lett. 95, 231109 (2009) [pdf] [link]
Erbium sensitization is observed in as-deposited Er3+ doped Si-rich SiO2, ruling out the involvement of Si nanocrystals in the Er3+ excitation in these samples. The Er3+ absorption cross section in this material is similar within a factor 3 to that of samples annealed at 600oC under 355nm and 532nm excitation. The density of excitable Er3+ ions is shown to be excitation wavelength independent, while the shape of the Er3+ excitation spectra is governed by a wavelength-dependent Er3+ absorption cross section. These findings enable the use of a broad range of wavelengths for the efficient excitation of this gain medium.
Structural control of nonlinear optical absorption and refraction in dense metal nanoparticle arrays, Dana C. Kohlgraf-Owens and Pieter G. Kik, Opt. Express 17, 15032 (2009) [pdf] [link]
The linear and nonlinear optical properties of a composite containing interacting spherical silver nanoparticles embedded in a dielectric host are studied as a function of interparticle separation using three dimensional frequency domain simulations. It is shown that for a fixed amount of metal, the effective third-order nonlinear susceptibility of the composite X(3) can be significantly enhanced with respect to the linear optical properties, due to a combination of resonant surface plasmon excitation and local field redistribution. It is shown that this geometry-dependent susceptibility enhancement can lead to an improved figure of merit for nonlinear absorption. Enhancement factors for the nonlinear susceptibility of the composite are calculated, and the complex nature of the enhancement factors is discussed.
Observation of temperature-independent internal Er3+ relaxation efficiency in Si-rich SiO2 films, Oleksandr Savchyn, Ravi M. Todi, Kevin R. Coffey, and Pieter G. Kik, Appl. Phys. Lett. 94, 241115 (2009) [pdf] [link]
Time-dependent photoluminescence measurements of low-temperature-annealed Er-doped Si-rich SiO2 were conducted at sample temperatures 15-300K. The erbium internal relaxation efficiency from the second (4I11/2) to the first (4I13/2) excited state upon luminescence-center-mediated Er3+ excitation is investigated. Despite the observation of temperature-dependent relaxation rates, the erbium internal relaxation efficiency is found to be remarkably temperature-independent, which suggests that the internal relaxation efficiency is near-unity. Internal relaxation is shown to account for 50-55% of the 4I13/2 excitation events in the entire temperature range. These results demonstrate that high pump efficiency and stable operation of devices based on this material will be possible under varying thermal conditions.
Excitation of propagating surface plasmons by a periodic nanoparticle array: trade-off between particle-induced near-field excitation and damping,
Amitabh Ghoshal and Pieter G. Kik, Appl. Phys. Lett. 94, 251102 (2009) [pdf] [link]
The excitation of propagating surface plasmons (SPs) on a silver-SiO2 interface by an array of ellipsoidal silver nanoparticles is investigated using numerical simulations as a function of particle volume for three different nanoparticle aspect ratios. We find that while the SP amplitude depends sensitively on particle volume for each selected aspect ratio, the maximum SP amplitude obtained for the different particle shapes is remarkably similar. These observations are explained in terms of particle-mediated SP excitation, counteracted by a size dependent particle-induced damping. An analytical model is presented that quantitatively describes the observed trends in SP damping.
Experimental observation of mode-selective anticrossing in surface-plasmon-coupled metal nanoparticle arrays, Amitabh Ghoshal, Ivan Divliansky, and Pieter G. Kik, Appl. Phys. Lett. 94, 171108 (2009) [pdf] [link]
Surface plasmon excitation using resonant metal nanoparticles is studied experimentally. Geometry dependent reflection measurements reveal the existence of several optical resonances. Strong coupling of the in-plane nanoparticle plasmon resonance and propagating plasmons is evident from clear anticrossing behavior. Reflection measurements at high numerical aperture demonstrate the excitation of surface plasmons via out-of-plane particle polarization. The thus excited plasmons do not exhibit anticrossing in the considered frequency range. The results are explained in terms of the known surface plasmon dispersion relation and the anisotropic frequency dependent nanoparticle polarizability. These findings are important for applications utilizing surface-coupled nanoparticle plasmon resonances.
Multi-level sensitization of Er3+ in low-temperature-annealed silicon-rich SiO2, Oleksandr Savchyn, Ravi M. Todi, Kevin R. Coffey, and Pieter G. Kik, Appl. Phys. Lett. 93, 233120 (2008) [pdf] [link] Abstract
The dynamics of Er3+ excitation in low-temperature-annealed Si-rich SiO2 are studied. It is demonstrated that Si-excess-related indirect excitation is fast (transfer time τtr < 27 ns) and occurs into higher lying Er3+ levels as well as directly into the first excited state (4I13/2). By monitoring the time-dependent Er3+ emission at 1535 nm the multi-level nature of the Er3+ sensitization is shown to result in two types of excitation of the 4I13/2 state: a fast excitation process (τtr < 27 ns) directly into the 4I13/2 level and a slow excitation process due to fast excitation into Er3+ levels above the 4I13/2 level, followed by internal Er3+ relaxation with a time constant τ32 > 2.3 ms. The fast and slow excitation of the 4I13/2 level account for an approximately equal fraction of the excitation events: 45 – 50 % and 50 – 55 % respectively.
Linear and Nonlinear Effective Medium Properties of Metallodielectric Composites of Interacting Spheres and Isolated Spheroids, Dana C. Kohlgraf-Owens and Pieter G. Kik, in Plasmonics and Metamaterials, OSA Technical Digest, paper MThB3 (2008) [pdf] [link] Abstract
We compute the effective medium properties for Kerr-type metallodielectric composites of interacting spheres and isolated spheroids. We show the nonlinear index enhancement increases significantly faster than the linear absorption as particles interact or become elongated.
Numerical study of surface plasmon enhanced nonlinear absorption and refraction, Dana C. Kohlgraf-Owens and Pieter G. Kik, Optics Express 16, 16823 (2008) [pdf] [link] Abstract
Maxwell Garnett effective medium theory is used to study the influence of silver nanoparticle induced field enhancement on the nonlinear response of a Kerr-type nonlinear host. We show that the composite nonlinear absorption coefficient, βc, can be enhanced relative to the host nonlinear absorption coefficient near the surface plasmon resonance of silver nanoparticles. This enhancement is not due to a resonant enhancement of the host nonlinear absorption, but rather due to a phase shifted enhancement of the host nonlinear refractive response. The enhancement occurs at the expense of introducing linear absorption, αc, which leads to an overall reduced figure of merit βc/αc for nonlinear absorption. For thin (<1μm) composites, the use of surface plasmons is found to result in an increased nonlinear absorption response compared to that of the host material.
Effect of hydrogen passivation on luminescence-center-mediated Er excitation in Si rich SiO2 with and without Si nanocrystals, Oleksandr Savchyn, Pieter G. Kik, Ravi M. Todi, and Kevin R. Coffey, Phys. Rev. B 77, 205438 (2008) [pdf] [link] Abstract
The influence of hydrogen passivation on luminescence-center-mediated excitation of Er3+ in Er-doped Si-rich SiO2 films with significantly different microstructures is studied. Photoluminescence measurements are presented for samples containing no detectable silicon nanocrystals (annealed at 600°C) and for samples containing silicon nanocrystals (annealed at 1100°C) as a function of hydrogen passivation temperature. Passivation is found to have little effect on the Er3+ photoluminescence intensity at 1535 nm in the samples that do not contain nanocrystals. In contrast, a pronounced increase in the Er3+ photoluminescence intensity is observed in the samples containing Si nanocrystals, which is accompanied by a similar increase in the nanocrystal photoluminescence intensity and a gradual increase in the Si nanocrystal emission lifetime. This observation is attributed to two interrelated effects, namely, (a) an increase in the density of fully passivated optically active nanocrystals due to the passivation-induced removal of silicon dangling bonds and (b) a concurrent reduction in nonradiative Er3+ relaxation from levels above the 4I13/2 level due to a direct interaction of excited Er3+ ions with silicon dangling bonds. In addition, the observed counterintuitive gradual increase in the nanocrystal photoluminescence decay time upon passivation is successfully explained taking into account a passivation-induced change in the concentration of optically active nanocrystals with different sizes and the inhomogeneous nature of the nanocrystal-related emission band. It is shown that the combination of luminescence-center-mediated Er3+ excitation and silicon-dangling-bond-induced Er3+ de-excitation can explain at least 14 experimental observations reported by independent authors.
Simultaneous excitation of fast and slow surface plasmon polaritons in a high dielectric contrast system, Grady Webb-Wood and Pieter G. Kik, Appl. Phys. Lett. 92, 133101 (2008) [pdf] [link] Abstract
Surface plasmon polaritons propagating in a high dielectric contrast system are investigated numerically. Using frequency domain simulations, we show that a three layer system consisting of air - silicon (7 nm) - silver supports two different modes at the Ag-Si interface: a fast mode, which exhibits normal dispersion, and a slow mode, which exhibits anomalous dispersion. Near the Ag-Si surface plasmon polariton resonance frequency, surface waves with a wavelength of 25 nm are observed at a vacuum wavelength of 595 nm, equivalent to lambda/24. The results show the possibility of exciting surface waves with extreme ultraviolet wavelengths using visible frequencies.
Theory and simulation of surface plasmon excitation using resonant metal nanoparticle arrays, Amitabh Ghoshal and Pieter G. Kik, J. Appl. Phys. 103, 113111 (2008) [pdf] [link] Abstract
We discuss a plasmonic coupling device consisting of an array of ellipsoidal silver nanoparticles embedded in SiO2 and placed near a silver surface. By tuning the shape of the particles in the array, the nanoparticle plasmon resonance is tuned. The resulting resonantly enhanced fields near the nanoparticles in turn excite surface plasmons on the metal film. We have performed Finite Integration Technique simulations of such a plasmon coupler, optimized for operation near a wavelength of 676 nm. Analysis of the frequency dependent electric field at different locations in the simulation volume reveals the separate contributions of the particle and surface resonance to the excitation mechanism. A coupled oscillator model describing the nanoparticle and the metal film as individual resonators is introduced and is shown to reproduce the trends observed in the simulations. Implications of our analysis on the resonantly enhanced excitation of surface plasmons are discussed.
Surface Plasmon Nanophotonics, Mark L. Brongersma and Pieter G. Kik (Eds.), Springer Series in Optical Sciences (2007) [chapter 1] [book website] Abstract
The development of advanced dielectric photonic structures has enabled tremendous control over the propagation and manipulation of light. This book covers an exciting new class of photonic devices, known as surface plasmon nanophotonic structures. Surface plasmons are easily accessible excitations in metals and semiconductors and involve a collective motion of the conduction electrons. These excitations can be exploited to manipulate light in new ways that are impossible in conventional dielectric structures. The field of plasmon nanophotonics is rapidly developing and impacting a wide range of areas including: electronics, photonics, chemistry, biology, and medicine. The book highlights several exciting new discoveries that have been made, and discusses the underlying physics, the nanofabrication issues, and the materials considerations involved in designing plasmonic devices with new functionality.
Luminescence center mediated excitation as the dominant erbium sensitization mechanism in Er-doped silicon-rich SiO2 films, Oleksandr Savchyn, Forrest R. Ruhge, Pieter G. Kik, Ravi M. Todi, Kevin R. Coffey, Haritha Nukala, and Helge Heinrich, Phys. Rev. B 76, 195419 (2007) [pdf] [link] Abstract
The structural and optical properties of erbium-doped silicon-rich silica samples containing 12 atomic % of excess silicon and 0.63 atomic % of erbium are studied as a function of annealing temperature in the range 600 - 1200°C. Indirect excitation of Er3+ ions is shown to be present for all annealing temperatures, including annealing temperatures well below 1000°C for which no silicon nanocrystals are observed. Two distinct efficient (htr > 60%) transfer mechanisms responsible for Er3+ excitation are identified: a fast transfer process ( ttr < 80 ns) involving isolated luminescence centers (LC), and a slow transfer process (ttr ~ 4 - 100 µs) involving excitation by quantum confined excitons inside Si nanocrystals. The LC-mediated excitation is shown to be the dominant excitation mechanism for all annealing temperatures. The presence of a LC-mediated excitation process is deduced from the observation of an annealing- temperature-independent Er3+ excitation rate, a strong similarity between the LC and Er3+ excitation spectra, as well as an excellent correspondence between the observed LC-related emission intensity and the derived Er3+ excitation density for annealing temperatures in the range of 600 - 1000ºC. The proposed interpretation provides an alternative explanation for several observations existing in the literature.
Optimization of surface plasmon excitation using resonant nanoparticle arrays above a silver film, Amitabh Ghoshal and Pieter G. Kik, Proc. SPIE 6641, 664119 (2007) [pdf] [link] Abstract
A plasmonic coupling device consisting of an array of ellipsoidal silver nanoparticles embedded in silica in close proximity to a silver surface is studied. By tuning the inter-particle spacing, the shape of the particles in the array, and the height of the array above the silver film, the array-mediated surface plasmon excitation is studied. Finite Integration Technique simulations of such a plasmon coupler optimized for operation at a free space wavelength of 676 nm are presented. Plane wave normal incidence excitation of the system results is seen to result in resonantly enhanced fields near the nanoparticles, which in turn excite surface plasmons on the metal film. The existence of an optimum particle-surface separation for maximum surface plasmon excitation efficiency is demonstrated. Analysis of the frequency dependent electric field in the simulation volume as a function of particle aspect ratio reveals the influence of the particle resonance and the surface plasmon resonance on the excitation efficiency.
Demonstration of three dimensional imaging of blood vessel using a no moving parts electronic lens-based optical confocal microscope, Nabeel A. Riza, Mumtaz Sheikh, Grady Webb-Wood, and Pieter Kik, Proc. SPIE 6510, 65100J (2007) [pdf] [link] Abstract
To the best of our knowledge, for the first time, biological Three Dimensional (3-D) imaging has been achieved using an electronically controlled optical lens to accomplish no-moving parts depth section scanning in a modified commercial 3-D confocal microscope. Specifically, full 3-D views of a standard CDC blood vessel (enclosed in a glass slide) have been obtained using the modified confocal microscope operating at the red 633 nm laser wavelength.
Optical and morphological properties of MBE grown wurtzite CdxZn1-xO thin films, J.W. Mares, F.R. Ruhge, A.V. Thompson, P.G. Kik, A. Osinsky, B. Hertog, A.M. Dabiran, P.P. Chow, and W.V. Schoenfeld, Opt. Mater. 30, 346 (2007) [pdf] [link] Abstract
Wurtzite CdxZn1-xO epilayers with cadmium concentrations ranging from x = 0.02 to 0.30 were investigated using photoluminescence, transmission / reflection spectroscopy, and atomic force microscopy. The CdxZn1-xO photoluminescence peak was found to shift through the visible region from 421 (2.95 eV) to 619 nm (2.0 eV) as the cadmium concentration was increased from 2% to 30%. An additional broad photoluminescence peak was observed and is attributed to deep levels - the center of the broad peak was found to shift from 675 to 750 nm as the cadmium concentration was increased. RMS roughness of the epilayers increased from 1.5 nm (x = 0.02) to 9.2 nm (x = 0.30), as determined from atomic force microscopy. The demonstrated visible wavelength tunability throughout the visible range verifies the viability of using wurtzite CdxZn1-xO compounds for visible light emission in future optoelectronic devices.
In-situ experimental study of a near-field lens at visible frequencies, Grady Webb-Wood, Amitabh Ghoshal, and Pieter G. Kik, Appl. Phys. Lett. 89, 193110 (2006) [pdf] [link] Abstract
We present frequency-dependent near-field scanning optical microscopy (NSOM) measurements of plasmon mediated near-field focusing using a 50 nm Au film. In these studies the tip aperture of an NSOM probe acts as a localized light source, while the near-field image formed by the metal lens is detected in-situ using nanoscale scatterers placed in the image plane. By scanning the relative position of object and probe we resolve the near-field image generated by the lens. NSOM scans performed at different illumination frequencies reveal an optimum near-field image quality at frequencies close to the localized surface plasmon frequency.
Coherent far-field excitation of surface plasmons using resonantly tuned metal nanoparticle arrays, Amitabh Ghoshal, Grady Webb-Wood, Clarisse Mazuir, and Pieter G. Kik, Proc. SPIE 5927, 592714 (2005) [pdf] [link] Abstract
Recent work in plasmon nanophotonics has shown the successful fabrication of surface plasmon (SP) based optical elements such as waveguides, splitters, and multimode interference devices. These elements enable the development of plasmonic integrated circuits. An important challenge lies in the coupling of conventional far-field optics to such nanoscale optical circuits. To address this coupling issue, we have designed structures that employ local resonances for far-field excitation of SPs. The proposed coupler structure consists of an array of ellipsoidal silver nanoparticles embedded in SiO2 and placed close to a silver surface. To study the performance of the coupler we have performed simulations using the Finite Integration Technique. Our simulations show that normal incidence illumination at a freespace wavelength of 676 nm leads to the resonant excitation of SP oscillations in the Ag nanoparticles, accompanied by coherent near-field excitation of propagating SPs on the Ag film. The excitation efficiency can by maximized by tuning the aspect ratio of the nanoparticles, showing optimum coupling at an aspect ratio of 3.0 with the long axis (75 nm) along the polarization of the excitation signal. We discuss the origin of these observations.
Silicon optical nanocrystal memory, R.J. Walters, P.G. Kik, J.D. Casperson, H.A. Atwater, R. Lindstedt, M. Giorgi, and G. Bourianoff, Appl. Phys. Lett. 85, 2622 (2004) [pdf] Abstract
We describe the operation of a silicon optical nanocrystal memory device. The programmed logic state of the device is read optically by the detection of high or low photoluminescence intensity. The suppression of excitonic photoluminescence is attributed to the onset of fast nonradiative Auger recombination in the presence of an excess charge carrier. The device can be programmed and erased electrically via charge injection and optically via internal photoemission. Photoluminescence suppression of up to 80% is demonstrated with data retention times of up to several minutes at room temperature.
Image resolution of surface-plasmon-mediated near-field focusing with planar metal films in three dimensions using finite-linewidth dipole sources, P.G. Kik, S.A. Maier, and H.A. Atwater, Phys. Rev. B 69, 45418 (2004) [pdf] [link] Abstract
We study the role of surface plasmons in the near-field focusing of a finite-linewidth point dipole by a planar silver film using three-dimensional finite element calculations. We find that the intensity distribution in the image plane at a distance of 60 nm from the source is narrowed by a factor of 1.4 in the presence of a 30-nm-thick silver film. The lateral field components are found to be focused significantly better. We show that the difference is caused by unavoidable stray fields normal to the lens surface due to the interface charge distribution induced by the presence of surface plasmons.
Optical pulse propagation in metal nanoparticle chain waveguides, S.A. Maier, P.G. Kik, and H.A. Atwater, Phys. Rev. B 67, 205402 (2003) [pdf] Abstract
Finite-difference time-domain simulations show direct evidence of optical pulse propagation below the diffraction limit of light along linear arrays of spherical noble metal nanoparticles with group velocities up to 0.06c. The calculated dispersion relation and group velocities correlate remarkably well with predictions from a simple point-dipole model. A change in particle shape to spheroidal particles shows up to a threefold increase in group velocity. Pulses with transverse polarization are shown to propagate with negative phase velocities antiparallel to the energy flow.
Local detection of electromagnetic energy transport below the diffraction limit in metal nanoparticle plasmon waveguides, S.A. Maier, P.G. Kik, H.A. Atwater, S. Meltzer, E. Harel, B.E. Koel, A.A.G. Requicha, Nature Materials 2, 229-232 (2003) [pdf] Abstract
Achieving control of light-material interactions for photonic device applications at nanoscale dimensions will require structures that guide electromagnetic energy with a lateral mode confinement below the diffraction limit of light. This cannot be achieved by using conventional waveguides or photonic crystals. It has been suggested that electromagnetic energy can be guided below the diffraction limit along chains of closely spaced metal nanoparticles that convert the optical mode into non-radiating surface plasmons. A variety of methods such as electron beam lithography and self-assembly have been used to construct metal nanoparticle plasmon waveguides. However, all investigations of the optical properties of these waveguides have so far been confined to collective excitations, and direct experimental evidence for energy transport along plasmon waveguides has proved elusive. Here we present observations of electromagnetic energy transport from a localized subwavelength source to a localized detector over distances of about 0.5 um in plasmon waveguides consisting of closely spaced silver rods. The waveguides are excited by the tip of a near-field scanning optical microscope, and energy transport is probed by using fluorescent nanospheres.
Cooperative upconversion as the gain-limiting factor in Er doped miniature Al2O3 optical waveguide amplifiers, P.G. Kik and A. Polman, J. Appl. Phys. 93, 5008 (2003) [pdf] [link] Abstract
Erbium doped Al2O3 waveguide amplifiers were fabricated using two different doping methods, namely Er ion implantation into sputter deposited Al2O3, and co-sputtering from an Er2O3 / Al2O3 target. Although the Er concentration in both materials is almost identical ~0.28 and 0.31 at.%, the amplifiers show a completely different behavior. Upon pumping with 1.48 um, the co-sputtered waveguide shows a strong green luminescence from the 4S3/2 level, indicating efficient cooperative upconversion in this material. This is confirmed by pump power dependent measurements of the optical transmission at 1.53 um and the spontaneous emission at 1.53 and 0.98 um. All measurements can be accurately modeled using a set of rate equations that include first order and second order cooperative upconversion. The first order cooperative upconversion coefficient C24 is found to be 3.5x10-16 cm3/s in the co-sputtered material, two orders of magnitude higher than the value obtained in Er implanted Al2O3 of 4.1x10^-18 cm3/s. It is concluded that the co-sputtering process results in a strongly inhomogeneous atomic scale spatial distribution of the Er ions. As a result, the co-sputtered waveguides do not show optical gain, while the implanted waveguides do.
Towards an Er-doped Si nanocrystal sensitized waveguide laser, P.G. Kik, and A.Polman, Proceedings NATO Workshop OASIS (2002) [pdf] Abstract
Important progress is being made in the development of a Si based waveguide laser operating at 1.5 um. The gain medium responsible for the recent progress is Er-doped Si nanocrystal co-doped SiO2, a composite material that can potentially be fabricated using a VLSI compatible process. The material combines the broad absorption spectrum of Si nanocrystals with the efficient narrow linewidth emission of Er ions. This combination promises to enable the fabrication of a broadband pumped integrated optical amplifier or laser in a Si based materials system. In this paper we systematically discuss the applicability of Si nanocrystals to serve as sensitizers for Er, relating the available data to key sensitizer requirements.
Metal nanoparticle arrays for near field optical lithography, P.G. Kik, S.A. Maier, and H.A. Atwater, SPIE Proceedings (2002) [pdf] Abstract
We have recently proposed a new approach to optical lithography that could be used to replicate arrays of metal nanoparticles using broad beam illumination with visible light and standard photoresist. The method relies on resonant excitation of the surface plasmon oscillation in the nanoparticles. When excited at the surface plasmon frequency, a resonantly enhanced dipole field builds up around the nanoparticles. This dipole field is used to locally expose a thin layer of photoresist, generating a replica of the original pattern in the resist. Silver nanoparticles on photoresist can be resonantly excited at wavelengths ranging from 410 nm to 460 nm, allowing for resonantly enhanced exposure of standard g-line photoresist. Finite Difference Time Domain (FDTD) simulations of isolated silver particles on a thin resist layer show that broad beam illumination with p-polarized light at a wavelength of 439 nm can produce features as small as 30 nm, or λ/14. Depending on exposure time lateral spot sizes ranging from 30 to 80 nm with exposure depths ranging from 12 to 45 nm can be achieved. We discuss the effect of particle-particle interactions in the replica formation process. Experiments on low areal density Ag nanoparticle arrays are discussed. Resist layers (thickness 75 nm) in contact with 40 nm Ag nanoparticles were exposed using 410 nm light and were subsequently developed. Atomic Force Microscopy on these samples reveals nanoscale depressions in the resist, providing evidence for plasmon-enhanced resist exposure.
Observation of coupled plasmon-polariton modes in Au nanoparticle chain waveguides of different lengths: Estimation of waveguide loss,
S. A. Maier, P. G. Kik, and H. A. Atwater, Appl. Phys. Lett. 81, 1714 (2002) [pdf] Abstract
Near-field interactions between closely spaced Au nanoparticles were characterized by studying the spectral position of the extinction bands corresponding to longitudinal (L) and transverse (T) plasmon-polariton modes of Au nanoparticle chains. Far-field spectroscopy and finite-difference time-domain simulations on arrays of 50 nm diameter Au spheres with an interparticle spacing of 75 nm both show a splitting dE between the L and T modes that increases with chain length and saturates at a length of seven particles at dE=65 meV. We show that the measured splitting will result in a propagation loss of 3 dB/15 nm for energy transport. Calculations indicate that this loss can be reduced by at least one order of magnitude by modifying the shape of the constituent particles.
Observation of near-field coupling in metal nanoparticle chains using far-field polarization spectroscopy, S. A. Maier, M. L. Brongersma, P. G. Kik, and H. A. Atwater, Phys. Rev. B 65, 193408 (2002) [pdf] Abstract
Far-field polarization spectroscopy on chains of Au nanoparticles reveals the existence of longitudinal (L) and transverse (T) plasmon-polariton modes. The experimental results provide support for the validity of a recently published dipole model for electromagnetic energy transfer below the diffraction limit along chains of closely spaced metal nanoparticles. The key parameters that govern the energy transport are determined for various interparticle spacings using measurements of the resonance frequencies of L and T modes, yielding a bandwidth of 1.4 x 10^14 rad/s and a maximum group velocity of vg=4.0 x 10^6 m/s for a 75 nm-spacing.
Plasmon printing - a new approach to near-field lithography, P.G. Kik, S.A. Maier, and H.A. Atwater, Mat. Res. Soc. Symp. Proc. 705, Y3.6 (2002) [pdf] Abstract
We propose a method for replicating patterns with a resolution well below the diffraction limit, using broad beam illumination and standard photoresist. In particular it is shown that visible exposure (λ=410 nm) of silver nanoparticles in close proximity to a thin film of g-line resist (AZ1813) can produce selectively exposed areas with a diameter smaller than λ/20. The technique relies on the local field enhancement around metal nanostructures when illuminated at the surface plasmon resonance frequency. The method can be extended to various metals, photosensitive layers, and particle shapes.
Design and Performance of an Erbium-Doped Silicon Waveguide Detector Operating at 1.5 um, P. G. Kik, A. Polman, S. Libertino, and S. Coffa, J. Lightwave Technol. 20, 862 (2002) [pdf] Abstract
A new concept for an infrared waveguide detector based on silicon is introduced. It is fabricated using silicon-on-insulator material, and consists of an erbium-doped p-n junction located in the core of a silicon ridge waveguide. The detection scheme relies on the optical absorption of 1.5 um light by Er3+ ions in the waveguide core, followed by electron-hole pair generation by the excited Er and subsequent carrier separation by the electric field of the p-n junction. By performing optical mode calculations and including realistic doping profiles, we show that an external quantum efficiency of 10^-3 can be achieved in a 4-cm-long waveguide detector fabricated using standard silicon processing. It is found that the quantum efficiency of the detector is mainly limited by free carrier absorption in the waveguide core, and may be further enhanced by optimizing the electrical doping profiles. Preliminary photocurrent measurements on an erbium-doped Si waveguide detector at room temperature show a clear erbium related photocurrent at 1.5 um.
Gain limiting processes in Er-doped Si nanocrystal waveguides in SiO2,
P. G. Kik and A. Polman, J. Appl. Phys. 91, 534 (2002) [pdf] [link] Abstract
Erbium-doped Si nanocrystal based optical waveguides were formed by Er and Si ion implantation into SiO2 . Optical images of the waveguide output facet show a single, well-confined optical mode. Transmission measurements reveal a clear Er related absorption of 2.7 dB/cm at 1.532 um, corresponding to a cross section of 8x10^-20 cm2. The Si nanocrystals act as sensitizers for Er but under high doping conditions (50 Er ions per nanocrystal) no pump-induced change in the Er related absorption is observed under optical pumping (λ=458 nm), which is ascribed to an Auger quenching effect. For very high pump powers, a broad absorption feature is observed, attributed to free carrier absorption.
Plasmonics - A route to nanoscale optical devices,
S.A. Maier, M.L. Brongersma, P.G. Kik, S. Meltzer, A.A.G. Requicha, and H.A. Atwater, Adv. Mater. 13, 1501 (2001) [pdf] Abstract
The further integration of optical devices will require the fabrication of waveguides for electromagnetic energy below the diffraction limit of light. We investigate the possibility of using arrays of closely spaced metal nanoparticles for this purpose. Coupling between adjacent particles sets up coupled plasmon modes that give rise to coherent propagation of energy along the array. A point dipole analysis predicts group velocities of energy transport that exceed 0.1c along straight arrays and shows that energy transmission and switching through chain networks such as corners (see Figure) and tee structures is possible at high efficiencies. Radiation losses into the far field are expected to be negligible due to the near-field nature of the coupling, and resistive heating leads to transmission losses of about 6 dB/lm for gold and silver particles. We analyze macroscopic analogues operating in the microwave regime consisting of closely spaced metal rods by experiments and full field electrodynamics simulations. The guiding structures show a high confinement of the electromagnetic energy and allow for highly variable geometries and switching. Also, we have fabricated gold nanoparticle arrays using electron beam lithography and atomic force microscopy manipulation. These plasmon waveguides and switches could be the smallest devices with optical functionality.
Pumping planar waveguide amplifiers using a coupled waveguide system,
L.H. Slooff, P.G. Kik, A. Tip, A. Polman, J. Lightwave Technol. 19, 1740 (2001) [pdf] Abstract
A novel scheme is presented that can be used to efficiently pump optical waveguide amplifiers. It is based on the coupling between two adjacent waveguides, where pump light is gradually coupled from a nonabsorbing pump waveguide into the amplifier waveguide. The coupling between the waveguides in such a configuration is calculated using an improved coupled mode theory (CMT). The proposed distributed coupling scheme can enhance the optical gain in systems that exhibit a reduced pumping efficiency at high pump power. A numerical example is given for a sensitized neodymium-doped polymer waveguide amplifier, in which the optical gain increases from 0.005 dB to 1.6 dB by changing from conventional butt-coupling to distributed coupling.
Exciton-erbium energy transfer in Si nanocrystal-doped SiO2,
P.G. Kik, and A. Polman, Mat. Sc. Eng. B 81, 3 (2001) [pdf] Abstract
Silicon nanocrystals were formed in SiO2 using Si ion implantation followed by thermal annealing. The nanocrystal-doped SiO2 layer was implanted with Er to peak concentrations ranging from 0.015 to 1.8 at.%. Upon 458 nm excitation, a broad nanocrystal-related luminescence spectrum centered around 750 nm and two sharp Er luminescence lines at 982 and 1536 nm are observed. By measuring the temperature-dependent intensities and luminescence dynamics at a fixed Er concentration, and by measuring the Er concentration dependence of the nanocrystal and Er photoluminescence intensity, the nanocrystal excitation rate, the Er excitation and decay rate, and the Er saturation with pump power we conclude that: (1) the Er is excited by excitons recombining within Si nanocrystals through a strong coupling mechanism; (2) the exciton-Er energy transfer rate is >10^6 s^-1; (3) the exciton-Er energy transfer efficiency is >60 %; (4) each nanocrystal can have at most _1-2 excited Er ions in its vicinity, which is attributed to either an Auger de-excitation or a pair-induced quenching mechanism; (5) at a typical nanocrystal concentration of 10^19 cm^-3, the maximum optical gain at 1.54 um of an Er-doped waveguide amplifier based on Si nanocrystal-doped SiO2 is _0.6 dB cm_1; (6) the effective Er excitation cross-section using this nanocrystal sensitization scheme is s_eff = 10^-15 cm^2 at 458 nm, which is a factor 10^5-10^6 larger than the cross-section for direct optical pumping of Er. This enables the fabrication of an Er-doped nanocrystal waveguide amplifier that can be pumped using a white light source.
Energy transfer in erbium doped optical waveguides based on silicon,
P.G. Kik, Ph.D. Thesis, Utrecht University, The Netherlands (2000) [pdf]
Energy backtransfer and infrared photoresponse in erbium doped silicon p-n diodes,
N. Hamelin, P.G. Kik, and A. Polman, J. Appl. Phys. 88, 5381 (2000) [pdf] Abstract
Temperature-dependent measurements of the photoluminescence (PL) intensity, PL lifetime, and infrared photocurrent, were performed on an erbium-implanted silicon p - n junction in order to investigate the energy transfer processes between the silicon electronic system and the Er 4f energy levels. The device features excellent light trapping properties due to a textured front surface and a highly reflective rear surface. The PL intensity and PL lifetime measurements show weak temperature quenching of the erbium intra-4f transition at 1.535 um for temperatures up to 150 K, attributed to Auger energy transfer to free carriers. For higher temperatures, much stronger quenching is observed, which is attributed to an energy backtransfer process, in which Er deexcites by generation of a bound exciton at an Er-related trap. Dissociation of this exciton leads to the generation of electron-hole pairs that can be collected as a photocurrent. In addition, nonradiative recombination takes place at the trap. It is shown for the first time that all temperature-dependent data for PL intensity, PL lifetime, and photocurrent can be described using a single model. By fitting all temperature-dependent data simultaneously, we are able to extract the numerical values of the parameters that determine the (temperature-dependent) energy transfer rates in erbium-doped silicon. While the external quantum efficiency of the photocurrent generation process is small (1.8x10^-6) due to the small erbium absorption cross section and the low erbium concentration, the conversion of Er excitations into free e - h pairs occurs with an efficiency of 70% at room temperature.
Selective modification of the Er3+ 4I11/2 branching ratio by energy transfer to Eu3+, C. Strohhöfer, P.G. Kik, and A. Polman, J. Appl. Phys. 88, 4486 (2000) [pdf] Abstract
We present an investigation of Er3+ photoluminescence in Y2O3 waveguides codoped with Eu3+. As a function of europium concentration we observe an increase in decay rate of the erbium 4I11/2 energy level and an increase of the ratio of photoluminescence emission from the 4I13/2 and 4I11/2 states. Using a rate equation model, we show that this is due to an energy transfer from the 4I11/2 to 4I13/2 transition in erbium to europium. This increases the branching ratio of the 4I11/2 state towards the 4I13/2 state and results in a higher steady state population of the first excited state of erbium. Absolute intensity enhancement of the 4I13/2 emission is obtained for europium concentrations between 0.1 and 0.3 at. %. In addition, the photoluminescence due to upconversion processes originating from the 4I11/2 state is reduced. Using such state-selective energy transfer the efficiency of erbium doped waveguide amplifiers can be increased.
Exciton-erbium interactions in Si nanocrystal-doped SiO2,
P.G. Kik and A. Polman, J. Appl. Phys. 88, 1992 (2000) [pdf] [link] Abstract
The presence of silicon nanocrystals in Er doped SiO2 can enhance the effective Er optical absorption cross section by several orders of magnitude due to a strong coupling between quantum confined excitons and Er. This article studies the fundamental processes that determine the potential of Si nanocrystals as sensitizers for use in Er doped waveguide amplifiers or lasers. Silicon nanocrystals were formed in SiO2 using Si ion implantation and thermal annealing. The nanocrystal-doped SiO2 layer was implanted with different doses of Er, resulting in Er peak concentrations in the range 0.015-1.8 at. %. All samples show a broad nanocrystal-related luminescence spectrum centered around 800 nm and a sharp Er luminescence line at 1536 nm. By varying the Er concentration and measuring the nanocrystal and Er photoluminescence intensity, the nanocrystal excitation rate, the Er excitation and decay rate, and the Er saturation with pump power, we conclude that: (a) the maximum amount of Er that can be excited via exciton recombination in Si nanocrystals is 1-2 Er ions per nanocrystal, (b) the Er concentration limit can be explained by two different mechanisms occurring at high pump power, namely Auger de-excitation and pair-induced quenching, (c) the excitable Er ions are most likely located in an SiO2-like environment, and have a luminescence efficiency <18%, and (d) at a typical nanocrystal concentration of 10^19 cm^-3, the maximum optical gain at 1.54 um of an Er-doped waveguide amplifier based on Si nanocrystal-doped SiO2 is ~0.6 dB/cm.
Strong exciton-erbium coupling in Si nanocrystal-doped SiO2,
P.G. Kik, M.L. Brongersma, A. Polman, Appl. Phys. Lett. 76, 2325 (2000) [pdf] Abstract
Silicon nanocrystals were formed in SiO2 using Si ion implantation followed by thermal annealing. The nanocrystal-doped SiO2 layer was implanted with Er to a peak concentration of 1.8 at. %. Upon 458 nm excitation the sample shows a broad nanocrystal-related luminescence spectrum centered around 750 nm and two sharp Er luminescence lines at 982 and 1536 nm. By measuring the excitation spectra of these features as well as the temperature-dependent intensities and luminescence dynamics we conclude that (a) the Er is excited by excitons recombining within Si nanocrystals through a strong coupling mechanism, (b) the Er excitation process at room temperature occurs at a submicrosecond time scale, (c) excitons excite Er with an efficiency >55%, and (d) each nanocrystal can have at most ~1 excited Er ion in its vicinity.
Size-dependent electron-hole exchange interaction in Si nanocrystals,
M.L. Brongersma, P.G. Kik, A. Polman, K.S. Min. and H.A. Atwater, Appl. Phys. Lett. 76, 351 (2000) [pdf] Abstract
Silicon nanocrystals with diameters ranging from ~2 to 5.5 nm were formed by Si ion implantation into SiO2 followed by annealing. After passivation with deuterium, the photoluminescence (PL) spectrum at 12 K peaks at 1.60 eV and has a full width at half maximum of 0.28 eV. The emission is attributed to the recombination of quantum-confined excitons in the nanocrystals. The temperature dependence of the PL intensity and decay rate at several energies between 1.4 and 1.9 eV was determined between 12 and 300 K. The temperature dependence of the radiative decay rate was determined, and is in good agreement with a model that takes into account the energy splitting between the excitonic singlet and triplet levels due to the electron-hole exchange interaction. The exchange energy splitting increases from 8.4 meV for large nanocrystals (~5.5 nm) to 16.5 meV for small nanocrystals (~2 nm). For all nanocrystal sizes, the radiative rate from the singlet state is 300-800 times larger than the radiative rate from the triplet state.
Optical and electrical doping of silicon with holmium,
J.F. Suyver, P.G. Kik, T. Kimura, A. Polman, G. Franzò, S. Coffa, Nucl. Instrum. Meth. B 148, 497 (1999) [pdf] Abstract
2 MeV holmium ions were implanted into Czochralski grown Si at a fluence of 5.5x10^14 Ho/cm^2. Some samples were co-implanted with oxygen to a concentration of (7+/-1)x10^19 cm^-3. After recrystallization, strong Ho segregation to the surface is observed, which is fully suppressed by co-doping with O. After recrystallization, photoluminescence peaks are observed at 1.197, 1.96 and 2.06 um, characteristic for the 5I6 to 5I8 and 5I7 to 5I8 transitions of Ho3+. The Ho3+ luminescence lifetime at 1.197 lm is 14 ms at 12 K. The luminescence intensity shows temperature quenching with an activation energy of 11 meV, both with and without O co-doping. The observed PL quenching cannot be explained by free carrier Auger quenching, but instead must be due to energy backtransfer or electron hole pair dissociation. Spreading resistance measurements indicate that Ho exhibits donor behavior, and that in the presence of O the free carrier concentration is enhanced by more than two orders of magnitude. In the O co-doped sample 20% of the Ho3+ was electrically active at room temperature.
Erbium doped optical-waveguide amplifiers on silicon,
P.G. Kik, A. Polman, Mat. Res. Soc. Bull. 23, 48 (1998) [pdf] [link]
Erbium-doped phosphate glass waveguide on silicon with 4.1 dB/cm gain at 1.535 mm, Y.C. Yan, A.J. Faber, H. de Waal, P.G. Kik, and A. Polman, Appl. Phys. Lett. 71, 2922 (1997) [pdf] Abstract
Erbium-doped multicomponent phosphate glass waveguides were deposited by rf sputtering techniques. The Er concentration was 5.3x10^20 cm^-3. By pumping the waveguide at 980 nm with a power of ~21 mW, a net optical gain of 4.1 dB at 1.535 um was achieved. This high gain per unit length at low pump power could be achieved because the Er-Er cooperative upconversion interactions in this heavily Er-doped phosphate glass are very weak [the upconversion coefficient is (2.0 +/- 0.5)x10^-18 cm^3/s], presumably due to the homogeneous distribution of Er in the glass and due to the high optical mode confinement in the waveguide which leads to high pump power density at low pump power.
Excitation and de-excitation of Er3+ in crystalline silicon,
P.G. Kik, M.J.A. de Dood, K. Kikoin, and A. Polman, Appl. Phys. Lett. 70, 1721 (1997) [pdf] Abstract
Temperature dependent measurements of the 1.54 um photoluminescence of Er implanted N codoped crystalline Si are made. Upon increasing the temperature from 12 to 150 K, the intensity quenches by more than a factor thousand, while the lifetime quenches from 420 to 3 us. The quenching processes are described by an impurity Auger energy transfer model that includes bound exciton dissociation and a nonradiative energy backtransfer process. Electron and hole trap levels are determined. Direct evidence for a backtransfer process follows from spectral response measurements on an Er-implanted Si solar cell.
Incorporation, excitation, and de-excitation of erbium in crystal silicon,
M.J.A. de Dood, P.G. Kik, J.H. Shin, and A. Polman, Mat. Res. Soc. Symp. Proc. 422, 219 (1996)
Concentration quenching in erbium implanted alkali silicate glasses, E. Snoeks, P.G. Kik, A. Polman, Opt. Mater. 5, 159 (1996) [pdf] Abstract
A comparison is made of photoluminescence properties of six sodalime and alkali-borosilicate glasses implanted with Er to concentrations as high as 1.4 × 10^21 at/cm3. Clear photoluminescence (PL) spectra around 1.54 um, due to the 4I13/2 -> 4I15/2 transition in Er3+ are observed, of which the shape depends on the host glass composition. PL lifetimes in the range of 0.9-12.6 ms are found, depending on glass and Er concentration. In borosilicate glass, implantation-induced defects remain after annealing and cause quenching of the Er luminescence due to a direct coupling to the Er. Such defects are not present in Er-implanted sodalime glass after annealing. In both types of glass the luminescence lifetime decreases strongly with concentration due to a concentration quenching effect in which energy migration takes place due to energy transfer between Er ions, followed by quenching at hydroxyl groups. Concentration quenching via this mechanism is less strong in the borosilicates than in the sodalime glasses, but because of the quenching effect of implantation-induced defects in borosilicates these glasses are not suitable for optical doping by ion implantation.
Ion beam synthesis of planar opto-electronic devices,
A. Polman, E. Snoeks, G.N. van den Hoven, M.L. Brongersma, R. Serna, J.H. Shin, P.G. Kik, E. Radius, Nucl. Instrum. Meth. B 106, 393 (1995) [pdf] Abstract
Photonic technology requires the modification and synthesis of new materials and devices for the generation, guiding, switching, multiplexing and amplification of light. This paper reviews how some of these devices may be made using ion beam synthesis. Special attention is paid to the fabrication of erbium-doped optical waveguides.