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Publications
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 Surface Plasmon Nanophotonics, Mark L. Brongersma and Pieter G. Kik (Eds.), Springer Series in Optical Sciences (2007) [chapter 1] [book website]
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.
NEW Numerical study of surface plasmon enhanced nonlinear absorption and refraction, Dana C. Kohlgraf-Owens and Pieter G. Kik, Optics Express 16, 16823 (2008) [download] [link]
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.
NEW 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) [download] [link]
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.
NEW 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) [download] [link]
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.
NEW Theory and simulation of surface plasmon excitation using resonant metal nanoparticle arrays, Amitabh Ghoshal and Pieter G. Kik, J. Appl. Phys. 103, 113111 (2008) [download] [link]
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.
NEW 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) [download] [link]
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) [download]
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) [download]
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) [download]
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) [download]
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) [download]
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) [download]
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) [download]
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) [download]
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) [download]
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) [download]
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) [download]
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) [download]
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) [download]
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) [download]
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) [download]
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) [download]
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) [download]
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) [download]
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) [download]
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) [download]
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) [download]
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) [download]
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) [download]
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.
Infrared detector device of semiconductor material and manufacturing process thereof, A. Polman, N. Hamelin, P.G. Kik, S. Coffa, M. Saggio, and F. Frisina, European Patent 98830592.6, US Patent 09/415,022
Exciton-erbium interactions in Si nanocrystal-doped SiO2, P.G. Kik and A. Polman, J. Appl. Phys. 88, 1992 (2000) [download]
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) [download]
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) [download]
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) [download]
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) [download]
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) [download]
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) [download]
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) [download]
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 4I(13/2) -> 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) [download]
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.