KEYWORDS: Atomic force microscopy, Silicon, Semiconductor lasers, Near field optics, Mode locking, Waveguides, Near field, Mirrors, Active optics, Raman spectroscopy
Optical characterization at the nanoscale currently requires instruments such as NSOM/TERS, or hybrid AFM with specialized far-field optical microscopes that are quite complicated and do not provide any time-resolved data. We have demonstrated a novel class of probes for Scanning Probe Microscopy (SPM) - an Ultrafast Pulsed Atomic Force Microscopy Optical Probe (UFP AAOP) that will enhance characterization capabilities at the nanoscale and provide an exciting opportunity for obtaining both space- and time-resolved chemical information simultaneously. In the UFP AAOP design, a two-section quantum-dot mode-locked laser is monolithically integrated with an SPM probe fabricated from GaAs, with a nanoscale opening at the apex of the tip as the output aperture. With UFP AAOP, the light is supplied through the tip; hence, there is no scattered far-field light and thus significantly reduced background. Furthermore, the difficulties associated with laser alignment onto the tip and with imaging the signal onto a detector are avoided with the UFP AAOP. The UFP AAOP provides pulses with less than 4 ps duration and spatial resolution better than 300 nm at 1240 nm wavelength. It is potentially possible to reduce the pulse width to ~ 0.3 ps and to improve lateral resolution to ~ 1 nm. These unique optical probes will perform the functions of conventional AFM probes and simultaneously provide information about chemical properties of the sample at the nanoscale together with time-resolved spectroscopy. UFP AAOP will facilitate the creation of a new microscopy/spectroscopy instrument with combined single-molecule spatial resolution and ultrafast time-resolved capability.
Localized surface plasmon resonances can increase the quantum efficiency of photon emitters through both absorption and spontaneous emission enhancement effects. Despite extensive studies, experimental results that clearly distinguish the two plasmonic enhancement effects are rarely available. Here, we present clear spectral signatures of the plasmonic enhancement effects on the absorption (excitation) and spontaneous emission (Purcell factor) by analyzing the temperature dependent photoluminescence (PL) properties of InGaAs/GaAs single quantum well (QW) coupled to colloidal gold nanorods (AuNRs) at different GaAs capping layer thickness (d). We find that when the emitting InGaAs layer is close to the AuNRs (d = 5 nm), the plasmonic enhancement effect on the QW PL is dominated by the Purcell factor that significantly increases the external quantum efficiency of the QW that otherwise barely emits. When d is increased to 10 nm, the temperature dependence of the PL enhancement factor (F) reflects absorption enhancement in the capping layer followed by carrier diffusion and capture by the well. First F increases with temperature and then decreases following the temperature dependence of the carrier diffusion coefficient in GaAs. By factoring out the contribution of the captured carriers to F, it is shown that carrier transfer to the well reaches saturation with increasing incident laser power. In addition to providing insight into the plasmonic enhancement mechanism, the results presented in this work suggest that colloidal plasmonic nanoparticles can be used as simple probes for understanding carrier transport phenomena in arbitrary semiconductor heterostructures.
Actoprobe team had developed custom Tip Enhancement Raman Spectroscopy System (TERS) with specially developed Ultra High Aspect Ratio probes for AFM and TERS measurements for small pixel infrared FPA sidewall characterization. Using this system, we report on stimulated Raman scattering observed in a standard tip-enhanced Raman spectroscopy (TERS) experiment on GaSb materials excited by 637-nm pump laser light. We explain our results by TERS-inherent mechanisms of enormous local field enhancement and by the special design and geometry of the ultrahigh-aspect-ratio tips that enabled conditions for stimulated Raman scattering in the sample with greatly enhanced resonance Raman gain when aided by a microcavity to provide feedback mechanism for the Raman emission. The approach has great potential for further, orders-of-magnitude, progress in TERS enhancement by significantly increasing its nonlinear component. We report development of novel class of probes for atomic force microscopy (AFM active optical probe - AAOP) by integrating a laser source and a photodetector monolithically into the AFM probe. The AAOPs are designed to be used in a conventional AFM and would enhance its functionality to include that of the instruments (NSOM, TERS, hybrid AFM).
We propose to utilize confocal Raman spectroscopy combined with high resolution atomic force microscopy (AFM) for nondestructive characterisation of the sidewalls of etched and passivated small pixel (24 μm×24 μm) focal plane arrays (FPA) fabricated using LW/LWIR InAs/GaSb type-II strained layer superlattice (T2SL) detector material. Special high aspect ratio Si and GaAs AFM probes, with tip length of 13 μm and tip aperture less than 7°, allow characterisation of the sidewall morphology. Confocal microscopy enables imaging of the sidewall profile through optical sectioning. Raman spectra measured on etched T2SL FPA single pixels enable us to quantify the non-uniformity of the mesa delineation process.
We compare an InAs quantum dot (QD) vertical external-cavity surface-emitting laser (VECSEL) design consisting of 4
groups of 3 closely spaced QD layers with a resonant periodic gain (RPG) structure, where each of the 12 QD layers is
placed at a separate field antinode. This increased the spacing between the QDs, reducing strain and greatly improving
device performance. For thermal management, the GaAs substrate was thinned and indium bonded to CVD diamond. A
fiber-coupled 808 nm diode laser was used as pump source, a 1% transmission output coupler completed the cavity. CW
output powers over 4.5 W at 1250 nm were achieved.
External quantum efficiency of semiconductor photonic devices is directly measured by
wavelength-dependent laser-induced temperature change (scanning laser calorimetry) with very high
accuracy. Maximum efficiency is attained at an optimum photo-excitation level that can be determined with
an independent measurement of power-dependent photoluminescence. Differential power-dependent
photoluminescence measurement is used to quickly screen the sample quality before processing.
We characterize high quantum efficiency double GaAs/InGaP heterostructures used in semiconductor laser cooling. To
identify potential samples for laser cooling, measuring the nonradiative recombination rate coefficient is necessary. We
describe a technique called power dependent photoluminescence measurement, which when combined with timeresolved
photoluminescence lifetime determines the nonradiative recombination coefficient.
Laser cooling of a semiconductor has been an elusive but highly desirable goal for several years. Although it is
theoretically possible, tedious and often time-consuming sample preparation, processing and testing has slowed
the progress on the experimental end. The work presented here focuses on a new approach to the first step, the
growth of high quality starting samples by molecular beam epitaxy (MBE).
MBE is believed to have an inherent advantage over chemical vapor deposition techniques since typically
material with higher purity can be grown by MBE, thereby reducing the chance for parasitic absorption and nonradiative
recombinations to occur. Additionally, with MBE very precise control over interfaces is possible,
where a significant portion of the non-radiative traps are usually located. The most promising material for laser
cooling is the binary compound GaAs. The lattice-matched material Ga0.515In0.485P is chosen for passivating the
surface as it has shown much longer radiative lifetimes in GaAs than, for example, AlxGa1-xAs. The present
study focuses on growth optimization of Ga0.515In0.485P/GaAs/Ga0.515In0.485P heterostructures and the influence of
growth conditions on sample suitability for laser cooling as measured by non-radiative lifetimes in GaAs. In
particular, parameters such as growth temperature, group V:III overpressure, substrate orientation, doping, and
interface composition on a monolayer length scale are varied and analyzed. The suitability of an optimized
sample for semiconductor laser cooling is discussed.
One of the challenges of laser cooling a semiconductor is its typically high index of refraction (greater than 3), which
limits efficient light output of the upconverted photon. This issue is addressed with a novel concept of coupling the
photon out via a thin, thermally insulating vacuum gap that allows light to pass efficiently by frustrated internal
reflection.
Although silicon technology is mature and inexpensive, the indirect nature of the bandgap of silicon makes it unsuitable
for laser cooling. The material of choice is the binary compound semiconductor GaAs, which can be fabricated with high
quality necessary for laser cooling experiments. Moreover, process technology exists that enables a relatively simple
fabrication of a thin vacuum gap in this material system.
This paper will present an investigation of heat transport and light transmission across a "nanogap" consisting of a thin
epitaxial film supported over a substrate by an array of nanometer-sized posts. The structure is manufactured by crystal
growth of a sacrificial Al0.98Ga0.02As layer on a single crystal GaAs substrate. After lithographically defining holes in the
Al0.98Ga0.02As layer, the holes are filled with GaAs and a top GaAs layer is deposited. Lateral selective etching of the
Al0.98Ga0.02As will create a nanogap between two GaAs layers separated by GaAs posts. We are demonstrating the
successful fabrication of various size nanogaps in this material system, as well as their properties with respect to reduced
heat transfer across the gap. We are also presenting data supporting that the interface quality is high enough to allow
evanescent tunneling of light at angles otherwise forbidden by total internal reflection. The implications for
semiconductor laser cooling will be discussed.
Doping of the clad layers in thin GaAs/GaInP heterostructures, displaces the band energy discontinuity, modifies
the carrier concentration in the active GaAs region and changes the quality of the hetero-interfaces. As a result,
internal and consequently external quantum efficiencies in the double heterostructure are affected. In this paper,
the interfacial quality of GaAs/GaInP heterostructure is systematically investigated by adjusting the doping
level and type (n or p) of the cladding layer. An optimum structure for laser cooling applications is proposed.
Understanding and quantifying nonradiative recombination is a critical factor for the successful laser cooling of semiconductors. The usual approach to measuring the nonradiative lifetime employs pulsed photoexcitation and monitors the luminescence decay via time-resolved photon counting. We present an alternative approach that
employs phase fluorometry with a lock-in amplifier. A sinusoidally modulated diode laser is used for excitation. Lifetime data are extracted from the frequency dependent phase shift and amplitude response of the photolumi-nescence signal, detected by a photomultiplier tube. Samples studied include high quality AlGaAs/GaAs/AlGaAs and GaInP/GaAs/GaInP double heterostructures, grown by MBE and MOCVD. Data over a temperature range from 10 to 300 K is compared with results obtained in time-domain measurements.
One of the challenges of laser cooling a semiconductor is the typically high index of refraction (greater than 3), which limits efficient light output of the upconverted photon. This challenge is proposed to be met with a novel concept of coupling the photon out via a thin, thermally insulating vacuum gap that allows light to pass efficiently by frustrated total internal reflection. This study has the goal of producing a test structure that allows investigation of heat transport across a 'nanogap' consisting of a thin film supported over a substrate by an array of nanometer-sized posts. The nanogap is fabricated monolithically by first creating a film of SiO2 on a silicon substrate, lithographically defining holes in the SiO2, and covering this structure including the holes with silicon. Selective lateral etching will then remove the SiO2, leaving behind a thin gap between two Si layers spaced apart by nanometer-scale Si posts. Demonstration of this final step by successfully undercutting the a-Si upper layer due to the hydrophobic nature of silicon and the slow etch rate of buffered oxide etch in the small gap has proved to be problematic. Arriving at a feasible solution to this conundrum is the current objective of this project in order to begin investigating the thermal conductivity properties of the structure.
Model consideration is given to explain observed multi-shell emission spectra from InAs quantum dots embedded in GaAs or InGaAs. The shell model is based on the quantization of kinetic energy of lateral motion of carrier in the dot. 2-D oscillator is calculated on the basis of effective mass approximation. Profiles of inter-level separation are classified into categories that are connected with the lateral confining potential. Comparison is carried with experimental data on InAs/InGaAs quantum dot structures of the DWELL type (dot-in-a-well).
Photoluminescence spectra are investigated of InAs/InGaAs QD structures prepared be MBE on GaAs substrates in a range of pumping power density up to 0.6 kW/cm2. Multiple spectra band are observed corresponding to electron shells in atom-like dots. Identification of shells is proposed on the basis of spherical oscillator model. Energy diagram of dots is proposed taking into account identical temperature dependence of PL intensity in three lowest spectral bands.
A theoretical model for the dependence on temperature of the carrier behavior in a semiconductor structure containing InAs quantum dots grown inside a Ga0.85In0.15As quantum well is presented. The conditions, that have to be imposed in order to obtain analytical solutions with obvious physical interpretation are kept to minimum. Two temperature domains are approached in this model. In the low temperature case the equation system that describes the carrier behavior can be reduced to a cubic equation. One of the solutions of the equation represents the quantum dot photoluminescence yield. Also, a solution is obtained for the dot emission yield in the high temperature domain, where the carrier thermal escape from dots cannot be neglected. The solution depends, on the probabilities for electron and hole capture and reemission, and on the number of dot states occupied by electrons and holes. Temperature dependent measurements of the quantum dot photoluminescence are performed and the results are fit with the theoretical model.
The migration of carriers in optically pumped semiconductor structures with quantum dots is investigated using a confocal setup. Experiments are performed in order to test the performances of the optical setup. The samples studied by optical confocal microscopy are the following: semi-insulating GaAs, InAs quantum dots grown directly in GaAs, InAs quantum dots grown inside a Ga0.85In0.15As quantum well and a Ga0.85In0.15As quantum well with GaAs barriers. Measurements are done in a range of temperatures starting from 78 K up to 295 K. A theoretical model for the migration length is presented. The solution of a diffusion-type equation is used to fit the experimental data.
Optical characteristics are investigated and compared of nanostructure semiconductor lasers with quantum dots and quantum dashes. Spectra of optical gain and of linewidth enhancement factor are obtained. Optical anisotropy in quantum dash structures is investigated.
We report our progress on the design and fabrication of electrostatically-actuated microelectromechanical (MEM) tunable wavelength filters and vertical cavity surface-emitting lasers (VCSELs). We investigate both an all-semiconductor monolithic approach and a hybrid approach based on the combination of conventional polysilicon microelectromechanical systems (MEMS) and III-V semiconductor thin-film distributed Bragg reflector (DBR) and VCSEL structures. In the tunable hybrid structures the III-V semiconductor layers are flip-bonded onto specially designed polysilicon foundry MEMS structures and separated from their lattice-matched parent substrates by a novel post-bonding lift-off process.
The goal of this study is to understand how to optimize the performance of micro-mirrors for a variety of optical microsystem applications. Our approach relies on a number of process variations and mirror designs to ultimately produce relatively large (500μm to mm-scale), smooth (for nm RMS), and flat mirrors (greater than 1m curvature). White-light interferometric measurements, and finite element models are discussed in support of these findings. Stress gradients and residual stresses have been measured for accurate modeling of micro-mirrors. Through this modeling study, we have identified relevant structural parameters that will optimize SUMMiT V MEMS mirrors for optical applications. Ways of mitigating surface topography, print-through effects, and RMS roughness are currently being investigated.
Remarkably few applications of boundary element techniques to the solution of Schroedinger's equation have been reported. However, use of boundary elements can reduce the dimensionality by one, and the increased computational efficiencies enable one to compute eigenstates and eigenvalues of 3D quantum dots on desktop PCs. In this work, we introduce the boundary element technique and describe the single band quantum mechanical properties of various quantum dot and quantum wire configurations. The observed behavior of coupled quantum structures results in the equivalent of molecular bonding and antibonding states. Extensions of the method are developed with a numerical perturbation technique for spatially varying potentials, such as the influence of an electric field on quantum wires. Excellent agreement with an exact solution for a quantum wire is reported.
We demonstrated that a simple flashlamp pumped Nd:YAG laser, with the insertion of solid state passive elements, can be made a source of trains of high power picosecond pulses with accurate pulse to pulse reproducibility. The combination of passive negative feedback using GaAs together with semiconductor quantum well saturable absorber in an actively mode-locked Nd:YAG laser led to generation of stretched 200 ns long trains with pulsewidth of 42 ps. Cavity dumping resulted in single pulses at energies of 500 (mu) J with a nearly Gaussian spatial profile at repetition rate of 5 Hz. Without passive negative feedback, stable 75 ns long trains of pulses with pulsewidth of 52 ps were generated.
We report on application of semiconductor quantum well saturable absorber as a passive mode-locker in flashlamp pumped Nd:YAP laser. In passive regime of modulation, reproducible single trains containg about 10 pulses were generated with probability higher than 90 %. The single pulse duration was ~ 50 ps with a nearly Gaussian spatial profile. Energy ofthe whole train was 3 mJ. In active passive-regime using an additional acousto-optic modulator the probability of generation of mode-locked trains increased to 98 %.
Quantum dots laser diodes using the dots-in-a-well (DWELL) structure (InAs dots in an InGaAs quantum wells) have exhibited significant recent progress. With a single InAs dot layer in In0.15Ga0.85As quantum well, threshold current densities are as low as 26 A cm-2 at 1.25 micrometer. Quantum dot laser threshold current densities are now lower than any other reported semiconductor laser. In this work, the threshold current density is reduced to 16 A cm-2 by HR coatings on the same device. Further investigation of performance reveals that use of multiple DWELL stacks improves the modal gain and internal quantum efficiency. It is suggested that carrier heating out of the quantum dots limits the TO value of these DWELL lasers.
Short period superlattices of the ternary InGaAs/InGaSb alloys are grown by molecular beam epitaxy with the intent of approximating a quaternary InGaAsSb alloy. This technique where a quaternary semiconductor alloy is replaced by building short period superlattices using its ternary constituents is referred to as 'digital alloying.' Three sets of 2 micrometer In0.1Ga0.9AsySb1-y bulk alloys are grown; the first set was grown conventionally with varying growth temperature, while the other two sets were grown digitally. The first set of digital alloys lattice matched to GaSb is grown at various substrate temperatures, the second set was grown at one constant growth temperature but with various superlattice periodicities. Using high-resolution x-ray diffraction and room temperature photoluminescence, the structural and optical properties of the bulk In0.1Ga0.9AsySb1-y alloys were investigated. It is observed that the digital alloys are less sensitive to changes in the growth temperature by a factor of approximately 3, and the maximum periodicity a digital alloy can be grown before the onset of relaxation is approximately 9 ML. An optically pumped approximately 2 micrometer laser structure with InGaAsSb quantum wells and AlGaAsSb barriers both grown using the digital alloy technique was characterized. Room temperature operation, a low threshold current density of 104 W/cm2 (at 80 K with 808 nm pump), and a high characteristic temperature (TO) of 104 K show the feasibility of applying digital alloying techniques to mid-infrared optical devices.
A systematic and comparative study of the temperature dependence of the threshold current, threshold voltage, and output power of vertical-cavity surface-emitting lasers (VCSELs) is presented to discuss the factors that limit their temperature range for cw operation. To achieve high-performance for VCSELs, the position of the lasing mode must be in close proximity to (within +/- 5nm of) the gain peak under cw operation. In addition, by introducing continuously graded hetero-interfaces throughout the VCSEL structure, the effect of thermal self-heating is reduced. The combination of low thermal dissipation and alignment of the cavity mode to the gain peak resulted in VCSELs with excellent operating characteristics over a broad range of temperatures, including a thermally stable threshold voltage, and a very wide temperature range for both pulsed (100K to 540K) and cw (100K to 410K) operation.
Methods that increase the bandwidth of an optical fiber communication link beyond what is achievable with a single high speed laser and detector are of great interest. Time division multiplexing, TDM, is an attractive method of increasing overall system bandwidth, without having to increase the bandwidth of the individual transmitters and receivers. Mode-locked and gain-switched lasers are attractive sources for TDM. However, due to inadequate wavelength control in the laser fabrication process and the relatively wide spectrum resulting from mode-locking and gain-switching, fiber dispersion can degrade TDM systems. We introduce a simulation package that is used to model an experimental TDM fiber link and investigate the limits of transmission distance due to dispersion in a high speed communication link.
We compare vertical-cavity surface emitting lasers grown by molecular beam epitaxial methods to those grown by metal organic chemical vapor deposition methods as sources for wavelength-division multiplexing systems.
Deep level transient spectroscopy (DLTS) of deep levels occurring at MOCVD grown and regrown interfaces is described as a function of surface preparation. We examine two types of interfaces: 1 ) nGaAs grown on SI-GaAs and 2) A10,1Ga0,9As regrowth on Alo.lGao,9As. Surface preparation includes both shallow and deep wet etching, passivation with (NH)2S, and in-situ heat treatments and HCL etching. A new electron trap with EcEa 0.1 1 eV and 1019 cm2 and a new minority carrier trap with Ea E 0.18 eV and 10-15 cm2 were found in GaAs samples. The minority carrier trap is related to sulphur passivation. Four traps were found in the AlGai..As regrown samples. It is demonstrated that (NH)2S passivation before MOCVD improves interface quality for both the GaAs and Al1Ga0,9As grown and regrown layers.
Ar+ ion milling of A1xGa1-xAs layers grown by Low Pressure MOCVD, with aluminum compositions from 10 to 80% and for ion energies from 300to 1200 eV is reported in this work. The etch rate decreases with Al composition and increases with ion energy. The ion milling rate was found to depend exponentially on the ion energy, with an activation energy of 0.02 eV. Results are compared with the milling of a GaAs control sample and device layers etched under similar conditions. Etching was also studied as a function of ion angle-of-incidence.
The use of a PLZT thin-film waveguide for surface normal second harmonic generation (SHG) has been theoretically investigated. The necessary conditions for surface normal second harmonic generation are described. The polycrystalline nature of the film poses no disadvantages for this application beyond the increase in scattering losses.
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