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Quantum Dots, Particles, and Nanoclusters IV

Listed below are the papers found in this volume. Click the paper title to view an abstract or to order an individual paper.

This PDF file contains the front matter associated with SPIE Proceedings Volume 6481, including the Title Page, Copyright information, Table of Contents, Introduction (if any) and the Conference Committee listing.

We exploit atomistic tight-binding theory to describe the effects of dot size, composition, internal strain and applied strain on the confined states in nanocrystals and quantum dots. Both types of strain are critical, so we include the local strain and the externally imposed strain on an equal footing via an atomistic valence force field approach. A tightbinding model including an sp³s*d⁵ orbital model and spin-orbit effects is used. Several examples are discussed, including GaAs nanocrystals, InP nanocrystals under pressure, and core-shell nanocrystals, to highlight the importance of using atomistic models, extending tight-binding models to include d states, and having both internal and applied strain.

Thermal Lens (TL) and spectroscopic characterizations were performed in CdSe/ZnS core-shell nanoparticles embedded into the underlying polymethyl methacrylate (PMMA) dissolved in chloroform (CHCl₃). TL measurements were accomplished in function of the CdSe/ZnS quantum-dot concentration (12-60 mg/ml). The average value obtained for the thermal diffusivity was (0.85 ± 0.02) x 10^-3 cm²/s. The absolute nonradiative quantum efficiency φ and the fluorescence quantum efficiency η were determined by the TL method. Higher values of φ were obtained with the increase of the core-shell nanoparticles concentrations in PMMA-chloroform solutions. These results are in agreement with the behavior of fluorescence spectra that decrease with CdSe/ZnS concentration increase.

Carrier-density-dependent internal optical loss sets an upper limit for operating temperatures and considerably reduces the characteristic temperature of a quantum dot laser. Such internal loss also constrains the shallowest potential well depth and the smallest tolerable size of a quantum dot at which the lasing can be attained. At the maximum operating temperature or when any parameter of the structure is equal to its critical tolerable value, the characteristic temperature drops to zero.

Excited-state-mediated capture of carriers from the waveguide into the lasing ground-state in quantum dots (QDs) is studied. Such a two-step capture places a fundamental limitation on ground-state lasing - the output power saturates at high injection currents. The saturation power is controlled by the transition time between the excited- and ground-state in a QD. The longest, cut-off transition time exists, beyond which no ground-state lasing is possible.

Cavity quantum electrodynamic (QED) effects are studied in semiconductor microcavities embedded with InGaAs quantum dots. Evidence of weak coupling in the form of lifetime enhancement (the Purcell effect) and inhibition is found in both oxide-apertured micropillars and photonic crystals. In addition, high-efficiency, low-threshold lasing is observed in the photonic crystal cavities where only 2-4 quantum dots exist within the cavity mode volume and are not in general spectrally resonant. The transition to lasing in these soft turn-on devices is explored in a series of nanocavities by observing the change in photon statistics of the cavity mode with increasing pump power near the threshold.

Multilayers of PbTe quantum dots embedded in SiO₂ were fabricated by alternatively use of Laser Ablation and Plasma Enhanced Chemical Vapor Deposition techniques. A set o samples containing different PbTe nanoparticles sizes was prepared for the study. The morphological properties of the nanostructured material were studied by means of grazing-incidence small-angle X-ray scattering (GISAXS) and x-ray reflectometry (XRR) techniques. A preliminary analysis of the GISAXS spectra provided information about the multilayer periodicity and its relationship to the size of the deposited PbTe nanoparticles.

We report on the modulation spectroscopy investigation (in a form of photoreflectance (PR)) of self-assembled InAs/GaInAsP quantum dash structures grown by gas source molecular beam epitaxy on InP (100) substrates and designed for laser applications at 1.55 μm wavelength range in two different architectures: dash-in-a-barrier and dash-in-a-well. The dashes parameters have been determined by cross-sectional and plane-view transmission electron microscopy to be of typical height and width of about 2 per 20 nm and length from 50 to more than 200 nm, depending on the growth conditions. The part of the PR spectra related to the quantum dash layer has revealed several well distinguished transitions with the energy separation between the ground state and first excited state ones of about 150 meV independently of the length of the dashes for the dash-in-a-barrier and twice less for dash-in-a-well-designs, respectively. Our theoretical analysis based on effective mass approximation calculations has shown that all the higher order state transitions are related to the InAs wetting layer quantum well, where its parameters, as WL thickness and conduction band offset ratio, have been estimated on the base of agreement with the experimental data. The PR-based optical properties have found confirmation in the PL thermal quenching and device characteristics.

We have investigated structures grown on InP substrate by gas source molecular beam epitaxy and composed of InAs/In_0.53Ga_0.23Al_0.24As quantum dash (QDash) layer and In_0.53Ga_0.47As/In_0.53Ga_0.23Al_0.24As quantum well (QW) separated by a thin barrier allowing the carriers tunneling between these parts. The growth conditions for QDashes have been optimized to achieve their ground state emission at 1.55 ?m suitable for telecommunication laser applications, whereas the well parameters have been chosen to get the QW ground state levels above the dash ones, i.e. a construction allowing the injection of carriers from the QW layer (injector) into the dashes (emitter). Basing on photoreflectance spectra, supported by effective mass calculations, we have studied the electronic structure of the system, determining the energy levels and band offsets. The QW-QDash energy transfer has been probed by temperature dependent photoluminescence with changed excitation wavelength, where an efficient tunneling has been evidenced directly in the photoluminescence excitation spectra up to 130 K.

Round and dome-shaped InAs/InGaAsP QDs on InP (100) substrate were used for semiconductor optical amplifier. The diameter, height and density of QDs were 32 nm, 3.4 nm and 1.1x10¹¹ cm^-2, respectively. The PL peak was controlled from 1.43 to 1.57 μm and the room temperature PL yield was quite high, about 25% of the low temperature value. We have fabricated SOAs with 2 ~ 4 μm ridge width using the above mentioned QDs. The QDSOAs had a barrier of 1.1 μm emitting InGaAsP and a simple separate confinement hetero-structure with InP cladding layer. The spacing between adjacent QD layers was 40 nm to ensure no vertical electronic coupling, confirmed by time resolved PL measurements. The small signal gain was about 20 dB and the typical gain peak was around 1.54 μm, which matches well with the optical communication band. We have also measured 3-dB gain bandwidth by using a broadband light source with 200 nm flat band and found that it was around 40 nm.

Quantum dot (QD) size distribution and limitations in carrier capture and thermalization rates are still limiting the maximum saturation gain in QD-based laser diodes and the utilization of QD-medium in all-epitaxial vertical cavity surface emitting lasers (VCSELs). To overcome these problems structures of tunnel coupled pairs consisting of InGaAs quantum wells grown on top of self-assembled InAs QDs (QW-on-QDs) were employed as a gain medium for VCSELs. Photoluminescence, transmission electron microscopy and electroluminescence were used to study the properties of the multiple-layer QW-on-QDs active medium. QW-on-QDs tunnel structures with 3 - 5 nm tunnel barrier thicknesses and with different ground state (GS) relative separations were grown with varying InGaAs QW while the QD growth process parameters were kept constant. We have developed a tunnel QW-on-QDs structure with a QD PL line red-shifted by 32 meV relative to QW GS line. The narrow linewidth (22 meV) of this QD transition likely indicates an efficient LOphonon assisted tunneling of carriers from QW into QD ensemble states. Optimized tunnel (with 3 nm barrier thickness) QW-on-QDs structures were evaluated in VCSELs. All-epitaxial VCSELs with triple-pair tunnel QW-on-QDs as active medium demonstrated continuous wave mode lasing. These QD-based VCSELs with n-doped AlGaAs/GaAs mirrors and tunnel n-p junction exhibited 1.8 mA (Jth ~ 800 A/cm2) minimum threshold current at QD GS emission wavelength, 1135 nm, with 0.7mW optical power and 12% slope efficiency.

A two-photon sequential absorption photocurrent generation process in modulation doped InAs/GaAs quantum dot (QD) is proposed and analyzed. Enhanced nonlinear absorption is expected due to the long excited sate lifetime. Ultra-low leakage current has been verified. A High photodetectivity of > 10¹¹cmHz^1/2/W can be obtained at 180K. The two-photon sequential absorption photocurrent generation process is promising to achieve thermal-electrically cooled long-wave infrared (LWIR 8-12μm) photodetector with high photodetectivity.

To investigate the nonlinear optical properties of metallic nanoparticles in dielectric composite materials, germano-silicate glass optical fibers incorporated with gold nanoparticles were made by using modified chemical vapor deposition technique and solution doping process. The incorporation of the gold nanoparticles was confirmed by the sharp absorption peak appeared near 498.4nm, which was due to the surface plamon resonance absorption of Au nanoparticles. Resonant optical nonlinearities of the fibers were estimated by measuring the peak shift of the fringes obtained from the long-period fiber grating pair upon pumping with Argon laser at 488nm. The resonant nonlinearity was found to be 5.00x10^-16m²/W by the incorporation of the gold metal concentration and with the addition of Al³⁺ ions.

Nanostructures, such as quantum dots, nanocrystals and nanowires, made of wurtzite ZnO have recently attracted attention due to their proposed applications in optoelectronic devices. Raman spectroscopy has been widely used to study the optical phonon spectrum modification in ZnO nanostructures as compared to bulk crystals. Understanding the phonon spectrum change in wurtzite nanostructures is important because the optical phonons affect the light emission and absorption. The interpretation of the phonon peaks in the Raman spectrum from ZnO nanostructures continues to be the subject of debates. Here we present a comparative study of micro-Raman spectra from ZnO quantum dots, nanocrystals and nanowires. Several possible mechanisms for the peak position shifts, i.e., optical phonon confinement, phonon localization on defects and laser-induced heating, are discussed in details. We show that the shifts of ~2 cm^-1 in non-Resonant spectra are likely due to the optical phonon confinement in ZnO quantum dots with the average diameter of 4 nm. The small shifts in the non-Resonant spectra from ZnO nanowires with the diameter ~20 nm - 50 nm can be attributed to either defects or large size dispersion, which results in a substantial contribution from nanowires with smaller diameters. The large red-shifts of ~10 cm^-1 in the resonant Raman spectrum from nanocrystals were proved to be due to local laser heating.

Recently quantum dots (QDs) have been the topic of extensive research. Unique properties arise in QDs due to a combination of the localized nature of their wavefunctions and a singularity in the associated density of states. Many strained III-V semiconductor film-substrate systems form QDs via a self-assembly process by means of a Stranski-Krastanov process. The strain relief responsible for the 3D nucleation causes a variation in the in-plane lattice constant which allows subsequent QD layers separated by thin spacer layers to be vertically stacked. Recently this concept has been extended to allow the formation of a heterojunction quantum dot (HeQuaD). In this structure an initial self-assembled QD (SAQD) is formed and then a different similarly strained material is nucleated on the initial SAQDs forming a crown on the underlying QD. This crown is also of a size appropriate to cause quantum confinement. In particular a stack of 4 layers of a HeQuaD structure of a GaSb crowned InAs SAQD on GaAs with GaAs spacer layers has been formed. The top HeQuaDs have been left uncapped to allow AFM analysis of the morphology. Photoluminescence of the HeQuaD has 3 peaks at ~0.95eV, 1.15eV, and 1.35eV. We have measured the intensity and temperature dependence of these PL peaks.