Semiconductor lasers and amplifiers were developed based on self-assembled quantum-dot gain material. This paper gives an overview about the recent work on GaAs- and InP-based quantum-dot devices mainly dedicated for telecom applications. The major advantage of quantum-dot like gain material, i.e. the possibility to tailor the spectral and spatial gain properties of an amplifying material, was used to optimize different device aspects, like low threshold current, broad band amplification or low temperature sensitivity. High performance GaAs-based continuous wave (cw) operating quantum-dot lasers could be fabricated with threshold currents of about 2 mA (L = 400 μm). Single mode emitting devices with emission wavelengths > 1.3 μm were realized by laterally coupled feedback gratings with threshold currents below 5 mA, output powers > 5 mW and cw operation temperatures up to 85 °C. Modulation frequencies of up to 7.5 GHz were obtained for standard device structures. For long wavelength telecom applications quantum-dot like material with dash geometry was developed on InP substrates with basic properties in the transition region between quantum-dot and -wire systems. A very large tuning range of the emission wavelength between 1.2 and 2.0 μm (room temperature) was obtained which allow the realization of material with ultra-wide gain bandwidth. Quantum-dash laser structures reaches threshold current densities < 1 kA/cm². Ridge waveguide lasers with a cavity length of 1.9 mm show cw threshold currents of about 100 mA and maximum output powers > 40 mW per facet. With 300 μm long facet coated devices cw threshold currents of 23 mA and maximum operation temperatures in pulsed mode of 130 °C were achieved. Semiconductor optical amplifiers were fabricated by using broad band quantum-dash material. For a 1.9 mm long device, up to 22 dB gain was obtained with a three times larger spectral range than in comparable quantum well devices. High speed nearly pattern free signal amplification up to 10 GBit/s could be demonstrated and wavelength conversion experiments were performed.

There has been great technological interest in the use of InAs quantum dots for InP-based lasers which can provide long wavelength emission in the 1.55-2 μm range. The atom-like densities of states of quantum dots provide low threshold current density, high differential gain, temperature insensitive operation and low chirp. However, to take advantage of these aspects, it is important to have dots with uniform size and shape. We report the atomic force microscope (AFM) and photoluminescence (PL) studies of self organized InAs quantum dots grown by molecular beam epitaxy on InGaAs and InAlAs lattice matched to InP. Our experiments confirm prior results that InAs forms quantum wires on InGaAs matrix layer. However, we find that depositing a thin buffer layer of InAlAs helps in the formation of well-shaped quantum dots. We believe that the aluminum in the buffer layer reduces the surface diffusion of indium adatoms and aids the formation of dots with high density. Our results show that formation of quantum dots depends strongly on the strain, surface energy and surface diffusion kinetics that are in turn dependent on the nature of buffer layer and growth conditions. We improve the quality of dots by optimizing the growth parameters such as growth temperature and arsenic overpressure.

We analyse the sensitivity of quantum dot semiconductor lasers to optical. While bulk and quantum well semiconductor lasers are usually extremely unstable when submitted to back reflection, quantum dot semiconductor lasers exhibit a reduced sensitivity. Using a rate equation approach, we show that this behaviour is the result of a relatively low but nonzero line-width enhancement factor and of strongly damped relaxation oscillations.

We report measurements of the near-field pattern of ridge-waveguide tunnel injection In_0.4Ga_0.6As/GaAs self-assembled quantum dot lasers and have compared the results with similar strained quantum well lasers. While no filamentation is observed in the quantum dot devices, significant filamentatin and side lobes are observed in the quantum well lasers. The trend is corroborated by measured linewidth enhancement factor, α, of the two types of devices. Values of α~3.8 are measured in the quantum well lasers, while α is ≤0.7 in the quantum dot lasers, suggesting a very small refractive index change with injection in the active region. Chirp ≤0.6Å is measured in the tunnel injection devices, while it varies in the 1.6-3 Å range in the quantum well lasers.

Interaction between strongly localized charge carriers in zero-dimensional systems like quantum dots (QD) depends sensitively on the geometrical roperties of the dots. The recently observed monolayer splitting with eight well resolved peaks (in low excitation photoluminescence (PL)) together with eight-band kp theory as the appropriate tool for modeling electronic and optical properties offers direct spectroscopic access to details of the QD morphology. By this achievement it became possible to link single-dot spectra obtained by cathodoluminescence measurements via the exciton transition energy to structural properties of the probed QD. In view of theory this situation constitutes an ideal starting point to study few-particle interactions for realistic InAs QDs as a function of their structural properties. This is done using the configuration interaction method. The wavefunctions are obtained from eight-band kp calculations of single-particle states including explicitly piezoelectric effects in the confinement potential.

In this work we present a method to obtain room temperature ground state emission beyond 1.3 μm from InGaAs QDs, grown by MOCVD, embedded directly into a binary GaAs matrix. The wavelength is tuned from 1.26 μm up to 1.33 μm by varying the V/III ratio during the growth of the GaAs cap layer, without using seeding layer or InGaAs wells. A line-shape narrowing (from 36 meV to 24 meV) and a strong reduction of the temperature dependent quenching of the emission (down to a factor 3 from 10K to 300K) are observed, that represent the best value reported for QD structures emitting at 1.3 μm. The results are explained in term different morphological evolution and surface reconstruction undergone by the InGaAs islands during the GaAs overgrowth that result in larger QD size and in lower In-Ga intermixing. Indeed, cross sectional TEM images show an increase in the QD size of more than 30% with decreasing the AsH₃ flow. The overall strain reduction due to the use of the GaAs matrix allows the fabrication of highly efficient staked QD layers. The single and multiple QDs samples show a systematic increase of the emission intensity and similar spectral shape.

The fabrication of semiconductor nanowires, in which composition, size and conductivity can be controlled in both the radial and axial direction of the wire is of interest for fundamental studies of carrier confinement as well as nanoscale device development. In this study, group IV semiconductor nanowires, including Si, Ge and Si_xGe_1-x alloy nanowires were fabricated by vapor-liquid-solid (VLS) growth using gaseous precursors. In the VLS process, gold is used to form a liquid alloy with Si and Ge which, upon supersaturation, precipitates a semiconductor nanowire. Nanoporous alumina membranes were used as templates for the VLS growth process, in order to control the diameter of the nanowires over the range from 45 nm to 200 nm. Intentional p-type and n-type doping was achieved through the addition of either trimethylboron, diborane or phosphine gas during nanowire growth. The electrical properties of undoped and intentionally doped silicon nanowires were characterized using field-assisted assembly to align and position the wires onto pre-patterned test bed structures. The depletion characteristics of back-gated nanowire structures were used to determine conductivity type and qualitatively compare dopant concentration. SiGe and SiGe/Si axial heterostructure nanowires were also prepared through the addition of germane gas during VLS growth. The Ge concentration in the wires was controllable over the range from 12 % to 25% by varying the inlet GeH₄/SiH₄ ratio.

The role of the size of amorphous silicon quantum dots in the Er luminescence at 1.54 µm was investigated. As the dot size was increased, the more Er ions were located near one dot due to its large surface area and more Er ions interacted with other Er ions. This Er-Er interaction caused a weak photoluminescence intensity despite the increase in the effective excitation cross section. The critical dot size, needed to take advantage of the positive effect on Er luminescence, is considered to be about 2.0 nm, below which a small dot is very effective in the efficient luminescence of Er. However, the hydrogenation is considered to suppress this Er-Er interaction.

The absorption or emission wavelength in optoelectronic devices such as quantum well infrared photodetectors, quantum cascade lasers, and type II superlattice photodiodes can be controlled by the thickness and composition of the quantum wells that constitute their active layers. By further confining the charge carriers, for instance in a quantum dot, even more control can be gained over energy transitions within the semiconductor crystal. We propose a method for manipulating the semiconductor band structure by confining carriers within nanopillar structures. Using electron beam lithography and dry plasma etching, we can precisely control the pillar placement, density and dimensions, and thus the performance characteristics, of the optoelectronic device. Furthermore, by patterning different size structures, it is possible to create arrays of multi-color devices on the same substrate, a technique that lends itself to large-scale monolithic integration. We demonstrate the fabrication of nanopillar arrays in the GaSb, GaInP, GaInAs, and type II InAs/GaSb superlattice material systems and show initial photoluminescence data, which seems to indicate quantum confinement within these structures.

Carrier and spin dynamics are measured in meutral, positively and negatively charged quantum dots using polarization-sensitive time-resolved photoluminescence. Carrier capture rates are observed to be strongly enhanced in charged quantum dots, suggesting that electron-hole scattering dominates this process. For positive quantum dots, the enhanced spin-polarized electron capture rate eliminates loss of electron spin information in the GaAs barriers prior to capture, resulting in strong circularly-polarized emission. Comparison of spin relaxation times in positively charged and neutral quantum dots reveals a negligible influence of the large built-in hole population, in contrast to measurements in higher-dimensional p-type semiconductors. The long spin life-time, short capture time, and high radiative efficiency of the positively charged quantum dots indicates that these structures are superior to both quantum wells and neutral quantum dots for spin detection using a spin light-emitting diode.

We discuss the potential applications of single photon sources based on a single quantum dot, emphasizing the crucial importance of the efficiency parameter in view of applications in the field of quantum computing and quantum communications. By inserting the single quantum dot in a pillar microcavity, an efficiency as high as 44% has been obtained by using the Purcell effect. We show that this approach is limited in practice by extrinsic cavity losses, such as those related to the scattering by the sidewalls roughness. We present novel design rules for micropillars in view of this application and show that for the well-mastered GaAs/AlAs system more than 70% of the emission can be concentrated into the collimated emission beam associated with the fundamental cavity mode. We show finally that a novel design, based on a state-of-the-art 2D photonic crystal microcavity, could permit to reach efficiencies in excess of 0.95.

The dephasing time in semiconductor quantum dots and quantum-dot molecules is measured using a sensitive four-wave mixing heterodyne technique. We find a dephasing time of several hundred picoseconds at low temperature in the ground-state transition of strongly-confined InGaAs quantum dots, approaching the radiative-lifetime limit. Between 7 K and 100 K the polarization decay has two distinct components resulting in a non-Lorentzian lineshape with a zero-phonon line and a broad band from elastic exciton-acoustic phonon interactions. On a series of InAs/GaAs quantum-dot molecules having different interdot barrier thicknesses a systematic dependence of the dephasing dynamics on the barrier thickness is observed. The results show how the quantum mechanical coupling of the electronic wavefunctions in the molecules affects both the exciton radiative lifetime and the exciton-acoustic phonon interaction.

The design of quantum logical elements based on nanocrystalline silicon was made. The optical properties of polycrystalline silicon films with oxygen incorporation in grain boundary were experimentally studied. The Raman scattering, photoluminescent and Fourier-transformed infrared spectra were measured. The different kinds of defects were detected. The E' and D centers was measured by electron-spin resonance method. The triplet state of oxygen was detected by fourier-transform infrared spectroscopy. The hyperfine structure of oxygen level in A-center was detected by laser picosecond spectroscopy. It is assumed that the splitting of oxygen level is caused due to the ultrasound oscillation of oxygen atom in A-center. We propose the qubit based on A-centers in polycrystalline oxidized silicon.

Mesostructured 2d-hexagonal sol-gel films were prepared by dip-coating method. Their structures were detected by X-ray diffraction. Silver nanoparticles reduced from Ag⁺ ion (silver nitrate) to Ag⁰ were deposited into the channels of the structure produced by the neutral surfactant Brij58. Surface enhanced raman spectroscopy technique was used to characterize the films. Spectra of films without metallic particles were compared to those with silver nanoparticles; the earliest exhibit an increased intensity on the 885, 955, 1061, 1129, 1230,1429, 1521, and 1796 cm^-1 bands. This enhancement due to SERS is the result of the surface plasmon excitation inside the silver particles causing a reactivation of the Raman scattering from the molecules on the surface colloids. Photoconductivity studies were performed on mesostructured films with silver colloids. φl0 and φµτ parameters are bigger than those from photorefractive crystals KnbO₃:Fe³⁺. The photovoltaic effect increases with AgNO₃ concentration. Mesostructured film without silver colloids shows a small photovoltaic parameter.

We report preparation of silver nanoparticles in the 10 nm size range by exhastive reduction of a well-known and well-characterized AgBr nanosol. These silver nanoparticles exhibit a second order hyperpolarizability tensor, β=(100±10) x 10^-30 esu per atom. The nanoparticles chmisorb a monolayer of iodide ions with concomitant bleaching of the surface plasmon absorption but no attenuation of Hyper-Rayleigh (second harmonic light) Scattering (HRS). Further addition of iodide leads to significant enhancement of HRS, which we attribute so surface-enhanced second harmonic scattering (SESHS) from iodide ion physisorbed on the AgI adlayer. A merocyanine dye, which does not exhibit HRS in solution, chemisorbs as a monolayer to the nanosilver, with formation of J-aggregates. These aggregates, like the free dye, are not active in second harmonic scattering. Further addition of dye leads to HRS enhancement, which we interpret as physisorption of dye onto the J-aggregate monolayer; the non-centrosymmetric surface environment allows observation of SESHS. A lower-limit estimate of the second order hyperpolarizability of the adsorbed dye is 200 X 10^-30 esu per molecule.

Biological labeling has been demonstrated with CdSe quantum dots in a variety of animal cells, but bacteria are harder to label because of their cell walls. We discuss the challenges of using minimally coated, bare CdSe quantum dots as luminescent internal labels for bacteria. These quantum dots were solubilized with mercaptoacetic acid and conjugated to adenine. Significant evidence for the internal staining of Bacillus subtilis (Gram positive) and Escherichia coli (Gram negative) using these structures is presented via steady-state emission, epifluorescence microscopy, transmission electron microscopy, and energy dispersive spectroscopy. In particular, the E. coli adenine auxotroph, and not the wild type, took up adenine coated quantum dots, and this only occurred in adenine deficient growth media. Labeling strength was enhanced by performing the incubation under room light. This process was examined with steady-state emission spectra and time-resolved luminescence profiles obtained from time-correlated-single-photon counting.

We describe the synthesis of glass incorporating photoluminescent semiconductor nanocrystals through a sol-gel processing and discuss the characterization of their properties. CdTe nanocrystals with a mean size between 3 and 6 nm are embedded in a glass matrix made from silane coupling agent functionalized with amine groups. The synthesis process was optimized to avoid nanocrystals agglomeration and to prevent surface deterioration of nanocrystals. The nanocrystals embedded in the glass matrix remained almost unmodified during preparation. Their photoluminescent spectra were about 41 to 65 nm in width at half maximum and can be tuned from green to red with luminescent efficiency up to 41%. The nanocrystals embedded in glass exhibit enhanced long-term stability over several months keeping. They also display high stability even after heat treatment.

Solution-synthesized nanocrystals which manifest strong quantum-confinement effects enable size-tunability of spectral properties and strong excitonic effects. Lead sulfide (PbS) nanocrystals are especially interesting for applications in telecommunication because their resonance is tunable to wavelengths from 1.3-1.55 μm and beyond. In other quantum dot systems, optically-induced bleaching of absorption has been shown to lead to a strong nonlinearity in the vicinity of the exciton peak wavelength [1][2-4][5-8]. We report herein results of picosecond-resolved transient absorption in spin-processible solution-synthesized PbS nanocrystals across the wavelength range 1100 nm to 1600 nm. The sample was synthesized using the solution phase organometallic method (hot injection technique), which provides good control over the size of the nanocrystals [9]. The sample consisted of nanocrystals with diameter around 5.2 nm resulting in an exciton peak at 1330 nm. Since the Bohr radius for the bulk PbS is 18 nm, these nanocrystals lay within the strong quantum-confinement regime [10]. Time-resolved absorption was studied using the single-wavelength collinear pump-probe setup. From the observed double-exponential decay trace of the transmission, fast and slow time constants were extracted. The fast component of few 10is of ps was attributed to Auger recombination. The slow component is on the order of ns. The saturation intensity was also measured in this wavelength range using the Z-Scan technique [11]. The open aperture signals were fit to the intensity-dependent absorption model. The value of the saturation intensity was found to be 0.6 GW/cm² around the exciton peak.