Laser mode competition is a well-known phenomenon in a multi-mode laser system. The competition between different lasing modes is considered inevitable in all kinds of lasers. However, our experiments show that laser mode anti-competition can be observed in lasers that combine either quantum dots (QD) of different sizes or quantum wells of different composition and width. Here we report the anti-competition experiment from QD lasers. The QD structure is grown on a GaAs substrate. Two types of QD layers for 1.24μm emission and 1.28μm emission are grown alternatively in the active layer. The anti-competition behavior is observed in an external cavity laser controlled by the grating, oscillating at two different wavelengths. Experimental results show that when short-wavelength light intensity increases, long-wavelength light intensity will also increase. This is the anti-competition behavior. Nonetheless, when short-wavelength light intensity is above a certain level, long-wavelength light intensity decreases. It means that the laser behavior changes to the usual competition situation at the large intensity.

We investigated the multiple cations intermixing in InAs/InGaAlAs quantum dot-in-well laser structure grown on InP substrate using impurity-free vacancy disordering (IFVD) technique. Selective control of the bandgap shifts has been achieved using SiO₂ and Si_xN_y annealing caps. A differential wavelength shift of 76 nm has been observed after a rapid thermal annealing step at 750 ^oC for 30 s. In contrast to most IFVD results in other materials, we observed a larger bandgap shift from the Si_xN_y capped samples than from the SiO₂ capped samples. Based on theoretical calculations, we attribute this to the different effective interdiffusion rates of group-III cations. The demonstrated intermixing process provides an effective method for fine tuning the bandgap of InAs QDs around 1.55 μm as an alternative to the growth manipulation, as well as for realizing photonics integrated circuits.

Feasibility is discussed of the conventional method of determining internal loss coefficient and internal quantum efficiency from a measured plot of the reciprocal slope efficiency versus the cavity length L in semiconductor edge-emitting quantum dot (QD) lasers. The limitations are imposed by the cavity-length-dependence of internal loss and quantum efficiency themselves. The effect of internal loss is quantitatively analyzed, which originates from the dependence of internal loss on the carrier density, with the latter being L-dependent. In short cavities, a plot of the reciprocal slope efficiency versus the cavity length can significantly deviate from a straight line; particularly, the reciprocal slope efficiency increases with reducing the cavity length in short cavities. This plot asymptotically approaches an inclined line only in long cavities. Correspondingly, only infinitely-long-cavity (no mirror loss) values of the internal loss coefficient and the internal quantum efficiency can be extracted using the standard procedure. These latter differ significantly from those in short-cavity devices, thus strongly limiting the practicality of the procedure. For L longer than several hundred microns, the limitations are strong in a single-QD-layer laser and moderate in a multiple-QD-layer laser.

We investigate optical properties of In(Ga)As/InGaAsP/InP quantum dots (QDs) operating at 1.5 μm and device characteristics of laser diodes (LDs) made of this QD system. Round and dome-shaped QDs are formed and emission wavelength can be adjusted from 1.4 μm to 1.6 μm by changing QD growth conditions with the same InGaAsP barrier. Even though relatively large QDs were formed, the areal density of QDs is quite high (1.1×10¹¹/cm²) compared to other QD systems. Time-resolved photoluminescence measurements were carried out at low temperature and revealed no evidence of electronic coupling between QDs in spite of this high QD density. We also fabricate LDs with these QDs as gain medium. LDs are operated in continuous-wave mode over 40°C. The lasing spectrum shows strong inhomogeneous characteristics at room temperature. This InP based QD system would be appropriate for multi-wavelength device applications such as semiconductor optical amplifier for optical communications.

GaN nanocolumn (sometimes called as nanorod, nanowire, and nanopillar) is a columnar single crystalline GaN nano-crystal having small diameter of from tens to hundreds nanometers. In this paper, photoluminescence (PL) study of GaN nanocolumns and InGaN multiple quantum disks (MQDs) embedded in the GaN nanocolumns grown by RF-plasma assisted molecular beam epitaxy are described. The room temperature PL peak intensity of GaN nanocolumns was several hundred times stronger than that of conventional GaN film with dislocation density of 3~5x10⁹ cm^-2. Stimulated emission with very low threshold optical power density of 198~290 kW/cm² was observed for the GaN nanocolumns. InGaN MQDs also showed intense PL emission with peak wavelength of from 436 (blue) to 614 nm (red). GaN nanocolumn based light emitting diodes (LEDs) with InGaN MQD active layer were successfully fabricated on n-type (111) Si substrates. The emission color of the nanocolumn LED was varied from violet to red by changing the growth conditions of InGaN MQD active layer. The full width at half maximum (FWHM) of emission spectrum of the LED emitting around 500 nm taken from whole semi-transparent top electrode area (Φ=500 μm) was 73.5 nm but from small area (Φ=3 μm) was 37.0 nm. This result suggests that emission spectrum of a single InGaN quantum disk have narrower FWHM but large area LED that contains huge number of nanocolumn LEDs have wider FWHM.

In this paper we report the use of a photonic crystal resonant cavity to increase the quantum efficiency, detectivity (D*) and the background limited infrared photodetector (BLIP) temperature of a quantum dot detector. The photonic crystal is incorporated in InAs/InGaAs/GaAs dots-in-well (DWELL) detector using Electron beam lithography. From calibrated blackbody measurements, the conversion efficiency of the detector with the photonic crystal (DWELL-PC) is found to be 58.5% at -2.5 V while the control DWELL detectors have quantum efficiency of 7.6% at the same bias. We observed no significant reduction in the dark current of the photonic crystal devices compared to the normal structure. The generation-recombination limited D* at 77K with a 300K F1.7 background, is estimated to be 6 x 10¹⁰ cmHz^1/2/W at -3V bias for the DWELL-PC which is a factor of 20 higher than that of the control sample. We also observed a 20% increase in the BLIP temperature for the DWELL-PCs.

We discuss a technique for tailoring the emission bandwidth of a quantum dot (QD) superluminescent light emitting diode (SLED). We utilize a multi-dot-in-well (DWELL) structure with different indium compositions within each well which we term dots in compositionally modulated well (DCMWELL) structures. One key aspect of our design is the overlap of the ground and excited state emission of different DWELL layers. Such SLED devices operate CW at room temperature with powers in excess of 2.5mW per facet, and exhibit a single peak almost 85 nm wide, which is almost flat topped.

Structures of tunnel coupled pairs consisting of InGaAs quantum wells grown on top of self-assembled InAs quantum dots (QW-on-QDs) were employed to improve the gain medium in laser diodes. Photoluminescence, transmission electron microscopy and electroluminescence were used to study the properties of multiple-layer QW-on-QDs active medium. QW-on-QDs tunnel structures with 4.5 nm tunnel barrier thickness and with different ground state (GS) relative separations were grown by variation of InGaAs QW while the QD growth process was kept constant. We have developed a tunnel QW-on-QDs structure with a resonance transition which is red-shifted ~35 meV relative to QW GS. This transition with narrow linewidth, 21.6 meV at T=77K, likely indicates an efficient LO-phonon assisted tunneling of carriers from QW into QD ensemble states. The highest gain was achieved with a QW-on-QDs active medium with GS relative separation of close to 35-40 meV. Optimized triple-pair tunnel QW-on-QDs laser diodes with cleaved mirrors emitting at 1145 nm (corresponding to QD GS) exhibited a saturated modal gain exceeding 80 cm^-1 with minimum cavity length of 0.14 mm. Small signal modulation characteristics of these lasers were measured. From the damping factor and resonance frequency dependence on driving current, the damping-limited cut-off frequency for this QW-on-QDs medium can be estimated as exceeding 30 GHz. All-epitaxial vertical cavity surface emitting lasers with triple-pair tunnel QW-on-QDs as active medium demonstrated continuous wave mode lasing with 5.7 mA minimum threshold current at QD GS emission wavelength, 1131 nm.

We have investigated the characteristics of GaAs-based 1.3 μm quantum-dot laser diode (QDLD) with Al_0.7Ga_0.3As cladding layers. The active region of QDLD consists of 3-stacked InAs quantum-dots (QDs) in an In_0.15Ga_0.85As quantum well (dots-in-a-well: DWELL), which was grown by molecular beam epitaxy (MBE). For advanced performances of QDLD, the high-growth-temperature spacer layer and p-type modulation doping were applied to QDLD active region. We fabricated ridge waveguide structure LDs which had 10 ~ 50 μm ridge width with several cavity lengths and applied a high reflection (HR) coating on one-sided mirror facet. The threshold current density was 155 and 95 A/cm² for a 2000 μm-long as-cleaved and a 1500 μm-long HR coated LDs, respectively. The lasing wavelength was 1.31 μm from the ground state transition, under a pulsed operation condition (0.1%) at room temperature. The QDLD showed simultaneous lasing at 1.31 μm and 1.23 μm from the ground state (GS) and the excited state (ES), respectively. The lasing wavelength switching from the GS to the ES depends on the cavity length, the injection current and operating temperature.

Quantum dots (QDs) have been receiving considerable attention due to the unique properties, which arise due to the confinement of the electron and holes in a lower band gap material. The InAs on GaAs material system is one of the most studied combinations in which quantum dots form during epitaxy. These QDs form in a Stranski Krastanov manner via a self-assembly process in which the dots nucleate at a critical adatom coverage on a wetting layer of InAs. QDs may be vertically aligned by using the residual strain above a buried dot layer to enhance the nucleation of the second layer of dots. In this work, we show the formation of QDs, which are composed of multiple materials, can be formed through a marriage of these two concepts. In this particular demonstration, we formed InAs dots on GaAs andcrowned the QDs with GaSb and encapsulated the entire structure with GaAs. Atomic Force Microscopy shows additional nucleation between the InAs layers has been minimized and cross-sectional transmission electron microscopy shows the formation the composite structure. Transmission electron microscopy indicated a clear boundary between the GaSb and InAs regions. AFM analysis of the HeQuaD structure shows that GaSb material grows mainly on the two (1 1 0) inclined facets. Thus, the HeQuaD is elongated along the (1 1 0) direction. We have also obtained preliminary photoluminescence (PL) from a 3 layer GaS/InAs HeQuaD structure with a peak around 1.3 microns.

Ge islands fabricated on Si(100) by molecular beam epitaxy at different growth temperatures, were studied using crosssectional scanning transmission electron microscopy and energy-dispersive X-ray spectrometry combined with electron energy loss spectrometry experiments. The island size, shape, strain, and material composition define the dot-related optical transition energies, but they are all strongly dependent on the growth temperature. We have performed quantitative investigations of the material composition of Ge/Si(001) quantum dots. The samples were grown at temperatures ranging from 430 to 730 ^oC, with one buried and one uncapped layer of Ge islands separated by 140 nm intrinsic Si. The measurements showed a Ge concentration very close to 100 % in the islands of samples grown at 430 ^oC. With a growth temperature of 530 ^oC, a ~20 % reduction of the Ge fraction was observed, which is due to intermixing of Si and Ge. This is consistent with our previous photoluminescence results, which revealed a significant blue shift of the Ge dot-related emission peak in this growth temperature range. The Ge concentration decreases more slowly when the growth temperature is increased above 600 ^oC, which can be explained by geometrical arguments. The longer distance between the interface and the core of these larger sized dome-shaped islands implies that less Si atoms reach the dot center. In general, the uncapped Ge dots have similar widths as the embedded islands, but the height is almost exclusively larger. Furthermore, the Ge concentration is slightly lower for the overgrown dots.

In this paper, we describe the results of using strain-compensation (SC) for closely-stacked InAs/GaAs quantum dot (QD) structures. The effects of the (In)GaP SC layers has been investigated using several methods. High-resolution x-ray diffractometry (XRD) quantifies the values of experimental strain reduction compared to calculations. Atomic force microscopy (AFM) indicates that the SC layer improves both QD uniformity and reduces defect density. Furthermore, increase in photoluminescence (PL) intensity has been observed from compensated structure. The use of Indium-flushing to dissolve large defect islands prevent further defect propagation in stacked QD active region. Room-temperature ground-state lasing at emission wavelengths of 1227-1249 nm have been realized with threshold current densities of 208-550 A/cm² for 15-20 nm spacing structures.

We present that size of Ge nanoparticle can be controlled by changing the angle between ultrafast laser polarization and crystal axis using ultrafast laser irradiation. The nanoparticle size dependence on the laser polarization with respect to the Ge crystal axis exhibits a sinusoidal function with a minimum size at (100) axis. Moreover, the measurement of transient reflection reveals the presence of large anisotropies in both its amplitude and its relaxation dynamics with a minimum at (100) crystal axis. This implies that the observed anisotropic dependence of nanostructure size of Ge is followed by a different carrier density as well as its relaxation process depending on the orientation of Ge crystal axis only at near and above threshold fluence.

We report the selective area molecular beam epitaxial (SAMBE) growth of quantum dot (QD) structures. The formation of polycrystalline deposits on dielectric masks is shown to be controlled by the growth rate and growth temperature. Furthermore, we report the size, areal density and energy control of QDs in the region of the dielectric mask. We show that for SAMBE, a reduction in InAs QD size and areal density is obtained close to a polycrstal covered dielectric mask, and that this effect is dependent upon the amount of polycrystalline GaAs coverage of the mask. We attribute this effect to the transport of indium from neighboring epitaxial areas to the polycrystalline GaAs covered mask.

We have measured the nonlinear optical response of Cadmium Sulfide quantum dots (CdS QD) in a poly(propyleneimine) dendrimer matrix having diaminobutane (DAB) core. Large refractive nonlinear coefficients and low absorption losses were observed at all wavelengths. Dendrimers are nanosize, highly branched, tree like monodisperse macromolecules that emanate from a central core with a branch occurring at each monomer unit. Dendrimers encapsulations convey stability, control of emission wavelengths by QD size. The branching points in the interior of the dendrimers are occupied by tertiary nitrogen to provide numerous nucleation sites to drive formation of QD clusters of small size. The dendrimer-stabilized CdS QDs were stable at room temperature, both in solution and in solid state for several weeks. Thin films were deposited by spin casting from methanol solutions. The resulting samples consisted of a 1mm thick quartz substrate with a 200-400 Å nonlinear optical film on one side. The Z-scan technique was used to characterize the NLO response. A mode-locked YAG laser provided the laser pulses with 30-ps duration at 355 nm, 532 nm and 1064 nm at a 20-Hz repetition rate with energies per pulse ranging from few microjoules to several millijoules. These results indicate relatively large values for the nonlinear response (> 10^-10 esu) at all three wavelengths. Our calculations indicate that quantum dot-organic systems have large optical nonlinearity due to interactions between excitons in the quantum dots and the organic medium. We calculate that an increase of the QD radius to ~4-8 nm will result in a substantial enhancement of the nonlinearity.

Multilayer of PbTe quantum dots embedded in SiO₂ were fabricated by alternatively use of Plasma Enhanced Chemical Vapor Deposition and Laser Ablation techniques. The optimal growing parameters for both the SiO₂ films and the PbTe quantum dots were obtained. The refractive index and optical absorption of the sample were studied. Multilayer X-ray diffraction patterns were used to estimate the nanoparticles diameter. Morphological properties of the nanostructured material were studied using Transmission Electron Microscopy. Both absorption spectra and X-ray diffraction patterns reveled the nanoparticles are 6-8 nm in diameter, consequently appropriate for developing optical devices in the infra red region. Finally the multilayer was grown inside a Fabry Perot cavity. The transmittance of the one-dimensional photonic crystal was measured.

Single photon sources are important components for future quantum communication networks. Lights emitting diodes with emission from an embedded self-organized quantum dot offer compact semiconductor sources that can be easily fabricated using standard photolithographic techniques. In this paper, progress towards an electrically driven 1300 nm quantum dot single photon emitter for fiber optic based applications are addressed. Low density longer wavelength emissions were achieved by exploiting the second critical growth threshold for large self-assembled InAs quantum dots on GaAs. The single photon collection efficiency was improved by incorporating the quantum dots between GaAs/AlxGa1-xAs distributed Bragg reflector mirror stacks and laterally confined inside etched micropillars. Resonance of the microcavity mode with the InAs quantum dot emission leads to an enhancement in the collection intensity. Emission from an active quantum dot was collected using a confocal microscope and coupled directly into a single mode fiber. Strong suppression in the multiphoton emission rate was verified by a custom Hanbury-Brown and Twiss interferometer set-up with optical fibers and InGaAs single photon avalanche photodetectors. Integration of electrical contacts with a planar resonant microcavity structure for a single photon light emitting diode is also discussed. Electroluminescence spectra recorded on such a device revealed sharp lines due to the charge recombination in a quantum dot. Correlation measurements on a single quantum dot line showed the suppression of multiphoton emission for an electrically driven source near 1300 nm for the first time.