We have realized an ultrafast diode containing a single quantum dot, whose transitions can be Stark shifted in a few hundred picoseconds. Carrying out resonant excitation of this sample we demonstrate pulsed single photon emission with high spectral purity, by rapidly sweeping the neutral exciton transition into and out of the laser energy. Detailed study of the detuning dependence of the fluorescence reveals sources of decoherence in this sample, which we show can be filtered out with narrow-band excitation.

An external-cavity laser with a quantum-dot (QD) gain medium is attractive because it combines the advantages of both QDs and the external-cavity configuration. Investigations of external-cavity QD lasers have revealed that these lasers demonstrate good performance with features such as a wide wavelength tuning range, stable lasing oscillation, and highspeed transmission. In this study, we employed an 800-GHz etalon filter inserted into an external cavity and obtained a four-channel oscillation spectrum that coincided with the local area network (LAN)-WDM grid. Each mode of the four channels oscillated stably at the single longitudinal mode defined by the external-cavity length. We sliced the four channels into a single channel using an inline band-pass filter. The filtered single channel has a high side-mode suppression ratio (SMSR) of 43.9 dB and a low relative intensity noise (RIN) of -137.9 dB/Hz in the frequency range of 0.5–20 GHz. For comparison with a multi-quantum well (MQW) gain medium, we obtained the four-channel spectrum using the same setup. However, each channel was multi-mode, and the four-channel simultaneous oscillation could not be maintained for a few dozen minutes. Furthermore, when we sliced the four-channels into a single channel, the spectrum intensity became changing; therefore, we could not measure the RIN. These results show that both the stable single longitude modes and the low RIN spectrum of the filtered mode are inherent in the QD medium, indicating that the external-cavity comb laser with the QD gain medium is promising as a light source for WDM transmission.

We fabricate an integrated photonic circuit with emitter, waveguide and detector on one chip, based on a hybrid superconductor-semiconductor system. We detect photoluminescence from self-assembled InGaAs quantum dots on-chip using NbN superconducting nanowire single photon detectors. Using the fast temporal response of these detectors we perform time-resolved studies of non-resonantly excited quantum dots. By introducing a temporal filtering to the signal, we are able to resonantly excite the quantum dot and detect its resonance fluorescence on-chip with the integrated superconducting single photon detector.

The performance of conventional Al(Ga)N planar devices decays drastically with increasing Al content, leading to low internal quantum efficiencies (IQEs) and high device operation voltages. In this paper, we show that these challenges can be addressed by utilizing epitaxially grown nitrogen polar (N-polar) Al(Ga)N nanowires. With a careful control of the growth conditions, a strong AlN band edge emission at 210 nm can be observed at room temperature, and an IQE of 80% was derived. Furthermore, the Mg incorporation can be drastically enhanced by controlling the growth rate. The hole concentrations of AlN:Mg nanowires were estimated to be on the order of 1016 cm-3, or higher at room temperature. 210 nm emitting AlN nanowire LEDs were achieved, which exhibit excellent electrical performance (at a forward current of 20 mA, the forward bias is about 8 V for a standard 300×300 μm2 device.). This can be ascribed to both efficient Mg doping and N-polarity induced internal electrical field that enhances hole injection. In the end, high performance AlGaN nanowire LEDs were demonstrated. This work provides a practical path for high efficiency DUV light sources with nanotechnology.

Ga assisted GaAs/GaAsSb core-shell structured nanowires were successfully grown on chemically etched p-type Si(111) substrate by molecular beam epitaxy (MBE). The morphology, structural and optical properties of the nanowires are found to be strongly influenced by the shell growth temperature and Sb% in the nanowires. The nanowires exhibit planar defects like twins and stacking faults, with more stacking faults and micro-twins found at the top section. Optical characteristics of the nanowires as measured by 4K photoluminescence (PL) exhibit a red shift to 1.2 eV with increasing Sb incorporation up to 12%. The Raman spectra of reference GaAs nanowires show TO and LO modes representative of the zinc blende structure at 291 cm^-1 and 267.8 cm^-1, respectively. Red shifts of both modes in conjunction with corresponding asymmetrical peak broadening observed in X-ray diffraction with increasing Sb incorporation are attributed to enhanced strain and disorder within the nanostructures. Nanowires of similar Sb composition but grown at different shell temperatures reveal straight nanowires with better microstructural and optical quality when grown at higher growth temperatures. The presence of GaAs passivation layer significantly enhanced the PL intensity such that PL was observed even at room temperature.

We realise growth of both GaAsP and GaAs core nanowires (NWs), as well as GaAsP core-shell NWs grown on (111) Si substrates using solid source molecular beam epitaxy (MBE). By modifying the growth conditions it is possible to change the dimensions of the GaAsP NWs and optimisation of these conditions yields high crystal quality structures. Scanning electron microscopy (SEM) as well as temperature, power and time resolved photoluminescence (PL) are used to study the optical and structural properties of the NWs. The incorporation of P into the NWs is used to shift the PL emission for ~ 810 nm to ~ 730 nm at 77 K, and also results in enhanced PL and an improved carrier lifetime. The addition of a p-doped GaAsP shell to a GaAsP core NW reduces the nonradiative recombination at surface states, as evidenced by x14 reduction of PL quenching with temperature, enhanced carrier lifetime, as well as a x3.5 increase in 77 K integrated PL intensity.

In this work, the optical properties and emission dynamics of core-shell InGaAs/GaAs nanopillars (NPs) have been in- vestigated using low-temperature photoluminescence (PL) and time-resolved photoluminescence (TRPL). These novel structures have recently attracted much interest within the silicon photonics scientific community due to their potential employment as gain medium for monolithically integrated lasers on silicon substrates. The optimization of the emission properties of these heterostructures is essential to obtain full compatibility with silicon photonics and requires an accurate tailoring of the pillar geometry (i.e. size, pitch) and composition. Therefore it is critical to gain deeper insight into the optical and dynamical properties of different NP designs if optimal device performance is to be achieved. The experimental characterization, carried out on a number of different NP structures with different geometries and compositions, shows that the time evolution of the emission peak exhibits a strong excitation-dependent blue-shift which can be attributed to the band-filling effect. Measured emission decay times were strongly geometry-dependent and varied from nanoseconds to tens of picoseconds. In addition, a dramatic reduction of the decay time was observed for the highest indium concentration due to the dominant contribution of the strain-induced non-radiative recombination processes.

In this work, we investigated how the blinking statistics and the photon antibunching behavior of single CdSe/CdS core/shell quantum dots(QDs) get modified in the presence of gold nanoparticles(Au NPs) overcoated with a silica shell of varying thickness.(Au@SiO2). The Au@SiO2 NPs have distinct plasmon resonance peaks which overlap with the absorption and emission of QDs, thereby effectively increasing the mutual plasmon-exciton interactions between them. From the second-order photoluminescence intensity cross-correlation measurements, we observed that in the regime of low excitation power, the relative ratio of the biexciton/exciton (BX/X) quantum yield (QY) and lifetimes of the single QDs in presence of the plasmonic substrates get significantly modified as compared to the QDs on glass. An electrodynamics model was developed to further quantify the effect of plasmons on the emission intensity, QY and lifetimes of X and BX of single QDs. The theoretical studies also indicated that the relative position of the QDs and orientation of the electric field are the critical factors regulating the emission properties of Xs and BXs.

We introduce the concept of using strained superlattice structures as defect filters, with their purpose to reduce the upwards propagation of dislocations that result from the lattice mismatch which occurs when III-V materials are grown on silicon substrates. Three samples with defect filter layers are grown on Si with and without in situ annealing and are compared to a similar structure grown on a GaAs substrate. Transmission electron microscopy is used to verify the effectiveness of the different designs grown on Si, with the twice-annealed sample reducing the number of defects present in the active region by 99.9%. Optical studies carried out exhibit brighter room temperature emission and reduced photoluminescence quenching with temperature in samples where annealing is performed. Photoluminescence excitation measurements reveal a ~20 meV redshift in the position of the GaAs exciton for the samples grown on Si compared to that of GaAs, indicating a residual inplane tensile strain ~0.35% in the GaAs of the active region for the samples grown on Si.

In quantum dot laser and solar cell structures, high temperature is required for the growth of cladding or window layer. Therefore to check the thermal stability in emission peak in high temperature strain coupled InAs/GaAs QDs are grown by MBE capped with two different type of spacer layer- InGaAs and InAlGaAs. Photoluminescence spectra shows multimodal distribution of QDs. Till 700^oC annealing temperature, no shift in peak emission wavelength is observed for InGaAs capped sample. The vertical strain prevents the inter-diffusion by maintaining a strain relaxed state due to coupling. For quaternary InAlGaAs capped QDs this stability is observed till 800^oC.

The application of static high pressure provides a method for precisely controlling and investigating many fundamental and unique properties of semiconductor nanocrystals (NCs). This study systematically investigates the high-pressure photoluminescence (PL) and time-resolved carrier dynamics of thiol-capped CdTe NCs of different sizes, at different concentrations, and in various stress environments. The zincblende-to-rocksalt phase transition in thiol-capped CdTe NCs is observed at a pressure far in excess of the bulk phase transition pressure. Additionally, the process of transformation depends strongly on NC size, and the phase transition pressure increases with NC size. These peculiar phenomena are attributed to the distinctive bonding of thiols to the NC surface. In a nonhydrostatic environment, considerable flattening of the PL energy of CdTe NCs powder is observed above 3.0 GPa. Furthermore, asymmetric and double-peak PL emissions are obtained from a concentrated solution of CdTe NCs under hydrostatic pressure, implying the feasibility of pressure-induced interparticle coupling.

Lead sulfide (PbS) quantum dots are attractive candidates for optical refrigeration due to the tuneability of their band gap as a function of size and the resulting quantum confinement. We focus on the simple and straightforward method of synthesis of PbS NPs coated with zein protein with well-defined morphologies which show remarkable pH responsive behavior and excellent photo physical properties. The synthesis of PbS NPs in the presence of zein depicts a fine shape control effect as indicated in TEM images. At 0.4 %, zein produces nanocubes of ~ 42 nm which convert into spheres of almost equal dimensions with 0.2 % and 0.1 % zein. Since all reactions contain equal amount of precursor with identical conditions except the amount of zein, the shape transformation from nanocubes to spheres is mainly due to the decrease in the amount of zein from 0.4 to 0.2 %. The presence of protein coating makes the NPs bioactive and pH responsive. An increase in the pH systematically increases the absorbance of PbS NPs with a significant red shift which is simultaneously shown by remarkable color change in the solution from light grey with orange tinge to dark brown. These photo physical properties may have implications on laser cooling and it will be discussed.

One dimensional superstructures are of great interest in the field of optics, opto-electronics and photonics. Over a period of time, researchers have developed various new and low cost techniques to prepare nanostructures of metals as well as semiconductors. We found that ionic liquid can be used to synthesis nanostructured film directly on the substrate by means of electrodeposition. It was found that spindle like CdTe superstructure assemblies over the commercial F:SnO₂ substrate gives a Schottky junction, resulting in the highest rectification ratio of 6000. It was also possible to manipulate the dimensions of structures by changing the precursor molar ratio in the ionic liquid bath. It was observed that by varying the structure size, electrical properties of the diodes improve significantly. The admittance measurements reveal that by increasing the length and diameter of the structures, the conductance of the diode increases linearly. We believe that proper arrangement of the fabricated CdTe thin films will find many applications in near future.

This paper presents a detailed morphological analysis of vertically strain-coupled InAs quantum dots with a fixed quaternary capping (In_0.21Al_0.21Ga_0.58As) of 3 nm and a GaAs barrier ranging in thicknesses from 9 to 18 nm. The coupled heterostructures were studied using cross-sectional transmission electron microscopy and compared with uncoupled heterostructures with 2-nm quaternary capping and 50-nm GaAs capping thickness. Power-dependent photoluminescence spectra showed that a minimum capping of 9 nm produced a multimodal dot-size distribution. Increasing the capping from 9 to 18 nm reduced the vertical correlation, thus increasing the dot uniformity. Increasing the capping thickness reduced the coupling and increased the dot size. At a maximum capping (18nm) coupled quantum dots exhibit a bimodal dot-size distribution compared to the mono-modal distribution of the uncoupled quantum dots. The coupled samples demonstrated superior optical properties to uncoupled samples.