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We present a compact, fibre-coupled single photon source using gradient-index (GRIN) lenses and an InAsP semiconductor quantum dot embedded within an InP photonic nanowire waveguide. A GRIN lens assembly is used to collect photons close to the tip of the nanowire, coupling the light immediately into a single mode optical fibre. The system provides a stable, high brightness source of fibre-coupled single photons. Using pulsed excitation, we demonstrate on-demand operation with a single photon purity of 98.5% when exciting at saturation in a device with a source-fibre collection efficiency of 35% and an overall single photon collection efficiency of 10%. We also demonstrate “plug and play” operation using room temperature photoluminescence from the InP nanowire for room temperature alignment.

Photonics-based quantum information technologies require efficient, high emission rate sources of single photons. Position-controlled quantum dots embedded within a broadband nanowire waveguide provide a fully scalable route to fabricating highly efficient single-photon sources. However, emission rates for single-photon devices are limited by radiative recombination lifetimes. Here, we demonstrate a multiplexed single-photon source based on a multidot nanowire. Using epitaxially grown nanowires, we incorporate multiple energy-tuned dots, each optimally positioned within the nanowire waveguide, providing single photons with high efficiency. This linear scaling of the single-photon emission rate with number of emitters is demonstrated using a five-dot nanowire with an average multiphoton emission probability of

We briefly review the features of interband cascade lasers (ICLs) and discuss rationales for using an InAs-based plasmon waveguide configuration for extending efficient operation of ICLs to long wavelengths. We then present our recent progress in InAs-based ICLs based on an advanced waveguide configuration. We demonstrated room temperature continuous wave operation of ICLs at wavelengths longer than 6 μm.

Through fluctuations in the local composition, InAsP quantum dots embedded within site-selected InP nanowires are observed to display biexciton binding energies spanning a range between -0.3 meV and +2.9 meV. From this range we select dots having energy-degenerate exciton and biexciton emission and observe an excitation rate-mediated transition from sub- to super-Poissonian second-order correlation statistics. Under pulsed excitation, g⁽²⁾ (τ=0) is found to increase from 0.5 at high excitation levels, rising to 28 as the excitation is reduced by two orders of magnitude. The observed second-order correlation statistics are interpreted using both a stochastic model and a rate equation model of the competition between the various excitonic emission processes. Our results demonstrate that nanowire quantum dots represent a promising approach to the efficient generation of twin-photon states.

Interband cascade lasers (ICLs) are becoming a leading semiconductor laser technology for the mid-infrared because of their high efficiency and low power consumption, especially as compared with conventional diode lasers and intersubband quantum cascade lasers (QCLs) in the wavelength range from 3-5 μm. Although a greater effort has been directed towards GaSb-based ICLs in the ~3-5μm range, recent work has highlighted the exciting potential for InAs-based ICLs for reaching longer emission wavelengths. In this work we report the development of low-threshold InAs-based ICLs with a room-temperature emission wavelength of 6.3μm. The devices were grown on n+-InAs (100) substrates by solid-source molecular beam epitaxy in a custom V90 system using valved crackers for Sb2 and As2. The ICL structures employ an improved waveguide design using intermediate AlAs/AlSb/InAs strain-balanced superlattice cladding layers surrounded by heavily-doped n+-InAs plasmonic claddings. The active region includes 15-stages with AlSb/InAs/In(0.35)Ga(0.65)Sb/InAs/AlSb type-II “W” quantum wells and optimized electron injector doping. In pulsed mode, broad-area devices lased at 300 K at a lasing wavelength of 6.26 μm and a threshold current density of 395 A/cm2 which is the lowest ever reported among semiconductor lasers at similar wavelengths. The broad-area devices lased up to 335K in pulsed mode at a wavelength of 6.45 μm. These results provide strong evidence of the potential for InAs-based ICLs as efficient sources in the mid-IR.

We demonstrate an in-situ technique to tune the emission energy of semiconductor quantum dots. The technique is based on laser-induced atomic intermixing applied to nanowire quantum dots grown using a site-selective process that allows for the deterministic tuning of individual emitters. A tuning range of up to 15 meV is obtained with a precision limited by the laser exposure time. A distinct saturation of the energy shift is observed, which suggests an intermixing mechanism relying on grown-in defects that are subsequently removed from the semiconductor material during annealing. The ability to tune different emitters into resonance with each other will be required for fabricating remote quantum dot-based sources of indistinguishable photons for secure quantum networks.

InAs-based interband cascade (IC) lasers with improved optical confinement have achieved high-temperature operation with a threshold current density as low as 333 A/cm2 at 300 K for emission at 6003 nm. The threshold current density is the lowest ever reported among semiconductor midinfrared lasers at similar wavelengths. These InAs-based IC devices lased in pulsed mode at temperatures up to 357 K near 6.28 μm. A narrow-ridge device was able to operate in continuous-wave mode at temperatures up to 293 K at 6.01 μm.

GaSb-based tunable single-mode diode lasers can enable rapid, highly-selective and highly-sensitive absorption spectroscopy systems for gas sensing. In this work, single-mode distributed feedback (DFB) laser diodes were developed for the detection of various trace gases in the 2-3.3um range, including CO2, CO, HF, H2S, H2O and CH4. The lasers were fabricated using an index-coupled grating process without epitaxial regrowth, making the process significantly less expensive than conventional DFB fabrication. The devices are based on InGaAsSb/AlGaAsSb separate confinement heterostructures grown on GaSb by molecular beam epitaxy. DFB lasers were produced using a two step etch process. Narrow ridge waveguides were first defined by optical lithography and etched into the semiconductor. Lateral gratings were then defined on both sides of the ridge using electron-beam lithography and etched to produce the index-grating. Effective index modeling was used to optimize the ridge width, etch depths and the grating pitch to ensure single-lateral-mode operation and adequate coupling strength. The effective index method was further used to simulate the DFB laser emission spectrum, based on a transfer matrix model for light transmission through the periodic structure. The fabricated lasers exhibit single-mode operation which is tunable through the absorption features of the various target gases by adjustment of the drive current. In addition to the established open-path sensing applications, these devices have great potential for optoelectronic integrated gas sensors, making use of integrated photodetectors and possibly on-chip Si photonics waveguide structures.

A method to integrate nanowire-based quantum dot single photon sources on-chip using evanescent coupling is demonstrated. By deterministically placing an appropriately tapered III-V nanowire, containing a single quantum dot, on top of a silicon-based ridge waveguide, the quantum dot emission directed toward the taper can be transferred to the ridge waveguide with calculated efficiencies close to 100%. As the evanescent coupling is bidirectional, the source can be optically pumped in both free-space and through the ridge waveguide. The latter configuration paves the way toward a self-contained, all-fiber, plug-and-play solution for applications requiring a bright on-demand single photon source. Using InAsP quantum dots embedded in InP nanowire waveguides, coupling efficiencies to a SiN ridge waveguide of 74% with a single photon purity of 97% are demonstrated. The technique to demonstrate deterministic placement of single quantum emitters onto pre-fabricated waveguides is used, an important step toward the fabrication of complex quantum photonic circuits.

Hexagonal lattice photonic crystal microcavities with modified single missing air-hole defects were fabricated in suspended InAs∕InP quantum dot membranes. The cavity modes predicted from finite-difference time-domain simulations are observed in photoluminescence measurements. The resonant energies of the defect modes are tuned across the band gap of the photonic crystal through modifications of the size and position of the inner ring holes surrounding the defect. Up to a 20-fold enhancement of the quality factor of the modes are observed as they are tuned across the band gap, with measured Q values of up to 6000.

Interband cascade (IC) lasers have been demonstrated based on type-I InGaAsSb/AlAsSb quantum well (QW) active regions. These type-I IC lasers are composed of 6-cascade stages and InAs/AlSb superlattice cladding layers. In contrast to the use of quinary AlGaInAsSb barriers for active region in previous type-I QW lasers, the type-I QW active region in each stage is sandwiched by digitally graded multiple InAs/AlSb QW electron injector and GaSb/AlSb QW hole injector. The fabricated type-I IC lasers were able to operate in continuous wave and pulsed modes at temperatures up to 306 and 365 K, respectively. The threshold current densities of broad-area lasers were around 300 A/cm2 at 300 K with a lasing wavelength near 3.2 μm. The implications and prospects of these initial results are discussed.

Silicon wafers varying in doping type and concentration have been probed using slow positrons. The variation of positron diffusion with doping is explained in terms of intrinsic electric fields extending from the native-oxide/Si interface into the crystalline bulk. Variation in the surface S parameter is correlated with the bulk Fermi level. Large variations between measured data from three differently doped samples of 70 nm amorphous Si overlayers created by ion irradiation are shown. The need for the inclusion of sample characteristics resulting from doping to obtain meaningful quantitative results is stressed.

We present optical characterization measurements from three-dimensional pillar microcavities comprising an InP spacer layer sandwiched between two SiO2/Ta2O5 Bragg reflectors. InAs quantum dots placed at the center of the InP spacer are used as broadband emitters. Emission into the lowest-order mode is observed at an energy of 820 meV in the wide-pillar limit, with shifts of approximately 30 meV in the ground state and more than 80 meV in higher-order modes for pillars of 2 μm diameter. Excellent agreement is found between the experimentally observed cavity mode energies as a function of pillar diameter and the predictions of guided-mode theory. Cavity quality factors in excess of 2200 are obtained for large diameter pillars, with a decrease to approximately 500 for pillars of 2 μm diameter.

Ghosh et al. recently proposed that positron stopping profiles in elements and elemental multilayers, calculated by Monte Carlo for incident positron energies in the range 1–10 keV, may be scaled onto energy independent curves using the mean implantation depth. It is shown here that Monte Carlo stopping profiles in elemental multilayer systems can be reproduced accurately in the incident energy range 1–25 keV using a modification of this scaling model that takes into account the backscattering effects of interfaces. The mean depth scaling approach represents a saving of several orders of magnitude in the computation time for multilayer stopping profiles and has been incorporated into a new defect profiling program POSTRAP6.

The ability to control the nucleation site of a single quantum dot will have a profound effect on the development of quantum dot-based photonic devices. The deterministic approach will provide a truly scalable technology that can take full advantage of conventional semiconductor processing for device fabrication. In this review, we discuss the progress towards the integration of deterministically nucleated single quantum dots with top-down quantum optical devices targeting telecommunication wavelengths. Advances in site-controlled quantum dot nucleation using selective-area epitaxy now makes it possible to position quantum dots at predetermined positions on a substrate in registry with alignment markers. This, in turn, has allowed for devices fabricated in subsequent processing steps to be aligned to individual quantum dots. The specific devices being targeted are gated-single dots and coupled dot-cavity systems which are key components of efficient sources of single photons and entangled photon pairs.

We report on the experimental demonstration and analysis of a new waveguide principle using subwavelength gratings. Unlike other periodic waveguides such as line-defects in a 2D photonic crystal lattice, a subwavelength grating waveguide confines the light as a conventional index-guided structure and does not exhibit optically resonant behaviour. Subwavelength grating waveguides in silicon-on-insulator are fabricated with a single etch step and allow for flexible control of the effective refractive index of the waveguide core simply by lithographic patterning. Experimental measurements indicate a propagation loss as low as 2.1 dB/cm for subwavelength grating waveguides with negligible polarization and wavelength dependent loss, which compares favourably to conventional microphotonic silicon waveguides. The measured group index is nearly constant n(g) ~1.5 over a wavelength range exceeding the telecom C-band.

Optoelectronic devices based on single, self-assembled semiconductor quantum dots are attractive for applications in secure optical communications, quantum computation and sensing. In this paper we show how it is possible to dictate the nucleation site of individual InAs/InP quantum dots using a directed self-assembly process, to control the electronic structure of the nucleated dots and also how to control their coupling to the optical field by locating them within the high field region of a photonic crystal nanocavity. For application within fiber networks, these quantum dots are targeted to emit in the spectral region around 1550 nm., Quantum Sensing and Nanophotonic Devices VII, January 24, 2010, San Francisco, California, USA

Site-selective epitaxy is used to deterministically control the nucleation site of a single quantum dot. A photonic crystal cavity is fabricated at the dot site for a true single quantum dot-cavity system which, by design, contains no background emitters. Cavity tuning at fixed temperature is used to measure the dot-cavity coupling over a large $(g15\text{ }\text{meV})$ detuning range using nonresonant excitation. The low-excitation spectra are modeled using a master equation model based on incoherent excitation. We find that pure phonon dephasing alone does not account for the observed nonresonant cavity emission and an additional cavity feeding mechanism, consistent with phonon-assisted dot-cavity coupling, must be included to reproduce the experimental spectra.

SPIE OPTO: Integrated Optoelectronic Devices, January 24, 2009, San Jose, California, United States, Series: SPIE Proceedings

Single-mode laser diodes on GaSb substrates were developed using InGaAsSb/AlGaAsSb triple quantum well active regions grown by molecular beam epitaxy. The devices were fabricated using lateral Cr gratings, with a grating pitch designed to coincide with a strong absorption feature of HF gas, deposited adjacent to a dry-etched narrow ridge waveguide. High sidemode suppression was achieved, and in 20°C continuous-wave operation, devices with a 400μm-long cavity provided 4.5mW total output power at the 2396nm target wavelength. Anti-reflection and high-reflection facet coatings exhibited no deleterious effects on the laser tunability or mode quality, thus allowing the preferential extraction of output power from a single laser facet.

We present the results of experimental measurements and numerical calculations of the layer uniformity of MBE grown GaAs layers on stationary and rotating wafers using different crucible geometries. We obtain excellent agreement between theory and experiment for the actual geometry used in the MBE deposition and describe the configuration required to obtain very good uniformity over a 3 inch rotating wafer. We also study the case of a trumpet crucible insert used as a simple way of improving layer uniformity.

A detailed study of the high-power pulsed operation of C-band optically-pumped GaInNAsSb vertical cavity surface emitting lasers is reported. The devices employ a resonant periodic gain structure grown by molecular beam epitaxy on a GaAs substrate with a 31-pair GaAs/AlAs bottom distributed Bragg reflector and a 4-λ, GaAs-based resonant cavity containing 10 GaInNAsSb quantum wells distributed among the 7 antinodes of the electric field. A dual-pump-band SiO2/TiO2 dielectric top mirror allows efficient optical pumping via low reflectivities at 808nm and 1064nm while providing very high reflectivity at the 1.55μm target emission wavelength. The laser characteristics were evaluated using both a Q-switched Nd:YAG 1064nm pump and a 20W-peak 180ns-pulsed 850nm diode laser. The importance of the gain-cavity detuning was evident from time-dependent spectral measurements of laser material subjected to post-growth annealing at different temperatures between 725 and 775°C. The highest annealing temperature produces the largest blue shift of the gain peak relative to the cavity resonance, resulting in the best power transfer characteristics as well as reduced temperature sensitivity.

We demonstrate highly efficient evanescent coupling via a silica loop-nanowire, to ultra-small (0.5 (lambda/n)(3) ), InAs/InP quantum dot photonic crystal cavities, specifically designed for single photon source applications. This coupling technique enables the tuning of both the Q-factor and the wavelength of the cavity mode independently, which is highly relevant for single photon source applications. First, this allows for the optimization of the extraction efficiency while maintaining a high Purcell factor. Second, the cavity mode can be matched with a spectrally misaligned quantum dot without changing the structure or degrading the Q-factor: a 3 nm resonance shift is reported.

The room-temperature 1.55 m m continuous-wave (CW) operation of single-lateral mode GaInNAsSb ridge waveguide lasers grown on GaAs is reported. Detailed measurements of the light output power and spectral properties were used to assess the device characteristics as a function of applied current and temperature in both CW and pulsed operation. An exemplary, 3 m ×750 m m, device with a 92% high-reflectivity back facet coating exhibited a record low CW threshold current of 63~mA, with a peak output power of 15~mW. High-resolution modal gain spectra were extracted from amplified spontaneous emission measurements yielding the internal loss (8.0~cm -1 , transparency current (50~mA) and the wavelength dependence of the differential gain. The latter was used with careful measurements of the Fabry-Perot mode shift with injection current to determine the linewidth enhancement factor of 2.8 at the transparency current. The first measurement of intrinsic modulation frequency in 1.55 m m GaInNAsSb lasers is reported, based on the observed relative intensity noise (RIN). The RIN measurements indicate a maximum modulation frequency of 7.2~GHz, which is a promising result for future telecommunications applications.

We demonstrate efficient coupling via a tapered silica nanowire micro-loop to a photonic crystal based nanocavity containing quantum dots, specifically designed for single photon source applications.

We demonstrate efficient coupling via a tapered silica nanowire micro-loop, to a photonic crystal based nanocavity containing quantum dots, specifically designed for single photon source applications.

Polarization-resolved second-harmonic spectra are obtained from the resonant modes of a two-dimensional planar photonic crystal microcavity patterned in a free-standing InP slab. The photonic crystal microcavity is comprised of a single missing-hole defect in a hexagonal photonic crystal host formed with elliptically-shaped holes. The cavity supports two orthogonally-polarized resonant modes split by 60 wavenumbers. Sum-frequency data are reported from the nonlinear interaction of the two coherently excited modes, and the polarization dependence is explained in terms of the nonlinear susceptibility tensor of the host InP., Comment: 7 pages, 8 Postscript figures, to be presented at Photonics West Jan. 25

The 1.55 mum pulsed operation of an optically-pumped GaInNAsSb vertical-cavity surface emitting laser (VCSEL) is reported. Full temperature (10-50degC) and wavelength-dependent (1540-1560 nm) laser measurements were recorded. The annealing effect on laser performance is also shown.

We examine the optical mode structure of two-dimensional, photonic crystal microcavities based on triple missing hole defects in hexagonal symmetry, etched air-hole, suspended InP membranes. Polarisation dependent photoluminescence from InAs/InP quantum dots embedded within the cavities is used to explore mode energies and quality factors, Q, as a function of cavity design parameters. Optimised Q values are in excess of 7,500 for the modified y-dipole mode. (© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

A method to fine-tune photonic crystal defect cavities is developed based on successive oxidation and wet etching cycles. Photonic crystal microcavities based on InP membranes are oxidized using an ultraviolet (UV)/ozone treatment, and the oxide is subsequently removed using a hydrofluoric acid solution. Each oxidation/etch cycle consumes a thin layer of InP directly exposed to the UV/ozone, enlarging the radius of holes in the photonic crystal and decreasing the membrane thickness. The method is applied to single missing air-hole defect cavities with embedded InAs quantum dots, permitting measurement of the resonant frequency tuning in emission. Defect mode energies were found to blueshift 1.74meVpercycle, consistent with finite-difference time-domain simulations. A tuning range of 33meV was obtained after 20cycles.

The resonant modes of two-dimensional planar photonic crystal microcavities patterned in a free-standing InP slab are probed in a novel fashion using a long working distance microscope objective to obtain cross-polarized resonant scattering and second-harmonic spectra. We show that these techniques can be used to do rapid effective assays of large arrays of microcavities that do not necessarily contain resonant light-emitting layers. The techniques are demonstrated using microcavities comprised of single missing-hole defects in hexagonal photonic crystal hosts formed with elliptically shaped holes. These cavities typically support two orthogonally polarized resonant modes, and the resonant scattering and harmonic spectra are well fitted using a coherent sum of Lorentzian functions. The well-defined coherence between the two resonant features is explained in terms of a microscopic harmonic oscillator model. The relative merits of these techniques are quantitatively compared with the more commonly used cavi...

Electrical and structural properties of platinum monosilicide (PtSi) in deep submicron lines are reported. The sheet resistance of the silicide films was found to be rather independent of the linewidth down to dimensions as small as 0.15 µm. Plan-view and cross-sectional transmission electron microscopy was performed to study the structural properties of these films, including their gain structures and lateral growth. The insensitive nature of the electrical properties of the silicide films to the linewidths is correlated with their structural properties.

A technique, based on quantum well (QW) intermixing, has been developed for the post growth, spatially selective tuning of the QW bandgap in a semiconductor laser structure. High energy (MeV) ion implementation is used to create a large number of vacancies and interstitials in the device. During high temperature processing (rapid thermal annealing), these defects simultaneously enhance the intermixing of the QW and the barrier materials, producing a blue shift of the quantum well bandgap, and are annealed out. Increases in the bandgap energy of greater than 100 nm at 1.55 (mu) m in InGaAs/InGaAsP/InP structures can be achieved, while absorption losses are unaffected or reduced. Absorption spectroscopy in the waveguide geometry is used to quantify any excess loss in the structure. Using a simple masking scheme to spatially modify the defect concentration, different regions of a wafer can be blue shifted by different amounts. This allows the integration of many different devices such as lasers, detectors, modulators, amplifiers and waveguides on a single wafer using only a single, post-growth processing step. The performance of both passive (waveguide) and active (laser and amplifier) devices produced using this technique will be described, as well as the practicality of this technique in the production of photonic integrated circuits.

The Stranski-Krastanov (S-K) self-assembly process produces quantum dots with a large distribution of sizes. In the S-K growth mode, the in-plane spatial distribution of dots is quasi-random. Different strategies have been used to narrow the size distribution and increase the dot-dot spatial correlation. Here, we report a simple method to reduce the height of taller dots by using the P/As exchange exhibited during CBE growth. The fabrication of InAs/InP quantum dots self-assembled into linear arrays through growth on patterned substrates is described. Photoluminescence from one-dimensional arrays is compared with the non-correlated case of quantum dots grown on planar substrates. The interplay between size and spatial distributions is discussed.

Time-resolved photoluminescence (PL), steady-state PL, and electro-luminescence techniques have been used to characterize the carrier relaxation processes and carrier escape mechanisms in a self-assembled AlInAs quantum dots (SAQD) p-i-n laser structure under reversed bias. The measurements were performed between 5 K and room temperature on a ring mesa sample as a function of bias. At 100 K, the PL decay time originating from the SAQD decreases with increasing reversed bias from approximately 450 ps under flat band condition to approximately 150 ps for biases of -4 V. The data can be explained by a simple model based on electron recombination in the quantum dots or escape our of the dots. The escape can occur by one of three possible routes: direct tunneling out of the distribution of electron excited state level, thermally assisted tunneling of ground state electrons through the upper electron excited states or thermionic emission to the wetting layer.

Although silicon is the paramount material for the microelectronic industry, bulk Si is of little use for photonic devices owing to its indirect band-gap, which prevents the all-important direct optical transitions. However, a new type of luminescent Si has opened up its future for photonic applications. This new material is light-emitting SiO 2 /Si superlattices, fabricated in our laboratory. Our theoretical calculations showed that the energy band within the silicon layer has direct band-gap character, a result of strong quantum-confinement caused by the large band-offset at the SiO 2 /Si interface, so that the direct optical transition is not only possible but also vigorous. For a quantum-confined amorphous silicon, the breakdown of angular momentum will naturally make all optical transition possible. Our experiments have shown that SiO 2 /Si superlattices can indeed emit bright light. Moreover, the band-gap or the wavelength can be tuned over the visible range by changing the Si layer thickness, in good agreement with quantum confinement theory. The luminescence intensity as a function of Si layer thickness is found to increase, reach a maximum, and then decrease. Theoretical studies show that this phenomena is caused by competition between an increased overlap of electron-hole wave functions in the normal direction to the quantum well and an increased exciton radius in the plane of the quantum well.

Polarization insensitive 1.5 micrometer QW optical amplifiers, modulators, and detectors were fabricated using a novel, simple, post-growth, integratable technique. The process utilizes ion-implantation-induced, spatially selective, quantum well (QW) shape modification. A simple model shows that if the interdiffusion rate of the anions is larger than that of the cations, the blue shift in the ground state heavy hole transition energy after implantation and annealing is greater than the light hole state blue shift, merging the two bands and thus eliminating the difference between the TE (transverse electric) and TM (transverse magnetic) waveguide propagation modes. Current- voltage measurements indicate that junction characteristics are well maintained after processing. This simple technique for fabricating discrete polarization insensitive optoelectronic devices is readily extended to the monolithic integration of such devices along with other passive and active optoelectronic devices and provides a pathway to practical photonic integrated circuits., Emerging Components and Technologies for All-Optical Photonic Systems II, Nov. 18, 1996, Boston, Massachusetts, United States, Series: Proceedings of SPIE; no. 2918

A technique for fabricating transparent waveguides on the same wafer as a quantum well (QW) DBR laser array has been developed. High [MeV] energy ion implantation is used to create a large number of vacancies and interstitials throughout the active region of the device. Upon annealing, these entities enhance the intermixing of the QW and barrier materials resulting in a blue shift of the QW bandgap. Energy shifts (measured using low temperature photoluminescence spectroscopy) of greater than 60 meV can be achieved. Room temperature waveguide absorption measurements verify the shift in the bandgap energy and confirm that the waveguide is now effectively transparent in the wavelength range of the QW lasers. This technique is being used in an eight wavelength WDM transmitter array in which the waveguiding region is selectively implanted and blue shifted.

We present the electronic properties of a triple quantum dot molecule embedded inside a sub-micron mesa, made from a quadruple-barrier triplequantum- well structure, and surrounded by a single gate electrode. We outline the design principles of the quadruple-barrier triple-quantum-well structure and calculate the energy of the three lowest states as a function of center well thickness. We observe regular and irregular shaped Coulomb diamond regions similar to those for double quantum dot devices. Variation in the Coulomb blockade region shape is introduced by fluctuation in the offset energies between the quantum dots likely associated with device processing and random impurity potential in the material. We also present Coulomb blockade patterns calculated with a constant interaction model for sequential tunneling through the three series-coupled quantum dots. © 2012 The Japan Society of Applied Physics.

Quantum well intermixing has been performed using ion implantation techniques to increase the optical bandgap in a spatially selective manner. We show that there is a maximum single dose beyond which further intermixing of the QWs is impeded by damage to the semiconductor surface. We overcome this problem by using a series of implants and rapid thermal anneals, with each rapid thermal anneal repairing the crystal surface. Using this technique we have demonstrated shifts in optical bandgap for multiple implants greater than seven times that observed for a single implant.© (1994) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

A process is described where the position, size, and cladding of an InP nanowire with an embedded InAsP quantum dot are determined by design through lithography, processing, and growth. The vapor-liquid-solid growth mode on a patterned substrate is used to grow the InP core and defines the quantum dot size to better than ±2 nm while selective-area growth is used to define the cladding thickness. The clad nanowires emit efficiently in the range λ=0.95–1.15 μm. Photoluminescence measurements are used to quantify the dependence of the excitonic energy level structure on quantum dot size for diameters 10–40 nm.

We describe a means to realize slow and fast electron channels by coherent mixing of single-particle levels in quantum dots. The underlying physics, which gives insight into state superposition, can potentially be realized in multi-dot structures with complex gate control. However, we employ vertical double dot structures and in our scheme the mixing of single-particle levels arises because of natural perturbations in the confining potential of the high-symmetry dots. Additionally, because of the intrinsic properties of a Fock–Darwin-like spectrum, we utilize a magnetic field to bring multiple single-particle energy levels into close proximity. We determine single-electron resonant tunneling times (effectively dwell times when on resonance) that are either extended in the slow channel or shortened in the fast channel. Most dramatically, for the slow channel, slow-down factors of ∼10 and single-electron resonant tunneling times extended into the µs range are demonstrated in all systems of two, three, and four mixed single-particle states investigated here.

A comparison is made between the conventional non-selective vapour-liquid-solid growth of InP nanowires and a novel selective-area growth process where the Au-seeded InP nanowires grow exclusively in the openings of a SiO(2) mask on an InP substrate. This new process allows the precise positioning and diameter control of the nanowires required for future advanced device fabrication. The growth temperature range is found to be extended for the selective-area growth technique due to removal of the competition between material incorporation at the Au/nanowire interface and the substrate. A model describing the growth mechanism is presented which successfully accounts for the nanoparticle size-dependent and time-dependent growth rate. The dominant indium collection process is found to be the scattering of the group III source material from the SiO(2) mask and subsequent capture by the nanowire, a process that had previously been ignored for selective-area growth by chemical beam epitaxy.

The authors study the quality factor Q of the x-dipole mode in single missing hole defect photonic crystal microcavities in InAs∕InP quantum dot membranes as a function of the structural design parameters. Photoluminescence experiments show an optimized Q in excess of 28 000 for a wavelength close to λ=1550nm. This is to be compared with a Q of 57 000 determined by finite difference time domain calculations. The fabrication tolerances necessary to achieve experimental Q values close to those predicted by theory are identified.

Over the last 10 years, positrons have been used as a tool to study and profile dilute concentrations of defects in solids. The two basic techniques involve the measurement of the positron lifetime and the Doppler broadening of the annihilation γ-rays. Only recently have positrons been used to profile defects in multilayered structures. We review the theory of positron–defect interactions and positron transport in silicon, and introduce a model used to profile defects and electric-field effects using the steady-state diffusion equation. An example is discussed that involves the modelling of defects and electric fields in homoepitaxial layers of silicon grown by molecular-beam epitaxy on Si(100) substrates. Finally we discuss the limitations of the modelling technique and future prospects.

We assess the non-radiative recombination mechanisms in GaInNAsSb structures by comparing p-doped, n-doped and un-doped material. Our results indicate a contribution from thermally activated leakage in 1.55 mum GaInNAsSb Quantum well lasers at elevated temperatures.