Robin L. Williams

Extremely low threshold current strained InGaAs/AlGaAs lasers by molecular beam epitaxy

Using solid source molecular beam epitaxy we have grown strained layer InGaAs/AlGaAs graded index separate confinement heterostructure lasers operating at 1.01 μm. For broad‐area, uncoated Fabry–Perot devices with cavity lengths in excess of 3000 μm, the threshold current density is 56 A/cm2, a value which we believe to be the lowest ever reported for laser diodes in any materials system. The internal quantum efficiency for these lasers is 88%, while the materials losses are 1.8 cm−1.

Optical spectroscopy of single, site-selected, InAs/InP self-assembled quantum dots

We present optical spectroscopy measurements on a single InAs/InP quantum dot emitting around λ=1.55 μm. The dot is produced using a nanotemplate deposition technique that allows precise a priori control of quantum dot position and electronic configuration. Clear evidence of excitonic shell structure and many-body renormalization effects are observed.

Monolithic, surface-emitting, semiconductor visible lasers and spectrometers for WDM fiber communication systems

Proof-of-concept devices to create surface-emitting visible lasers are described and demonstrated. A visible, surface-emitting, solid-state diode laser for InGaAs, InP, and GaAs geometries, as well as for several other devices, is discussed. The use of a multilayer system to enhance the waveguide harmonic mixing over a large bandwidth is demonstrated. The resulting surface-emitted light can then be used to measure frequency differences of multiple optical channels in a fiber communication system. It can also be used for nonblocking switching systems. Since phase is preserved by the harmonic mixing, the device is also usable in coherent communication schemes. Experimental measurements of harmonic generation coefficients for a large Al alloy content in AlGaAs are presented. >

Resonant scattering and second-harmonic spectroscopy of planar photonic crystal microcavities

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...

Metal-insulator transition atB=0inp-type SiGe

Observations are reported of a metal-insulator transition in a 2D hole gas in asymmetrically doped strained SiGe quantum wells. The metallic phase, which appears at low temperatures in these high mobility samples, is characterised by a resistivity that decreases exponentially with decreasing temperature. This behaviour, and the duality between resistivity and conductivity on the two sides of the transition, are very similar to that recently reported for high mobility Si-MOSFETs.

Nanowire coupling to photonic crystal nanocavities for single photon sources

We demonstrate highly efficient evanescent coupling via a silica loop-nanowire, to ultra-small quantum-dot photonic-crystal cavities. It enables the tuning of both the Q-factor and the wavelength of the cavity mode independently.

Using As/P exchange processes to modify InAs/InP quantum dots

We have used low temperature photoluminescence and atomic force microscopy to study the growth, by chemical beam epitaxy, of self-assembled InAs/InP quantum dots. By modifying the procedure for capping the dots we have been able to control their emission energy by adjusting their height. This process relies on the As/P exchange process that occurs when an InAs surface is exposed to a phosphorus flux. This exchange is shown to occur for both continuous and discontinuous capping procedures.

InGaAs/InP quantum well intermixing studied by cross-sectional scanning tunneling microscopy

Cross-sectional scanning tunneling microscopy (STM) is used to study lattice matched InGaAs/InP quantum well (QW) intermixing induced by ion implantation and thermal annealing. Different strain development in QWs (determined by STM topography of elastic relaxation in cross sectionally cleaved samples) is found to be dependent upon the range of the implanted ions relative to the QWs. It is found that the quantum wells remain latticed matched to the barrier layers after intermixing when ions are implanted through the multiple quantum well (MQW) stack. A shallow implantation in which ions are implanted into the cap layer above the MQW stack leads to tensilely strained wells and compressively strained interfaces between wells and barriers. The strain development in the latter case is attributed to different degrees of interdiffusion on the group III and group V sublattices. Finite element elastic computations are used to extract the group V and group III interdiffusion length ratio, and results using differen...

Selective-area vapor-liquid-solid growth of tunable InAsP quantum dots in nanowires

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.

Fabrication of nanostructures in strained InGaAs/GaAs quantum wells by focused‐ion‐beam implantation

Low‐temperature photoluminescence (PL) spectroscopy was used to estimate the compositional disordering induced by Ga focused‐ion‐beam implantation in a series of strained InGaAs/GaAs quantum wells (QW). A simple formalism was derived to estimate the indium concentration and the degree of interdiffusion of the QWs from the observed shifts in PL transition energies. A study of nanostructures indicated that the lateral extent of compositional disordering was significantly larger than predicted from the lateral spreading of the ion beam.