Robust single frequency index-patterned laser design using a Fourier design method.

We use a Fourier-transform based method to investigate the magnitude and robustness of mode selectivity in as-cleaved discrete-mode semiconductor lasers, where a small number of refractive index perturbations are introduced into a Fabry-Pérot laser cavity. Three exemplar index perturbation patterns are considered. Our results demonstrate the capability to significantly improve modal selectivity by choosing a perturbation distribution function that avoids placing perturbations near to the cavity centre.

Impact of random alloy fluctuations on inter-well transport in InGaN/GaN multi-quantum well systems: an atomistic non-equilibrium Green's function study.

Recent experimental studies indicate the presence of ballistic hole transport in InGaN multi quantum well (MQW) structures. Widely used drift-diffusion models cannot give insight into this question, since quantum mechanical effects, such as tunneling, are not included in such semi-classical approaches. Also atomistic effects, e.

GeSn alloys: Consequences of band mixing effects for the evolution of the band gap Γ-character with Sn concentration.

In this work we study the nature of the band gap in GeSn alloys for use in silicon-based lasers. Special attention is paid to Sn-induced band mixing effects. We demonstrate from both experiment and ab-initio theory that the (direct) Γ-character of the GeSn band gap changes continuously with alloy composition and has significant Γ-character even at low (6%) Sn concentrations.

GaAsBi/GaNAs type-II quantum wells: novel strain-balanced heterostructures for GaAs-based near- and mid-infrared photonics.

The potential to extend the emission wavelength of photonic devices further into the near- and mid-infrared via pseudomorphic growth on conventional GaAs substrates is appealing for a number of communications and sensing applications. We present a new class of GaAs-based quantum well (QW) heterostructure that exploits the unusual impact of Bi and N on the GaAs band structure to produce type-II QWs having long emission wavelengths with little or no net strain relative to GaAs, while also providing control over important laser loss processes. We theoretically and experimentally demonstrate the potential of GaAsBi/GaNAs type-II QWs on GaAs and show that this approach offers optical emission and absorption at wavelengths up to ~3 µm utilising strain-balanced structures, a first for GaAs-based QWs.

Optical gain in GaAsBi/GaAs quantum well diode lasers.

Electrically pumped GaAsBi/GaAs quantum well lasers are a promising new class of near-infrared devices where, by use of the unusual band structure properties of GaAsBi alloys, it is possible to suppress the dominant energy-consuming Auger recombination and inter-valence band absorption loss mechanisms, which greatly impact upon the device performance. Suppression of these loss mechanisms promises to lead to highly efficient, uncooled operation of telecommunications lasers, making GaAsBi system a strong candidate for the development of next-generation semiconductor lasers. In this report we present the first experimentally measured optical gain, absorption and spontaneous emission spectra for GaAsBi-based quantum well laser structures.

Self-consistent Green's function method for dilute nitride conduction band structure.

We present a self-consistent Green's function (SCGF) approach for the Anderson many-impurity model to calculate the band dispersion and density of states near the conduction band edge in GaN(x)As(1-x) dilute nitride alloys. Two different models of the N states have been studied to investigate the band structure of these materials: (1) the two-band model, which assumes all N states have the same energy, EN; (2) a model which includes a full distribution of N states obtained by allowing for direct interaction between N sites. The density of states, projected onto extended and localised states, calculated by the SCGF two-band model, are in excellent agreement with those previously obtained in supercell calculations and reveal a gap in the density of states just above E(N), in contrast with the results of previous non-self-consistent Green's function calculations.

Internal photoemission from plasmonic nanoparticles: comparison between surface and volume photoelectric effects.

We study the emission of photoelectrons from plasmonic nanoparticles into a surrounding matrix. We consider two mechanisms of electron emission from the nanoparticles--surface and volume ones--and use models for these two mechanisms which allow us to obtain analytical results for the photoelectron emission rate from a nanoparticle. Calculations have been carried out for a step potential at the surface of a spherical nanoparticle, and a simple model for the hot electron cooling has been used.

Optical absorption of dilute nitride alloys using self-consistent Green's function method.

We have calculated the optical absorption for InGaNAs and GaNSb using the band anticrossing (BAC) model and a self-consistent Green's function (SCGF) method. In the BAC model, we include the interaction of isolated and pair N levels with the host matrix conduction and valence bands. In the SCGF approach, we include a full distribution of N states, with non-parabolic conduction and light-hole bands, and parabolic heavy-hole and spin-split-off bands.

Electronic properties of site-controlled (111)-oriented zinc-blende InGaAs/GaAs quantum dots calculated using a symmetry-adapted k·p Hamiltonian.

In this work, we present and evaluate a (111)-rotated eight-band k ⋅p Hamiltonian for the zinc-blende crystal lattice to investigate the electronic properties of site-controlled InGaAs/GaAs quantum dots grown along the [111] direction. We derive the rotated Hamiltonian including strain and piezoelectric potentials. In combination with our previously formulated (111)-oriented continuum elasticity model, we employ this approach to investigate the electronic properties of a realistic site-controlled (111)-grown InGaAs quantum dot.

Comparison of stress and total energy methods for calculation of elastic properties of semiconductors.

We explore the calculation of the elastic properties of zinc-blende and wurtzite semiconductors using two different approaches: one based on stress and the other on total energy as a function of strain. The calculations are carried out within the framework of density functional theory in the local density approximation, with the plane wave-based package VASP. We use AlN as a test system, with some results also shown for selected other materials (C, Si, GaAs and GaN).

The polarization response in InAs quantum dots: theoretical correlation between composition and electronic properties.

III-V growth and surface conditions strongly influence the physical structure and resulting optical properties of self-assembled quantum dots (QDs). Beyond the design of a desired active optical wavelength, the polarization response of QDs is of particular interest for optical communications and quantum information science. Previous theoretical studies based on a pure InAs QD model failed to reproduce experimentally observed polarization properties.

Dynamics of light propagation in spatiotemporal dielectric structures.

Propagation, transmission and reflection properties of linearly polarized plane waves and arbitrarily short electromagnetic pulses in one-dimensional dispersionless dielectric media possessing an arbitrary space-time dependence of the refractive index are studied by using a two-component, highly symmetric version of Maxwell's equations. The use of any slow varying amplitude approximation is avoided. Transfer matrices of sharp nonstationary interfaces are calculated explicitly, together with the amplitudes of all secondary waves produced in the scattering.

Unification of the band anticrossing and cluster-state models of dilute nitride semiconductor alloys.

We show that a quantitative description of the conduction band in Ga(In)NAs is obtained by combining the experimentally motivated band anticrossing model with detailed calculations of nitrogen cluster states. The unexpectedly large electron effective mass values observed in many GaNAs samples are due to hybridization between the conduction band edge E- and nitrogen cluster states close to the band edge. Similar effects explain the difficulty in observing the higher-lying E+ level at low N composition.

Inverted electron-hole alignment in InAs-GaAs self-assembled quantum dots.

New information on the electron-hole wave functions in InAs-GaAs self-assembled quantum dots is deduced from Stark effect spectroscopy. Most unexpectedly it is shown that the hole is localized towards the top of the dot, above the electron, an alignment that is inverted relative to the predictions of all recent calculations. We are able to obtain new information on the structure and composition of buried quantum dots from modeling of the data.