N. Babazadeh

Electronic control of coherence in a two-dimensional array of photonic crystal surface emitting lasers

We demonstrate a semiconductor PCSEL array that uniquely combines an in-plane waveguide structure with nano-scale patterned PCSEL elements. This novel geometry allows two-dimensional electronically controllable coherent coupling of remote vertically emitting lasers. Mutual coherence of the PCSEL elements is verified through the demonstration of a two-dimensional Young’s Slits experiment. In addition to allowing the all-electronic control of the interference pattern, this type of device offers new routes to power and brightness scaling in semiconductor lasers, and opportunities for all-electronic beam steering.

Development of high temperature AlGaAs soft X-ray photon counting detectors

New types of detectors based on the wide band gap material AlGaAs have been developed for soft X-ray spectroscopy applications. We report on the spectroscopic performance of simple p-i-n diodes and avalanche photodiodes (APDs). A number of diode types with different layer thicknesses have also been characterised. X-ray spectra from 55Fe and 109Cd radioactive sources show these diodes can be used for spectroscopy with promising energy resolution (1.0–1.25 keV) over a -30 to +90 °C temperature range. The temperature dependence of the avalanche multiplication process at soft X-ray energies in Al0.8Ga0.2As APDs was also investigated at temperatures from -20 to +80 °C. The temperature dependence of the pure electron initiated multiplication factor (Me) and the mixed carrier initiated avalanche multiplication factor (Mmix) were extracted from the X-ray spectra. The experimental results are compared with a spectroscopic Monte Carlo model for Al0.8Ga0.2As diodes from which the temperature dependence of the pure hole initiated multiplication factor (Mh) is determined. Monte Carlo simulations for the avalanche gain of absorbed X-ray photons have also been developed to study the relationship between avalanche gain and energy resolution for semiconductor X-ray avalanche photodiodes. The model showed that the distribution of gains, which directly affects the energy resolution, depends on the number of injected electron-hole pairs (and hence the photon energy), the relationship between the two ionization coefficients, and the overall mean gain. Our model showed that the conventional notion of APD gains degrading energy resolution significantly is incomplete. We compare the Monte Carlo simulations with experimental data from a number of different Al0.8Ga0.2As diodes.

Coherently Coupled Photonic-Crystal Surface-Emitting Laser Array

The realization of a 1 × 2 coherently coupled photonic crystal surface emitting laser array is reported. New routes to power scaling are discussed and the electronic control of coherence is demonstrated.

GaAs-based superluminescent diodes with window-like facet structure for low spectral modulation at high output powers

We demonstrate a GaAs-based superluminescent diode (SLD) based on the incorporation of a window-like back facet into a self-aligned stripe structure in order to reduce the effective facet reflectivity. This allows the realisation of SLDs with low spectral modulation depth (SMD) at high power spectral density (PSD), without the application of anti-reflection coatings to either facet. This approach is therefore compatible with ultra-broadband gain active elements. We show that 30 mW output power can be attained in a narrow bandwidth, corresponding to 2.2 mW nm−1 PSD with only 5% SMD, centred about 990 nm. We discuss the design criteria for high power and low SMD and the deviation from a linear dependence of SMD on output power, resulting from Joule heating in the self-aligned stripe.

A GaAs-based self-aligned stripe distributed feedback laser

We demonstrate operation of a GaAs-based self-aligned stripe (SAS) distributed feedback (DFB) laser. In this structure, a first order GaInP/GaAs index-coupled DFB grating is built within the p-doped AlGaAs layer between the active region and the n-doped GaInP opto-electronic confinement layer of a SAS laser structure. In this process no Al-containing layers are exposed to atmosphere prior to overgrowth. The use of AlGaAs cladding affords the luxury of full flexibility in upper cladding design, which proved necessary due to limitations imposed by the grating infill and overgrowth with the GaInP current block layer. Resultant devices exhibit single-mode lasing with high side-mode-suppression of >40 dB over the temperature range 20 °C–70 °C. The experimentally determined optical profile and grating confinement correlate well with those simulated using Fimmwave.

Radiation tolerant DC characteristics of InAs/GaAs quantum-dot diodes

Effects of 137Cs gamma irradiation on the DC electrical characteristics of InAs/GaAs quantum dots (QDs) mesa diodes are reported. The devices were irradiated with gamma-rays for different doses ranging from 100 rad to about 1 Mrad (GaAs). The QDs mesa diodes are found to be tolerant to γ radiation. No enhanced leakage current and shift in the turn-on voltage were observed in the InAs/GaAs QD devices after exposure to γ-radiation. When irradiated by γ-rays continuously, there seemed to be a small degradation trend in the forward-bias current after irradiating the mesa diode for about six hours.

Development of broad spectral bandwidth hybrid QW/QD structures from 1000-1400 nm

We describe the development of hybrid quantum well (QW)/quantum dot (QD) active elements to achieve broad spectral bandwidth spontaneous emission and gain. We have previously reported that the placement of the QW within the active element is a critical factor in obtaining broad spectral bandwidth emission. We now present new designs to further broaden the spontaneous emission from hybrid structures by increasing the number of QD layers and dot density, and by using QDs with wider state-separation. Introducing chirped QD layers reduced the modulation in the spontaneous emission spectra, and by utilising self-heating effects and state-filling, a spontaneous emission with 3dB line-width of 350nm is obtained.

Broad bandwidth emission from hybrid QW/QD structures

We previously demonstrated a hybrid quantum well/quantum dot structure to enhance the gain and spontaneous emission bandwidth of a quantum dot active region. We now present new designs to further broaden the spontaneous emission from hybrid QW/QD structure. Utilizing a high junction temperature, and chirped quantum dot layers with higher areal densities, increased layer number, and increased state separation a spontaneous emission FWHM of ~350nm is achieved.

Incorporating structural analysis in a quantum dot Monte-Carlo model

We simulate the shape of the density of states (DoS) of the quantum dot (QD) ensemble based upon size information provided by high angle annular dark field scanning transmission electron microscopy (HAADF STEM). We discuss how the capability to determined the QD DoS from micro-structural data allows a MonteCarlo model to be developed to accurately describe the QD gain and spontaneous emission spectra. The QD DoS shape is then studied, with recommendations made via the effect of removing, and enhancing this size inhomogeneity on various QD based devices is explored.

Size Anisotropy inhomogeneity effects in state-of-the-art quantum dot lasers

We describe a high angle annular dark field scanning transmission electron microscopy study of a self-assembled InAs-GaAs quantum dot (QD) laser sample providing insight into the micro-structure of the QD ensemble. A size distribution anisotropy of the QDs is observed in the two orthogonal (110) planes, and this structural information is used to develop a density of states model for the QD ensemble which is shown to be in strong agreement with a range of optical spectroscopic measurements. This link between the micro-structure and optical properties allows routes to QD device simulation. We go on to discuss how changes to the micro-structure would affect the density of states and hence laser performance.We describe a high angle annular dark field scanning transmission electron microscopy study of a self-assembled InAs-GaAs quantum dot (QD) laser sample providing insight into the micro-structure of the QD ensemble. A size distribution anisotropy of the QDs is observed in the two orthogonal (110) planes, and this structural information is used to develop a density of states model for the QD ensemble which is shown to be in strong agreement with a range of optical spectroscopic measurements. This link between the micro-structure and optical properties allows routes to QD device simulation. We go on to discuss how changes to the micro-structure would affect the density of states and hence laser performance.