Since 2016 working as a Teaching Fellow in the Physics Department.
From 2016-2018 also working as a project facilitator helping researchers to develop new techniques to address to antimicrobial resistance challenge.
From January 2011 working as a Research Fellow on the optical characterisation of bismuth containing semiconductors. Applications for this range from more temperature sensitive telecommunications lasers to novel solar cell designs. Recently also getting involved into research of solid state lighting, not only from the technology side but also with respect to the human response.
2005-2008 Research Assistant (University of Surrey) working on GaAsSb based lasers for emission at 1.3 microns.
2000-2003 University of Surrey. The PhD work (Surrey 2003) concerned the spectroscopy (photomodulated reflectance) and device measurements of Resonant Cavity LEDs emitting at 650nm.
1994-2000 Universität Dortmund. Working on the spectroscopy of single excitonic states in II-VI semiconductor structures for the Diplom obtained in January 2000.
Improving the performance of LEDs and lasers.
Influence of light on humans.
New ways of addressing the antimicrobial resistance challenge.
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Level 1 Properties of Matter
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In spite of the almost ideal variation of the radiative current of 1.3 mu m GaAsSb/GaAs-based lasers, the threshold current, J(th), is high due to non-radiative recombination accounting for 90% J(th) near room temperature. This also gives rise to low T-0 values similar to 60K close to room temperature, similar to that for InGaAsP/InP.
GaInAsSb/GaSb based quantum well vertical cavity surface emitting lasers (VCSELs) operating in mid-infrared spectral range between 2 and 3 micrometres are of great importance for low cost gas monitoring applications. This paper discusses the efficiency and temperature sensitivity of the VCSELs emitting at 2.6 μm and the processes that must be controlled to provide temperature stable operation. We show that non-radiative Auger recombination dominates the threshold current and limits the device performance at room temperature. Critically, we demonstrate that the combined influence of non-radiative recombination and gain peak – cavity mode de-tuning determines the overall temperature sensitivity of the VCSELs. The results show that improved temperature stable operation around room temperature can only be achieved with a larger gain peak – cavity mode de-tuning, offsetting the significant effect of increasing non-radiative recombination with increasing temperature, a physical effect which must be accounted for in mid-infrared VCSEL design.
The potential to extend the emission wavelength of photonic devices further into the near- and midinfrared 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 GaAs1−xBix/GaNyAs1−y 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. Experimental measurements on a prototype GaAs0.967Bi0.033/GaN0.062As0.938 structure, grown via metal-organic vapour phase epitaxy, indicate good structural quality and exhibit both photoluminescence and absorption at room temperature. The measured photoluminescence peak wavelength of 1.72 μm is in good agreement with theoretical calculations and is one of the longest emission wavelengths achieved on GaAs to date using a pseudomorphically grown heterostructure. These results demonstrate the significant potential of this new class of III-V heterostructure for longwavelength applications.
We investigate the temperature and pressure dependence of carrier recombination processes occurring in various GaAsSb/GaAs QW laser structures grown under similar growth conditions. Thermally activated carrier leakage via defects is found to be very sensitive to the strain induced interface imperfections. Nonradiative recombination is found to be sensitive to the number of QWs. A strain compensated MQW structure leads to a reduced contribution of non-radiative recombination to the threshold current density (Jth) and a high characteristic temperature (T0) of 73K at room temperature.
Replacing small amounts of As with Bi in InGaBiAs/InP induces large decreases and increases in the bandgap, E, and spin-orbit splitting, Δ, respectively. The possibility of achieving Δ>E and a reduced temperature (T) dependence for E are significant for suppressing recombination losses and improving performance in mid-infrared photonic devices. We measure E (x, T) and Δ(x, T) in InGa BiAs/InP samples for 0≤x≤0.032 by optical spectroscopy. While we find no clear evidence of a decreased dE /dT (≈0.33±0.07meV/K in all samples) we find Δ>E for x>3.3-4.3%. The predictions of a valence band anti-crossing model agree well with the measurements. © 2012 IEEE.
This paper investigates the contributions of leakage current and self-heating to the temperature sensitivity. The amount of self-heating and leakage current is estimated and plotted for a constant light output with increasing temperature for both CW and pulsed operation, and the radiative current density for 650 nm emitting GaInP based resonant cavity light emitting diodes (RCLEDs) is calculated using a drift diffusion model (DDM).The model simulates the distribution of charge carriers within the structure and their recombination mechanisms. The radiative current in GaInP quantum wells is also extracted and plotted. © 2003 IEEE.
Intrinsically photosensitive retinal ganglion cells (ipRGCs), whose photopigment melanopsin has a peak of sensitivity in the short wavelength range of the spectrum, constitute a common light input pathway to the olivary pretectal nucleus (OPN), the pupillary light reflex (PLR) regulatory centre, and to the suprachiasmatic nuclei (SCN), the major pacemaker of the circadian system. Thus, evaluating PLR under short wavelength light (λmax 500 nm) and creating an integrated PLR parameter, as a possible tool to indirectly assess the status of the circadian system, becomes of interest. Nine monochromatic, photon-matched light stimuli (300 s), in 10 nm increments from λmax 420 to 500 nm were administered to 15 healthy young participants (8 females), analyzing: i) the PLR; ii) wrist temperature (WT) and motor activity rhythms (WA), iii) light exposure (L) pattern and iv) diurnal preference (Horne- Östberg), sleep quality (Pittsburgh) and daytime sleepiness (Epworth). Linear correlations between the different PLR parameters and circadian status index obtained from WT, WA and L recordings and scores from questionnaires were calculated. In summary, we found markers of robust circadian rhythms, namely high stability, reduced fragmentation, high amplitude, phase advance and low internal desynchronization, were correlated with a reduced PLR to 460–490 nm wavelengths. Integrated circadian (CSI) and PLR (cp-PLR) parameters are proposed, that also showed an inverse correlation. These results demonstrate, for the first time, the existence of a close relationship between the circadian system robustness and the pupillary reflex response, two non-visual functions primarily under melanopsin-ipRGC input.
This paper reports on progress in the development of GaAsBi/(Al)GaAs based lasers grown using metal-organic vapour phase epitaxy and focuses on the underlying processes governing their efficiency and temperature dependence. Room temperature lasing has been achieved in devices with 2.2% Bi and lasing in devices with 4.4% Bi was observed up to 180 K. We show that the device performance can be improved by optimizing both electrical and optical confinement in the laser structures. Analysis of the temperature dependence of the threshold current together with pure spontaneous emission and high hydrostatic pressure measurements indicate that device performance is currently dominated by non-radiative recombination through defects (>80% of the threshold current at room temperature in 2.2% Bi samples) and that to further improve the device performance and move towards longer wavelengths for optical telecommunications (1.3-1.5 μ m) further effort is required to improve and optimize material quality. © 2014 IOP Publishing Ltd.
The pupillary light reflex (PLR) is a neurological reflex driven by rods, cones, and melanopsin-containing retinal ganglion cells. Our aim was to achieve a more precise picture of the effects of 5-min duration monochromatic light stimuli, alone or in combination, on the human PLR, to determine its spectral sensitivity and to assess the importance of photon flux. Using pupillometry, the PLR was assessed in 13 participants (6 women) aged 27.2 ± 5.41 years (mean ± SD) during 5-min light stimuli of purple (437 nm), blue (479 nm), red (627 nm), and combinations of red+purple or red+blue light. In addition, nine 5-min, photon-matched light stimuli, ranging in 10 nm increments peaking between 420 and 500 nm were tested in 15 participants (8 women) aged 25.7 ± 8.90 years. Maximum pupil constriction, time to achieve this, constriction velocity, area under the curve (AUC) at short (0–60 s), and longer duration (240–300 s) light exposures, and 6-s post-illumination pupillary response (6-s PIPR) were assessed. Photoreceptor activation was estimated by mathematical modeling. The velocity of constriction was significantly faster with blue monochromatic light than with red or purple light. Within the blue light spectrum (between 420 and 500 nm), the velocity of constriction was significantly faster with the 480 nm light stimulus, while the slowest pupil constriction was observed with 430 nm light. Maximum pupil constriction was achieved with 470 nm light, and the greatest AUC0−60 and AUC240−300 was observed with 490 and 460 nm light, respectively. The 6-s PIPR was maximum after 490 nm light stimulus. Both the transient (AUC0−60) and sustained (AUC240−300) response was significantly correlated with melanopic activation. Higher photon fluxes for both purple and blue light produced greater amplitude sustained pupillary constriction. The findings confirm human PLR dependence on wavelength, monochromatic or bichromatic light and photon flux under 5-min duration light stimuli. Since the most rapid and high amplitude PLR occurred within the 460–490 nm light range (alone or combined), our results suggest that color discrimination should be studied under total or partial substitution of this blue light range (460–490 nm) by shorter wavelengths (~440 nm). Thus for nocturnal lighting, replacement of blue light with purple light might be a plausible solution to preserve color discrimination while minimizing melanopic activation.
Thermally activated carrier leakage via defects is found to be very sensitive to the growth temperature of GaAsSb quantum wells. Optimization of the growth temperature leads to a low Jth/QW of 138A/cm2 at RT.
Using a combination of experimental and theoretical techniques we present the dependence of the bandgap Eg and the spin orbit splitting energy so, with Bi concentration in GaAsBi/GaAs samples. We find that the concentration at which so,> Eg occurs at 9%. Both spectroscopic as well as first device results indicate a type I alignment.
GaAsBi QWs have the potential to remove inherent recombination losses thereby increasing the efficiency and reducing the temperature sensitivity of near-infrared telecommunications lasers. GaAsBi QW lasers are reported and prospects for 1550nm operation are discussed.