QWIPs for Gas Sensing in the Mid-IR

Dr. K. T. Lai, Dr. R. Gupta

We are developing QWIPs (Quantum Well Intersubband Photodetectors) which operate in the 2-5 μm wavelength range, for incorporation into gas sensors. Pollutant gases or contaminants in the air absorb electromagnetic radiation at characteristic frequencies. Minute concentrations of such molecules may, in principle, be detected optically. However, it happens that many of these wavelengths fall in the mid-infrared range (2-6 μm) where detectors have been scarce since they require semiconductors with a small energy band gap (0.1-0.2eV). These are more difficult to grow, process and fabricate. Here we propose to investigate detectors based on transitions within the conduction band of a quantum well. This offers the advantages of long wavelength detection from a large band gap material such as GaAs with established growth and processing technologies. High uniformity, excellent reproducibility and higher yield can thus be achieved. Although much work has been done on 5-10 μm QWIPs, the distinctive feature of this project is the use of stepped wells and strained layers to achieve shorter wavelength (2-5 μm) operation than usual and also enhanced normal incidence absorption.

Blue-Shifting Optical Modulators

Dr. A. Lim, Dr. R. Gupta

The enhanced exciton absorption observed in quantum well structures can be utilised in many device types, including electro-absorption modulators. MQW electro-absorption devices typically make use of the quantum-confined Stark effect (QCSE) to produce an absorption change at the band edge. Intensity modulation of an incident beam of appropriate wavelength can then be achieved.

Conventionally in a MQW electro-absorption device, the QCSE manifests itself as a reduction in peak height and a redshift of the e1-hh1 exciton transition with applied electric field. The absorption at the bandedge therefore decreases with applied field. However, with the use of our novel InP/GaInAs/InAsP/InP structure, a MQW electro-absorption modulator based on a blueshift of the e1-hh1 transition can be realised. The structure can be compared to a pre-biased conventional QW device. In such a device, removing the applied bias would cause the excitonic band edge to return to its zero field value. The key difference in our novel structures is that the field across the device must be increased in order to produce the blueshift.

Blue-shifting devices offer several advantages over their red-shifting counterparts; for example, blue-shifting devices allow operation at smaller detuning energies from the zero-applied field energy, and offer improvements in terms of insertion loss.

Theoretical Semiconductor Physics and Optoelectronic Device Modelling

Dr. R. Gupta

Our research work has centered mostly on the study of electronic, optical and transport properties of III-V semiconductors, with close collaboration with experimentalists. Some of our current research is briefly described below:

• Design and development of normal-incident QWIPs
  GaAs/InGaAs/AlGaAs Quantum Well Infrared/Intersubband Photodetector (QWIPs) operating at 3-5mm and under conditions of normal incidence of radiation have been designed (theoretically) and studied (both theoretically and experimentally). This has involved the optimisation of the device with view to applications such as gas sensing etc.

• Modelling of novel blue-shifting modulaters
  A novel design of InP/GaInAs/InAsP/InP quantum-well structure leads to the blue-shifting of the hh1-e1 transition under the application of an electric field. This structure has been modelled in order to understand the processes involved.

• Tunnelling-induced transparency and lasers without population inversion
  Quantum interference phenomena such as lasing without population inversion (LWI) and absorption cancellation (EIT/TIT) in semiconductor quantum wells have been studied theoretically. We aim to understand the physical processes, such as quantum-mechanical decoherence, involved in the observation of these effects.

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