Research

This page will give you a short introduction to my research and its significance. You can find more technical details under the research menu for each project.
Here’s a 1-page dissertation summary, if you would like. Dissertation Summary

The elevator pitch version of my research: “My work involves the development of intersubband optical devices like Quantum Cascade Lasers (QCLs) and Infrared (IR) detectors like Quantum Well Infrared Photodetectors (QWIPs) and Quantum Cascade Detectors (QCDs) using the II-VI ZnCdSe and ZnCdMgSe material system in the mid-IR wavelength range, from about 3 μm to about 16 μm for applications ranging from environmental trace-gas sensing to medical analysis.”

Why mid-IR? Virtually all gas molecules of particular interest in environmental monitoring and health like carbon-dioxide, sulfur-dioxide, nitrous oxides, ammonia, etc. absorb strongly in the mid-IR wavelength range, thanks to strong molecular vibrations as shown below. This fact can be exploited by developing sensor system that can detect these harmful trace gases down to ppb (parts per billion) and ppt (parts per trillion) levels. Developing such powerful, low cost and compact sensors systems is dependent on the availability of high performance mid-IR light sources like QCLs and detectors like QWIPs or QCDs.

Mol absorption

Why II-VI materials? Currently, most commercial QCLs are based on III-V based GaAs systems. Engineering has allowed these QCLs to be operated from about 5 μm to about 10 μm. Extending the wavelength range of these lasers is one of the most important goals for looking at alternative materials. When it comes to detectors, MCT (Mercury-Cadmium-Telluride) and InSb (Indium Antimonide) based detectors rule the market. However, many of them work only at cryogenic temperatures and have fairly narrow detection windows. II-VI materials, in principle, can deliver short wavelength room temperature AND broadband IR detection due to favorable material properties. We’ve already developed high performance II-VI based IR detectors that work at room temperature. The figure below more clearly illustrates the power of II-VI materials: the large conduction band offset that’s available while still being lattice matched to Indium Phosphide that result in defect-free devices.

II-VI materials

You can find more information about each of my individual projects on these II-VI systems through the links given below.

II-VI High Performance QWIPs: In this work, I’ve invented the first long-wave (~ 9 μm) and mid-wave (~ 4 μm) photoconductive QWIPs. The mid-wave detector worked up to room temperature and displayed a record responsivity of over 30 A/W (300 K) compared to similar mid-wave IR detectors.

II-VI High Performance QCDs: IR imaging systems, often using in night vision and defense applications, require very low noise to improve resolution and image quality under low light conditions. In this context, we’ve developed the first photovoltaic II-VI based QCD that exhibits ultra-low noise up to room temperature. Work is currently to improve the sensitivity of these detectors.

Broadband Infrared Detectors: This is one of my latest projects, and one that I consider the poster child for the power and flexibility of the II-VI material system. Utilizing the unique band structure properties of II-VI materials, I am working towards developing the first broadband (4 μm to 12 μm) IR detector that can work up to room temperature. This research represents the true competitor to MCT based detectors in terms of a broad spectral range.

Quantum Cascade Lasers: II-VI materials lend themselves to the development of short-wave QC lasers that are useful for applications in methane detection. Current techniques like strain balanced systems and Sb based materials suffer from electron transport issues. I have already demonstrated intersubband emission from QC structures at 5 μm, and currently working towards making the first QCLs from II-VI systems at 8 μm.