OP: Terahertz Lasers Using Intersubband Transitions in non-polar III-nitrides

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Title: Novel compact terahertz lasers utilizing polarization-free nitride semiconductors for sensing and imaging AbstractNon-technical description: This project investigates a new class of compact, efficient laser light sources operating in the long-wavelength spectral range between the infrared and the microwave ranges, known as the terahertz (THz) gap. Due to fundamental limitations of commonly used semiconductors, this range is currently inaccessible with any other semiconductor lasers. This project exploits the unique properties of nitride semiconductors. However, the nitrides also present some unique challenges related to their atomic structure and built-in electric fields. To overcome these challenges the researchers employ sophisticated modeling tools to design new laser structures. The materials are then grown using molecular beam epitaxy, a technique that allows control of the nitride thicknesses at the nanoscale, and characterized with structural and optical techniques. Finally, laser devices are fabricated and tested to study their performance under different operating conditions and to further refine the design parameters.These novel lasers have the potential to impact a number of technological applications with broad benefits to society. The applications loosely fit into one of two main categories: THz spectroscopy, and THz imaging. THz spectroscopy is currently used in fields ranging from astronomy, and atmospheric science, to plasma fusion diagnostics and bio-chemical weapons detection. THz imaging has broad applications from airport security to medical imaging. This research program also provides unique interdisciplinary research opportunities to a diverse group of students at Purdue University. Special attention is given to providing hands-on research experience to under-represented students, especially women. Outreach activities will increase the awareness and exposure of Grade 7-12 students and teachers from economically disadvantaged backgrounds in Central Indiana to the scientific content and methods of photonics.Technical description: This project develops and studies a new class of far-infrared semiconductor emitters for the 1-10 THz range, specifically operating in the reststrahlen band of GaAs (30-40 micron wavelength). These lasers utilize intersubband transitions in the conduction band of non-polar III-nitride heterostructures and employ the general operating principles of quantum cascade lasers. The technical approach involves using low Al-composition, non-polar nitride heterostructures (AlInGaN/GaN) grown on high quality free-standing m-plane GaN substrates to mitigate material quality and design issues that have, so far, impeded progress of nitride intersubband devices. The choice of polarization-free m-plane heterostructures has the tremendous advantage of eliminating the effect of built-in electric fields at hetero-interfaces. The research effort is interdisciplinary in nature and involves material design and growth, structural and optical material characterization, waveguide design and fabrication, and finally device testing.This research will enable a novel compact, coherent, tunable THz light source with power output suitable for technological applications (milliwatt level). In addition to broader wavelength flexibility, the THz nitride lasers are expected to have superior performance in terms of operating temperature and efficiency at the longer wavelengths currently accessible with GaAs THz quantum cascade lasers. The research focuses on THz lasers using non-polar nitrides, but the knowledge acquired is also relevant to nitride optoelectronic devices operating in other spectral ranges such as the near-infrared (telecom) range. Moreover, the acquired knowledge is valuable for other types of devices, such as transistors, and to other material systems. This research program also brings about important contributions to the physics of optical transitions and vertical charge transport in nitride materials. Elaborate modeling tools and techniques are developed to predict and explain real device behavior. Moreover, this project considerably advances knowledge regarding molecular beam epitaxy of III-nitride materials with the atomic-layer precision and exact reproducibility necessary for complex infrared devices.