Nanopillar quantum cascade lasers

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Abstract Title: Nanopillar quantum cascade lasersNon-technical descriptionThis research addresses the challenge of making terahertz semiconductor laser sources that operate at room temperature in the 1-5 THz range. Compact chip-scale sources of terahertz radiation that operate with both reasonably high output power (milliwatts or more) are desired for a range of spectroscopy and imaging applications. Examples include molecular gas sensing in the field of astrophysics and atmospheric science (for example investigation of star formation), imaging in the biological and medical sciences (for example burn and skin tumor imaging), security screening and illicit material detection (for example explosive and drug identification), and non-destructive evaluation (for example corrosion monitoring, delamination and void detection in films and coatings). Existing THz quantum-cascade lasers only operate at cryogenic temperatures which requires extra cooling, larger size, and increased power consumption. The intellectual merit of this proposal lies in the development of a new material system for such lasers "nanopillar quantum dots" that has the potential to increase operating temperatures to room temperature by suppressing unwanted interactions of the electrons with lattice vibrations. The broader impacts are addressed at several levels including undergraduate and graduate research experiences, dissemination of results, technology advancement. Outreach to underrepresented minorities will specifically occur through the development of research projects for a course designed for the recruitment and retention of underrepresented minority engineering undergraduates. Technical DescriptionThe intellectual merit of this proposal resides in two innovative components: the use of nanopillar quantum-dots for intersubband cascade lasers, and catalyst-free selective area semiconductor nanopillar epitaxy of quantum dot heterostructures. Use of quantum dots to create discrete energy levels can dramatically suppress nonradiative scattering of electrons by optical-phonon. Hence, a terahertz laser based discrete states in quantum-dots is expected to solve a fundamental limitation of conventional planar THz quantum-cascade lasers, where non-radiative phonon-assisted relaxation prevents room temperature operation. In this proposed concept, carriers would flow longitudinally down the length of a nanowire ensemble in a cascaded dot-to-dot tunneling regime without coupling with two-dimensional states. Selective-area MOCVD epitaxy without metal catalysts will be used to grow high-aspect ratio InAs/InAsP semiconductor nanopillar arrays using lithographically defined oxide growth masks. Lateral quantum confinement is determined by the mask feature dimension and the presence of an InP passivating shell heterostructure; longitudinal confinement is determined by the axial heterostructure. A collaborative research effort is proposed in three overlapping stages: (i) growth of axial and core/shell heterostructures in InAs/InAsP nanopillars for the formation of coupled-quantum wells and dots, (ii) investigation of the intersubband optical and transport properties, and (iii) investigation of nanopillar cascade designs for electroluminescence, stimulated emission, and quantum cascade laser demonstration.