Development of Continuous-Wave Terahertz Lasers

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The proposed research seeks to develop continuous-wave (CW) solid-state THz lasers based on intersubband transition in multiple quantum-well (MQW) structures. If carried out successfully, this project will have a major impact on the science and technology of the THz (1-10 THz) or far-infrared spectrum, where currently no coherent and intense solid-sate sources operating in the CW mode are available. Towards this goal, extensive studies have already been performed on the radiation properties of intersubband transition, scattering, and magneto-tunneling spectroscopy; the simulation-aided design of suitable MQWs; and spectrally resolved measurements of THz spontaneous emissions from MQWs. Detailed analysis indicates that electrically pumped lasing can be achieved at a reasonable population inversion density of 2'109/cm2. A three-year project is proposed to start on August 1, 2002. The first year will focus on two parallel tasks: improvement of the yield rate in fabrication of a novel metal waveguide structure for THz mode confinement, and investigation of several promising MQW structures. The metal waveguide structure will be very useful in achieving CW laser operation in two important ways, reducing the lasing threshold and thus the power dissipation and improving heat removal. The fabrication process involves wafer bonding and selective etching, and it has already been developed in the principal investigator's group. The promising MQW structures include a structure using electron-LO-phonon scattering for depopulation of the lower lasing level, a structure using electron-electron scattering for depopulation, and a chirped superlattice structure in which the radiative transition is to take place between two minibands. The design of these MQW structures will be aided by sophisticated computer simulations, including Monte Carlo simulation for transport analysis. Based on the results from this investigation, the second and third years will focus on the most promising structure and on the improvement of robustness of population inversion, in order to achieve CW lasing operations. Efforts will also be made to develop single-mode and frequency-stabilized THz lasers for local-oscillator applications in THz heterodyne receivers.