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Optical frequency conductance model of terahertz/infrared emission and detection in semiconductors

The optical frequency conductance is derived for quantum wells and quantum dots, and the optical frequency conductivity of bulk narrow-gap semiconductors is revisited. The teraHertz (THz) and infrared (IR) response of these semiconductor structures, in both free-space and guided-wave geometries, is described in a simple manner within the optical frequency conductance formalism. Familiar concepts form the microwave domain, including transmission lines and impedance matching, are extended into the THz and IR domains. We show that the fine structure constant of quantum electrodynamics sets the natural scale for the optical conductance of semiconductor structures, from which rules of thumb and physical limits to THz/IR gain and absorption can be derived. The optical conductance formalism can be applied to MCT photodetectors, quantum well IR photodetectors, quantum dot IR photodetectors, and quantum cascade lasers.