Theoretical electromagnetic analysis of a grating-gated double quantum well FET terahertz detector

We analyze resonant behavior in terahertz absorption, in connection with resonant photoconductance recently observe in field effect transistors with a double-quantum-well channel (DQW-FET). This phenomenon is potentially important for fast, tunable, terahertz detectors. Important features are the several sharp photoconductance resonances that tune with grating gate voltage and the fact that the multiple resonance response grows and narrows at elevated temperatures becoming more pronounced as temperature increases up to 40 K. Our electrodynamic analysis of the THz absorption spectrum compared to the experimental data establishes that the resonance is determined by standing plasma waves in the DQW channel, with multiple-resonances corresponding to spatial harmonics of standing plasmons under the metallic grating gate. This is due to static spatial modulation of the electron density in the channel induced by the gate. Higher order plasmon modes become more optically active as the depth of the DQW density modulation approaches unity. We find the maximum potential absorbance, at plasma resonance, to be 50%. Moreover, the strongest absorption also occurs when the standing plasmon resonance coincides with the fundamental dipole mode of the ungated portion of the channel.