Room-temperature terahertz spectroscopy of optically excited plasma waves in HEMTs

We report on systematic measurements of resonant plasma waves oscillations in several gate-length InGaAs HEMTs and compare them with numerical results from a specially developed model. A great concern of experiments has been to ensure that HEMTs were not subject to any spurious electronic oscillation that may interfere with the desired plasma-wave spectroscopy excited via a terahertz optical beating. The influence of geometrical HEMTs parameters such as gate-length or cap-layer length as well as biasing conditions is then explored extensively owing to many different devices. Plasma resonances up to the THz are observed.