"Hearing" terahertz electromagnetic radiation

The enhancement of acoustic waves, from audible into the ultrasonic range, is a linear function of the THz intensity incident on a laser-induced plasma, making THz-enhanced acoustics (TEA) useful for THz detection. Using a dual-color laser field to control electron trajectory inside the plasma, THz spectroscopic information can be encoded into the acoustic emission, making it possible to obtain the electric field profile of a THz pulse by simply “listening” to the plasma. A nearly instantaneous heating of a gas emits a shock wave through photoacoustics that quickly relaxes into an acoustic wave. The acoustic pulse contains a broad spectrum of frequencies that extend well beyond the range of human hearing into the ultrasonic range. When a broadband THz pulse is focused collinearly and simultaneously onto the plasma region, the THz field serves to accelerate freed electrons, giving them additional kinetic energy. Subsequent electron acceleration and collision between neighboring air molecules results in a THz field-induced energy transfer which gives rise to terahertz-enhanced acoustics (TEA).