Terahertz (far-infrared) spectroscopy is becoming an increasingly valuable tool in science and industry. It is widely used to study conductive materials on the macroscopic and nano-scale, and to perform non-destructive imaging. The efficient detection of light at terahertz (THz) frequencies remains a challenge, as signals are often weak and can be swamped by background infrared light. Often, cumbersome and expensive liquid helium-cooled detectors are required to observe incoherent THz radiation. In this article, Dean et al. report a study of a novel way to detect incoherent terahertz radiation via a photothermoelastic effect in zinc-blende crystals. Importantly, all the detection components required operate at room temperature, and are robust and inexpensive.

In the technique of THz time-domain spectroscopy zinc-blende crystals, such as ZnTe, can be utilized to generate and detect broadband pulses of THz radiation using femtosecond infrared pulses. In an electro-optic crystal, which lacks an inversion centre, the electric field of the THz radiation creates a birefringence via the linear electro-optic (Pockels) effect. This can then be measured optically using an infrared gate pulse synchronized to the THz pulse. Recently, the detection of continuous-wave THz radiation with unsynchronized infrared pulses has been reported for the zinc-blende crystals CdTe and ZnTe. The mechanism for the detection of THz radiation was found to be a thermally-induced change in birefringence, rather than an electro-optic effect.

The paper by Dean et al. substantially extends upon earlier studies, by quantifying the magnitude of this effect for ZnTe and GaP detection crystals, and identifying the underlying mechanism. A temperature-dependent refractive index is ruled out as the origin of the observed birefringence change; rather, a photothermoelastic mechanism is proposed. In this scheme a focussed THz beam is absorbed in a zinc-blende crystal and heats it, creating a locally-stressed region. The photoelastic effect, in which a stress field creates a birefringence, then alters the polarization state of the near-infrared detection beam. A comprehensive and quantitative model is developed and reported by Dean et al. for the thermally-induced photoelastic detection of THz radiation. This model is validated experimentally by spatially-resolving the detected signal.

Excitingly, Dean et al. performed their measurements with a near-infrared (788nm) continuous-wave beam with 20mW power. These powers are readily accessible with cheap solid-state diode lasers, rather than the expensive ultrafast lasers used in the previous studies. Intriguingly, the photothermoelastic detection method does not place stringent constraints on the crystal structure of the detection material, suggesting that it may be witnessed in optical media other than zinc-blende crystals. The findings are an important step forward in the understanding of this class of THz detector, lower its cost and complexity, and provide a route to try to enhance its responsivity.

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