Terahertz time-domain measurements by electro-optic crystals with various symmetries

Terahertz wave radiation (TWR) is attractive due to its applications in spectroscopy, communication technologies, biology, medical science and other fields. One of the popular ways of TWR time-domain measurements is based on its nonlinear-wave interaction with femtosecond laser pulses in electro-optic (EO) crystals [1]. This technique utilizes ellipticity changes of the laser pulse polarization (the Pockels effect) and is widely used because of its simplicity, high sensitivity and broad operating band of EO crystals. In the report, we present our recent theoretical and experimental study [2–6] of alternative EO detection techniques and compare them with the standard one in terms of their sensitivity and compatibility with different crystals. When linearly polarized laser pulse propagates in isotropic crystal (for example ZnTe or GaP) then in the absence of terahertz electric field its polarization doesn’t change. However, in the presence of electric field a refractive index tensor of the crystal is changed and the crystal becomes anisotropic. As a result, orthogonal polarization components of the laser pulse transmit through the matter with different optical densities and the laser pulse polarization becomes elliptic. This polarization ellipticity is measured by an appropriate optical scheme (figure 1, a) and terahertz electric field overlapped with the laser pulse is determined. This technique can be used with TWR generated by sources directed by femtosecond laser pulses [1] and from external sources (not synchronized with the laser system) emitting both continuous [7] and pulsed radiation [8]. In spite of high sensitivity of the technique, we have shown that sensitivity of EO measurements can be increased. The sensitivity improvement can be achieved by use of laser pulses with an edge-cut spectrum [4–6]. A new experimental scheme is presented in figure 1, b. An edge filter is positioned before the EO crystal to produce a high gradient of the spectral energy density of the laser pulse and the TWR interacted with the laser pulse that had previously passed through an edge filter. It was shown theoretically [5] that changes of polarization ellipticity at the frequencies corresponding to the edge-cut of the laser pulse spectrum could be much higher than those could when laser pulse with Gaussian spectrum is used. To utilize the ellipticity changes at the frequencies corresponding to the edge-cut of the laser pulse spectrum, the laser pulse after the EO crystal can be decomposed in the frequency spectrum by a diffraction grating or can be passed through another edge filter transmitting only the optical waves with frequencies corresponding to the slope of the first filter [5, 6] (figure 1, b). The ellipticity changes are measured using an appropriate optical scheme. In works [5,6] the scheme depicted in figure 1, b was implemented with GaP crystal. This crystal has no natural birefringence and the new technique demonstrated a much stronger response on the same TWR than the standard one (figure 2, a). The method based on ellipticity changes of the laser pulse with an edge-cut spectrum was also successfully used to improve the frequency band of the near-field microscopy technique [9].