Terahertz Generation via Optical Rectification in Reflection

Abstract :
Nonlinear optical techniques such as second- and third-harmonic generation are widely used in biological microscopy, but they give no access to the terahertz (THz) range. THz imaging via optical rectification (OR) could fill this gap, yet because water strongly absorbs THz radiation, transmission geometries are unsuitable for biological samples – a reflection geometry is needed instead, since it avoids bulk absorption and probes the biologically relevant surface. We present a combined experimental–theoretical study of THz OR generation in reflection, free of the propagation and phase-matching constraints that limit transmission-based OR. Using a double-pulse autocorrelation scheme, we retrieve THz and second-harmonic generation (SHG) waveforms with sensitive but slow, broadband detectors (photomultipliers, bolometers). An analytical model linking the nonlinear response to dispersion, detector bandwidth, and the crystal's linear optical constants reproduces the measured waveforms and spectra in ZnTe, GaSe, GaP, and LiNbO₃, with deviations at high pump power attributed to two-photon absorption. These results establish reflection-based THz OR as a sensitive probe of surface nonlinearities and a promising step toward THz microscopy of strongly absorbing samples, such as biological tissue.