Lasers, Optics & Photonics

THz spectroscopy has been playing a crucial role in the characterization of materials and chemical/biological sensing because of the abundance of   formation can be obtained through the excitations and resonant interactions in THz frequency range. Recent development of bright THz sources made it  ossible to explore the interaction of THz field and materials beyond the linear regime. So far, most of nonlinear THz spectroscopy has been demonstrated  sing solid state THz emitter in either low frequency or high frequency side of so called THz gap. This is mainly due to the bandwidth limitations of these  olid state THz sources. The laser-induced gas plasma source has intense and broad bandwidth covering entire THz gap region without hindered by the  honon absorption in THz emitter itself. In this presentation, frequency resolved THz z-scan spectroscopy and two-dimensional THz spectroscopy using laser induced gas plasma source are demonstrated and discussed. Electronic and phononic contributions were resolved in broadband THz transmission spectra. The field dependence of the spectra showed the apparent existence of THz nonlinear contributions, and these contributions are attributed to the combinational mode of zone boundary LA phonons. Two-dimensional THz spectroscopy in THz gap-region was demonstrated using much lower THz field (100 kV/cm) than previous reported (1MV) for higher frequency range (>20THz) in InSb. The utility of broadband nonlinear THz spectroscopy using laser-induced gas plasma provides a way to inspect and evaluate materials in more details.