We report on the experimental demonstration of nonlinear spectroscopy of crystalline quartz in the terahertz regime. Using accumulated time shift method in the time domain, we observe that with increasing the THz pulse intensity, the experienced delay increases. At higher field intensities, the delay increases with a smaller rate, demonstrating a phase saturation. Analysing the frequency response, we estimate a nonlinear refractive index of the order of 10−13 m2/W which exceeds its value in the visible range by seven orders of magnitude. Furthermore, a negative fifth-order susceptibility of the order of 10−30 m4/V4 is obtained.
Terahertz (THz) nonlinear optics is an emerging field thanks to recent developments in the generation of intense THz pulses. THz radiation interacts with the vibrational/rotational resonances of molecules. For example, water molecules in the atmosphere show a very rich absorption spectrum with hundreds of sharp resonances below 3 THz. We report on the first experimental demonstration of the nonlinear interaction of THz pulses with water vapor. We observed a strong nonlinear response of the vibration/rotation resonances to the THz pulses. The frequency response of the nonlinear Kerr coefficient is extracted from the experimental results.
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