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.
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