We report on the first experimental observation of a very strong nonlinear response in crystalline quartz in the terahertz (THz) frequency region through THz time-domain spectroscopy (THz-TDS). Theoretical modelling is presented and predicts a Kerr coefficient n2 equal to 5.17×10 −14 m 2 W −1 . The time-domain analysis of the measured data shows that with increasing of the THz peak amplitude, the pulse experiences a larger time delay in the sample. As the THz amplitude increases to values higher than 110 kV cm −1 , the growth rate of the delay decreases, indicating a saturation process. The value of the nonlinear refractive index calculated through the frequencyresponse analysis is estimated to be on the order of 10 −13 m 2 W −1 , which is several orders of magnitude larger than typical values of the nonlinear refractive index of solids in the visible region. Furthermore, a negative fifth-order susceptibility on the order of 10 −30 m 4 V −4 is measured.

This study reports on the experimental observation of a very strong nonlinear response in crystalline quartz in the terahertz (THz) frequency region through THz time‐domain spectroscopy (THz‐TDS). A previously established theoretical model is modified and presented and predicts a Kerr coefficient n2 equal to 5.17 × 10−14 m2 W−1. The time‐domain analysis of the measured data shows that with increasing of the THz peak amplitude, the pulse experiences a larger time delay in the sample. As the THz amplitude increases to values higher than 110 kV cm−1, the growth rate of the delay decreases, indicating a saturation process. The value of the nonlinear refractive index calculated through the frequency‐response analysis is estimated to be on the order of 10−13 m2 W−1, which is several orders of magnitude larger than typical values of the nonlinear refractive index of solids in the visible region. Furthermore, a negative fifth‐order susceptibility on the order of 10−30 m4 V−4 is measured.