Strong Nonlinear Response in Crystalline Quartz at THz Frequencies

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 n(2) equal to 5.17 x 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 frequency-response 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.