Credit: Light: Science & Applications
a & b, THz waves strongly couple with phonons in ionic crystals, taking lithium niobate for an example, resulting in a remarkable increase in the Kerr nonlinearity, which modulates the refractive index, leading to a frequency shift. For a Kerr medium, the refractive index should consist of a linear component and a nonlinear component, which indicates that the resonant frequency of a Kerr microcavity would change with the input THz power. c, Spectrum of a single-mode microcavity at different pump powers. The third-order nonlinear optical susceptibility can be calculated from the frequency shift.