The first harmonic generation and linear susceptibility tensor variation at crystal boundary

摘要

It’s widely agreed that optical characteristics at crystal boundary may change comparing to the internalpart of bulk crystal but, as far as we know, sometimes the phenomenon, for example, the variation ofsusceptibility usually can’t be intuitively, simply presented in experiments. Recently, we observe akind of special harmonic generation which is at the same wavelength as incident light. Besides, thiskind of harmonic generation behaves in a similar way with nonlinear Cherenkov radiation, thus we callit linear Cherenkov radiation (LCR). We theoretically predict and calculate the phase-matching schemeand radiation path of LCR. In our experiment, we employ a polished KDP to verify our theory aboutthe conversion of polarization in this process, and the phenomenon also help to rule out the possibilityof birefringence at boundary. Combining with the coupling wave equation, we can derive new elementsin linear susceptibility tensor according to the polarization states of incident light and LCR. The resulttells us the linear susceptibility at the KDP boundary is assuredly different from that in bulk crystal,and this is mainly because of the breakdown of crystal symmetry at boundary, in our opinion. Theexistence of LCR is evidence of the variation of linear susceptibility. And in return, we could use thisphenomenon to probe the non-zero elements in the tensor.