The First Harmonic Generation And Linear Susceptibility Tensor Variation At Crystal Boundary

It's widely agreed that optical characteristics at crystal boundary may change comparing to the internal part of bulk crystal but, as far as we know, sometimes the phenomenon, for example, the variation of susceptibility usually can't be intuitively, simply presented in experiments. Recently, we observe a kind of special harmonic generation which is at the same wavelength as incident light. Besides, this kind of harmonic generation behaves in a similar way with nonlinear Cherenkov radiation, thus we call it linear Cherenkov radiation (LCR). We theoretically predict and calculate the phase-matching scheme and radiation path of LCR. In our experiment, we employ a polished KDP to verify our theory about the conversion of polarization in this process, and the phenomenon also help to rule out the possibility of birefringence at boundary. Combining with the coupling wave equation, we can derive new elements in linear susceptibility tensor according to the polarization states of incident light and LCR. The result tells 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. The existence of LCR is evidence of the variation of linear susceptibility. And in return, we could use this phenomenon to probe the non-zero elements in the tensor.