Orientation-dependent depolarization of supercontinuum in BaF₂ crystal*

Abstract

We present a systematic investigation of the depolarization properties of a supercontinuum accompanied with femtosecond laser filamentation in barium fluoride (BaF2) crystal. It is found that the depolarization of the supercontinuum depends strongly on the crystal orientations with respect to the incident laser polarization. At most crystal orientations, the depolarization of the supercontinuum rises with the increase of the input laser energies and finally saturates. While at 45°, the depolarization of the supercontinuum is not changed and keeps nearly negligible with the increase of the input laser energies. These peculiar depolarization properties of the supercontinuum can be ascribed to the orientation dependence of the cross-polarized wave (XPW) generation and ionization-induced plasma scattering in the BaF2 crystal.

Significant intensity dependence of anisotropic two-photon absorption in ZnSe and GaP

“…The results of figure 6 clearly depict that in a wide intensity range exceeding three orders of magnitude before the laser damage threshold, the modulation amplitudes of orientation-dependent TPA in both ZnSe and GaP have obvious intensity dependence, with the maxima occurring around 10 11 W cm −2 . In general, our results confirm that the orientation-dependent TPA is directly linked with the ionic-bonding symmetrical characteristic of the specific crystal plane, which might be explained based on the theory of the anisotropic third-order optical nonlinearity [16][17][18] or the anisotropic SFI [5,[8][9][10][11][12]15]. Considering the significant intensity dependence of the third-order optical nonlinearity or SFI, it is intuitional to speculate that the modulation amplitude of orientation-dependent TPA should increase with the increase of irradiation intensity from the linear optical regime to the nonlinear optical regime, as predicted by some theoretical calculations [12].…”

“…Interestingly, our results further show that with the intensity approaching the laser damage threshold, instead of the normal ascending, a significant descent of modulation amplitude appears after the maximum-to a certain extent, which is somewhat similar to the experimental results of [9,10] in the high intensity regime. Such a declining process indicates that the crystal orientation dependence of TPA will weaken when the irradiation intensity approaches the laser damage threshold, which may be related to the saturation of TPA coefficient [20], the depletion of photons [11], the plasma-induced depolarization [15], or the dominance of avalanche ionization [5]-considering these different mechanisms, the origin of the phenomenon is still open to debate at present.…”

“…In addition to the crystal orientation dependence of laser-induced damage morphologies, a similar crystal orientation dependence for light transmission caused by SFI can also be observed below the laser damage threshold [9][10][11][12], indicating the crystal orientation dependence of the multiphoton absorption process (photon number n ⩾ 3 in [9][10][11][12]) in solids irradiated by polarized lasers. Furthermore, the high-order harmonics [13,14] or supercontinuum [15] generated by the strong laser irradiation of solids also exhibit significant crystal orientation dependence-these phenomena have essentially similar physical origins. In fact, even the nonlinear absorption process with photon number n = 2 may also illustrate an obvious crystal orientation dependence, such as two-photon absorption (TPA) [16][17][18] and two-photon electron emission [19].…”