Quill and Nonreciprocal Ultrafast Laser Writing

Abstract

Since the discovery of lasers, it was believed that a Gaussian mode of a laser beam interacting with an isotropic medium can produce only centrosymmetric material modifications. However, recent experiments provide the evidence that it is not always true. A remarkable phenomenon in ultrafast laser processing of transparent materials has been reported manifesting itself as a change in material modification by reversing the writing direction. The phenomenon has been interpreted in terms of plasma anisotropic trapping and heating by a tilted front of the ultrashort laser pulse. It has been experimentally demonstrated that indeed the pulse front tilt can be used to control material modifications and in particular as a new tool for laser processing and optical manipulation. Additionally, a new type of light-induced modification in a solid, namely an anisotropic cavitation, was observed in the vicinity of the focus at high fluences. The bubbles, formed in the bulk of the glass, can be trapped and manipulated in the plane perpendicular to the light propagation direction by controlling the laser writing direction relative to the tilt of the pulse front. Another common belief was that in a homogeneous medium, the photosensitivity and corresponding light-induced material modifications do not change on the reversal of light propagation direction. Recently, it was demonstrated that when the direction of the femtosecond laser beam is reversed from + Z to − Z directions, the structures written in a lithium niobate crystal are mirror images when translating the beam along the + Y and − Y directions. In contrast to glass, the directional dependence of writing in lithium niobate depends on the orientation of the crystal with respect to the direction of the beam movement and the light propagation direction. A theoretical model was created to demonstrate how interplay of the crystal anisotropy and light-induced heat flow can lead to a new nonreciprocal nonlinear optical phenomenon, nonreciprocal photosensitivity. In the lithium niobate, the nonreciprocal photosensitivity manifests itself as a changing the sign of the light-induced current when the light propagation direction is reversed. Therefore, in a non-centrosymmetric medium, modification of the material can be different when light propagates in opposite directions. Nonreciprocity is produced by magnetic field (Faraday effect) and movement of the medium with respect to the direction of light propagation: parallel (Sagnac effect) or perpendicular (KaYaSo effect).