Formation mechanism of femtosecond laser induced guiding structures in y-cut LiNbO₃ crystal

Abstract

In this work, we have used femtosecond laser direct writing to inscribe guiding structures in y-cut LiNbO3 crystal and explored the mechanism of laser modification. Experimental results reveal the fabrication-parameter windows for the modifications with refractive index increase and decrease. We investigate the guiding properties under different polarizations at 632.8 nm. The spectral shifts of Raman modes have also been analyzed to clarify the laser-induced modifications on the crystal lattice. This study demonstrates that the lattice expansion and compression, and the damage induced by the laser energy are the key factors affecting the structural vibration and refractive index modification types.

Optical Waveguides Fabricated via Femtosecond Direct Laser Writing: Processes, Materials, and Devices

“…Similar to the above, Yang et al. [ 66 ] fabricated the Type‐I and II modifications with different processing parameters inside y ‐cut LiNbO 3 . The Raman intensity at the written area decreases by only 5% compared with the substrate for the Type‐I modification, which demonstrates relatively complete lattices in the guiding area were well preserved during the FsDLW process.…”

“…One is the photophysical process that Fs‐laser is focused inside the materials and produces micro–nanoscale modification or ablation, resulting in stable refractive index contrast. [ 66–68 ] The difference between modification mechanism and ablation mechanism is whether to modify the properties of the material or to cause the loss of the material itself. There are four forms of waveguides that can be categorized according to the formation of waveguide cores ( Figure a).…”

“…Reproduced with permission. [ 66 ] Copyright 2020, Elsevier. g,h) The Raman spectrum at different region of Type‐I and Type‐II configurations.…”

“…Reproduced with permission. [ 66 ] Copyright 2020, Elsevier. i) Optical microscope images of the cross‐sections of depressed cladding and dual‐line BGO waveguides.…”

Type-Ⅰ waveguide and Y-branch splitter fabricated by femtosecond laser direct writing in X-cut LiNO3 crystal

“…Usually, when the strong laser pulses act on the dielectric materials, there will be several kinds of intense nonlinear phenomena, such as two-photon absorption, multi-photon absorption, and the avalanche ionization [3][4][5][6] . A great deal of plasma with high energy will be induced immediately while the femtosecond laser is focused into the transparent material, transferring energy to the crystal lattice, which will cause lattice distortion, volume expansion, or change the stress field 7,8 . Generally, the types of waveguides include Type-Ⅰ waveguide with positive refractive index modification and Type-Ⅱ waveguide with negative refractive index modification.…”