Laser-writing inside uniaxially birefringent crystals: fine morphology of ultrashort pulse-induced changes in lithium niobate

Abstract

This work presents a detailed analysis of the morphology of femtosecond laser-induced changes in bulk lithium niobate (LiNbO₃) - one of the most common host materials in photonics - using second-harmonic generation microscopy and scanning electron microscopy. It is shown that focused linearly polarized near-infrared pulses can produce two or three distinct axially separated regions of modified material, depending on whether the pulse propagation is along or perpendicular to the optical axis. When laser writing in LiNbO₃ is conducted in multi-shot irradiation mode and the focused light intensity is kept near the bulk damage threshold, periodic planar nanostructures aligned perpendicular to the laser polarization are produced inside the focal volume. These results provide a new perspective to laser writing in crystalline materials, including the fabrication of passive and active waveguides, photonic crystals, and optical data storage devices.

High-temperature stable π-phase-shifted fiber Bragg gratings inscribed using infrared femtosecond pulses and a phase mask

“…Recently, periodic planar nanostructures [44,45] were found in Type II Bragg gratings produced in SMF-28 fiber by side-illuminating it with multiple linearly polarized IR fs-laser pulses through a phase mask [37]. Similar nanostructures were also observed in many different transparent bulk materials [46][47][48][49][50][51][52][53][54][55].…”

Local domain inversion in MgO-doped lithium niobate by pyroelectric field-assisted femtosecond laser lithography

“…21 The domain length is limited in this process because the laser focus is elongated in the depth of the crystal due to spherical aberrations induced by the high refractive index n of LiNbO 3 , and it also splits into multiple foci because of birefringence. 24,25 Here, we present a domain inversion approach in MgOdoped LiNbO 3 that is different from the methods mentioned above. It is based on a versatile combination of fs-laser lithography and global thermal treatment.…”

“…Cerenkov SHG microscopy is also able to detect structural modifications in the crystal volume written by focused fslaser pulses. 8,25 The difference is that only domain boundaries give rise to Cerenkov SHG and not the inner part, 32,33 whereas a laser-induced filament is completely visible in SHG microscopy. 8,25 We have systematically investigated filaments created below the crystal surface with respect to different pulse energies and also changed the investigated length of the filaments (Fig.…”

Counter‐Propagating Optical Trapping of Resonant Nanoparticles Using a Uniaxial Crystal

“…The separation of the two foci thus turns out to depend only on the birefringence and the thickness of the crystal. [22] To obtain the intensity distribution in the trapping region indicated in Figure 1b,c we superpose the field of Equation (1a) with a counter-propagating field described by the same equation but with z = −z and including a π phase shift due to reflection from a mirror with reflectivity 100%. Figure 1b shows the result for the case of a MgO doped LiNbO 3 crystal (n o = 2.24, n e = 2.15) of thickness d = 0.3 mm and an incident circularly polarized beam with λ = 1064 nm and beam waist w 0 = 0.48 μm, corresponding to the experimental realization described below.…”