Overview: With the development of modern manufacturing, the size of optical devices is gradually developing towards miniaturization, and integrated optics is also developing to become a topical area of research for many scholars. One of the methods used for producing micro/nano optical devices is femtosecond laser direct writing, a fine three-dimensional processing technique that has been extensively studied by many scholars for its applicability to most materials and can be applied to the fabrication of a wide range of optical devices. Micro/nano-optical devices prepared by femtosecond laser direct writing in crystals have been applied in a broad range of applications in different wavelengths. PMN-PT crystal with relaxed ferroelectric has attracted much attention in recent years for its superior piezoelectric property and large electromechanical coupling coefficient, and its application in the infrared band is more prominent, so the fabrication of the optical devices based on PMN-PT crystal has gradually become a relevant research hotspot. The LIPSS is one of the micro/nano-structures that can be processed by femtosecond laser direct writing. The LIPSS is prevalent in many materials and has been found in metals, semiconductors, dielectrics, etc. Similarly, LIPSS can be induced by femtosecond lasers in PMN-PT crystal. The LIPSS has a wide range of applications in the fields of anti-reflectivity, permanent coloration, and wettability. Nevertheless, the physical processes and the mechanisms involved in the formation of LIPSS have different interpretations in different materials. In this paper, we describe the LIPSS induced by femtosecond laser on the surface of the PMN-PT crystal and characterize it theoretically. We have achieved a change in the period of the LIPSS from 750 nm to 3000 nm after experimenting with different laser parameters. Afterward, we simultaneously obtained the phase transition of the LIPSS in PMN-PT crystal through temperature modulation, and this phase transition can be analyzed by the variation of the Raman spectra. At the same time, we have obtained the Curie temperature for the LIPSS structure that is approximately 10 ℃ lower than that of the PMN-PT crystal and have analyzed the phase transition process through the structural properties of the PMN-PT crystal. The results of our experiments and analyzes on the LIPSS in PMN-PT crystal reported in this paper can provide some experience for the subsequent development of the optical devices related to the LIPSS in PMN-PT crystal.

作为三维超构材料的衍生物，具有亚波长厚度的人工超构表面结构能够在紧凑的平台上灵活操纵光与物质的相互作用，有利于多功能、超紧凑光子器件的研发，对于微纳光子学和集成光子学具有重要意义。铁电晶体铌酸锂凭借其跨越可见光至中红外波段的宽透明窗口以及较大的非线性光学、电光系数，被认为是最有前途的多功能集成光子平台之一。近年来，基于铌酸锂薄膜(lithium-niobate-on-insulator，LNOI)的集成光子学器件研究也得到了迅猛发展。本文总结了几种有潜力制备高质量铌酸锂超构表面的微纳加工技术，同时介绍了近年来铌酸锂超构表面结构的研究进展，并对其未来的研究方向进行了展望。

Depressed cladding waveguides are fabricated in Pr:LiYF₄ (YLF) crystal by femtosecond laser inscription following a helical scheme. With the optimized parameters, the propagation loss of the waveguide is around 0.12 dB/cm for multimode guiding. Under optical pumping with InGaN laser diodes at 444 nm, efficient waveguide lasers in the orange around 604 nm (π-polarized) are achieved with minimum lasing threshold of 119.8 mW, maximum slope efficiency of 16.6%, and maximum output power of 120.6 mW. Benefiting from their optimized performances, the waveguides produced in this work are promising for applications as compact orange laser sources.

We report an interesting study of electric-field-induced transformation from a single domain ferroelectric state to the multiple domain ferroelectric state in a KTa_1-xNb_xO₃ (KTN) crystal. Experimental results obtained using the confocal μ-Raman spectroscopy confirm the dynamic change of lattice structures induced by an external electric field. Furthermore, the dependence of relative permittivity on the applied voltage also indicates the transformation of ferroelectric states involving the processes of splintering, inversion, and re-formation of ferroelectric domains.

Optical channel waveguides with depressed cladding configurations have been produced in Nd,Gd:CaF2 laser crystals by using ultrafast laser inscription. Waveguide properties are investigated in terms of guiding behaviors and localized laser-induced lattice damages. Under an optical pump of 808 nm light, continuous-wave waveguide lasing at 1.06 μm is achieved, with a single-mode operation and a minimum lasing threshold of 98.8 mW. Furthermore, the visible emissions of Nd3+ with short wavelengths ranging from 415 nm to 550 nm and long wavelengths from 550 nm to 625 nm are observed upon 808 nm laser excitation via the up-converted process. The intensity ratios of two wavelength ranges are proved to be tunable through changing the pumping polarizations.

In this Letter, we report the existence and relaxation properties of a critical phenomenon on called a 3D super crystal that emerges at T = T_C ? 3.5°C, that is, in the proximity of the Curie temperature of a Cu:KTN sample. The dynamics processes of a 3D super crystal manifest in its formation containing polarized nanometric regions and/or polarized clusters. However, with strong coupling and interaction of microcomponents, the characteristic relaxation time measured by dynamic light scattering demonstrates a fully new relaxation mechanism with a much longer relaxation time. As the relaxation mechanism of a relaxator is so-far undetermined, this research provides a novel perspective. These results can help structure a fundamental theory of ferroelectric relaxation.