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.

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.

We report on the fabrication of circular cladding waveguides with cross-section diameters of 60-120 μm in Pr:YAG crystal by applying femtosecond laser inscription. The fabricated waveguides present 2D guidance on the cross-section and fairly low propagation losses. Multiple high-order guiding modes are observed in waveguides with different diameters. Corresponding simulation results reveal the origin of a specific kind of guiding modes. Confocal micro-Raman (μ-Raman) experiments demonstrate the modification effects in femtosecond laser affected areas and ascertain the refractive index induced guiding mechanism. In addition, luminescence emission properties of Pr³⁺ ions at waveguide volume are well preserved during the femtosecond laser inscription process, which may result in a potential high-power visible waveguide laser.

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.