We use the semianalytical Cartesian multipole method to investigate the light transmission and reflection spectra of anisotropic dielectric metasurfaces, and extend the multipole decompositions method to account for cross-polarization conversion effects. We observe sharp high- Q resonances arising from a distortion of symmetry-protected bound states in the continuum in asymmetric dielectric metasurfaces, i.e., quasibound states in the continuum. In addition, by further introducing in-plane symmetric breaking perturbation, the polarization conversions of linearly polarized light can be achieved through quasibound states in the continuum. With the aid of temporal coupled-mode theory, we can obtain the limit of cross-conversion under single high- Q resonance, i.e., 0.25. Our work will help to design dielectric metasurfaces to control the polarization states of light.

Saturable absorber (SA) based harmonic mode-locking (HML) techniques at 2 µm waveband are much less reported than those at 1.5 µm waveband, the maximum repetition rate of the harmonic pulse generated by such techniques at 2 µm waveband is also much lower than those generated at 1.5 µm waveband. In this paper, the 39th harmonic with the repetition rate of 908.6 MHz is realized in a Bi₂S₃-based thulium-doped fiber laser. The fundamental mode-locked pulse has a central wavelength of 1954.2 nm and a 3-dB bandwidth of 5.1 nm. The repetition rate is 23.27 MHz and the pulse width is 902 fs. The characteristics of the material and harmonic mode-locked pulse are investigated. To the best of our knowledge, this is the highest and the closest resonance frequency to GHz among the reported SA-based harmonic mode-locked fiber lasers operating at 2 µm waveband.

Solvothermal reactions of copper salts with NaN₃ and carboxylate led to the formation of a new magnetic compound, [Cu(3-fba)(N₃)(C₂H₅OH)]_n (3-fba = 3-fluorobenzoic acid) (1). Single crystal X-ray analysis indicates that only one crystallographically independent Cu(II) ion in the asymmetric unit of 1 displays stretched octahedral geometry in which two azido N atoms and two carboxylic O atoms locate in the equatorial square, while two ethanol O atoms occupy the apical positions, yielding a 1D Cu(II) chain with an alternating triple-bridge of EO-azido, syn,syn-carboxylate, and μ2-ethanol. Magnetic measurements reveal that the resulting compound is composed of well-isolated 1D Cu(II) chains with strong intrachain ferromagnetic interactions. The dominant ferromagnetic couplings between neighbouring Cu(II) ions occurring in the compound (J = 59.74 cm⁻¹) are due to the counter-complementarity of the multiple superexchange pathways, contributing to inspiring plots of ferromagnetic order (T_c = 11.0 K) and slow magnetic relaxation that are rarely observed in most reported azido-Cu(II) architectures. Heat-capacity experiments further emphasize the characteristic long-range ferromagnetic ordering in 1. Moreover, density functional theory (DFT) calculations (using different methods and basis sets) have been performed on the compound to obtain the theoretical interpretation of the magnetic behaviors.

We experimentally demonstrate a fiber laser with different linewidths based on self-injection locking (SIL) and the stimulated Brillouin scattering effect. Based on the homemade fiber laser, the error origin, resolution, and applicable range of delayed self-heterodyne interferometry (DSHI), self-correlation envelope linewidth detection (SCELD) and Voigt fitting are investigated numerically and experimentally. The selection of the linewidth measuring method should meet the following conclusions: an approximately Lorentzian self-heterodyne spectrum without the pedestal and high-intensity sinusoidal jitter is a prerequisite for DSHI; the SCELD needs a suitable length of delay fiber for eliminating flicker noise and dark noise of the electrical spectrum analyzer; a non-Lorentzian self-heterodyne spectrum without a pedestal is an indispensable element for Voigt fitting. According to the experimental results, the laser Lorentzian linewidth of SIL changes from 1.7 kHz to 587 Hz under different injection powers. When the Brillouin erbium fiber laser is utilized, the Lorentzian linewidth is measured to be 60 ± 5 Hz.

We propose a Yb-doped fiber laser with an all-fiber beam shaper based on a single-mode–graded-index multimode–few-mode fiber (SMF-GIMF-FMF) structure. The excitation coefficients of the mode can be adjusted continuously by changing the GIMF length. Numerical simulations are performed to investigate the beam shaping dynamics in the fiber structure. Through adding the simple device geometry in the laser cavity, the switchable output between the fundamental transverse (LP₀₁) mode and the second-order transverse (LP₁₁) mode can be achieved. Cylindrical vector beams with high mode purity are also shown by removing the degeneracy of the LP₁₁ mode.