Zeming Qi

NbOCl2 is an emerging layered van der Waals material with prominent anisotropy. In this work, we systematically investigate the anisotropic optical response and structural evolution of NbOCl2 under pressure up to 50 GPa by using polarized Raman spectroscopy, synchrotron polarized far-infrared spectroscopy, and polarized visible light absorption spectroscopy. A series of pressure-induced structure changes are identified, which are demonstrated to be closely correlated with the evolution of octahedral distortions. The initial ferroelectric phase transforms into an antiferroelectric phase within 2–8 GPa, remains in the antiferroelectric phase between 8 and 16 GPa, and further undergoes a transition to the paraelectric phase in the 16–21 GPa range. Meanwhile, the [NbO2Cl4] octahedral ordering undergoes a transition from a parallel to an antiparallel configuration, followed by a gradual reduction in octahedral distortion. A dramatic collapse of the optical bandgap is observed at pressures above 38 GPa, suggesting that the octahedral distortion may have been suppressed. This study provides an in-depth understanding of the anisotropic optical properties of NbOCl2 and offers important implications for the design and applications of future anisotropic optoelectronic devices based on this material.

Simultaneously exciting phonon polaritons with distinct topologies in a single crystal at a unified excitation frequency offers a route to combine their different propagation characteristics. In this work, by selecting an appropriate crystal plane in Lu2SiO5, we investigated the gold-disk-patterned Lu2SiO5 crystal using scattering-type scanning near-field optical microscopy (s-SNOM) and achieved the simultaneous excitation of two phonon polaritons with hyperbolic and elliptic-like isofrequency contours at a single frequency. The hyperbolic and elliptic-like phonon polaritons are launched by the fabricated gold disks and the SNOM tip, respectively. The propagation wave of the elliptic-like phonon polaritons is reflected by the gold disks and then is detected by the SNOM tip again, resulting in a momentum that is twice the intrinsic momentum, which is further confirmed by attenuated total reflection spectroscopy measurements using a coupled ZnSe prism for excitation. Our finding significantly enriches the anisotropic propagation behavior of phonon polariton in a single crystal.

For decades, γ-N2 has been known to exist at very low temperatures and pressures, located in a tiny area of the nitrogen phase diagram. Recently, it was shown that γ-N2 occupies most of the P–T space usually associated with molecular phases such as δ, ɛ, and ζ, and that it plays a pivotal role in shaping nitrogen’s phase diagram. Using powder synchrotron X-ray diffraction, Raman and infrared spectroscopy, and density function theory calculations, we have investigated the structural and optical properties of γ-N2 in a wide P–T range. The combined X-ray diffraction and infrared spectroscopy results unequivocally demonstrate that γ-N2 adopts the monoclinic (P21/c space group) configuration with two N2 molecules per unit cell. It appears that the γ-N2 is structurally closely related to θ-N2, leading to both phases having very similar Raman signatures. Additionally, the Raman spectroscopy reveals a vibrational mode intensity resonance effect in both phases, caused by a strong vibrational coupling between the isotopic 15N14N and 14N2 vibrational excitations.

In this study, SrSn(BO ₃ ) ₂ – x wt% LiF microwave ceramics were successfully prepared at 950 °C via conventional solid-state sintering.

We demonstrate that N-sites on N-doped graphene/TiO ₂ act as nitrogen carriers, sustaining NH ₃ generation via a N-cycle. Nitrogen is detached from graphene for NH ₃ synthesis and replenished by atmospheric N ₂ .

Ca₃(BO₃)₂ microwave dielectric ceramics with space group R-3c (#167) were prepared by cold sintering, and their properties were systematically investigated. Phonon density of state diagrams for the Ca₃(BO₃)₂ lattice were obtained based on first-principles calculations to provide a more comprehensive understanding of the lattice vibrational properties of the material. Raman scattering and infrared reflectance spectroscopy were employed to investigate the lattice vibrational characteristics, identifying two types of vibrational modes: internal modes associated with the planar bending and symmetric stretching vibrations of the [BO₃] group, and external modes linked to the vibrations of the [CaO₆] octahedron. The intrinsic dielectric properties were determined by fitting the experimental data using a four-parameter semi-quantum model. The results demonstrate that the dielectric properties of Ca₃(BO₃)₂ ceramics are primarily influenced by the external vibrational modes. The sample under 800 MPa exhibits optimal dielectric performance, with a dielectric constant (ε_r) of 5.95, a quality factor (Q × f) of 11,836 GHz, and a temperature coefficient of resonant frequency (τ_f) of −39.89 ppm/°C. A simulation of this Ca₃(BO₃)₂ sample as a dielectric substrate was conducted using HFSS to fabricate a microstrip patch antenna operating at 14.97 GHz, which exhibits a return loss (S₁₁) of −25.5 dB and a gain of 7.15 dBi.

Interfacial second harmonic generation (SHG) switching mediated by insulator–metal phase transition of VO2 can serve as switchable and ultracompact nonlinear optical elements for on-chip logical optical operations. We propose that enhanced conversion efficiency and switching ratio of interfacial SHG in telecom band can be achieved with multilayer InSe/VO2 heterostructure, which has been comprehensively validated by a theoretical model of interfacial SHG and SHG spectra measurements. Our results provide solid guidance for designing interfacial SHG switching devices of high performance.