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Frequency‐dependent nonlinear process in microresonators is widely acknowledged, but there is no theory available to calculate the conversion efficiency for each resonance of the ring, except for the phase‐matching one. Similarly for azimuth‐dependent nonlinear process in birefringent rings, there is a lack of theory to analysis the conversion efficiency for each azimuth of the ring. Consequently, it leads to old‐fashioned or ill‐considered coupling position and inefficient energy conversion in birefringent microrings. Here, the study introduces spatio‐temporal coupled‐mode equation to describe mode spatial properties in the cavity, compensating for the deficiency of temporal coupled‐mode equation in describing sophisticated responses. By this equation, it is found that over a wide frequency range, the extremely strong second‐harmonic generation of TE modes can be achieved at different azimuth under different pumps in an X‐cut lithium niobate microring, as proved by a 440 000%/W conversion efficiency in a non‐poling ring recently. This theory is important for realizing an efficient entangled quantum light source, for example. Meanwhile, it will provide new ideas and guidelines for design and applications of monolithic birefringent photonic integrated circuits with high energy efficiency. [ABSTRACT FROM AUTHOR]

Violet phosphorus (VP), a novel two-dimensional (2D) nanomaterial, boasts structural anisotropy, a tunable optical bandgap, and superior thermal stability compared with its allotropes. Its multifunctionality has sparked widespread interest in the community. Yet, the VP's air susceptibility impedes both probing its intrinsic features and device integration, thus making it of urgent significance to unveil the degradation mechanism. Herein, we conduct a comprehensive study of photoactivated degradation effects on VP. A nitrogen annealing method is presented for the effective elimination of surface adsorbates from VP, as evidenced by a giant surface-roughness improvement from 65.639 nm to 7.09 nm, enabling direct observation of the intrinsic morphology changes induced by photodegradation. Laser illumination demonstrates a significant thickness-thinning effect on VP, manifested in the remarkable morphological changes and the 73% quenching of PL intensity within 160 s, implying its great potential for the efficient selected-area etching of VP at high resolution. Furthermore, van der Waals passivation of VP using 2D hexagonal boron nitride (hBN) was achieved. The hBN-passivated channel exhibited improved surface roughness (0.512 nm), reduced photocurrent hysteresis, and lower responsivity (0.11 A/W @ 450 nm; 2 μW), effectively excluding adsorbate-induced electrical and optoelectrical effects while disabling photodegradation. Based on our experimental results, we conclude that three possible factors contribute to the photodegradation of VP: illumination with photon energy higher than the bandgap, adsorbed H2O, and adsorbed O2. [ABSTRACT FROM AUTHOR]

Studies into scatterings of photonic structures have been so far overwhelmingly focused on their dependencies on the spatial and spectral morphologies of the incident waves. In contrast, evolutions of scattering properties through another parameter space of incident directions (momentum space) has not attracted sufficient attention, with the underlying general governing rules still unrevealed. Here, from the perspective of quasi‐normal modes (QNMs), the momentum‐space scattering extremizations with respect to varying incident directions of plane waves are investigated. It is revealed that for effective single‐QNM excitations, scatterings are maximized exactly along those directions where the QNM radiation reaches its maximum, with matched incident and radiation polarizations. For an arbitrary direction, when the incident polarization is tuned to be orthogonal to that of the mode radiation, the QNM cannot be excited and thus the scatterer becomes invisible with null scatterings. The principles revealed are protected by fundamental laws of reciprocity and energy conservation (optical theorem), which can be further expanded and applied for other branches of wave physics. [ABSTRACT FROM AUTHOR]