Quasi-phase-matching has long been a widely used approach in nonlinear photonics, enabling efficient parametric frequency conversions such as second-harmonic generation. However, in silicon photonics the task remains challenging, as materials best suited for photonic integration lack second-order susceptibility (χ(2)), and means for achieving momentum conservation are limited. Here we present optically reconfigurable quasi-phase-matching in large-radius silicon nitride microresonators, resulting in up to 12.5-mW on-chip second-harmonic generated power and a conversion efficiency of 47.6% W−1. Most importantly, we show that such all-optical poling can occur unconstrained from intermodal phase-matching, leading to broadly tunable second-harmonic generation. We confirm the phenomenon by two-photon imaging of the inscribed χ(2) grating structures within the microresonators as well as by in situ tracking of both the pump and second-harmonic mode resonances during all-optical poling. These results unambiguously establish that the photogalvanic effect, responsible for all-optical poling, can overcome phase mismatch constraints, even in resonant systems.

Packing asymmetry between head-group and tail-chain of amphiphiles may induce buckling modulations in monolayers at air-water or oil-water interfaces. We consider three different cases associated with the head-tail asymmetry: (i) spontaneous curvature, (ii) molecular tilt divergence, and (iii) local composition variation in mixed monolayers. For a pure monolayer with non-zero spontaneous curvature, we find that, below some critical surface tension, an hexagonal array of “long-fingers" becomes more stable than the flat surface. This “long-finger" structure is not expected to remain stable against multilayer formation in the case of a Langmuir monolayer, but is relevant for monolayers at the oil-water interface. When the molecular tilt is non-zero, as often is the case in the liquid condensed phase of Langmuir monolayers, the coupling between curvature and tilt can also give rise to a first-order buckling transition. Considering a binary mixture monolayer, we find that it can easily buckle to periodic structures following composition modulations. For the latter case we find two kinds of buckling structures. One involves a very large amplitude, a counterpart of the “long-finger" structure, and is dominated by the average spontaneous curvature. The other structure is of much smaller amplitude, and results from the curvature-composition coupling. Implications for the process of spontaneous emulsification are also briefly discussed

The rapidly maturing integrated Kerr microcombs show significant potential for microwave photonics. Yet, state-of-the-art microcomb-based radiofrequency filters have required programmable pulse shapers, which inevitably increase the system cost, footprint, and complexity. Here, by leveraging the smooth spectral envelope of single solitons, we demonstrate microcomb-based radiofrequency filters free from any additional pulse shaping. More importantly, we achieve all-optical reconfiguration of the radiofrequency filters by exploiting the intrinsically rich soliton configurations. Specifically, we harness the perfect soliton crystals to multiply the comb spacing thereby dividing the filter passband frequencies. Also, the versatile spectral interference patterns of two solitons enable wide reconfigurability of filter passband frequencies, according to their relative azimuthal angles within the round-trip. The proposed schemes demand neither an interferometric setup nor another pulse shaper for filter reconfiguration, providing a simplified synthesis of widely reconfigurable microcombbased radiofrequency filters.

We demonstrate all-normal dispersion supercontinuum generation in chalcogenide photonic crystal fibers pumped at 2070-2080 nm with a femtosecond fiber laser. At 2.9 kW peak power, the generated supercontinuum has a 3 dB bandwidth of 27.6 THz and -20 dB bandwidth of 75.5 THz. We experimentally investigated the supercontinuum evolution inside our sample fiber at various peak powers and fiber lengths and study the impact of fiber water absorption on the generated supercontinuum spectrum. Multiple tests with fiber length- ranging from 0.34 to 10 cm-provide insight on pulse evolution along fiber length. Our simulations, which utilizes the generalized nonlinear Schrodinger equation model, match perfectly the experiments for all tested pump powers and fiber lengths, and confirm that the output pulse has a linear chirp, allowing linear pulse compression.

The coherent photogalvanic effect leads to the generation of a current under the absorption interference of coherent beams and allows for the inscription of space‐charge gratings leading to a second‐order susceptibility (χ(2)). The inscribed grating automatically results in quasi‐phase‐matching between the interfering beams. Theoretical and experimental studies, considering the degenerate case of second‐harmonic generation, show significant conversion efficiency enhancements. However, the link between the theory and experiment is not fully established such that general guidelines and achievable conversion efficiency for a given material platform are still unclear. In this work, the phenomenological model of coherent photogalvanic effect in optical waveguides is theoretically analyzed. This model predicts the existence of non‐degenerate sum‐frequency generation quasi‐phase‐matching gratings, which is confirmed experimentally for the first time. Furthermore, the time dynamics of the space‐charge grating inscription in coherent photogalvanic process is formulated. Based on the developed theoretical equations, the material parameters governing the process for stoichiometric silicon nitride are extracted. The results obtained provide a basis to compare the performances and potentials of different platforms. This work not only supplements the theory of coherent photogalvanic effect, but also enables us to identify critical parameters and limiting factors for the inscription of χ(2) gratings.