arXiv subjects

Jianqi Hu

Publications and source records attributed to Jianqi Hu.

22 records · Page 2Linked to original sources

arXiv2022

Photo-induced cascaded harmonic and comb generation in silicon nitride microresonators

Silicon nitride (Si$_3$N$_4$) is an ever-maturing integrated platform for nonlinear optics. Yet, due to the absence of second-order ($χ^{(2)}$) nonlinearity, Si$_3$N$_4$ is mostly considered for third-order ($χ^{(3)}$) nonlinear interactions. Recently, this limitation was overcome by optical poling in both Si$_3$N$_4$ waveguides and microresonators via the photogalvanic effect, resulting in the inscription of quasi-phase-matched $χ^{(2)}$ gratings. Here, we report cascaded nonlinear effects in a normal dispersion Si$_3$N$_4$ microresonator with combined $χ^{(2)}$ and $χ^{(3)}$ nonlinearities. We demonstrate that the photo-induced $χ^{(2)}$ grating also provides phase-matching for the sum-frequency generation process, enabling the initiation and successive switching of primary combs at pump wavelength. Additionally, the doubly resonant pump and second-harmonic fields allow for cascaded third-harmonic generation, where a secondary optically written $χ^{(2)}$ grating is identified. Finally, we reach a low-noise, broadband microcomb state evolved from the sum-frequency coupled primary comb. These results expand the scope of cascaded effects in $χ^{(2)}$ and $χ^{(3)}$ microresonators.

physics.optics↗

arXiv2022

Coherent Photogalvanic Effect for Second-Order Nonlinear Photonics

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 an effective second-order susceptibility ($χ^{(2)}$). The inscribed grating automatically results in quasi-phase-matching between the interfering beams. Theoretical and experimental studies have been carried out, mostly focusing on the degenerate case of second-harmonic generation, showing significant conversion efficiency enhancements. However, the link between the theory and experiment was not fully established such that general guidelines and achievable conversion efficiency for a given material platform are still unclear. In this work, we theoretically analyze the phenomenological model of coherent photogalvanic effect in optical waveguides. Our model predicts the existence of non-degenerate sum-frequency generation quasi-phase-matching gratings, which is confirmed experimentally for the first time. In addition, we rigorously formulate the time dynamics of the space-charge grating inscription in coherent photogalvanic process. Based on developed theoretical equations for the time dynamics of the space-charge grating formation, we extract the material parameters governing the process for our experimental platform, stoichiometric silicon nitride. The results obtained provides 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.

physics.optics↗

arXiv2021

Optically reconfigurable quasi-phase-matching in silicon nitride microresonators

Bringing efficient second-order nonlinear effects in integrated photonics is an important task motivated by the prospect of enabling all possible optical functionalities on chip. Such task has proved particularly challenging in silicon photonics, as materials best suited for photonic integration lack second-order susceptibility ($χ^{(2)}$). Methods for inducing effective $χ^{(2)}$ in such materials have recently opened new opportunities. Here, we present optically reconfigurable quasi-phase-matching in large radius Si$_3$N$_4$ microresonators resulting in mW level on-chip second-harmonic generated powers. Most importantly we show that such all-optical poling can occur unconstrained from intermodal phase-matching, leading to widely 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 dynamic tracking of both the pump and second-harmonic mode resonances. These results unambiguously establish that the photogalvanic effect, responsible for all-optical poling, can overcome phase mismatch constraints even in resonant systems, and simultaneously allow for the combined record of output power and tunability.

physics.optics↗

arXiv2020

Reconfigurable radiofrequency filters based on versatile soliton microcombs

The rapidly maturing integrated Kerr microcombs show significant potential for microwave photonics. Yet, state-of-the-art microcomb based radiofrequency (RF) 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 for the first time microcomb based RF filters free from any additional pulse shaping. More importantly, we achieve all-optical reconfiguration of the RF 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, a completely novel approach based on the versatile interference patterns of two solitons within one round-trip, enables wide reconfigurability of RF passband frequencies according to their relative azimuthal angles. The proposed schemes demand neither an interferometric setup nor another pulse shaper for filter reconfiguration, providing a practical route towards chip-scale, widely reconfigurable microcomb based RF filters.

physics.optics↗