Generalized Coherent Photogalvanic Effect in Coherently Seeded Waveguides

Abstract

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.

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

“…Because of the minimal out-coupling of TH light from the microresonator, the conversion efficiency of effective TH generation here is in the same order of what has been measured in Si 3 N 4 waveguides ( 36 ) but is much lower than poled lithium niobate microresonators ( 61 ). The efficiency can be improved significantly with proper chip design to maximally extract the generated effective TH.…”

“…To distinguish the process from the abovementioned SF generation relying on the same χ (2) QPM grating for SH generation, we term the process here as cascaded harmonic generation or effective TH generation interchangeably to avoid ambiguity. Reference ( 36 ) has highlighted that three arbitrary coherent waves that are frequency matched could yield a periodic electric field to compensate for their unmatched momenta. Hence, as shown in Fig.…”

“…To bring χ (2) into these complementary metal-oxide semiconductor–compatible materials, diverse approaches have been pursued ( 29 – 37 ). Paradigm demonstrations include symmetry breaking in silicon waveguides induced by applied electric fields ( 29 , 30 ) or via charged defects ( 31 ) and all-optical poling of Si 3 N 4 waveguides based on the photogalvanic effect ( 32 – 36 ). With regard to resonator structures, effective χ (2) can also be optically induced in Si 3 N 4 microresonators manifesting in efficient SH generation ( 38 – 40 ).…”

“…Now, with the better understanding of χ (2) phase-matching mechanism in Si 3 N 4 microresonators ( 40 ), it is worth coming back to these cascaded nonlinear effects. In addition, we have recently generalized the photogalvanic effect to nondegenerate arbitrary three-wave condition, whose interference could inscribe the required QPM for the corresponding sum-frequency (SF) conversion process ( 36 ). This can be beneficial, for instance, in frequency tripling through cascaded SF generation, which is now a standard and commercially available technology to generate TH light ( 50 ).…”

“…This can be beneficial, for instance, in frequency tripling through cascaded SF generation, which is now a standard and commercially available technology to generate TH light ( 50 ). Although frequency tripling has been experimentally shown in Si 3 N 4 waveguides ( 36 ), the responsible SF grating is rather weak, and its full potential in resonator structures remains unexplored.…”