Optically reconfigurable quasi-phase-matching in silicon nitride microresonators

Abstract

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.

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

“…In our system, the TH generation is also a χ (2) process resulting from the photogalvanic effect, but in this case based on the coherent interference among the FH, the generated SH and the seed TH waves. We confirm this by using two-photon microscopy (TPM) imaging of the poled microresonator 38,39,44 , where a secondary χ (2) grating responsible for the QPM of the underlying FH-SH-TH process is clearly identified. Additionally, we observe the generation of a primary comb in the FH band together with sum-frequency (SF) generation in the SH band.…”

“…Paradigm demonstrations include symmetry-breaking in silicon waveguides induced by applied electric fields 33,34 or via charged defects 35 and all-optical poling of silicon nitride (Si 3 N 4 ) waveguides based on the photogalvanic effect [36][37][38][39][40] . With regard to resonator structures, effective χ (2) can also be optically induced in Si 3 N 4 microresonators manifesting in efficient SH generation [42][43][44] .…”

“…SH generation in microresonators without periodically poled 15,16 or crystallographic 18 quasi-phase-matching (QPM) structures, had long been achieved by intermodal phase-matching between involved fundamental-harmonic (FH, i.e., pump) and SH resonances. Opposed to this convention, we have recently observed in Si 3 N 4 that periodic space-charges are self-organized along the circumferences of microresonators when the pump and its SH are doubly resonant 44 . The alternating electric field together with the χ (3) nonlinearity of Si 3 N 4 results in an effective χ (2) grating [45][46][47] , with a periodicity following the interference of FH and SH, and thus automatically compensating their phase-mismatch.…”

“…By varying the pump detuning, the doubly resonant condition of the pump mode and one of the several SH modes is easily matched, thanks to the small free spectral range (FSR ∼ 25 GHz) and multimode nature of the microresonator used. This gives rise to the photo-induced χ (2) grating inside the microresonator enabling QPM for SH generation 44 . Following the FH-SH grating inscription, we here show that the combined χ (2) and χ (3) nonlinearities can lead to the cascaded nonlinear processes like the generation of TH and frequency combs.…”

Compact, spatial-mode-interaction-free, ultralow-loss, nonlinear photonic integrated circuits

“…In addition, stimulated Brillouin scattering 2,52 and indirect-interband transition 6,53 among different spatial modes are the basis of magneticfree non-reciprocal devices [54][55][56][57] . Equally important, harmonic generation is facilitated by employing phase matching among different spatial modes [58][59][60][61][62] . For Kerr nonlinear photonics, anomalous GVD is required, which is realized via geometric dispersion engineering 45 using multi-mode waveguides.…”