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

Abstract

Silicon nitride (Si 3 N 4 ) is an ever-maturing integrated platform for nonlinear optics but mostly considered for third-order [χ (3) ] nonlinear interactions. Recently, second-order [χ (2) ] nonlinearity was introduced into Si 3 N 4 via the photogalvanic effect, resulting in the inscription of quasi-phase–matched χ (2) gratings. However, the full potential of the photogalvanic effect in microresonators remains largely unexplored for cascaded effects. Here, we report combined χ (2) and χ (3) nonlinear effects in a normal dispersion Si 3 N 4 microresonator. 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. In addition, the doubly resonant pump and second-harmonic fields allow for effective third-harmonic generation, where a secondary optically written χ (2) grating is identified. Last, we reach a broadband microcomb state evolved from the sum-frequency–coupled primary comb. These results expand the scope of cascaded effects in microresonators.

UV, visible, and near-infrared light generation through cascaded nonlinear processes in silicon-nitride microring resonators pumped at telecom wavelength

“…2 (b). The χ (2) eff lattice inscribed by the dislocated free charges has a spatial periodicity (Λ) which is inversely proportional to the difference in the effective indices (n eff ) of the fundamental harmonic at ω and of the second harmonic at 2ω: 8,12…”

Coupling to multi-mode waveguides with space-time shaped free-space pulses

“…Beyond optical fibers, integrated waveguides that are generally rectangular and smaller cross-section can still support multiple modes due to their large index contrast. The higherorder modes have been utilized for supercontinuum generation in straight waveguides [75] and nonlinear optics in microresonators [76][77][78], along with other parametric processes [79][80][81]. Due to the high level of confinement of the field the nonlinear optical modelling of these systems is much more complicated, where longitudinal fields can become important [82][83][84].…”

Versatile photonic molecule switch in multimode microresonators

“…Another method involves direct modulation of the coupling strength by mechanically changing the microresonators’ geometric factors 13 – 16 , which increases the complexity of fabrication processes. The microring resonator (MRR) can also be tailored by Bragg grating to enable coupling between clockwise (CW) and counter-clockwise (CCW) modes 17 – 20 to achieve PMs, where the coupling strength is either fixed once fabricated, or relies on the modulated refractive index via high power induced nonlinear effect 19 , 20 .…”