Large second harmonic generation enhancement in SiN waveguides by all-optically induced quasi phase matching

TL;DR

This paper demonstrates a significant enhancement in second harmonic generation in silicon nitride waveguides through all-optical quasi phase matching, enabling reconfigurable nonlinear interactions for integrated photonics.

Contribution

The authors introduce an all-optical method to induce quasi phase matching in SiN waveguides, achieving over 30 dB SHG enhancement and reconfigurable phase matching.

Findings

Over 30 dB SHG enhancement achieved.

Reconfigurable phase matching via pump wavelength and polarization.

Estimated second order nonlinearity of ~0.3 pm/V.

Abstract

Integrated waveguides exhibiting efficient second-order nonlinearities are crucial to obtain compact and low power optical signal processing devices. Silicon nitride (SiN) has shown second harmonic generation (SHG) capabilities in resonant structures and single-pass devices leveraging intermodal phase matching, which is defined by waveguide design. Lithium niobate allows compensating for the phase mismatch using periodically poled waveguides, however the latter are not reconfigurable and remain difficult to integrate with SiN or silicon (Si) circuits. Here we show the all-optical enhancement of SHG in SiN waveguides by more than 30 dB. We demonstrate that a Watt-level laser causes a periodic modification of the waveguide second-order susceptibility. The resulting second order nonlinear grating has a periodicity allowing for quasi phase matching (QPM) between the pump and SH mode. Moreover, changing the pump wavelength or polarization updates the period, relaxing phase matching constraints imposed by the waveguide geometry. We show that the grating is long term inscribed in the waveguides, and we estimate a second order nonlinearity of the order of 0.3 pm/V, while a maximum conversion efficiency (CE) of 1.8x10-6 W-1 cm-2 is reached.