Visible octave frequency combs in silicon nitride nanophotonic waveguides driven by Ti:sapphire lasers

TL;DR

This paper demonstrates visible octave frequency combs generated in silicon nitride nanophotonic waveguides driven by Ti:sapphire lasers, enabling efficient broadband comb access in the visible spectrum on CMOS-compatible platforms.

Contribution

It introduces dispersion-engineered air-clad silicon nitride waveguides that produce octave-spanning combs from visible to near-infrared using low pulse energies and tunable bandwidth.

Findings

Octave-spanning combs achieved from visible to near-infrared.

Waveguide width lithographically tunes the comb bandwidth.

Supports octave combs at 1 GHz repetition rate with Ti:sapphire pump.

Abstract

Nonlinear nanophotonic waveguides have opened a route to compact frequency combs for precision metrology, spectroscopy and astronomy, yet broadband comb access to the visible remains challenging on CMOS-compatible platforms. Silicon nitride is widely accessible and low loss into the visible, but most demonstrations rely on telecom pumping and thick stress-managed films, where the large spectral gap to the visible dispersive wave raises the soliton order and power required for efficient conversion. Here we show that pumping closer to the visible provides a complementary route. Starting from crack-free 400 nm SiN films, we implement dispersion-engineering with air-clad nanophotonic waveguides whose enhanced geometric dispersion opens an anomalous-dispersion window across the Ti-sapphire tuning range. Femtosecond Ti-sapphire pulses then drive octave-spanning combs from the visible to the near-infrared, with the visible edge and overall bandwidth lithographically tuned by the waveguide width at pulse energies of only tens of picojoules. The air-clad geometry also produces strong polarization-dependent dispersion, enabling switching between all-normal and soliton-dominated broadening in the same device, and support octave-spanning combs at 1 GHz repetition rates directly driven by a compact diode-pumped Ti-sapphire oscillator. These results position air-clad SiN nanophotonic waveguides as an efficient interface between emerging short-wavelength integrated gain platforms and fully integrated visible frequency-comb engines.