Third Harmonic Generation in Highly-doped Silica Glass Micro Ring Resonator

Visible emissions in microresonators by high-harmonic generation with input at telecom wavelengths have been demonstrated in a number of platforms, such as silica [1], silicon photonic-crystal [2], and silicon nitride [3]. In this work, we report the first observation of green emission by third-harmonic generation (THG) in a highly doped glass micro ring-resonator and show that the measured pump and emission dependence agrees with the theoretical prediction of the THG process. The ring resonator used in the experiment is composed of highly-doped silica glass with core refractive index of 1.7 and a radius of 135 μm. With waveguide dimensions of 2μm × 1μm, it corresponds to a free-spectral range (FSR) of 201.5 GHz. Fig. 1(a) shows the effective index mismatch between the pump mode and the various THG modes. With a measured Q factor of ∼5.5×10⁵ at 1550 nm and small mode volume, it contributes to the high conversion efficiency of the THG where the threshold pump power for THG is approximately 2.4 mW. The captured green emission from directly above the ring resonator (insert of Fig. 1(b)) when pumped with a continuous-wave (CW) laser at 1550 nm centred at 516.7 nm demonstrates that the green light is attributed to the THG process.