Resonant four-wave-mixing in microcavities has recently proven to be particularly interesting for obtaining ultra-efficient nonlinear wavelength conversion, parametric and frequency combs generation. Contrarily to the commonly used microring or whispering gallery mode cavities, photonic crystal nanocavities have not revealed yet their full potential in this direction. Despite their high-Q and ultra-small modal volume, they are not evidently suited for resonant four wave mixing as they do not naturally exhibit modes at equally spaced frequencies, a necessary condition for energy conservation.
In this work, we designed and fabricated 1D photonic crystal nanobeam cavities which exhibit ultra- high Q modes around 1.55µm equally spaced in frequency. These nanocavities are made of InGaP material bonded on top of a SOI waveguide optical circuitry. The evanescent wave coupling between the cavities and the waveguides can be controlled at will by changing the SOI waveguide width. The large electronic bandgap of InGaP inhibits 2 photon absorption at 1.55µm and allows us to exploit pure Kerr nonlinearity.
The electromagnetic potential inside the cavity is shaped to be spatially parabolic by engineering the hole position along the cavity. Thus, by construction the resonant modes supported by the cavity are equispaced in frequency.
The measured loaded Q factors exceed 105 and the free spectral range (FSR) goes from 150GHz to 1THz depending on the size of the cavity. We demonstrate that the FSR remains quasi constant (flat dispersion). Four wave mixing and parametric generation is observed using CW pump power of few mWs.
Gabriel Marty, Dorian Sanchez, Sylvain Combrié, Alfredo De Rossi, and Fabrice Raineri, "Four-wave mixing in hybrid InGaP on SOI multimodal nanocavities (Conference Presentation)," Proc. SPIE 10541, Photonic and Phononic Properties of Engineered Nanostructures VIII, 105411F (Presented at SPIE OPTO: February 01, 2018; Published: 14 March 2018); https://doi.org/10.1117/12.2291638.5751531023001.
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