Ultra-low mode volume on-substrate silicon nanobeam cavity.

We design and fabricate an on-substrate bowtie photonic crystal (PhC) cavity in silicon. By optimizing the bowtie shapes in the unit cells of the PhC cavity, the maximum of the electric field can be highly confined in the bowtie tips. Due to such confinement, an ultra-low mode volume of ∼0.1(λ/nSi)3 is achieved, which is more than an order of magnitude smaller than the previous on-substrate nanobeam cavities. An ultra-high quality (Q) factor as large as 106 is predicted by simulation, and up to 1.4×104 is measured in experiment. The observation of pronounced thermo-optic bistability is consistent with the strong confinement of light in the cavities.