Plasmonically Enhanced Kerr Frequency Combs

Optical frequency combs are high repetition rate, broad spectral bandwidth coherent light sources. These devices have numerous applications in many fields, ranging from fundamental science to defense. Recently, low-threshold and small-footprint frequency combs have been demonstrated using ultrahigh quality factor (Q) whispering gallery mode resonant cavities. The majority of research in cavity-based combs has focused on optimizing the Q. An alternative strategy is to engineer the cavity material to enhance the underlying nonlinear process for comb generation. In this work, we demonstrate that gold nanorods coated with a nonlinear material reduce the comb generation threshold when decorated on the surface of the resonant cavities. The enhancement mechanism is explored with finite element method modeling and can be explained in terms of photonic–plasmonic mode hybridization. A comb span of ∼300 nm in the near-IR range is observed with incident intensity <2 GW cm^–2. The required threshold for parametric oscillation directly scales with nanorod concentration and ranges from 148 μW to 1.5 mW, which is 15 times lower than uncoated silica devices with similar optical performance.