Efficient Nonlinear Metasurface Based on Nonplanar Plasmonic Nanocavities

Since their discovery in the 1960s, nonlinear optical effects have revolutionized optical technologies and laser industry. Development of efficient nanoscale nonlinear sources will pave the way for new applications in photonic circuitry, quantum optics and biosensing. However, nonlinear signal generation at dimensions smaller than the wavelength of light brings new challenges. The fundamental difficulty of designing an efficient nonlinear source is that some of the contributing factors involved in nonlinear wave-mixing at the nanoscale are often hard to satisfy simultaneously. Here, we overcome these limitations by developing a new type of nonplanar plasmonic metasurfaces, which can greatly enhance the second harmonic generation (SHG) at visible frequencies and achieve conversion efficiency of ∼6 × 10^–5 at a peak pump intensity of ∼0.5 GW/cm². This is 4–5 orders of magnitude larger than the efficiencies observed for nonlinear thin films and doubly resonant plasmonic antennas. The proposed metasurface consists of an array of metal-dielectric-metal (MDM) nanocavities formed by conformally cross-linked nanowires separated by an ultrathin nonlinear material layer. The nonplanar MDM geometry minimizes the destructive interference of nonlinear emission into the far-field, provides strongly enhanced independently tunable resonances both for fundamental and harmonic frequencies, a good mutual overlap of the modes and a strong interaction with the nonlinear spacer. Our findings enable the development of efficient nanoscale single photon sources, integrated frequency converters, and other nonlinear devices