Large optical nonlinearity of nanoantennas coupled to an epsilon-near-zero material

The size and operating energy of a nonlinear optical device are fundamentally constrained by the weakness of the nonlinear optical response of common materials(1). Here, we report that a 50-nm-thick optical metasurface made of optical dipole antennas coupled to an epsilon-near-zero material exhibits a broadband (similar to 400 nm bandwidth) and ultrafast (recovery time less than 1 ps) intensity-dependent refractive index n(2) as large as -3.73 +/- 0.56 cm(2) GW(-1). Furthermore, the metasurface exhibits a maximum optically induced refractive index change of +/- 2.5 over a spectral range of similar to 200 nm. The inclusion of low-Q nanoantennas on an epsilon-nearzero thin film not only allows the design of a metasurface with an unprecedentedly large nonlinear optical response, but also offers the flexibility to tailor the sign of the response. Our technique removes a longstanding obstacle in nonlinear optics: the lack of materials with an ultrafast nonlinear contribution to refractive index on the order of unity. It consequently offers the possibility to design low-power nonlinear nano-optical devices with orders-of-magnitude smaller footprints.