ABSTRACT
Freely controlling wavefronts with metasurfaces has been widely studied in linear optical systems. By constructing phase gradient meta-atoms with nonlinear responses, the wavefronts of high-harmonic fields in nonlinear metasurfaces can be arbitrarily steered by following nonlinear generalized Snell's law (NGSL). However, for incident angles above the critical angle, NGSL fails to predict the generated nonlinear waves. In this work, by involving the reciprocal lattice effect of the nonlinear metasurface, we show a modified diffraction law to completely describe the nonlinear diffraction phenomena. This law is numerically demonstrated and confirmed by designed graphene-based nonlinear metasurfaces in the terahertz regime. Moreover, based on the diffraction law, we designed a nonlinear retroreflector and realized tunable control over a nonlinear wavefront in a single nonlinear metasurface. Our work provides a way to manipulate nonlinear waves and provides a better design of functional nonlinear metadevices.
ABSTRACT
Inspired by the concept of phase-gradient metasurfaces (PGMs), we present a way to design a multi-functional PGM-based light beam splitter (LBS) operating in the optical regime by engineering the anomalous diffraction properties. As an example of a proof of concept, the designed LBS is a purely metallic slit array with gradient slit width, termed metagrating. It is shown that the designed LBS can simultaneously achieve high-efficiency light beam splitting on both energy and polarization, and it has broadband and wide-angle response. In addition, we also show that the Ohmic loss of metals plays an important role in determining the diffraction efficiency of each diffraction order, which is the physics for designing the LBS that can deliver the incident energy equally into the reflection and refraction sides. Our work enriches the existing methods of designing LBSs and particularly provides a route for the design of multi-functional LBSs with high performance.
