High-efficiency radiation beyond the critical angle via phase-gradient antireflection metasurfaces.

Total internal reflection generally occurs at incident angles beyond the critical angle, confining electromagnetic waves in dielectrics with higher refractive indices. In this work, we present a metasurface-based solution to transform such total reflection into high-efficiency transmission. We demonstrate that a phase-gradient antireflection metasurface designed on the dielectric surface not only compensates for the transverse wave vectors of the incident and transmitted waves but also addresses the impendence mismatch between the two media, eventually achieving high-efficiency transmission with flexibly-controlled wavefronts beyond the critical angle. The design of this unique metasurface is enabled by applying the reciprocity principle to circumvent the traditional limitation of total internal reflection. The theory and functionalities of the phase-gradient antireflection metasurfaces are verified through both simulations and microwave experiments. Our work opens a new avenue for high-efficiency radiation manipulation beyond the critical angle, enabling rich applications such as high-efficiency waveguide-to-free-space couplers, high-radiation-efficiency quantum dots, and high-radiation-efficiency light-emitting diodes.