High-quality-factor space–time metasurface for free-space power isolation at near-infrared regime

Abstract

.Space–time metasurfaces are promising candidates for breaking Lorentz reciprocity, which constrains light propagation in numerous practical applications. There is a substantial difference between carrier and modulation frequencies in space–time photonic metasurfaces that leads to negligible spatial pathway variation of light and weak nonreciprocal response. To surmount this obstacle, herein, the design principle of a high-quality-factor space–time gradient metasurface is demonstrated at the near-infrared regime that increases the lifetime of photons and allows for strong power isolation by lifting the adiabaticity of modulation. The all-dielectric metasurface consists of an array of silicon subwavelength gratings (SWGs) that are separated from distributed Bragg reflectors by a silica buffer. The resonant mode with ultrahigh quality-factor exceeding 104 is excited within the SWG, which is characterized as magnetic octupole and features strong field localization. The SWGs are configured as multijunction p–n layers, whose multigate biasing with time-varying waveforms enables modulation of carriers in space and time. The proposed nonreciprocal metasurface is exploited for free-space optical power isolation by virtue of modulation-induced phase shift. It is shown that under time reversal and by interchanging the directions of incident and observation ports, power isolation of ≈35 dB can be maintained between the two ports in free space.