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.
Space-time metasurfaces are great candidates for breaking the Lorentz reciprocity thorough inducing the desired momentum for photonic transitions between two modes. However, the significant difference between the carrier and modulation frequencies in photonic metasurfaces leads to negligible spatial pathway variation of light at different sidebands and weak power isolation. To surmount this obstacle, herein the design principle of the high Q-factor space-time metasurface is demonstrated that increases the lifetime of photons such that the optical cycle becomes comparable with the modulation cycle and strong power isolation is maintained by lifting the adiabaticity of modulation. It is shown that under time-reversal and by the virtue of modulation induced phase shift strong free space power isolation of ≈35dB is achieved between the two arbitrary ports at near-infrared regime.
Optical metasurfaces are periodic or graded pattern arrays of ultra-thin plasmonic and/or dielectric nanostructures, which are intended to scatter light in manners that cannot be achieved with conventional stratified media. Recent advancements in the theoretical knowledge and fabrication methods of two-dimensional materials, such as graphene, have provided the opportunity to scale down the principles of metasurfaces to atomic dimensions and to offer graded pattern meta-sheets. We present here engineered nanostructures to tailor the beaming pattern of light scattered through such meta-sheets. We obtain designs to precisely control both the in-plane scattering of surface waves associated with the sheets and also out-of-plane scattered far-field beams into a desired direction. We then determine a set of conductivity-balancing conditions to completely confine the surface waves to the meta-sheets at highly scattering sites and demonstrate that under such criterion the propagation of guided surface waves can be described simply using Fresnel equations of plane waves. Furthermore, we cascade three sinusoidally modulated reactance surfaces to realize a broad-beam leaky-wave antenna to completely scatter the surface waves to far-field and also control the steering direction. In addition, conformal patterned 2D sheets will be explored for the first time and how to successfully design and manipulate the light wavefront. For fast and accurate designs of the flat and conformal meta-sheets, we take advantage of our superior auxiliary differential equation finite-difference time-domain (ADE-FDTD) method. Also, an integral equations (IE) model will be applied for large-area system platforms design investigation.
A dual-band multilayer shared aperture antenna (SAA) is presented, which can recognize anomalous two-dimensional beam steering simultaneously at two distinct operating wavelengths lie in near-infrared (NIR) (λ1=1055 nm) and visible (λ2=700 nm) spectra. The supercell consists of one large cross-shaped resonator antenna (top layer) and a 2×2 square-shaped patches (bottom layer). This compact bifunctional dual-band reflectarray SAA can steer the main beam toward relatively large angles (≈40 deg) in both θ- and ϕ-planes at dual-frequency bands. The state-of-the-art techniques in antenna design are exploited to attain the minimized aperture size and negligible coupling among NIR and visible arrays. The overall height of the antenna is 275 nm (≈0.25λ1). The presented structure is customizable in form and can easily be scaled to other frequency ratios.
This paper presents an engineered metasurface which can serve functionalities such as anomalous bending, focusing, and beam shaping over the circularly polarized (CP) incident beam. The building block is a bilayer double split-loop resonators (DSLRs) where it can fully transmit the impinging light and control phase only by rotation of unit-cell and not by changing the structural parameters which can greatly facilitate the fabrication process. The mechanism behind this fascinating feature can be described as the conversion of an impinging CP incident beam into the opposite handedness and obtaining a geometrical phase shift equal to twice the rotating angle of DSLRs. It is illustrated that full transmission with 2π phase shift can be achieved with the proposed metasurface. Unique designs with helicity dependency to realize anomalous bending, bifunctional convergence/divergence, and flat-top beam creation with applying lossless beam shaping approach are presented.