distribution and adaptive beamforming. Toward the realization of these active engineered structures various mechanisms have been utilized including mechanical reconfiguration, [4,5] photoswitching of dye molecules, [6] nonlinear optical effects, [7] thermal phase transitions, [8,9] and electrooptical field-effect modulation. [10][11][12][13][14][15][16][17][18] Among the aforementioned techniques, electro-optical controllable devices offer continuous tunability, shorter response time, and relatively wide tuning range comparing to thermal and mechanical tunable stimulations. Furthermore, the possibility of direct and independent electrical biasing of each inclusion within an optical platform makes this approach more favorable than all-optical tunability approaches. [19,20] Wide range of electrooptical materials have recently emerged like graphene, liquid crystals, [11] doped semiconductors (InSb and GaAs), [12,13] and transparent conducting oxides materials (TCOs) [14,15] including indium tin oxide (ITO), doped zinc oxide (ZnO), and aluminum-, gallium-, and indium-doped zinc-oxide (AZO, GZO, and IZO). In the mid-infrared and far-infrared spectra, the surface conductivity of graphene is widely tunable by the change of its electrochemical potential via applying an external gate voltage. Due to the extreme thinness, conformable to diverse patterning schemes, and broadband operation, it has been exploited in reconfigurable metadevices for manipulation of the surface plasmons and dynamical tuning of the geometrical resonances. [16][17][18] In the near-infrared (NIR) regime, TCOs are of particular interest due to the short response time (≈ns), fabrication feasibility, the controllable optical and electrical properties through the pre-and post-depositional processes, and large variation of complex refractive index (unity-order index change) in the charge accumulation/depletion regime. Also, the epsilon-near-zero (ENZ) property has been observed at the NIR regime and telecommunication frequency range when the carrier concentration of TCO is in the range of 10 20 -10 21 cm −3 . [21][22][23][24] ITO as the most well-known TCO has been exploited in the design of various optoelectronic devices. [25][26][27][28][29][30][31][32][33][34][35] Due to the fact that the ultrathin active layer of ITO has limited interaction length with the normal impinging wave, two approaches have been proposed to overcome the weak interaction between light and ITO. First, integration of ITO into a subwavelength grating A. Forouzmand, M. M. Salary, Dr.

In this work, an all‐dielectric photonic metasurface with a flat macroscopic geometry is shown to provide a viable solution toward realization of a relativistic light sail driven by radiation pressure from high‐power lasers offering passive beam‐riding stability, efficient acceleration, and radiative cooling is demonstrated. A critical challenge that is addressed is sustaining acceleration and stability over the Doppler‐broadened propulsion band which is crucial for achieving relativistic velocities and requires the metasurface to maintain a high reflectivity and wide phase coverage over a broad bandwidth. For this purpose, a zero‐contrast dielectric metasurface consisting of a graded pattern of c‐Si nanodisks connected by a thin matched sublayer on top of a thin silica layer, featuring an average areal mass density of 0.54 g m−2 is used. The nanostructured silicon layer is mainly responsible for efficient acceleration and self‐stabilization of beam‐riding while the thin silica layer enhances the thermal emissivity to preserve the integrity of meta‐sail under intense illumination power via radiative cooling. The role of phase gradient, nanocraft center of mass, Doppler shift, and chromatic dispersion on the interplay between stability and acceleration of the meta‐sail is identified. Moreover, motion trajectory and local steady‐state temperature of the meta‐sail during acceleration are estimated.

We propose an electrically tunable metasurface, which can achieve relatively large phase modulation in both reflection and transmission modes (dual-mode operation). By integration of an ultrathin layer of indium tin oxide (ITO) as an electro-optically tunable material into a semiconductor-insulator-semiconductor (SIS) unit cell, we report an approach for active tuning of all-dielectric metasurfaces. The proposed controllable dual-mode metasurface includes an array of silicon (Si) nanodisks connected together via Si nanobars. These are placed on top of alumina and ITO layers, followed by a Si slab and a silica substrate. The required optical resonances are separately excited by Si nanobars in reflection and Si nanodisks in transmission, enabling highly confined electromagnetic fields at the ITO-alumina interface. Modulation of charge carrier concentration and refractive index in the ITO accumulation layer by varying the applied bias voltage leads to 240° of phase agility at an operating wavelength of 1696 nm for the reflected transverse electric (TE)-polarized beam and 270° of phase shift at 1563 nm for the transmitted transverse magnetic (TM)-polarized light. Independent and isolated control of the reflection and transmission modes enables distinctly different functions to be achieved for each operation mode. A rigorous coupled electrical and optical model is employed to characterize the carrier distributions in ITO and Si under applied bias and to accurately assess the voltage-dependent effects of inhomogeneous carrier profiles on the optical behavior of a unit cell.

Modulation of metasurfaces in time gives rise to several exotic space-time scattering phenomena by breaking the reciprocity constraint and generation of higher-order frequency harmonics. We introduce a new design paradigm for time-modulated metasurfaces, enabling tunable engineering of the generated frequency harmonics and their emerging wavefronts by electrically controlling the phase delay in modulation. It is demonstrated that the light acquires a dispersionless phase shift regardless of incident angle and polarization, upon undergoing frequency conversion in a timemodulated metasurface which is linearly proportional to the modulation phase delay and the order of generated frequency harmonic. The conversion efficiency to the frequency harmonics is independent of modulation phase delay and only depends on the modulation depth and resonant characteristics of the metasurface, with the highest efficiency occurring in the vicinity of resonance, and decreasing away from the resonant regime. The modulation-induced phase shift allows for creating tunable spatially varying phase discontinuities with 2π span in the wavefronts of generated frequency harmonics for a wide range of frequencies and incident angles. Specifically, we apply this approach to a time-modulated metasurface in the Teraherz regime consisted of graphene-wrapped silicon microwires. For this purpose, we use an accurate and efficient semi-analytical framework based on multipole scattering. We demonstrate the utility of the design rule for tunable beam steering and focusing of generated frequency harmonics giving rise to several intriguing effects such as spatial decomposition of harmonics, anomalous bending with full coverage of angles and dual-polarity lensing. Furthermore, we investigate the angular and spectral performance of the time-modulated metasurface in manipulation of generated frequency harmonics to verify its constant phase response versus incident wavelength and angle. The nonreciprocal response of the metasurface in wavefront engineering is also studied by establishing nonreciprocal links with large isolations via modulationinduced phase shift. The proposed design approach enables a new class of high-efficiency tunable metasurfaces with wide angular and frequency bandwidth, wavefront engineering capabilities, nonreciprocal response and multi-functionality.

metasurface to achieve a certain functionality in reflection or transmission mode. Recent years have witnessed the development of phase-gradient metasurfaces with different configurations for a wide range of applications such as beam-steering, focusing, and holography. [1,2] These structures allow for integration of various functionalities into a flat surface thus leading to a dramatic reduction in the footprint of optical systems by replacing conventional bulky optical components.Spatiotemporal control over the phase of transmitted and reflected lights across the 2π span is the key to achieve the desired functionalities. The phase tuning mechanisms of metasurfaces have been mostly based on varying the size of nanoantennas within the resonant scattering regime or rotating half-wave plate elements to imprint a geometric phase shift on the circularly polarized light scattered with the opposite handedness. [3,4] For a nanoantenna supporting a single isolated electric or magnetic resonance, the maximal resonant phase agility is π which hinders full control over the light wavefront without employing a back mirror. Metasurfaces with both electric and magnetic responses can be used to overcome this limitation via spectrally overlapping [5] and interleaving [6] electric and magnetic resonances for operation in transmission and reflection modes, respectively. The spectral overlap of electric and magnetic dipole resonances in a Huygens' metasurface satisfies the first Kerker condition leading to suppressed backward scattering from the elements thus eliminating the reflection and giving rise to maximal transmission with 2π phase agility. Although Huygens' metasurfaces have been constructed using metallic elements in microwave regime, bringing plasmonic Huygens' metasurfaces into optical frequencies is challenging due to their complex geometries and arrangements, and is further hindered by the significant increase in the ohmic loss of plasmonic materials at higher frequencies. [7] All-dielectric metasurfaces consisted of high-index subwavelength elements embedded in a low-index environment can eliminate these limitations in that they can support both electric and magnetic resonances with simple geometries and do not suffer from significant dissipative losses. [8][9][10] Moreover, they are fully compatible with standard In this paper, an electrically tunable all-dielectric metasurface doublet is proposed operating at mid-infrared frequency regime with dynamic 2π phase span in transmission mode. Each layer of the metasurface consists of a periodic array of silicon nanobars configured into p-i-n junctions in which the double carrier injection into the intrinsic region under forward bias allows for tuning of the silicon refractive index. The physical mechanism is based on the spectral overlap of high quality factor guided mode resonances supported by each constituent layer establishing an extreme Huygens' operation regime of nearly reflectionless transmission with steep phase spectrum. The short response time of field-driven car...

Phase-only modulators are of great importance for dynamic control over the wavefront of light in a wide range of applications where high efficiency and uniform amplitude are required. Electro-optical tuning approaches based on electro-refraction induced by free carrier effects are of particular interest for developing phase-only modulators due to offering high speed and low power consumption. Here, an electro-optically tunable all-dielectric metasurface is proposed operating in the near-infrared frequency regime for dynamic control over the phase retardation of transmitted light while maintaining a high amplitude with minimal variations over the phase modulation range. The metasurface consists of a zigzag array of elliptical silicon nanodisks connected in each column via silicon nanobars serving as biasing lines. The constituent elements of the metasurface are configured as multijunction p–n structures with moderate doping levels whose multigate biasing enables modulation of carrier concentrations. Due to broken symmetry in the zigzag arrangement, the symmetry-protected bound states in the continuum supported by the metasurface collapse into Fano resonances with extremely high quality-factors under normal incidence. The spectral overlap of excited electric and magnetic quasi-bound states in the continuum is exploited to establish a Huygens’ regime with maximal transmission and highly steep spectral phase agility of 2π. The electro-optical shift of the Huygens mode via the electro-refraction induced by carrier accumulation in multijunction p–n structures under applied bias voltage yields a wide dynamic phase span of 240° while maintaining an average transmission amplitude of 0.77. The performance manifests a substantially enhanced tunability afforded by a weak electro-refraction of Δn = 4 × 10–3 which is attributed to the ultrahigh Q-factors of the quasi-bound states in the continuum leading to the significant increase in the lifetime of photons and field confinement within the active regions of resonators. The potential application of such a multifunctional transmittive metasurface is numerically demonstrated in two different areas, namely dynamic polarization control and tunable pulse compansion.

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We present novel design approaches for metasurfaces and metamaterials with electrical tunability offering real-time manipulation of light and serving as multifunctional devices in near-infrared frequency regime (at the specific wavelength of 1.55 μm). For this purpose, we integrate indium-tin-oxide (ITO) as a tunable electro-optical material into multimaterial nanowires with metal-oxide-semiconductor and metal-insulator-metal configurations. In particular, an active metasurface operating in the transmission mode is designed which allows for modulation of the transmitted light phase over 280 degrees. This large phase modulation is afforded in the cost of low transmission efficiency. We demonstrate the use of such active metasurfaces for tunable bending and focusing in free-space. Moreover, we investigate the implementation of this material in deeply subwavelength multimaterial nanowires, which can yield strong variations in the effective refractive index by the virtue of internal homogenization enabling tunability of the performance in gradient refractive index metamaterials. In the theoretical modeling of these structures, we adopt a hierarchical multiscale approach by linking drift-diffusion transport model with the electromagnetic model which rigorously characterizes the electro-optical effects.