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.

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.

approaches based on only varying the inner/outer radius or the opening angle are proposed which can facilitate the design complexity, the encoding of the desired phase distribution for specific functionality, and easing the fabrication procedure. The proposed approaches are leveraged to design several highly transparent graded-pattern metasurfaces with the capability of beam steering, focusing, flat-top generation, and holography.