All-dielectric metasurfaces are unique component to control optical wavefront with high transmission or reflection coefficient. Recently, accelerating beam, which propagates along curved arbitrary trajectories, has been realized with conventional diffractive optical elements (DOE). However, DOE suffer from low sampling ratio of rapid phase gradients and its diffraction efficiency drops quickly when the wavelength is switched to another wavelength which is different than the designed wavelength.. In this study, we show accelerating beam which is generated by highly efficient and polarization insensitive all-dielectric metasurfaces in the visible wavelength. The acceleration beam is numerically generated with the proposed metasurfaces which are composed of TiO2 nanopillars residing on glass substrate using finite difference time-domain computational method. It is shown that this beam has the ability to propagate curved trajectories in air medium. Transmission efficiency of the proposed structure is above 65% and desired arbitrary trajectories have been achieved. Generating highly efficient accelerating beam can be used in photonic applications in optical imaging, spectroscopy, optical micromanipulation and nonlinear optics.
In modern optic and photonic applications, tunability of the asymmetric transmission has become important due to its adjustable unidirectional transmission. In this study, we design a three-dimensional trapezoidal metallic nano structure on a stretchable substrate. It shows broadband tunable asymmetric light transmission in the visible spectrum. The proposed structure is made of a periodic nano array of a trapezoidal shaped aluminum on a stretchable substrate. The transmission properties of the proposed structure with respect to the geometric parameters were systematically investigated employing finite-difference time-domain computations. It was shown that the intensity and the bandwidth of the asymmetric light transmission between 400 nm and 800 nm wavelengths change when the flexible substrate is stretched. The period of the designed structure varies depending on the stretch of the substrate. For example, when the substrate is stretched, the period of the structure is 450 nm and when it is unstretched, the period is 350 nm. This increase in the period causes a red shift in the wavelength range of the asymmetric transmission. While the asymmetric transmission under unstretched case starts at 350 nm and stops at 514.5 nm, under stretched case it starts at 450 nm and stops at 661.5 nm. In addition, the performance of our structure is insensitive the polarization of the incoming radiation in both forward and backward illumination directions. This study provides a path toward the realization of tunable optical devices for the applications which require dynamic tunability.
We propose an asymmetric to quasisymmetric behavioral tunability of a structured filter operating in the mid-infrared (IR) spectral region, where the atmospheric transparency is high. The structure is designed through electromagnetic wave analysis using finite-difference time-domain computations. The behavioral tunability provides dynamic control of the optical behavior of the IR filter without geometric and structural changes and opens up a field of research area in the tunable IR devices. It is shown that optical characteristic can be switched from an optical diode (unidirectional isolator or asymmetric light transmission) to the bidirectional isolator (quasisymmetric behavior) based on the phase transition of vanadium dioxide in the entire mid-IR spectrum. The proposed structure can be fabricated with the current nano and microfabrication techniques and can be utilized in smart front IR windows for protecting delicate sensors in the IR imaging systems and IR missile seekers under strong IR laser radiation.
Refractive and conventional optical elements such as prisms and lenses are heavy, large-sized and have limited performance in light-material interactions. Due to these severe constraints, new types of structures called metasurfaces, which are composed of subwavelength structural elements with subwavelength thicknesses, are used instead of conventional and refractive based optical elements. Metasurfaces enable unprecedented control of phase, polarization, amplitude and impedance of incident light. Thanks to these very effective features, metasurfaces have gathered remarkable attention in wavefront manipulation of photons for various applications. Earlier attempts have deployed plasmonic metasurfaces in the designs. However, the light coupled to plasmons suffers from great optical loss, which restricts high transmission efficiency, at visible wavelengths due to intrinsic heat dissipation. This problem can be overcome using all dielectric structures operating mainly in the transmission mode. Here, we numerically demonstrate vortex beam generation having donut-like intensity profile and 60% transmission efficiency. In this study, we use all dielectric metasurface that is composed of thick glass substrate and crystalline silicon which is shaped as trapezoid structure at 532 nm visible wavelength. The refractive indices of glass substrate and crystalline silicon are 1.46 and 4.15 with height of 220 nm, respectively at the designed wavelength. We have achieved 0-2π phase distribution by scaling trapezoid shaped silicon at fixed height. The interface of metasurface segmented 8 regions is filled with trapezoid shaped silicon with a π/4 phase increment in an azimuthal pattern. The obtained vortex beam can be used in various applications such as light trapping, optical tweezers, and laser beam forming.
Intensive researches in the area of metasurfaces have provided a new insight to obtain flat and compact optical systems.
In this letter, we numerically show that, highly efficient tunable beam steering effect in transmission mode is achieved at
wavelength λ = 550 nm using nematic liquid crystals (LCs) infiltrated into double sided metasurfaces. Using the electrooptical
feature of LCs, the phase profile of the metasurfaces is controlled and thus, the transmitted beam is deflected
within the range from -15° to 15° steering angles. Transparent dielectric materials are used in the designed structure that
provides highly efficient beam-steering; the corresponding transmission efficiency is above 83% in the visible spectrum,
which is another superiority of the proposed hybrid tunable structure over present plasmonic/metamaterial approaches.
The designed metasurface still preserves its beam deflection property covering the visible spectrum and hence, such
hybrid structure can be implemented for broadband electro-optically controllable beam steering applications.