Multi-wavelength voltage-coded metasurface based on indium tin oxide: independently and dynamically controllable near-infrared multi-channels.

In this paper, we present a design principle for achieving an electrically tunable, multi-wavelength device with multiple functionalities over a single metasurface platform with minimized footprint. This concept is realized based on the integration of four metal-insulator-metal (MIM) inclusions inside a unit cell, which is configured to support four independently controllable operating channels lying in near-infrared (NIR) regime. Incorporation of newly emerged, electrically tunable indium tin oxide (ITO) into such metasurface leads to a dynamical phase modulation over the reflected light. As a result, the phase tunability of almost 285°, 230°, 300°, and 280° are captured at T, O, C, and U optical communications bands, under applying external bias voltages. A digital coding strategy, consisting of "0" and "1" binary bits, is employed to represent the applied biasing configuration to the sub-units. Independently controlled, decoupled gap plasmon resonators, with the ability of eliminating the interference between channels, are enabled thanks to the geometry optimization and careful selection of materials. A meta-array configuration is implemented, in which electrically addressing the groups of MIM sub-units opens a pathway to the tunable applications, namely Airy beam generation, beam splitting, steering, and focusing.

Dynamic beam control via Mie-resonance based phase-change metasurface: a theoretical investigation.

Here, a non-volatile optically controllable metasurface is theoretically investigated at the operating wavelength of 1.55 µm by utilizing low loss phase-change Ge2Sb2Se4Te1 (GSST) as the constituent material of high-index resonant element. The GSST nanobar as the proposed building block supports both the magnetic and electric resonances whose strength and spectral positions can be governed by varying the GSST crystallization level. The possibility of operating at off-resonance regime (middle of geometrical resonances) and preventing from the concurrence of high field confinement and large dissipative loss provide the opportunity to obtain high reflection level (varying between 0.6 and 0.8) and wide phase agility (≈270°). The phase distribution at the interface of an array of GSST nanobars can be tailored by selective modification of the crystallization level of nanobars leading to active control over the wave-front of reflected beam with numerically calculated reflection efficiency higher than 45%.

Dielectric metasurfaces in transmission and reflection modes approaching and beyond bandwidth of conventional blazed grating.

Beyond the wave manipulation at a single frequency, efficiency bandwidth control and functional dispersion engineering over metasurfaces are key challenges towards practical applications. Here we propose a type of wideband dielectric metasurfaces made of ultra-thin and layered high-index dielectric patches. The inclusions can be considered as effective material with designable effective refractive index and dispersion. Beam-deflection metasurfaces composed of such inclusions are characterized with the bandwidth approaching and surpassing the limit of conventional blazed gratings in transmission and reflection manners. The bandwidths are more than twice of that in popular single-layer dielectric metasurfaces made of pillar and disk building blocks. In addition, the proposed design benefits from operation over wide range of incident angles and with large tolerance to fabrication errors. More complicated beam manipulation can be fulfilled similarly with great potential for wideband planar optics.

Graphene-based near-field optical microscopy: high-resolution imaging using reconfigurable gratings.

High-resolution and fast-paced optical microscopy is a requirement for current trends in biotechnology and materials industry. The most reliable and adaptable technique so far to obtain higher resolution than conventional microscopy is near-field scanning optical microscopy (NSOM), which suffers from a slow-paced nature. Stemming from the principles of diffraction imaging, we present fast-paced graphene-based scanning-free wide-field optical microscopy that provides image resolution that competes with NSOM. Instead of spatial scanning of a sharp tip, we utilize the active reconfigurable nature of graphene's surface conductivity to vary the diffraction properties of a planar digitized atomically thin graphene sheet placed in the near field of an object. Scattered light through various realizations of gratings is collected at the far-field distance and postprocessed using a transmission function of surface gratings developed on the principles of rigorous coupled wave analysis. We demonstrate image resolutions of the order of λ0/16 using computational measurements through binary graphene gratings and numerical postprocessing. We also present an optimization scheme based on the genetic algorithm to predesign the unit cell structure of the gratings to minimize the complexity of postprocessing methods. We present and compare the imaging performance and noise tolerance of both grating types. While the results presented in this article are at terahertz frequencies (λ0=10 µm), where graphene is highly plasmonic, the proposed microscopy principle can be readily extended to any frequency regime subject to the availability of tunable materials.

Light manipulation with flat and conformal inhomogeneous dispersive impedance sheets: an efficient FDTD modeling.

An efficient auxiliary differential equation method for incorporating 2D inhomogeneous dispersive impedance sheets in the finite-difference time-domain solver is presented. This unique proposed method can successfully solve optical problems of current interest involving 2D sheets. It eliminates the need for ultrafine meshing in the thickness direction, resulting in a significant reduction of computation time and memory requirements. We apply the method to characterize a novel broad-beam leaky-wave antenna created by cascading three sinusoidally modulated reactance surfaces and also to study the effect of curvature on the radiation characteristic of a conformal impedance sheet holographic antenna. Considerable improvement in the simulation time based on our technique in comparison with the traditional volumetric model is reported. Both applications are of great interest in the field of antennas and 2D sheets.

Real-time two-dimensional beam steering with gate-tunable materials: a theoretical investigation.

A leaky-wave antenna is proposed that furnishes two-dimensional (2-D) beam scanning in both elevation and azimuth planes via electrical control in real time, and at a single frequency. The structure consists of a graphene sheet on a metal-backed substrate. The 2-D beam-scanning performance is achieved through the proper biasing configuration of graphene. Traditional pixel-by-pixel electrical control makes the biasing network a huge challenge for chip-scale designs in the terahertz regime and beyond. The method presented here enables dynamic control by applying two groups of one-dimensional biasing on the sides of the sheet. They are orthogonal and decoupled, with one group offering monotonic impedance variation along one direction, and the other sinusoidal impedance modulation along the other direction. The conductivity profile of the graphene sheet for a certain radiation angle, realized by applying proper voltage to each pad underneath the sheet, is determined by a holographic technique and can be reconfigured electronically and desirably. Such innovative biasing design makes real-time control of the beam direction and beamwidth simple and highly integrated. The concept is not limited to graphene-based structures, and can be generalized to any available gate-tunable material system.

Surface plasmon engineering in graphene functionalized with organic molecules: a multiscale theoretical investigation.

Graphene was recently shown to support deep subwavelength surface plasmons at terahertz frequencies characterized by low energy loss and strong field localization, both highly desirable. The properties of graphene can be locally tuned by applying an external gate voltage or by the adsorption of organic molecules that lead to doping through charge transfer. Local tuning of the electronic features of graphene opens the possibility to realize any desired gradient index profile and thus brings large flexibility to control and manipulate the propagation of surface plasmons. Here, we explore this possibility created by functionalizing graphene with organic molecules. We employ a multiscale theoretical approach that combines first-principles electronic structure calculations and finite-difference time-domain simulations coupled by surface conductivity. We show that by patterning two types of organic molecules on graphene, a plasmonic metasurface can be realized with any gradient effective refractive index profile to manipulate surface plasmon beams as desired. The special properties of such devices based on functionalized graphene are compared to the similar metamaterials based on metallic films on top of a gradient index dielectric substrate. Using this idea, we design and analyze an ultrathin broadband THz plasmonic lens as proof-of-concept, while more sophisticated index profiles can also be realized and various plasmonic applications are readily accessible.

Optical metasurfaces for beam scanning in space.

The concept of a layered metasurface constructed from loop nanoantennas for beam scanning in space is explored. Each layer of the metasurface can be envisioned as a shunt impedance sheet designable by modifying the loop configuration cell by cell. The single and concentric loop nanoantennas made of silver provide capacitive and inductive impedances, respectively, with negligible loss at 1.5 µm, managing full control of the beam phase and amplitude. A telecom metasurface for beam scanning in 3D space is presented. The complex structure is modeled with an in-house-developed finite-difference time-domain method considering interactions among elements, in contrast to many designs in which isolated elements are simulated by assuming local periodicity.

Wave manipulation with designer dielectric metasurfaces.

The concept of an ultra-thin metasurface made of single layer of only-dielectric disks for successful phase control over a full range is demonstrated. Conduction loss is avoided compared to its plasmonic counterpart. The interaction of the Mie resonances of the first two modes of the dielectric particles, magnetic and electric dipoles, is tailored by the dimensions of the disks, providing required phase shift for the transmitted beam from 0° to 360°, together with high transmission efficiency. The successful performance of a beam-tilting array and a large-scale lens functioning at 195 THz demonstrates the ability of the dielectric metasurface that is thin and has also high efficiency of more than 80%. Such configurations can serve as outstanding alternatives for plasmonic metasurfaces especially that it can be a scalable design.

Asymmetric imaging through engineered Janus particle obscurants using a Monte Carlo approach for highly asymmetric scattering media.

The advancement of imaging systems has significantly ameliorated various technologies, including Intelligence Surveillance Reconnaissance Systems and Guidance Systems, by enhancing target detection, recognition, identification, positioning, and tracking capabilities. These systems can be countered by deploying obscurants like smoke, dust, or fog to hinder visibility and communication. However, these counter-systems affect the visibility of both sides of the cloud. In this sense, this manuscript introduces a new concept of a smoke cloud composed of engineered Janus particles to conceal the target image on one side while providing clear vision from the other. The proposed method exploits the unique scattering properties of Janus particles, which selectively interact with photons from different directions to open up the possibility of asymmetric imaging. This approach employs a model that combines a genetic algorithm with Discrete Dipole Approximation to optimize the Janus particles' geometrical parameters for the desired scattering properties. Moreover, we propose a Monte Carlo-based approach to calculate the image formed as photons pass through the cloud, considering highly asymmetric particles, such as Janus particles. The effectiveness of the cloud in disguising a target is evaluated by calculating the Probability of Detection (PD) and the Probability of Identification (PID) based on the constructed image. The optimized Janus particles can produce a cloud where it is possible to identify a target more than 50% of the time from one side (PID > 50%) while the target is not detected more than 50% of the time from the other side (PD < 50%). The results demonstrate that the Janus particle-engineered smoke enables asymmetric imaging with simultaneous concealment from one side and clear visualization from the other. This research opens intriguing possibilities for modern obscurant design and imaging systems through highly asymmetric and inhomogeneous particles besides target detection and identification capabilities in challenging environments.

Birefringent reflectarray metasurface for beam engineering in infrared.

An infrared reflectarray metasurface with engineered birefringent behavior is demonstrated. The array reradiates incoming light into two orthogonal, linearly polarized reflections. The reflectarray is composed of rectangular metallic patch nanoantennas placed on top of a grounded dielectric stand-off layer. The patches are designed to locally manipulate the phase front of the incoming wave. They tailor the reflection phase to transform the phase front on the surface to the one desired for both orthogonal polarizations at the same time. The proposed nanoantenna metasurface can find applications in many optical devices, such as birefringent modulators, waveplates, polarizers, and splitters.

Inverse design of perimeter-controlled InAs-assisted metasurface for two-dimensional dynamic beam steering.

The current commercially viable light detection and ranging systems demand continuous, full-scene, and dynamic two-dimensional point scanning, while featuring large aperture size to ensure long distance operation. However, the biasing architecture of large-area arrays with numerous individually controlled tunable elements is substantially complicated. Herein, inverse design of a perimeter-controlled active metasurface for two-dimensional dynamic beam steering at mid-infrared regime is theoretically presented. The perimeter-control approach simplifies biasing architecture by allowing column-row addressing of the elements. The metasurface consists of a periodic array of plasmonic patch nanoantennas in a metal-insulator-metal configuration, wherein two active layers of indium arsenide are incorporated into its building block. The metasurface profile facilitates wide phase modulation of ≈ 355 ° on the reflected light at the individual element level through applying independent voltages to its respective columns and rows. The multi-objective genetic algorithm (GA) for optimizing user-defined metrics toward shaping desired far-zone radiation pattern is implemented. It is demonstrated that multi-objective GA yields better results for directivity and spatial resolution of perimeter-controlled metasurface by identifying the design tradeoffs inherent to the system, compared to the single-objective optimizer. A high directivity and continuous beam scanning with full and wide field-of-view along the azimuth and elevation angles are respectively maintained.

Increasing the stability margins using multi-pattern metasails and multi-modal laser beams.

Laser-driven metasails can enable reaching velocities far beyond the chemically propelled spacecrafts, which accounts for precise engineering of the acceleration and the stability degree of the lightsail across the Doppler-broadened band. All-dielectric metasurfaces have shown great promise toward the realization of low-weight photonic platforms suitable for integrating multiple functionalities. The most paramount factor in the stability analysis of lightsail is the coupling between displacement and rotation, which mainly determines the durability of the nanocraft against displacement and rotation offsets. In this work, the marginal stability conditions of laser-propelled lightsails have been extended by replacing the reflective elements near the edges portions of the sail with broad-band transmissive elements and applying a multi-objective genetic algorithm (GA) optimization to the proposed configuration. The presented design not only remarkably suppresses the amplitude of the oscillatory motion but also can decrease the center of the mass requirement of the lightsail while maintaining an acceptable acceleration time. Next, a configuration where the payload is at the non-illuminating side of the dual-portion sail is proposed to protect the payload from the intense laser beam. In this case, a spherical phase profile is imprinted across the reflective elements while it is being propelled by a multi-modal beam.

Multifunctional metasails for self-stabilized beam-riding and optical communication.

The photonic propulsion of lightsails can be used to accelerate spacecraft to relativistic velocities, providing a feasible route for the exploration of interstellar space in the human lifetime. Breakthrough Starshot is an initiative aiming to launch lightsail-driven spacecrafts accelerated to a relativistic velocity of 0.2c via radiation pressure of a high-power laser beam in order to probe the habitable zone of Alpha Centauri, located 4.2 light years away from the Earth, and transmit back the scientific data collected in the flyby mission to an Earth-based receiver. The success of such a mission requires the lightsail to provide maximal acceleration while featuring beam-riding stability under the illumination of an intense laser beam during the launch phase. Moreover, the large-area lightsail can be harnessed to improve the margin in the photon-starved downlink channel throughout the communication phase by maximizing the gain of the transmitter despite extending the acceleration period and reducing the stability margin due to the elimination of a portion of the propulsion segments. Owing to the potential of metasurfaces to serve as low-weight versatile multifunctional photonic components, metasurface-based lightsails or metasails are deemed to be ideal candidates to simultaneously address the requirements of photonic propulsion and optical communication in laser-driven deep-space probes. Here, we demonstrate the design of a multifunctional metasail for providing high acceleration and enabling the self-stabilized beam-riding of a spacecraft with a detached payload from the sail while maximizing the transmission gain in the downlink optical communication. The metasail consists of two interleaved sub-arrays of dielectric unit cells operating based on the Pancharatnam-Berry geometric phase, optimized to meet the propulsion and communication requirements, respectively. The beam-riding stability of the sail is analyzed through simulation of the motion trajectory during the acceleration phase, while taking into account the effect of the relativistic Doppler shift, and the downlink communication performance is enabled by providing the required conjugate phase by the metasail elements, resulting in beam collimation. The obtained results verify the multifunctionality of the platform and point toward the promise of metasails for extended mission applications.

Characterization of large array of plasmonic nanoparticles on layered substrate: dipole mode analysis integrated with complex image method.

In this paper, an efficient analytical method for characterizing large array of plasmonic nanoparticles located over planarly layered substrate is introduced. The model is called dipole mode complex image (DMCI) method since the main idea lies in modeling a subwavelength spherical nanoparticle at its electric scattering resonance with an induced electric dipole and representing the electromagnetic (EM) fields of this electric dipole over the layered substrate in terms of finite complex images. The major advantages of the proposed method are its accuracy and rapid calculation in characterizing various kinds of large periodic and aperiodic arrays of nanoparticles on layered substrates. The computational time can be reduced significantly in compared to the traditional methods. The accuracy of the theoretical model is validated through comparison with numerical integration of Sommerfeld integrals. Moreover, the analytical results are compared well with those determined by full-wave finite difference time domain (FDTD) method. To demonstrate the capability of our technique, the performances of large arrays of nanoparticles on layered silicon substrates for efficient sunlight energy incoupling are studied.

Array of planar plasmonic scatterers functioning as light concentrator.

In this Letter, the concept of transmit array antenna enabling light concentration in the near-IR region is explored. As for the elements of the transmit array, concentric loop scatterers are chosen due to their design flexibility and potential for providing high phase variation by changing the dimensions of the loops. Periodic behaviors of the concentric loop elements are obtained by means of the finite-difference time-domain method. Using these results, a transmit array configuration is designed, and a focused beam at a desired distance is achieved. To model the electromagnetic fields of the finite array, the field equivalence principle and proper dyadic Green's functions are applied.

An optical reflectarray nanoantenna: the concept and design.

This paper presents the concept and design of a reflectarray nanoantenna at optical frequencies whose elements are nano-sized concentric spherical particles with the core made of ordinary dielectrics and the shell made of a plasmonic material. Modeling approaches based on finite difference time domain (FDTD) numerical method and dipole-modes scattering theory are used to characterize and tune the reflectarray design. A 6x6 elements reflectarray nanoantenna operating at wavelength 357.1nm with narrow beamwidth is presented, and its scanned radiation characteristics for 15 degrees and 30 degrees are demonstrated.

Plasmonic nanoloop array antenna.

In this Letter, we create an optical nanoantenna array composed of parasitic plasmonic loops that can enhance radiation characteristics and direct the optical energy successfully. Three metallic loops inspired by the concept of the Yagi-Uda antenna are optimized around the region where they feature high scattering performance to control the radiation beam. The loop geometry, when compared to the dipole configuration, has the benefit of using the available aperture in an effective way to provide higher directivity. The angular emission of the nanoloop array antenna is highly directive, and a directivity of 8.2 dB for upward radiation is established.

Plasmonic array nanoantennas on layered substrates: modeling and radiation characteristics.

In this paper, we theoretically characterize the performance of array of plasmonic core-shell nano-radiators located over layered substrates. Engineered substrates are investigated to manipulate the radiation performance of nanoantennas. A rigorous analytical approach for the problem in hand is developed by applying Green's function analysis of dipoles located above layered materials. It is illustrated that around the electric scattering resonances of the subwavelength spherical particles, each particle can be viewed as an induced electric dipole which is related to the total electric field upon that particle by a polarizability factor. Utilizing this, we can effectively study the physical performance of such structures. The accuracy of our theoretical model is validated through using a full-wave finite difference time domain (FDTD) numerical technique. It is established that by novel arraying of nano-particl and tailoring their multilayer substrates, one can successfully engineer the radiation patterns and beam angles. Several optical nanoantennas designed on layered substrates are explored. Using the FDTD the effect of finite size substrate is also explored.

Adaptive Genetic Algorithm for Optical Metasurfaces Design.

As optical metasurfaces become progressively ubiquitous, the expectations from them are becoming increasingly complex. The limited number of structural parameters in the conventional metasurface building blocks, and existing phase engineering rules do not completely support the growth rate of metasurface applications. In this paper, we present digitized-binary elements, as alternative high-dimensional building blocks, to accommodate the needs of complex-tailorable-multifunctional applications. To design these complicated platforms, we demonstrate adaptive genetic algorithm (AGA), as a powerful evolutionary optimizer, capable of handling such demanding design expectations. We solve four complex problems of high current interest to the optics community, namely, a binary-pattern plasmonic reflectarray with high tolerance to fabrication imperfections and high reflection efficiency for beam-steering purposes, a dual-beam aperiodic leaky-wave antenna, which diffracts TE and TM excitation waveguides modes to arbitrarily chosen directions, a compact birefringent all-dielectric metasurface with finer pixel resolution compared to canonical nano-antennas, and a visible-transparent infrared emitting/absorbing metasurface that shows high promise for solar-cell cooling applications, to showcase the advantages of the combination of binary-pattern metasurfaces and the AGA technique. Each of these novel applications encounters computational and fabrication challenges under conventional design methods, and is chosen carefully to highlight one of the unique advantages of the AGA technique. Finally, we show that large surplus datasets produced as by-products of the evolutionary optimizers can be employed as ingredients of the new-age computational algorithms, such as, machine learning and deep leaning. In doing so, we open a new gateway of predicting the solution to a problem in the fastest possible way based on statistical analysis of the datasets rather than researching the whole solution space.