Amr Shaltout

Optically Active Metasurface with Non-Chiral Plasmonic Nanoantennas

We design, fabricate, and experimentally demonstrate an optically active metasurface of λ/50 thickness that rotates linearly polarized light by 45° over a broadband wavelength range in the near IR region. The rotation is achieved through the use of a planar array of plasmonic nanoantennas, which generates a fixed phase-shift between the left circular polarized and right circular polarized components of the incident light. Our approach is built on a new supercell metasurface design methodology: by judiciously designing the location and orientation of individual antennas in the structural supercells, we achieve an effective chiral metasurface through a collective operation of nonchiral antennas. This approach simplifies the overall structure when compared to designs with chiral antennas and also enables a chiral effect which quantitatively depends solely on the supercell geometry. This allows for greater tolerance against fabrication and temperature effects.

Photonic spin Hall effect in gap–plasmon metasurfaces for on-chip chiroptical spectroscopy

Chiral structures possessing differential optical responses to light circular polarization are very common in biological and organic compounds. Attaining chiroptical effects is of great biochemical importance, yet requires complicated structures. Circular dichroism (CD) spectrometers measuring the differential absorption between left- (LCP) and right-circular (RCP) polarizations involve complex hardware to switch laser polarization and manage data acquisition sequentially. Here, we present compact and power-efficient metasurface-based chiroptical spectroscopy solutions based on gap–plasmon metasurfaces (GPMSs). First, a minimalistic design of a real-time CD spectrometer is obtained by using the photonic spin Hall effect (PSHE) in a single GPMS, which spatially separates LCP and RCP spectra. It is the smallest CD spectrometer to our knowledge. Another GPMS-based device built with the same approach rotates light polarization by 45° through adding a phase shift between LCP and RCP. Thus, PSHE in GPMS can provide efficient solutions to vital applications including biosensing, DNA structural analysis, and stereochemistry.

Time-varying metasurfaces and Lorentz non-reciprocity

A cornerstone equation of optics, Snells law, relates the angles of incidence and refraction for light passing through an interface between two media. It is built on two fundamental constrains: the conservation of tangential momentum and the conservation of energy. By relaxing the classical Snell law photon momentum conservation constrain when using space-gradient phase discontinuity, optical metasurfaces enabled an entirely new class of ultrathin optical devices. Here, we show that by eradicating the photon energy conservation constrain when introducing time-gradient phase discontinuity, we can further empower the area of flat photonics and obtain a new genus of optical devices. With this approach, classical Snell relations are developed into a more universal form not limited by Lorentz reciprocity, hence, meeting all the requirements for building magnetic-free optical isolators. Furthermore, photons experience inelastic interaction with time-gradient metasurfaces, which modifies photonic energy eigenstates and results in a Doppler-like wavelength shift. Consequently, metasurfaces with both space- and time-gradients can have a strong impact on a plethora of photonic applications and provide versatile control over the physical properties of light.

Evolution of photonic metasurfaces: from static to dynamic

The realization of commercial photonic devices based on three-dimensional optical metamaterials is challenged by large absorptive losses as well as complex and costly fabrication. Metasurfaces—two-dimensional metamaterials—have been introduced to overcome these major challenges. They provide simple, compact, and power efficient solutions to problems of immense importance in photonics design. In addition to the strong potential of metasurfaces to enable next generation optical devices, their reduced dimensionality also allows for new physical effects which do not have volumetric counterparts. Furthermore, various technologies are presently emerging to provide modulation of metasurface response using mechanical, electrical, or optical control. Although the general purpose of these approaches is to obtain tunable versions of static metasurfaces, recent studies have uncovered that the impact of dynamic metasurfaces far exceeds tunability alone and comprises new physical effects such as Lorentz nonreciprocity. Here we start with reviewing recent developments of static metasurface devices and their applications. Then we discuss the burgeoning area of dynamic metasurfaces and demonstrate current evolving technologies to achieve time-varying properties and their conceivable applications.

Controlling hybrid nonlinearities in transparent conducting oxides via two-colour excitation

Nanophotonics and metamaterials have revolutionized the way we think about optical space (ε,μ), enabling us to engineer the refractive index almost at will, to confine light to the smallest of the volumes, and to manipulate optical signals with extremely small footprints and energy requirements. Significant efforts are now devoted to finding suitable materials and strategies for the dynamic control of the optical properties. Transparent conductive oxides exhibit large ultrafast nonlinearities under both interband and intraband excitations. Here we show that combining these two effects in aluminium-doped zinc oxide via a two-colour laser field discloses new material functionalities. Owing to the independence of the two nonlinearities, the ultrafast temporal dynamics of the material permittivity can be designed by acting on the amplitude and delay of the two fields. We demonstrate the potential applications of this novel degree of freedom by dynamically addressing the modulation bandwidth and optical spectral tuning of a probe optical pulse.

Homogenization of bi-anisotropic metasurfaces.

Ultrathin metamaterial layers are modeled by a homogeneous bi-anisotropic film to represent various kinds of broken symmetries in photonic nanostructures, and specifically in optical metamaterials and metasurfaces. Two algorithms were developed to obtain the electromagnetic (EM) wave response from a metasurface (direct solver) or the metasurface parameters from the EM wave response (inverse solver) for a bi-anisotropic, subwavelength-thick nanostructured film. The algorithm is applied to two different metasurfaces to retrieve their effective homogeneous bi-anisotropic parameters. The effective layer of the same physical thickness is shown to produce the same response to plane wave excitation as the original metasurface.

Dynamic nanophotonics [Invited]

The field of integrated plasmonics is multifaceted in a way that few other disciplines in applied science are, mainly due to its intrinsic “hybrid” nature of combining materials and strategies borrowed from electronics and photonics. Because of the multitude of angles under which the plasmonic world could be analyzed, and also because of the intrinsic interest behind this branch of physics, numerous review papers have been recently published with the attempt to exhaustively describe this subject and its possible future developments. However, despite the considerable literature already available, a few important aspects deserve a deeper investigation. Among these dark spots we find the lack of a general overview of active plasmonics, specifically focused on the possibility to dynamically alter the optical properties of the constituent plasmonic materials in order to gain full active control over the overall desired functionality. The present review focuses on the possibility to tune the optical properties of said components, deliberately neglecting those strategies relying on the dynamic properties of the dielectric constituent. The present work also will attempt to outline experimental and multidisciplinary aspects of tunable plasmonic devices, giving only a marginal overview of telecom applications, for which considerable literature is already available.

Experimental validation of a new bianisotropic parameter retrieval technique using plasmonic metasurfaces made of V-shape antennas

We report on a numerical study of a new bianisotropic parameter retrieval technique for both regular and complementary V-shape antenna metasurfaces. Each antenna element with a discrete phase shift is modeled by a homogenous bianisotropic film to represent the optical response. For the complementary design, the retrieval implies a complementary behavior of effective material properties and predicts the analogous functionalities. Further, FDFD solver is developed to integrate the bianisotropic descriptions of each antenna and describes a fully functional metasurface. The computational burden is significantly reduced, because effective material properties replace the detailed meshing of the antennas. Experimentally, large dimension arrays of nano‐voids are fabricated using electron beam lithography. It is demonstrated that cross-polarized light is diffracted towards the same direction. Furthermore, the complementary design greatly increases the extinction ratio of functional fields to background fields.

Ultrathin and multicolour optical cavities with embedded metasurfaces

Over the past years, photonic metasurfaces have demonstrated their remarkable and diverse capabilities in advanced control over light propagation. Here, we demonstrate that these artificial films of deeply subwavelength thickness also offer new unparalleled capabilities in decreasing the overall dimensions of integrated optical systems. We propose an original approach of embedding a metasurface inside an optical cavity—one of the most fundamental optical elements—to drastically scale-down its thickness. By modifying the Fabry–Pérot interferometric principle, this methodology is shown to reduce the metasurface-based nanocavity thickness below the conventional λ/(2n) minimum. In addition, the nanocavities with embedded metasurfaces can support independently tunable resonances at multiple bands. As a proof-of-concept, using nanostructured metasurfaces within 100-nm nanocavities, we experimentally demonstrate high spatial resolution colour filtering and spectral imaging. The proposed approach can be extrapolated to compact integrated optical systems on-a-chip such as VCSEL’s, high-resolution spatial light modulators, imaging spectroscopy systems, and bio-sensors.Achieving miniature and versatile nanophotonic devices with metasurfaces is of great interest. The authors embed a metasurface inside an optical cavity to reduce thickness and provide degrees of freedom for controlling resonant wavelengths, enabling multi-band filtering, structural colouration and spectral imaging.

Engineered nonlinearities in transparent conducting oxides

Towards the fabrication of all-dielectric nanophotonic devices with tunable capabilities, we combined interband and intraband nonlinearities in aluminum-doped zinc oxide thin films thus enlarging the material bandwidth and gaining ultra-fast control over the transmitted spectrum.