Zhaxylyk Kudyshev

Spinning light on the nanoscale.

Light beams with orbital angular momentum have significant potential to transform many areas of modern photonics from imaging to classical and quantum communication systems. We design and experimentally demonstrate an ultracompact array of nanowaveguides with a circular graded distribution of channel diameters that coverts a conventional laser beam into a vortex with an orbital angular momentum. The proposed nanoscale beam converter is likely to enable a new generation of on-chip or all-fiber structured light applications.

Virtual hyperbolic metamaterials for manipulating radar signals in air

Microwave beam transmission and manipulation in the atmosphere is an important but difficult task. One of the major challenges in transmitting and routing microwaves in air is unavoidable divergence because of diffraction. Here we introduce and design virtual hyperbolic metamaterials (VHMMs) formed by an array of plasma channels in air as a result of self-focusing of an intense laser pulse, and show that such structure can be used to manipulate microwave beams in air. Hyperbolic, or indefinite, metamaterials are photonic structures that possess permittivity and/or permeability tensor elements of opposite sign with respect to one another along principal axes, resulting in a strong anisotropy. Our proof-of-concept results confirm that the proposed virtual hyperbolic metamaterial structure can be used for efficient beam collimation and for guiding radar signals around obstacles, opening a new paradigm for electromagnetic wave manipulation in air.

Asymmetric Positive-Negative Index Nonlinear Waveguide Couplers

Nonlinear optical couplers have attracted significant attention largely due to the many advantages they offer for optical communications and signal processing. Combined with the unique properties of an emerging new class of materials, metamaterials, nonlinear directional couplers open fundamentally new opportunities for the development of ultracompact signal processing functionalities for on-chip applications. In this paper, we explore theoretically nonlinear transmission properties of asymmetric nonlinear couplers with one channel made of positive index material and the other channel made of negative index material while only one of the channels is nonlinear. We demonstrate that such asymmetric couplers can be bi- and multistable, and we optimize the design with respect to the bistability threshold.

Experimental Demonstration of Anomalous Field Enhancement in All-Dielectric Transition Magnetic Metamaterials.

Anomalous field enhancement accompanied by resonant absorption phenomenon was originally discussed in the context of plasma physics and in applications related to radio-communications between the ground and spacecraft returning to Earth. Indeed, there is a critical period of time when all communications are lost due to the reflection/absorption of electromagnetic waves by the sheath of plasma created by a high speed vehicle re-entering the atmosphere. While detailed experimental studies of these phenomena in space are challenging, the emergence of electromagnetic metamaterials enables researchers exceptional flexibility to study them in the laboratory environment. Here, we experimentally demonstrated the strong localized field enhancement of magnetic field for an electromagnetic wave propagating in Mie-resonance-based inhomogeneous metamaterials with magnetic permeability gradually changing from positive to negative values. Although these experiments were performed in the microwave frequency range, the proposed all-dielectric approach to transition metamaterials can be extended to terahertz, infrared, and visible frequencies. We anticipate that these results, besides most basic science aspects, hold the potential for numerous applications, including low-intensity nonlinear transformation optics, topological photonics, and the broader area of surface and interface science.

Second harmonic generation in transition metamaterials

We show that the resonant field enhancement of light obliquely incident on a quadratically nonlinear metamaterial with refractive index gradually changing from positive to negative values enables ultracompact geometry for second harmonic generation at significantly reduced input powers.

Twisted light in a nonlinear mirror.

Opposite directionality of the Poynting vector and the wave vector, an inherent property of negative index metamaterials (NIMs), was predicted to enable backward phase-matching condition for a second harmonic generation (SHG) process. As a result, such a nonlinear negative index slab acts as a nonlinear mirror. In this Letter, we predict that SHG with structured light carrying orbital angular momentum (OAM) and propagating in NIMs results in a possibility of generating a backward propagating beam with simultaneously doubled frequency, OAM, and reversed rotation direction of the wavefront. These results may find applications in high-dimensional communication systems, quantum information processing, and optical manipulation on a nanoscale.

Temperature evolution of optical properties in plasmonic metals (Conference Presentation)

Understanding the temperature evolution of optical properties in thin metals is critical for rational design of practical metal based nanophotonic components operating at high temperatures in a variety of research areas, including plasmonics and near-field radiative heat transfer. In this talk, we will present our recent experimental findings on the temperature induced deviations in the optical responses of single- and poly-crystalline metal films – gold, silver and titanium nitride thin films - at elevated temperatures upto 900 0C, in the wavelength range from 370 to 2000 nm. Our findings show that while the real part of the dielectric function changes marginally with temperature, the imaginary part varies drastically. Furthermore, the temperature dependencies were found to be strongly dependent on the film thickness and microstructure/crystallinity. We attribute the observed changes in the optical properties to predominantly three physical processes: 1) increasing electron-phonon interactions, 2) reducing free electron densities and, 3) changes in the electron effective mass. Using extensive numerical simulations we demonstrate the importance of incorporating the temperature induced deviations into numerical models for accurate multiphysics modeling of practical high temperature plasmonic components. We also provide experiment-fitted models to describe the temperature-dependent metal dielectric functions as a sum of Drude and critical point/Lorentz oscillators. These causal analytical models could enable accurate multiphysics modeling of nanophotonic and plasmonic components operating at high temperatures in both frequency and time domains.

Kubo-equivalent closed-form graphene conductivity models(Conference Presentation)

The optical response of graphene is described by its surface conductivity - a multivariate function of frequency, temperature, chemical potential, and scattering rate. A Kubo formula that accounts for both interband and intraband transitions with two Fermi-Dirac-like integrals is conventionally used to model graphene. The first (intraband) integral can be reduced analytically to a Drude term. The second (intraband) term requires computationally expensive numerical integration over the infinite range of energies, and thus it is usually either neglected or substituted with a simpler approximation (typically valid within a limited range of parameters). Additional challenge is an integral-free time-domain (TD) formulation that would allow efficient coupling of the interband conductivity term to TD electromagnetic solvers. We propose Kubo-equivalent models of graphene surface conductivity that offer closed-form computationally efficient representations in time and frequency domains. We show that in time domain Kubo’s formula reduces to a combination of rational, trigonometric, hyperbolic, and exponential functions. In frequency domain the integral term is equivalent to an expression with digamma and incomplete gamma functions. The accuracy and improved performance of our integral-free formulations versus the direct integration of Kubo’s formula is critically analyzed. The result provides efficient broadband multivariate coupling of graphene dispersion to time-domain and frequency-domain solvers. To reinforce theory with practical examples, we use obtained closed-form frequency-domain model to retrieve the optical properties of graphene samples from variable angle spectroscopic ellipsometry (VASE) measurements. . We present ellipsometry fitting cases that are built on an in-the-cloud tool freely available online (https://nanohub.org/resources/photonicvasefit).

Probing metamaterials with structured light (Presentation Recording)

We propose and demonstrate a reliable and inexpensive tool for optical characterization of photonics metamaterials and metasurfaces. Existing characterization methods of metamaterials (or more precisely negative index metamaterials), including conventional interferometry and ellipsometry, are rather complex and expensive. The “measurable” difference between, for example, positive index materials and negative index materials is that the former introduces a phase delay to transmitted light beam and the latter one introduces a phase advance. Here, we propose to use optical vortex interferometry to directly “visualize” phase delay or phase advance. In the proposed setup a laser beams at the wavelength of 633 nm is separated in two by a beam splitter. One beam is transmitted through a spiral phase plate in order to generate a beam with an orbital angular momentum, and the second beam is transmitted through a nanostructured sample. Two beams are subsequently recombined by a beam splitter to form spiral interferogram. Spiral patterns are then analyzed to determine phase shifts introduced by the sample. In order to demonstrate the efficiency of the proposed technique, we fabricated four metasurface samples consisting of metal nano-antennas introducing different phase shifts and experimentally measured phase shifts of the transmitted light using the proposed technique. The experimental results are in good agreement with numerical simulations. In summary, we report a novel method to characterize metasurfaces and metamaterials using optical vortex interferometry. The proposed characterization approach is simple, reliable and particularly useful for fast and inexpensive characterization of phase properties introduced by metamaterials and metasurfaces.

Nonlocal Response in Transition Metamaterials

We investigate resonant enhancement of light in transition metamaterials under the local and nonlocal response function approximations, and analyze the influence of nonlocality on the field distribution in the near-zero region.