Browsing by Author "Asadchy, Viktar S."

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  • Bianisotropic metasurfaces: Physics and applications
    (2018) Asadchy, Viktar S.; Díaz-Rubio, Ana; Tretyakov, Sergei A.
     A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
    Metasurfaces as optically thin composite layers can be modeled as electric and magnetic surface current sheets flowing in the layer volume in the metasurface plane. In the most general linear metasurface, the electric surface current can be induced by both incident electric and magnetic fields. Likewise, magnetic polarization and magnetic current can be induced also by external electric field. Metasurfaces which exhibit magnetoelectric coupling are called bianisotropic metasurfaces. In this review, we explain the role of bianisotropic properties in realizing various metasurface devices and overview the state-of-the-art of research in this field. Interestingly, engineered bianisotropic response is seen to be required for realization of many key field transformations, such as anomalous refraction, asymmetric reflection, polarization transformation, isolation, and more. Moreover, we summarize previously reported findings on uniform and gradient bianisotropic metasurfaces and envision novel and prospective research directions in this field.
  • Broadband Reflectionless Metasheets: Frequency-Selective Transmission and Perfect Absorption
    (2015) Asadchy, Viktar S.; Faniayeu, Ihar A.; "Ra'di", Y.; Khakhomov, Sergei A.; Semchenko, Igor V.; Tretyakov, Sergei A.
     A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
    Energy of propagating electromagnetic waves can be fully absorbed in a thin lossy layer, but only in a narrow frequency band, as follows from the causality principle. On the other hand, it appears that there are no fundamental limitations on broadband matching of thin resonant absorbing layers. However, known thin absorbers produce significant reflections outside of the resonant absorption band. In this paper, we explore possibilities to realize a thin absorbing layer that produces no reflected waves in a very wide frequency range, while the transmission coefficient has a narrow peak of full absorption. Here we show, both theoretically and experimentally, that a thin resonant absorber, invisible in reflection in a very wide frequency range, can be realized if one and the same resonant mode of the absorbing array unit cells is utilized to create both electric and magnetic responses. We test this concept using chiral particles in each unit cell, arranged in a periodic planar racemic array, utilizing chirality coupling in each unit cell but compensating the field coupling at the macroscopic level. We prove that the concept and the proposed realization approach also can be used to create nonreflecting layers for full control of transmitted fields. Our results can have a broad range of potential applications over the entire electromagnetic spectrum including, for example, perfect ultracompact wave filters and selective multifrequency sensors.
  • Concept of an asymmetric metasurface absorber
    (2018-01-01) Wang, Xuchen; Díaz-Rubio, Ana; Asadchy, Viktar S.; Tretyakov, Sergei A.
     A4 Artikkeli konferenssijulkaisussa
    In this talk we show how to break the angular symmetry of electromagnetic response of thin absorbers. Based on our recent results on multichannel metasurfaces, we propose a new concept of asymmetric absorbers in which the absorption coefficient for waves impinging from a given oblique angle is extraordinarily different from that for waves incident from the oppositely tilted direction. The proposed asymmetric structure realizes controllable reflectance (from 0 to 0.99) for waves incident from one direction, exhibiting total absorption when the sign of the incidence angle is reversed. We provide a theoretical analysis for the asymmetric absorber as well as a numerical verification.
  • Eliminating Scattering Loss in Anomalously Reflecting Optical Metasurfaces
    (2017-05-17) Asadchy, Viktar S.; Wickberg, Andreas; Díaz-Rubio, Ana; Wegener, Martin
     A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
    Emerging gradient metasurfaces represent a new class of diffraction optical components. Through elaborate engineering of planar arrangements of subwavelength optical antennas, metasurfaces are capable of imparting arbitrary phase profiles on to the incident light, thereby enabling devices such as holograms, complex lenses, and beam splitters. However, the traditional approach for designing reflective gradient metasurfaces fails for simple beam deflection if the angle included by the incident and the anomalously reflected beam is large. Recently, it has been shown that this shortcoming, which results from the impedance mismatch at the interface and parasitic reflections, can be eliminated by proper metasurface engineering. Here, we report the design, fabrication, and characterization of an optical metasurface, which reflects normally impinging light at around 1550 nm wavelength at an angle of 80° with respect to the surface normal. Under these conditions, the calculated and measured results show a reflection efficiency that exceeds previous results by a factor of 2. We believe that our findings are an important step toward high-efficiency devices for general wavefront manipulation.
  • Extreme Asymmetry in Metasurfaces via Evanescent Fields Engineering: Angular-Asymmetric Absorption
    (2018-12-19) Wang, Xuchen; Díaz-Rubio, Ana; Asadchy, Viktar S.; Ptitcyn, Grigorii; Generalov, Andrey A.; Ala-Laurinaho, Juha; Tretyakov, Sergei A.
     A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
    On the quest towards full control over wave propagation, the development of compact devices that allow asymmetric response is a challenge. In this Letter, we introduce a new paradigm for the engineering of asymmetry in planar structures, revealing and exploiting unilateral excitation of evanescent waves. We test the idea with the design and experimental characterization of a metasurface for angular-asymmetric absorption. The results show that the contrast ratio of absorption (the asymmetry level) can be arbitrarily engineered from zero to infinity for waves coming from two oppositely tilted angles. We demonstrate that the revealed asymmetry effects cannot be realized using conventional diffraction gratings, reflectarrays, and phase-gradient metasurfaces. This Letter opens up promising possibilities for wave manipulation via evanescent waves engineering with applications in one-side detection and sensing, angle-encoded steganography, flat nonlinear devices, and shaping the scattering patterns of various objects.
  • Functional Metamirrors Using Bianisotropic Elements
    (2015) Asadchy, Viktar S.; "Ra'di", Younes; Vehmas, Joni; Tretyakov, Sergei A.
     A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
  • Light control with Weyl semimetals
    (2023) Guo, Cheng; Asadchy, Viktar S.; Zhao, Bo; University, Stanford
     A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
    Weyl semimetals are topological materials whose electron quasiparticles obey the Weyl equation. They possess many unusual properties that may lead to new applications. This is a tutorial review of the optical properties and applications of Weyl semimetals. We review the basic concepts and optical responses of Weyl semimetals, and survey their applications in optics and thermal photonics. We hope this pedagogical text will motivate further research on this emerging topic.
  • Metasurface-based realization of photonic time crystals
    (2023-04-05) Wang, Xuchen; Mirmoosa, Mohammad Sajjad; Asadchy, Viktar S.; Rockstuhl, Carsten; Fan, Shanhui; Tretyakov, Sergei A.
     A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
    Photonic time crystals are artificial materials whose electromagnetic properties are uniform in space but periodically vary in time. The synthesis of these materials and experimental observation of their physics remain very challenging because of the stringent requirement for uniform modulation of material properties in volumetric samples. In this work, we extend the concept of photonic time crystals to two-dimensional artificial structures-metasurfaces. We demonstrate that time-varying metasurfaces not only preserve key physical properties of volumetric photonic time crystals despite their simpler topology but also host common momentum bandgaps shared by both surface and free-space electromagnetic waves. On the basis of a microwave metasurface design, we experimentally confirmed the exponential wave amplification inside a momentum bandgap and the possibility to probe bandgap physics by external (free-space) excitations. The proposed metasurface serves as a straightforward material platform for realizing emerging photonic space-time crystals and as a realistic system for the amplification of surface-wave signals in future wireless communications.
  • Nanoscale optical nonreciprocity with nonlinear metasurfaces
    (2024-12) Tripathi, Aditya; Ugwu, Chibuzor Fabian; Asadchy, Viktar S.; Faniayeu, Ihar; Kravchenko, Ivan; Fan, Shanhui; Kivshar, Yuri; Valentine, Jason; Kruk, Sergey S.
     A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
    Optical nonreciprocity is manifested as a difference in the transmission of light for the opposite directions of excitation. Nonreciprocal optics is traditionally realized with relatively bulky components such as optical isolators based on the Faraday rotation, hindering the miniaturization and integration of optical systems. Here we demonstrate free-space nonreciprocal transmission through a metasurface comprised of a two-dimensional array of nanoresonators made of silicon hybridized with vanadium dioxide (VO2). This effect arises from the magneto-electric coupling between Mie modes supported by the resonator. Nonreciprocal response of the nanoresonators occurs without the need for external bias; instead, reciprocity is broken by the incident light triggering the VO2 phase transition for only one direction of incidence. Nonreciprocal transmission is broadband covering over 100 nm in the telecommunication range in the vicinity of λ = 1.5 µm. Each nanoresonator unit cell occupies only ~0.1 λ3 in volume, with the metasurface thickness measuring about half-a-micron. Our self-biased nanoresonators exhibit nonreciprocity down to very low levels of intensity on the order of 150 W/cm2 or a µW per nanoresonator. We estimate picosecond-scale transmission fall times and sub-microsecond scale transmission rise. Our demonstration brings low-power, broadband and bias-free optical nonreciprocity to the nanoscale.
  • Nihility in non-reciprocal bianisotropic media
    (2015) "Ra'di", Younes; Asadchy, Viktar S.; Tretyakov, Sergei A.
     A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
  • Non-scattering Systems for Field Localization and Emission Enhancement
    (2019-05-01) Asadchy, Viktar S.; Cuesta, Francisco S.; Mirmoosa, Mohammad S.; Tretyakov, Sergei A.
     A4 Artikkeli konferenssijulkaisussa
    We propose invisible cavities which do not scatter electromagnetic waves under normal incidence but strongly enhance or suppress the fields inside. They can be used for cloaking sensors and emission enhancement of wave sources.
  • Space-Time Metasurfaces for Power Combining of Waves
    (2021-10-20) Wang, Xuchen; Asadchy, Viktar S.; Fan, Shanhui; Tretyakov, Sergei A.
     A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
    In passive, linear, and monochromatic systems, complete and phase-insensitive combining of powers carried by waves from several input channels into a single output channel is forbidden by the energy conservation law. Here, we demonstrate that the complete combination of both coherent and incoherent plane waves can be achieved using metasurfaces with properties varying in space and time. The proposed structure reflects waves of the same frequency but incident at different angles toward a single direction. The frequencies of the output waves are shifted by the metasurface, ensuring perfect incoherent power combining. The proposed concept of power combining is general and can be applied for electromagnetic waves from the microwave to optical domains, as well as for waves of other physical natures.
  • Time-modulated reactive elements for control of electromagnetic energy
    (2019-05-01) Ptitcyn, Grigorii A.; Mirmoosa, Mohammad S.; Asadchy, Viktar S.; Tretyakov, Sergei A.
     A4 Artikkeli konferenssijulkaisussa
    Accumulation of electromagnetic field energy in classical passive structures such as resonators and reactive circuit elements is limited by the amplitude of time-harmonic external sources. In the steady-state regime, all incident power which is not absorbed is fully re-radiated (reflected), and the time-averaged stored energy is constant, even though the external source continuously supplies energy. Recently, we have shown that lossless objects can continuously accumulate energy if their properties are properly modulated in time. Here, we discuss this possibility and study means to release this accumulated energy in the desired moment or moments of time in form of pulses of the desired shape. Furthermore, we discuss a possibility of keeping the energy inside the load, turning off the incident wave and changing the modulation function of the load. Finally, instead of a single time-reactive element we consider a parallel time-varying LC circuit, that gives the same functionality, in addition allowing to engineer currents flowing through the load circuit elements.
  • Tunable localization of light using nested invisible metasurface cavities
    (2023-03-02) Cuesta, Francisco S.; Kosulnikov, Sergei; Asadchy, Viktar S.
     A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
    An invisible cavity is an open resonant device that confines a localized field without producing any scattering outside of the device volume. By exploiting the scatter-less property of such device, it is possible to nest two invisible cavities, as the outer cavity would simply not notice the presence of the inner one, regardless of their relative position. As a result, the position of the inner cavity becomes a means to easily control the field localized inside the cavity and its quality factor. In this paper, we discuss the properties of nested invisible cavities as a simple method to achieve stronger localized fields and high tunable quality factor. Furthermore, we show that in optics, these cavities can be implemented using nanodisk-based dielectric metasurfaces that operate near their electric resonances.
  • Tunable magnetless optical isolation with twisted Weyl semimetals
    (2023-08-01) Chistyakov, Vladislav A.; Asadchy, Viktar S.; Fan, Shanhui; Alù, Andrea; Krasnok, Alex
     A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
    Weyl semimetals hold great promise in revolutionizing nonreciprocal optical components due to their unique topological properties. By exhibiting nonreciprocal magneto-optical effects without necessitating an external magnetic field, these materials offer remarkable miniaturization opportunities and reduced energy consumption. However, their intrinsic topological robustness poses a challenge for applications demanding tunability. In this work, we introduce an innovative approach to enhance the tunability of their response, utilizing multilayered configurations of twisted anisotropic Weyl semimetals. Our design enables controlled and reversible isolation by adjusting the twist angle between the anisotropic layers. When implemented in the Faraday geometry within the mid-IR frequency range, our design delivers impressive isolation, exceeding 50dB, while maintaining a minimal insertion loss of just 0.33dB. Moreover, the in-plane anisotropy of Weyl semimetals eliminates one or both polarizers of conventional isolator geometry, significantly reducing the overall dimensions. These results set the stage for creating highly adaptable, ultra-compact optical isolators that can propel the fields of integrated photonics and quantum technology applications to new heights.