Browsing by Author "Lakhtakia, A."

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Open Access
Characteristic attributes of multiple cascaded terahertz metasurfaces with magnetically tunable subwavelength resonators
(Wiley-VCH Verlag, 2018) Serebryannikov, A. E.; Lakhtakia, A.; Özbay, Ekmel
The characteristics of multiple cascaded metasurfaces comprising H-shaped, magnetostatically controllable, subwavelength terahertz (THz) resonators made of InAs were systematically investigated, using a commercial solver based on the finite-integration method, for the design of tunable filters. Three configurations of the biasing magnetostatic field were compared with each other as well as with the bias-free configuration for filtering of normally incident linearly polarized plane waves. A close study of only one metasurface was found sufficient to broadly determine the sensitivity to the direction of the magnetostatic field and the bandwidth of a stopband. Furthermore, the effects of metasurface geometry and biasing field can be considered separately for initial design purposes. All features in the transmittance spectra for the bias-free configuration that are related to the number of cascaded metasurfaces are also observed when the biasing magnetostatic field is applied. The coupling of adjacent metasurfaces in a cascade is strongly affected by the relative permittivity and the thickness of the spacer between the two metasurfaces. The spectral locations of stopbands scale with respect to the spacer's relative permittivity, the scaling rule being different from a classical one. The stopbands are redshifted when the spacer thickness is increased, with the redshift dependent on the polarization of the incident plane wave. Inter-metasurface coupling and inter-resonator coupling on the same metasurface affect the spectral location of a stopband in opposite ways. On-off type switching can be obtained by changing the orientation of magnetostatic field. The elucidated characteristics are expected to be important for not only filters but also other tunable THz devices.
Open Access
Single and cascaded, magnetically controllable metasurfaces as terahertz filters
(Optical Society of America OSA, 2016) Serebryannikov, A. E.; Lakhtakia, A.; Özbay, Ekmel
Transmission of a normally incident, linearly polarized, plane wave through either a single electrically thin metasurface comprising H-shaped subwavelength resonating elements made of magnetostatically controllable InAs or a cascade of several such metasurfaces was simulated in the terahertz regime. Stop bands that are either weakly or strongly controllable can be exhibited by a single metasurface by proper choice of the orientation of the magnetostatic field, and a ∼19%downshift of stop bands in the 0.1-5.5 THz spectral regime is possible on increasing the magnetostatic field strength from 0 to 1 T. Better controllability and wider bandwidths are possible by increasing the number of metasurfaces in a cascade, although increase of the total losses can lead to some restrictions. ON/OFF switching regimes, realizable either by applying/removing the magnetostatic field or just by changing its orientation, exist.
Open Access
Temperature-mediated invocation of the vacuum state for switchable ultrawide-angle and broadband deflection
(Nature Publishing Group, 2018) Serebryannikov, A. E.; Lakhtakia, A.; Aalizadeh, Majid; Özbay, Ekmel; Vandenbosch, G. A. E.
Temperature-mediated appearance and disappearance of a deflection grating in a diffracting structure is possible by employing InSb as the grating material. InSb transits from the dielectric state to the plasmonic state in the terahertz regime as the temperature increases, this transition being reversible. An intermediate state is the vacuum state in which the real part of the relative permittivity of InSb equals unity while the imaginary part is much smaller. Then the grating virtually disappears, deflection being impossible as only specular reflection can occur. This ON/OFF switching of deflection and relevant angular filtering are realizable over wide ranges of frequency and incidence angle by a temperature change of as low as 20 K. The vacuum state of InSb invoked for ON/OFF switching of deflection and relevant angular filtering can also be obtained for thermally tunable materials other than InSb as well as by using non-thermal mechanisms.
Open Access
Thermally sensitive scattering of terahertz waves by coated cylinders for tunable invisibility and masking
(OSA - The Optical Society, 2018) Serebryannikov, A. E.; Alıcı, K. B.; Özbay, Ekmel; Lakhtakia, A.
Temperature-sensitive scattering of terahertz (THz) waves by infinitely long, cylindrical core-shell structures was theoretically studied. Each structure is a dielectric cylinder coated with an InSb shell illuminated by either a transverse-electric (TE) or a transverse-magnetic (TM) plane wave. InSb is a thermally tunable semiconductor showing a transition from dielectric to plasmonic state at THz frequencies. Accordingly, the total scattering efficiency (TSE) can be thermally tuned for both polarization states of the incident plane wave. The spectral locations of the maxima and minima of the TSE of an InSb-coated cylinder can be exploited for cloaking the core. At least three scenarios lead to the strong suppression of scattering by a single core-shell structure in different spectral regimes when the temperature is fixed. The excitation of localized surface-plasmon resonances is the feature being common for two of them, while the effect of volumetric resonance dominates in the third scenario. Regimes that are either highly or weakly sensitive to the core material were identified. Weak sensitivity enables masking, i.e., the core material cannot be identified by a far-zone observer. The TSE minima are usually significantly sensitive to the polarization state, but ones with weak sensitivity to the polarization state also exist.