In this study we experimentally demonstrate the formation of a near-IR tunable narrow transparency window within a plasmonic absorptive band at the photonic regime of split-ring resonators array.

    Coherent interaction of scattered light serves the basis to numerous optical phenomena from openings of optical bandgaps in photonic crystals to the century-years old Rayleigh-anomaly (RA) in metallic diffraction gratings [1]. Recently it was shown that in two-dimensionally arrays of nanoparticles the coherent buildup of scattered light at the array plane can significantly alter the optical response compared to the case of phase canceled scattered fields. In the case of metallic nanoparticles, where the collective oscillations of the conduction electrons govern the absorption and scattering behavior, the simultaneous existence of the metallic resonances, or the so called localized surface plasmons resonances (LSPRs), along with coherent scattered light at the surface of the array, i.e. at the RAs condition, can lead to hybridized photonic-plasmonic resonances known as surface lattice resonances (SLRs) [2]. These unique resonances have attracted much attention over the last decade due to their potential to surpass many of the cons set by the inherent high dissipation rate of metals. It was shown that narrow spectral features that accompanies the SLRs along with their photonic bandgaps behavior make them attractive to various potential processes and applications as enhanced nonlinearity of nanostructured materials [3], passing the lasing threshold in fluorophore-plasmonic metasurfaces and for nonlocal coupling of the magneto-optical response of magnetic nanoparticles [2].

    In the last decade a quantum coherent effect from atomic physics, electromagnetic induced transparency (EIT), has found its counterpart in nanostructured optical materials. This phenomenon, so called EIT-like effect, renders the material a narrow transparency window within a broader absorptive band due to the coupling of superradiant and subradiant modes. The enhanced transmission is accompanied by reduced group velocity of the light, which altogether pave the way toward implementation of on-chip optical signal processing in the time domain and enhanced linear and nonlinear light–matter interactions. To date, most of the experimental demonstrations of EIT-like effect in plasmonic nanostructures rely on the simultaneous spectral and spatial coexistence of two bright and dark near field coupled localized modes. Whereas this proposed prescription has shown the attractive features associated with EIT it involves delicate nanofabrication of extremely packed nanostructures, the Q-factors of the transparency widows are limited by the relatively broad linewidth of the dissipative dark plasmonic mode and active tunability of the transparency window is not straight forward enabled.

    In this study we experimentally demonstrate, for the first time to our knowledge, the formation of a near-infrared tunable narrow transparency window within a plasmonic absorptive band at the photonic regime of gold split-ring resonators array. We show, both by white light interferometric phase measurements and by finite difference time domain simulations, that the enhanced transmission is accompanied by high anomalous dispersion which leads to reduced group velocity, as low as v_g≈c/50. We use the coupled dipole approximation to study the non-local coupling dynamics on the ordered array and show that the intriguing effects of strong absorption-less interaction on the resonant plasmonic surface is attributed to a special case of photonic-plasmonic hybridization. We find that exactly at the condition of coherent scattering of the nanoparticles at the array plane, i.e. at the RAs, equal magnitudes and opposite phases of the incident and scattered light leads to full electric field cancellation at the nanoparticles positions. Moreover, by angle and wavelength resolved transmission and phase measurements we demonstrate the observed effects at the spectral range of two distinct counter-propagating surface waves and show tunability of the observations over a wide spectral range of ~200 nm. In addition, we show that the observed effects occur only for TE polarized light, which therefore renders this phenomenon the complementary metamaterial behavior of the conventional Brewster's angle.

    We believe that this work will promote future studies of nonlocal coherent interaction in nanostructured geometries and facilitate the route toward tunable, integrable, ultra-small slow light devices with high delaying capabilities. The presented study may also be beneficial and find direct functionality in the fields of sensing, displays, polarizers, optical buffers, filters and enhanced nonlinear interaction.

References

[1] Lord Rayleigh, “On the Dynamical Theory of Gratings,” Proceedings of the Royal Society of London A: Mathematical, Physical and Engineering Sciences, vol. 79, no. 532, 1907.

[2] V. G. Kravets, A. V. Kabashin, W. L. Barnes, and A. N. Grigorenko, “Plasmonic Surface Lattice Resonances: A Review of Properties and Applications,” Chemical Reviews, vol. 118, no. 12.

     pp. 5912–5951, 2018.

[3] L. Michaeli, S. Keren-Zur, O. Avayu, H. Suchowski, and T. Ellenbogen, “Nonlinear Surface Lattice Resonance in Plasmonic Nanoparticle Arrays,” Physical Review Letters, vol. 118, no. 24,

     p. 243904, Jun. 2017.

[4] N. Liu et al., “Plasmonic analogue of electromagnetically induced transparency at the Drude damping limit,” Nature Materials, vol. 8, no. 9, pp. 758–762, Sep. 2009.