Abstract
We investigate second harmonic generation, low-threshold multistability, all-optical switching, and inherently nonlocal effects due to the free-electron gas pressure in an epsilon-near-zero (ENZ) metamaterial slab made of cylindrical, plasmonic nanoshells illuminated by TM-polarized light. Damping compensation in the ENZ frequency region, achieved by using gain medium inside the shells' dielectric cores, enhances the nonlinear properties. Reflection is inhibited and the electric field component normal to the slab interface is enhanced near the effective pseudo-Brewster angle, where the effective \epsilon-near-zero condition triggers a non-resonant, impedance-matching phenomenon. We show that the slab displays a strong effective, spatial nonlocality associated with leaky modes that are mediated by the compensation of damping. The presence of these leaky modes then induces further spectral and angular conditions where the local fields are enhanced, thus opening new windows of opportunity for the enhancement of nonlinear optical processes.
Highlights
Even though we show that favorable conditions for second harmonic generation (SHG) conversion efficiency are met near the pseudo-Brewster angle where local fields are maximized because of a forced excitation of the pseudo-Brewster mode, interestingly, we predict that optimal conditions for SHG conversion efficiency are rather met when effective nonlocal effects induced by leaky modes dominate the slab response
We have investigated the field enhancement capabilities of an ENZ slab illuminated at oblique incidence with transverse magnetic (TM)-polarized light
Low-damping (i) favors impedance matching and electric field enhancement near the pseudo-Brewster angle of the slab and (ii) triggers an effective nonlocal response mediated by additional leaky modes observable as narrow, resonant, Fano-like states in the angular-frequency transmission or absorption spectra
Summary
Recent interest in ENZ materials has been motivated by the possibility of controlling antenna directivity[1,2] and achieving perfect couplers through electromagnetic tunneling in subwavelength, low permittivity regions.[3,4] ENZ materials may be used to achieve enhanced harmonic generation,[5,6] optical bistability,[7,8] and soliton excitation .9 The efficiency of harmonic generation is boosted in subwavelength ENZ slabs because the electric field is enhanced at the interface with a higher-index substrate.[5,6] This non-resonant enhancement occurs at oblique incidence for transverse magnetic (TM) polarization and is triggered by the continuity of the component of the displacement field normal to the interface.[10]. The phenomenology of nonlocal contributions of free electrons on the optical response of nanoscale plasmonic structures has been widely discussed in literature.[22,23,24,25,26,27,28] Typical manifestations of the nonlocal, free-electron gas pressure are blue shift and broadening of plasmonic resonances, anomalous absorption,[29] unusual resonances above the plasma frequency,[25] and limitation of field enhancements.[28] These effects are more pronounced when the electron wavelength (~1 nm) becomes comparable to the radius of curvature of metallic nanostructures or to the distance between the metal boundaries of larger structures We use both analytical and full-wave tools to show that these phenomena are magnified in ENZ arrays of metallic nanoshells, in the low-damping regime. We find that additional damping and limitations on field enhancements due to the inherent nonlocality arising from free-electron gas pressure may be mitigated by slightly increasing the gain in the nanoshells’ cores
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