Resonance order-dependent plasmon-induced transparency in orthogonally-arranged nanoscale cavities

In this study, we investigate plasmon-induced transparency (PIT) in a resonator structure consisting of two orthogonally-arranged metal-insulator-metal (MIM) nanocavities with the aim of spectral modulation of a specific resonant order of the resonator. Our FDTD simulations demonstrate that when both cavities in this structure resonate at the same frequency, the PIT effect can be used to induce spectral modulation. This spectral modulation depends on the resonance order of the cavity coupled directly to the external field, occurring when first-order resonance is exhibited, but not with second-order resonance. We confirmed that this behavior is caused by the discrepancies between odd-order and even-order resonances using classical mechanical models analogous to the nanocavities. By tuning the resonance frequency and resonance order of the cavities, one can modulate the spectrum of the resonator structure in an order-selective manner

Spatiotemporal control of surface plasmon polariton wave packets with nanocavities

Modulation of the optical index by means of atomic and material resonances provides a basis for controlling light propagation in natural and artificially fabricated materials. In addition, recent advances in the tuning of spatiotemporal couplings of ultrashort laser pulses have enabled almost arbitrary control over the group velocity of light. Here, using femtosecond time-resolved microscopy and numerical calculations, we investigate the spatiotemporal dynamics of a surface plasmon polariton wave packet (SPP WP) that interacts with a plasmonic nanocavity. The nanocavity consists of metal-insulator-metal multilayer films that function as subwavelength meta-atom possessing tunable discretized eigenmodes. When a chirp-induced femtosecond SPP WP is incident on a nanocavity, only the spectral component matching the resonance energy is transmitted. This spectral clipping effect is accompanied by a spatial shift of the WP. The shift can be adjusted in either the positive or negative direction by controlling the resonance energy or the chirp. If this spatial shift is regarded as a modulation of the apparent group velocity in the nanocavity, the range of modulation includes superluminal, subluminal, and negative group velocities

Experimental realization of Lorentz boosts of space-time wave packets

It is now well-understood that a Lorentz boost of a spatially coherent monochromatic optical beam yields a so-called space-time wave packet (STWP): a propagation-invariant pulsed beam whose group velocity is determined by the relative velocity between the source and observer. Moreover, the Lorentz boost of an STWP is another STWP, whose group velocities are related by the relativistic law for addition of velocities typically associated with massive particles. We present an experimental procedure for testing this prediction in both the subluminal and superluminal regimes that makes use of spatio-temporal Fourier synthesis via a spatial light modulator. Our approach enables realizing the change in temporal bandwidth, the invariance of the spatial bandwidth, the concomitant change in the spatio-temporal wave-packet envelope, and the change in group velocity that all accompany a Lorentz boost of a monochromatic optical beam. The only consequence of the Lorentz boost not captured by this methodology is the Doppler shift in the optical carrier. This work may provide an avenue for further table-top demonstration of relativistic transformations of optical fields

Rotationally displaced electric field intensity distribution around square nanoantennas induced by circularly polarized light

An optical field around regular polygon metal nanostructures excited by circularly polarized light can exhibit rotationally displaced intensity distributions. Although this phenomenon has been recognized, its underlying mechanisms has not been sufficiently explained. Herein, finite-difference time-domain simulations and model analyses reveal that the rotationally displaced optical intensity distribution can be generated when each of the linear polarization components that constitute circular polarization excites a superposition of multiple modes. The proposed model reasonably explains the rotationally displaced patterns for a square nanoantenna and other regular-polygon nanoantennas

Exciting space-time surface plasmon polaritons by irradiating a nanoslit structure

Space-time (ST) wave packets are propagation-invariant pulsed optical beams that travel freely in dielectrics at a tunable group velocity without diffraction or dispersion. Because ST wave packets maintain these characteristics even when only one transverse dimension is considered, they can realize surface-bound waves (e.g., surface plasmon polaritons at a metal-dielectric interface, which we call ST-SPPs) that have the same unique characteristics of their freely propagating counterparts. However, because the spatio-temporal spectral structure of ST-SPPs is key to their propagation invariance on the metal surface, their excitation methodology must be considered carefully. We show here using finite-difference time-domain (FDTD) simulations that an appropriately synthesized ST wave packet in free space can be couples to a ST-SPP via a single nano-scale slit inscribed in the metal surface. Our calculations confirm that this excitation methodology yields surface-bound ST-SPPs that are locarized in all dimensions (and can thus be considered as plasmonic 'bullets'), which travel rigidly at the metal-dielectric interface without diffraction or dispersion at a tunable group velocity

Visualization2.mp4

Time evolution of the electric field (Ey) distribution (upper) and the profile taken along the z-axis (x=0) (lower) of a subluminal space-time light wave packet (ST-WP) with a group velocity of 0.8c

Visualization1.mp4

Time evolution of the electric field (Ey) distribution (upper) and the profile taken along the z-axis (x=0) (lower) of a superluminal space-time light wave packet (ST-WP) with a group velocity of 1.2c

Visualization4.mp4

Time evolution of the out-of-plane electric field (Ey) distribution (upper) and the profile taken along the z-axis (x=0, y=0) (lower) of a subluminal space-time surface plasmon polariton wave packet (ST-SPP WP) with a group velocity of 0.8c

Visualization3.mp4

Time evolution of the out-of-plane electric field (Ey) distribution (upper) and the profile taken along the z-axis (x=0, y=0) (lower) of a superluminal space-time surface plasmon polariton wave packet (ST-SPP WP) with a group velocity of 1.2c

Transverse spin angular momentum of space-time surface plasmon polariton wave packet

In addition to longitudinal spin angular momentum (SAM) along the axis of propagation of light, spatially structured electromagnetic fields such as evanescent waves and focused beams have recently been found to possess transverse SAM in the direction perpendicular to the axis of propagation. In particular, the SAM of SPPs with spatial structure has been extensively studied in the last decade after it became clear that evanescent fields with spatially structured energy flow generate threedimensional spin texture. Here we present numerical calculations of the space-time surface plasmon polariton (ST-SPP) wave packet, a plasmonic bullet that propagates at an arbitrary group velocity while maintaining its spatial distribution. ST-SPP wave packets with complex spatial structure and energy flow density distribution determined by the group velocity are found to propagate with accompanying three-dimensional spin texture and finite topological charge density. Furthermore, the spatial distribution of the spin texture and topological charge density determined by the spatial structure of the SPP is controllable, and the deformation associated with propagation is negligible. ST-SPP wave packets, which can stably transport customizable three-dimensional spin textures and topological charge densities, can be excellent subjects of observation in studies of spinphotonics and optical topological materials