We investigate space-time surface plasmon polariton (ST-SPP) wave packet, a conceptual correspondence of a surface-electromagnetic wave to the space-time wave (ST wave) that is excited on a metal surface through a light-SPP coupling at a nano-scaled ridge. In a framework of the finite-difference time-domain (FDTD) method, a pulsed excitation light was constructed by using a couple of hundred plane waves with different frequencies whose intensity and incident angle were determined to satisfy the conditions for generating ST wave according to the dispersion relation of SPP on the metal surface. The ST-SPP WP launched from the nano-ridge exhibited propagation invariance and tunability of the group velocity.
We investigate spatial control of femtosecond surface plasmon polariton wave packets (SPP WPs) induced by a nanoscaled metal-insulator-metal structure by using time-resolved two-photon excitation microscopy and numerical calculations. Irradiations of 10 fs laser pulses to a ridge structure placed on a Au film launch SPP WPs that propagate on the surface to interact with a MIM nano-cavity. When the SPP WP reaches the MIM nano-cavity, the field intensity of the WP of which frequency coincides with the cavity resonance is enhanced in the MIM structure, and then transmits the cavity to continue the propagation. The time-resolved micrographs reveal deformations of the spatial shape of WPs after passing through the cavity. Because the SPP WP possesses a down-chirped spatial frequency while propagating on the Au surface, the spectral modulation of the WP owing to the cavity transmission is reflected as a spatial deformation of the envelope shape. When the cavity transmits low (high)-frequency components, the peak position of the transmitted WPs shows an advance (delay) sift compared to a reference WP propagated on the plane surface. The numerical model based on complex dispersion relations of the SPP reasonably reproduced the deformation of the SPP WP.
We study optical interactions between nano-scaled cavities consist of metal-insulator-metal structures (MIM nano-cavity) with femtosecond surface plasmon polariton (SPP) wavepackets in terms of numerical simulations and time-resolved microscopy methods. Finite-difference time-domain (FDTD) simulations show that when the eigen-frequency of the nanocavity falls within the spectral width of the SPP wave, the incident wavepacket causes resonant excitations of the cavity. The resonance effect of the SPP-cavity interaction is reflected as strong modulations in the spectra of transmitted and reflected wavepackets. Because the MIM nanocavity separates the wavepacket into resonance frequencies as the transmission and others as the reflection, both the envelope and the spectral shapes of the transmitted/reflected components are largely deformed. Particularly, when the spectrum width of the incident wave was equivalent to or narrower than the resonance linewidth of the cavity, the whole intensity of the transmitted wavepacket varied largely. The time-resolved imaging by using a 10 fs laser pulse, of which spectral width is comparable to the linewidth of cavity resonance, shows reasonable agreement with the simulation. In the case the length of the MIM nano-cavity is chosen so that the eigen-modes do not have spectral overlap with that of the 10 fs laser, the intensity of the transmitted wavepackets showed a considerable attenuation. These features can be interpreted in terms of the functionality of the MIM cavity as a Fabry–Pérot etalon.
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