Plasmonic nanostructures with local temporal response: a platform for time-varying photonics

Time-varying media open up unprecedented abilities for controlling electromagnetic fields. This is due to novel forms of light-matter interactions that arise when one of the optical parameters of the medium is abruptly switched in time. The latter means that the switching time is shorter than the period of wave oscillations. This requirement makes it challenging to implement time-varying materials suitable for applications in the optical spectral range. This work is devoted to the development of an approach for implementing time-varying media with a large depth and speed of modulation of the refractive index. We exploit a system, consisting of plasmonic antennas coupled to nonlinear material with epsilon-near-zero (ENZ) properties. Hybrid plasmonic-ENZ structures modulated by pulsed laser excitation have been recently proposed as a promising platform for time-varying photonics. However, the optical response of plasmonic components is typically slow, on the order of 10-100 fs. This stems from the long lifetime of plasmonic excitations. Here, we propose a mechanism that allows one to achieve an ultrashort, less than 1 fs, plasmon lifetime. The mechanism is based on reducing of the spectral dispersion of the real part of permittivity. For this purpose, we used titanium oxynitride (TiO
x
N
y
), which represents a plasmonic material with tunable optical properties. We synthesized a series of TiO
x
N
y
thin films using various parameters of magnetron sputtering. This allowed us to achieve a unique behavior, such as broadband flat dispersion of the dielectric function in the near-infrared range. Under these conditions, the optical response becomes local in time. The results of this study provide novel opportunities for designing and practical implementation of photonic devices based on time-varying media.