Resonant femtosecond laser pulse dynamics in metal nanorod-based hyperbolic metamaterials in epsilon-near-zero regime

Wealth of fascinating phenomena provided by artificial photonic structures sparks extensive research efforts in the studies of metamaterials, a special and perspective class of them being the hyperbolic metamaterials (HMMs). Strong HMMs optical anisotropy attained by the combination of metal and dielectric constituents underpins unusual hyperbolic dispersion of light in these artificial media, which marvels plethora of far-reaching applications in photonics. However the dynamics of ultrashort optical pulses in HMMs remains almost unexplored. In this work we experimentally investigate the interaction of femtosecond laser pulses with metal-nanorod based HMMs exhibiting the epsilon-near-zero (ENZ) spectral point, when extraordinary dielectric permittivity goes to zero yielding transition between topologically distinct elliptic and hyperbolic light HMM dispersion. We demonstrate a pronounced superluminal and slow propagation of laser pulses in the HMM, with the transition between these regimes and resonant character of these phenomena in the spectral vicinity of the ENZ point. We put forward a theoretical model of the superluminality and slow light, which leverages unusual case of laser pulse with spectral components in elliptic and hyperbolic dispersion at once. We believe our findings bring to life superior applications for future linear and nonlinear ultrafast photonics of HMMs harnessing of revealed exotic dynamics of optical field in these media.