Nonlinear light-matter interaction in epsilon-near-zero media

Since coining the term metamaterial, tailoring electromagnetic properties of media has been a stimulating field of research. The advance in fabrication techniques has made the design of matter along any spatial dimensions possible. By shaping materials in length, width, and height, at the nanoscale, extreme scenarios have been realized, such as media with a vanishing dielectric permittivity, the so-called epsilon-near-zero (ENZ) media, and a near-zero or negative values of the refractive index. With metamaterials, the variation of a propagating wave along spatial dimensions has been widely investigated and has brought to light phenomena beyond the common restrictions imposed by the diffraction limit, such as metalenses for subwavelength resolution imaging. Similar to spatial dimensions, time can also be considered as an additional degree of freedom for material optical properties. The possibility to use time as a 4th dimension opens the route to investigate unique phenomena, such as photon acceleration and time-reversed field, with a possible impact on current technology specific to non-reciprocal devices and temporal bandgap structures. Until now, only a few experimental proofs have been reported in subwavelength homogeneous materials or surfaces due to the extremely large and fast temporal modulation of the refractive index required. In this thesis, we demonstrate a high-efficiency generation of optically induced negative refraction (NR) and time-reversal phase-conjugation (PC) in which the refractive index is modulated in time by using optical excitation. The nonlinear media consist of transparent conductive oxide (TCO) thin films optically pumped in their ENZ and near-zero-index of refraction spectral range. Recent studies demonstrate that TCO films exhibit a uniform, large, and ultrafast Kerr nonlinear optical response, making these natural ENZ media the right platform for time-varying optical phenomena. We demonstrate that dynamically tunable coherent perfect absorbers that rely on efficient light-with-light modulation may benefit from the intensity-dependent refractive index change of TCOs in their ENZ spectral range. We also prove that the efficiency of phenomena arising from time-dependent media, such as PC and NR, increases across the bandwidth in which the ENZ feature is expressed. In the direction of a low optical power footprint, we investigate a time-varying, strongly coupled system based on plasmonic antennas on top of deeply subwavelength ENZ film. The enhancement of the optical energy density within the ENZ layer in such a system with respect to the unpatterned case could pave the way to the implementation of time-varying ENZ media in nanophotonic devices