Nonlinear plasmonic response in atomically thin metal films and generation of entangled photon pairs in optical waveguides

In his talk Álvaro will address two subjects:

A) Nonlinear plasmonic response in atomically thin metal films:

Nanoscale nonlinear optics is limited by the inherently weak nonlinear response of conventional materials and the small light–matter interaction volumes available in nanostructures. Plasmonic excitations can alleviate these limitations through subwavelength light focusing, boosting optical near fields that drive the nonlinear response, but also suffering from large inelastic losses that are further aggravated by fabrication imperfections. Here, we theoretically explore the enhanced nonlinear response arising from extremely confined plasmon polaritons in few-atom-thick crystalline noble metal films. Our results are based on quantum-mechanical simulations of the nonlinear optical response in atomically thin metal films that incorporate crucial electronic band structure features associated with vertical quantum confinement, electron spill-out, and surface states. We predict an overall enhancement in plasmon-mediated nonlinear optical phenomena with decreasing film thickness, underscoring the importance of surface and electronic structure in the response of ultrathin metal films.

B) Direct generation of entangled photon pairs in nonlinear optical waveguides

We introduce two schemes to generate entangled photons pairs into the guided modes of dielectric and metal waveguides:

B1) Direct light illumination in dielectrics: We study this scheme of pair production by employing a rigorous theoretical method relying on the intrinsic second-order optical nonlinearly of the fiber to down-convert a direct incident electric field into guided modes, where energy and momentum conservation restricts the allowed excited modes. We demonstrate that for optical fibers made of a suitable nonlinear material such as LiNbO3, the rate of photon pair production can be ~10^5 under attainable illumination conditions, thus supporting the feasibility of this disruptive approach to directly generate entangled and waveguided photon pairs. Structuring the external light illumination enables a selection of the guided modes in which the original photon down-converts, such that higher-order modes can be selected on demand.

B2) Free electrons in metals: We investigate the excitation of plasmon polaritons in metal strip waveguides that, within specific frequency regimes, strongly enhance light-matter interactions that lead to two-plasmon generation in comparison to the probability of single-plasmon excitation. We demonstrate that, under appropriate conditions, an electron energy loss detected in an optimal frequency range can reliably signal the generation of a plasmon pair entangled in energy and momentum.