Abstract
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.
Highlights
The search for materials that exhibit a large nonlinear optical response at reduced light intensity thresholds has been a prominent theme in the optical sciences ever since the laser was introduced [1,2,3,4,5,6]
Plasmonic excitations can alleviate these limitations through subwavelength light focusing, boosting optical near fields that drive the nonlinear response, and suffering from large inelastic losses that are further aggravated by fabrication imperfections
We introduce rigorous theory based on a quantum–mechanical description of the SPP-mediated nonlinear optical response in crystalline metal thin films, and in the spirit of motivating experimental investigations, we simulate the signal produced by evanescent fields encountered in near-field characterization techniques such as the Kretschmann configuration, scanning near-field optical microscopy (SNOM), or optical gratings
Summary
The search for materials that exhibit a large nonlinear optical response at reduced light intensity thresholds has been a prominent theme in the optical sciences ever since the laser was introduced [1,2,3,4,5,6]. The situation can be partly alleviated through electronic band structure engineering [9], boosting the intrinsic nonlinear response of a material, or by exploiting the near-field enhancement supplied by subwavelength optical resonances [10]. Propagating surface plasmon polaritons (SPPs), characterized in ultrathin films by extremely compressed wavelengths compared to those of freely propagating photons with the same frequency, must be launched by evanescent fields to satisfy energymomentum conservation [30], and the nonlinear optical response associated with SPPs is less commonly probed in experiments [31], despite the appeal of nonlocal control over nonlinear interactions of propagating SPPs. many of the exciting properties of plasmons in ultrathin films that emerge from vertical electron confinement rely on the preservation of 2D translational symmetry, which becomes crucial for high-quality crystalline samples to exhibit lower losses than their amorphous counterparts [18, 32]. We theoretically explore the nonlinear optical response associated with plasmons in few-atom-thick crystalline noble metal films, an emerging high-quality material platform for nanophotonics [18]. We expect that our results will inspire future explorations in nonlinear nano optics using crystalline noble metal films that are currently available in experiment [18]
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