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Thin films have attracted a great deal of attention within the nanophotonics community due to their ability to strongly confine and manipulate light on extreme subwavelength scales. Recent advances in nanotechnology now enable the fabrication of films with nanometer-scale thickness, which can potentially be used to further miniaturize the next generation of electronic and photonic devices. Nowadays, there exist many kinds of materials that can be structured into thin films. Noble metals (Cu, Ag, Au, ...) are a particularly interesting class of material for nanotechnology due to their appealing chemical and physical properties. However, in very small metallic structures, quantum mechanical effects play an important role in the dynamics of electrons and their interaction with external electromagnetic fields (light). These effects can become even more significant when dealing with nonlinear optical phenomena, which necessitate extremely intense optical fields.
Here we investigate the linear and nonlinear optical response of thin metallic films consisting of a finite number of atomically-thin layers by adopting a quantum-mechanical description of the electron dynamics in the atomic planes of the metal layers. We consider films of noble metals characterized by a potential that captures important electronic band features, including the atomic periodicity across the films, electron spill-out, and surface states, all of which contribute significantly to the optical response associated with plasmon resonances. Self-consistent field calculations of the optical response are performed by solving the single-electron density matrix equation of motion perturbatively, which yields the linear optical response, while non-perturbative simulations in the time-domain enable study of extreme nonlinear optical phenomena, including saturable absorption and high-order harmonic generation. We identify a strong dependence on quantum-finite size effects in the optical response associated with plasmon resonances, both in the linear and nonlinear domains.
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