We explore the outcomes of detailed microscopic models by calculating second- and third-harmonic generation from thin film surfaces with discontinuous free-electron densities. These circumstances can occur in structures consisting of a simple metal mirror, or arrangements composed of either different metals or a metal and a free electron system like a conducting oxide. Using a hydrodynamic approach we highlight the case of a gold mirror, and that of a two-layer system containing indium tin oxide (ITO) and gold. We assume the gold mirror surface is characterized by a free-electron cloud of varying density that spills into the vacuum, which as a result of material dispersion exhibits epsilon-near-zero conditions and local field enhancement at the surface. For a bylayer consisting of a thin ITO and gold films, if the wave is incident from the ITO side the electromagnetic field is presented with a free-electron discontinuity at the ITO/gold interface, and wavelength-dependent, epsilon-near-zero conditions that enhance local fields and conversion efficiencies, and determine the surface's emission properties. We evaluate the relative significance of additional nonlinear sources that arise when a free-electron discontinuity is present, and show that harmonic generation can be sensitive to the density of the screening free-electron cloud, and not its thickness. Our findings also suggest the possibility to control surface harmonic generation through surface charge engineering.
Origin, strength, and speed of the nonlinear optical response of transparent conducting oxides to single-cycle optical pulses
South China Normal University（华南师范大学）；Ben-Gurion University of the Negev（内盖夫本-古里安大学）；Shiv Nadar Institution of Eminence Deemed to be University（希夫纳达尔卓越机构大学）
Orientation-dependent high harmonic generation in ZnO
Institute for Nuclear Research and Nuclear Energy, Bulgarian Academy of Sciences（保加利亚科学院核研究与核能研究所）；Institute for Advanced Physical Studies, New Bulgarian University（新保加利亚大学高级物理研究所）；ELI-ALPS, ELI-HU Ltd（ELI-ALPS，ELI-HU有限公司）；ELI-ERIC；Sofia University（索非亚大学）
