Macroscopic Size Effects in Second Harmonic Generation from Si(111) Coated by Thin Oxide Films: The Role of Optical Casimir Nonlocality

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Optical second harmonic generation from Si(111)-Si${\mathrm{O}}_{2}$ interface shows a strong nonmonotonic dependence on the oxide thickness between 2 and 300 nm. The Brewster angle of incidence, $p$-in, $p$-out combination of polarizations and strong, uniform suppression of this effect by near-index matching fluid exclude trivial multiple reflections and microscopic interface effects. The observation can be interpreted to originate from the optical (Casimir) nonlocality stemming from the thickness-dependent electron-electron interaction via virtual photons of the quantized electromagnetic field.