A metallic nanoparticle surrounded by an amplifying medium results in a boundary condition that creates a singularity in the particle's dynamic polarizability at a critical value of the gain. When this boundary condition is time dependent, parametric excitation of the electromagnetic vacuum results in photon emission. Estimates of the vacuum emission from nanostructures embedded in high-gain laser dyes excited by femtosecond lasers predict energies nearly 2 orders of magnitude larger than the spontaneous emission background.
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