Luminescence response of synthetic opal under femtosecond laser pumping

Synthetic opal is an artificial photonic metamaterial composed from spherical globules of amorphous silica (SiO2) about 300 nm in diameter. We report, for the first time to our knowledge, the origin of a narrow luminescence spectral peak (4 nm HWHM) and optical second and third harmonic generation in synthetic opal samples under femtosecond laser excitation (800 nm) at liquid-nitrogen temperature. Stimulated-emission effects are discussed related to the possibility of nanocavity lasing at the condition of the first Mie resonance in a dielectric sphere.