Reflected optical harmonics from dielectric mirrors

Optical second and third harmonics were generated in reflection from commercially available laser dielectric mirrors. Although most dielectric coatings on laser mirrors are centrosymmetric, the inversion symmetry is broken at the dielectric-air interface. When intense femtosecond laser pulses are reflected off the dielectric mirrors, a second-order dipole contribution to surface second-harmonic generation can exist and is detected. More dramatically, at a laser fluence of ∼10 GW/cm(2) the normally much weaker electric-dipole-allowed third-harmonic generation that originated in the bulk accentuates the second-harmonic generation. These reflected harmonics are, in some cases, nonnegligible in many sensitive experiments.