Third harmonic generation at 223 nm in the metallic regime of GaP

We demonstrate second and third harmonic generation from a GaP substrate 500 mu m thick. The second harmonic field is tuned at the absorption resonance at 335 nm, and the third harmonic signal is tuned at 223 nm, in a range where the dielectric function is negative. These results show that a phase locking mechanism that triggers transparency at the harmonic wavelengths persists regardless of the dispersive properties of the medium, and that the fields propagate hundreds of microns without being absorbed even when the harmonics are tuned to portions of the spectrum that display metallic behavior. (C) 2011 American Institute of Physics. [doi: 10.1063/1.3565240]