We theoretically investigate second harmonic generation in extremely narrow, sub-wavelength semiconductor and dielectric waveguides. We discuss a novel guiding mechanism characterized by the inhibition of diffraction and the suppression of cut-off li
mits in the context of a light trapping phenomenon that sets in under conditions of general phase and group velocity mismatch between the fundamental and the generated harmonic.
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 335nm, and the third harmonic signal is tuned at 223nm, in a range where the dielectric functio
n 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.
In this letter we experimentally demonstrate second harmonic conversion in the opaque region of a GaAs cavity with efficiencies of the order of 0.1% at 612nm, using 3ps pump pulses having peak intensities of order of 10MW/cm2. We show that the conver
sion efficiency of the inhomogeneous, phase-locked second harmonic component is a quadratic function of the cavity factor Q.
