Generation of Second-Harmonics Near Ultraviolet Wavelengths from Femtosecond Pump Pulses

Second-harmonic generation (SHG) near ultraviolet wavelengths is experimentally demonstrated by coupling femtosecond pump pulses into the normal dispersion region far away from the zero-dispersion wavelength of the fundamental mode in a silica photonic crystal fiber (PCF) fabricated in our laboratory. When the pump pulses with average input power Pavof 500 mW and center wavelength λpof 820 nm are used, the maximum conversion efficiency ηSHof the second harmonics centered at 410 nm can be up to 1.6 × 10-6, corresponding to the output power PSH of 520 nW. By measuring PSH at different PCF lengths and studying the temporal dependence of PSH, it is confirmed that the physical mechanism of SHG is dominated by surface nonlinearity polarization, which is resulted from the local inhomogeneities in the silica core region and at the coreair-silica cladding interface of PCF. Finally, a theoretical model is established to analyze the nonlinear optical process.