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
$P_{\mathrm {av}}$
of 500 mW and center wavelength
$\lambda _{\mathrm {p}}$
of 820 nm are used, the maximum conversion efficiency
$\eta _{\mathrm {SH}} ^{^{^{}}}$
of the second harmonics centered at 410 nm can be up to
$1.6\times 10^{-6}$
, corresponding to the output power
$P_{\mathrm {SH}}$
of 520 nW. By measuring
$P_{\mathrm {SH}}$
at different PCF lengths and studying the temporal dependence of
$P_{\mathrm {SH}}$
, 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 core-air-silica cladding interface of PCF. Finally, a theoretical model is established to analyze the nonlinear optical process.