Deep ultraviolet supercontinuum generation in optical nanofibers by femtosecond-pulses at 400 nm wavelength

Freestanding nanofibers with submicron diameter exhibit a strong waveguide dispersion and field localization caused by both the extreme small core diameter compared to common optical standard fibers and the large refractive index difference of Delta n = 0.45 between the silica core and air cladding. These are promising characteristics for nonlinear applications like e. g. supercontinuum generation that require waveguides with large nonlinear coefficients and specific dispersion behavior in dependence on the pump wavelength. Optical nanofibers allow a shift of the zero dispersion wavelength and in addition anomalous dispersion down to a wavelength of about 460 nm. We report on deep ultraviolet broadband supercontinuum generation in optical nanofibers pumped with femtosecond pulses from a frequency doubled titan-sapphire oscillator at 400nm wavelength. Numerical simulations of the generated spectra show a remarkable broadening in the deep ultraviolet wavelength range below 250 nm after only a few millimeter of propagation distance, while a single recompressible pulse is maintained in the time domain. The spectrum can be influenced by nanofiber diameter, pulse duration, and pulse energy. In experiment these nanofibers are situated in an optical fiber taper configuration where a waist of constant submicron diameter is located between two taper transitions with varying diameter. For this reason the generated supercontinuum is not only influenced by the nanofiber diameter but by the taper transitions.