Supercontinuum Emission from Water using 40 fs Pulses in the External Tight Focusing Limit

Abstract

From the initial observation of self-channeling of high-peak power femtosecond (fs) laser pulses in air, propagation of intense ultrashort laser pulses in different media has become one of the most investigated research areas. The supercontinuum emission (SCE), a spectral manifestation of the spatio-temporal modifications experienced by a propagating ultrashort laser pulse in a nonlinear medium, has many practical applications. However, the extent of blue shift of SCE is reported to be constant due to the phenomenon of "intensity clamping". To further explore the recently observed regime of filamentation without intensity clamping, we measured the evolution of spectral blue shift of SCE resulting from the propagation of fs pulses (800 nm, 40 fs, 1 kHz) in distilled water under different focusing geometries. The efficiency of SCE from tight focusing (f/6) geometry was always higher than the loose focusing (f/12) geometry for both linear and circular polarized pulses. The blue edge of the SCE spectrum (λ min ) was found to be blue shifted for f/6 focusing conditions compared to f/12 focusing geometry. The lower bound of the intensity deposited in the medium measured from the self-emission from the filament demonstrated the existence of intensities ~ 6 ×10 13 Wcm -2 , far beyond the clamping intensities achieved erstwhile.