Supercontinua extending from the ultraviolet to the infrared are observed from high-pressure (1-40 atm) Ar, Kr, Xe, H2, N2, or CO2 illuminated with 2-psec or 70-fsec, 0.6-/xm pulses with an energy ;$500 /xJ. The blue spectral component is shown to display a nearly universal behavior for all gases and pulse durations. Although the maximum intensity of the focused, femtosecond pulse in an evacuated cell was -10 B W/cm 2 , continuum generation was only observed with the femtosecond pulse when the threshold for self-focusing was exceeded. PACS numbers: 42.65.Jx, Discovered in 1970, supercontinuum generation has now been demonstrated in a wide variety of solids and liquids. Self-phase modulation, four-wave mixing, and plasma production (for the blue spectral component) are the processes most commonly invoked to explain continuum generation. 2 Recent plane-wave theoretical models based on cubic nonlinearities 2 ' 3 describe continuum generation in terms of a parameter Q :as rf2E 2 z/cr, where 172 is the nonlinear refractive index, E is the incident laser field amplitude, z is the interaction length, T is the pulse duration, and c is the speed of light. For small Q, the continuum bandwidth is given by Aco -Qco. A value of Q ^ 1 is required for the generation of a broad (8v~~ v) supercontinuum. 2,3 Experimentally, self-focusing (the spatial analog of self-phase modulation) is difficult to avoid. Self-focusing limits the validity of current rj2 continuum theories to e<(4/r)(r ?2 P/cW) 1/2 , (1) where P is the laser power. Equation (1) is obtained by the requirement that the interaction length be less than the self-focusing length for a Gaussian beam and by the assumption of only full-beam self-focusing. Mierofilamentation lowers Q even further. 4 Although no continuum-generation experiments have clearly satisfied the inequality in Eq. (1), the parameter Q is used as a guide to estimate the expected spectral broadening. Based on this parameter, gases are not likely to be sources of supercontinua. For example, xenon, a highly nonlinear gas, has a nonresonant 772 "^x 10~~2 6 m 2 /V 2 at atmospheric pressure 5 (r] 2 of water is 1.5x 10~2 2 m 2 /V 2 ). If we consider the maximum field that can be applied to Xe in the absence of breakdown 6 (/ -10 13 W/cm 2 ) then, even with a 100-fsee (2 psec) pulse, an interaction length of 2=6 xlO 3 cm (1.2xl0 5 cm) is required for
