Supercontinuum Generation in Gases

Abstract

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 <? = 1 in 1-atm xenon. It is not surprising that supercontinuum generation has not been predicted for gaseous media.This Letter is the first report of supercontinua from gases. Significant spectral broadening has been previously observed by use of 350-fs, 308-nm pulses, and, in independent work, supercontinua have recently been produced in high-pressure gases by this group as well. 7 We observe continua extending from the ultraviolet to the infrared when picosecond or femtosecond 0.6-/xm pulses are focused into high-pressure gases. Continua are produced in the rare gases Ar, Kr, and Xe and the molecular gases H 2 , N2, and CO2. Supercontinua are seen with input po...

Nanowire formation under femtosecond laser radiation in liquid

“…These photos clearly indicate the generation of white-light filaments. It is well known that when a high intense femtosecond laser pulse is launched in gases (Braun et al, 1995;Corkum et al, 1986), liquids (Liu et al, 2002), and solids (Tzortzakis et al, 2001), it self-focuses and self-transforms into a white-light emission during femtosecond laser filamentation. Filamentation is commonly explained in terms of a balance between two nonlinear effects: Kerr self-focusing inducing a change of the refractive index of the medium proportional to the laser intensity; plasma defocusing through multiphoton and/or tunnel ionization of the medium in the highintensity zone.…”

Novel aspects of pulse propagation in photonic crystal fibers

“…Supercontinua have been generated in various materials, including condensed matter [52,61,62], water [63,64] and gases [65][66][67]. Since the advent of the PCF, the availability of supercontinua increased dramatically.…”

Theoretical investigation of noncollinear phase-matched parametric four-photon amplification of ultrashort light pulses in isotropic media

“…It should be noted that media with a large figure of merit for parametric four-photon amplification (small Xn\ responsible for self-phase modulation, small dispersion characterized by large Abbe number, high damage threshold intensity) and low Raman gain factor are widely used for picosecond and femtosecond light continuum generation (H 2 0 [11,12,56,72], D 2 0 [11,12,15,56], mixtures of H 2 0 and D 2 0 [73], ethylene glycol [74,75], mixtures of ethylene glycol and glycerin [76], fused silica [11,12], optical silica fibres [10,11,13,14,77], NaCl crystal [12], compressed xenon gas [78,79], compressed nitrogen gas [78,79]). …”