Kerr self-defocusing of multiple filaments in TW peak power UV laser beam

Abstract

An effective suppression of multiple filamentation of the sub-TW peak power supercritical laser beam in xenon gas was demonstrated in direct amplification of subpicosecond UV pulses at Ti:sapphire/KrF laser facility GARPUN-MTW. A large negative nonlinear refractive index due to a two-photon resonance of KrF laser radiation with Xe state ensured Kerr self-defocusing of a few hundred filaments with a local peak intensity of ~0.2 TW cm−2, 200-fold higher than the average one over the beam cross section, and thus homogenized the laser beam. UV filaments in Xe produced a narrow-angle monochromatic coherent cone emission at 828 nm wavelength due to stimulated hyper-Raman scattering and amplified spontaneous emission at the transition .

Monochromatic Conical IR Emission from Decaying KrF Laser Filaments in Xenon as Coherent Stimulated Four-Wave Mixing Process

“…We have recently observed, in contrast to the mentioned above experiments [30,31], narrowly directed monochromatic CE at the 828 nm wavelength arising from the filaments of KrF laser radiation at intensities ∼10 11 W/cm 2 [32]. Here, we treat this monochromatic CE as a resonance-enhanced coherent stimulated four-wave mixing process, in which two pump UV photons are converted into a coupled pair of phase-correlated IR and VUV photons corresponding to the 5p 5 [19][20][21][22][23][24][25][26], we study the coherent interaction regime when the relevant timescale is less than characteristic transverse relaxation time T 2 in the system.…”

“…These spots look unfocused and off-axis aberatted which is due to the large filament length (compared to the focal length) and their inclination to the lens axis. The IR interference patterns observed in the near-field zone indicate phase matching within the UV filament multitude and a coherent nature of the generated IR CE [32]. The energy of IR CE is found to be below the calorimeter sensitivity, i.e., less than 1% of the UV laser pulse energy.…”

“…To describe the dynamics of the three-level system under consideration, we need to find the elements of the effective density matrix a i a * j , i, j = 1, 2, 3. In the experiment [32], the intensity of pump KrF laser pulse is rather high and well exceeds intensity of the IR conical emission as well as VUV radiation. This allows us to apply the perturbation theory assuming the amplitude E 0 is much larger than the amplitudes of IR and VUV light fields,…”

“…The pump pulse area is assumed to be constant over the amplification length L amp and equals to the above threshold value, tan τ = 0.47. Note, measuring of the limiting spectrum width of the amplified pulse can be considered as an independent method for experimental determination of the transverse relaxation time for a given transition: so, as a result of our experiment [32], one can conclude that the transverse relaxation time of 5p 5 ( 2 P 3/2 )6p[1/2] 0 → 5p 5 ( 2 P 3/2 )6s[3/2] o 1 transition in xenon at 1 atm pressure is determined to be 10 ps. Figure 6.…”

“…Visualization of UV radiation is carried out by installing a UV-absorbing K8 glass in the needed position (see Figure 1a). Glass luminescence in the visible range is displayed onto a Spiricon SP620U Beam Profiler (Ophir Photonics) [32]. A filamented UV beam is introduced into a 2.5 m-long gas cell filled with Xe at a pressure varied in the range 0.1-2 atm (Figure 1b).…”