Self-trapped filaments

Using a high-resolution spectrograph and a Fabry-Perrot interferometer, Gunther eta/. 727) resolved a number of equidistant components with a mean separation of 0.04 cm" in the stimulated Raman spectrum of CSj, which can be attributed to resonant modes in single medium-scale filaments of self-trapped radiation within the liquid. 

One of the properties of Cherenkov radiation is its characteristic of a surface phenomenon which does not conserve the transverse component of the linear momentum. Thus, the ring emission should originate mainly at the surfaces of the self-trapped filaments, and not in their bulk. 

Our experimental apparatus consists of a sodium containing heat-pipe cell with an active length of 20 cm the buffer gas is argon at a few torr pressure. The sodium density is in the range of 10 -10 cm". The dye laser is Hansch type and is pumped by a 30 kW peak power copper-halide laser with a 25 nsec duration pulse. The dye laser bandwidth is 0.5 cm . The laser beam is spatially filtered and focused into the sodium cell. The laser intensity at the focus is about 10 Mw/cm and is sufficient to form self-trapped filaments. The spectrum of the emission is monitored by 1.2 cm resolution McPearson 0.3m monochromator. The forward emission is photographed by an Alphax B216 camera placed after the sodium cell without any imaging optics. The laser beam is blocked with an on-axis disk. 

We have verified experimentally, that the ring emission is a surface radiation and occurs at the boimdary of self-trapped filaments, confirming that it is a Cherenkov type radiation. To this end, we examined the special coherence of the ring emission. The laser radiation was focused into the sodium cell by a cylindrical lens and the produced pattern is shown in Fig. 1. The off-axis ring emission preserves its symmetry around the laser axis, while the laser beam spot has an ellipse form. Should the ring emission be produced in the interior of the saturated filaments, where n=l, the interference between different filament emissions would result in a pattern representing the spatial distribution of the filaments, i.e. ellipse. We thus conclude, that the ring emission is produced mainly at the interior of the filaments. 

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