Conical emission in dense cesium vapor

Damir Aumiler, Ticijana Ban, Goran Pichler
Institute of Physics, Zagreb, Croatia

When strong, near-resonant laser light propagates through gaseous medium, conical emission is observed. The effect was originally observed in potassium vapor [1 ] and has been the subject of intense theoretical and experimental investigations ever since. Continuous wave (cw) and pulsed nanosecond (ns) conical emission was observed in sodium [2], potassium, barium [3], cesium [4], calcium [5] and strontium [6] vapor. Picosecond (ps) and femtosecond (fs) conical emission was also observed in glasses [7].
As a physical phenomenon of strong light field interaction with nonlinear medium, conical emission involves a whole range of classical nonlinear-optical effects, such as self-focusing, self-phase modulation, supercontinuum generation, four-wave mixing, stimulated Raman scattering, multiphoton ionization and many others, which add up together to produce emission in the form of a cone of broad spectra.
We present experimental results of femtosecond laser generated conical emission in dense cesium vapor. The problem of interplay of different nonlinear-optical effects leading to the formation of the conical emission is accessed through the measurements of cone emission angle dependence on laser wavelength, laser power and vapor optical characteristics.
The results show that the maximum cone angle is observed at approximately 750 nm laser wavelength, which is very close to the maximum of the Cs2 X-B absorption band. This is rather surprising since the conical emission is usually connected to atomic resonances. However, our findings suggest the molecular origin of the conical emission.