Bessel beams possess a nondiffractive property that makes them suitable for propagating optical energy over long distances. We have utilized an axicon-generated Bessel beam to carry out a systematic study of the physics that governs propagation of intense, femtosecond-long pulses in a transparent crystal (barium fluoride). Filamentation and supercontinuum generation have been studied using axicon lenses of different cone angles. Emission induced by six-photon absorption of 800-nm photons is utilized to visualize the filaments within the crystal. Our results show that higher incident energy, $E_{\text{in}}$ ($\ge$ 600 $\mu$J), is required to generate filaments inside the crystal when an axicon is used, as compared to a spherical lens (6 $\mu$J). Increase in cone angles increases the value of $E_{\text{in}}$ for filament formation. The size of the filaments is found to be independent of whether an axicon or a spherical lens is used. For a given axicon cone angle, variation of the input energy enables us to observe the transition from steady Bessel filamentation to a nonlinear, unbalanced, Bessel regime. Varying the separation distance between the axicon and the crystal enables differences in periodicity (distance between two focusing-refocusing events) in the filaments to be quantified. Supercontinuum generation is also probed as a function of distance of sample from the tip of the axicon and with different incident beam sizes. The input beam diameter is shown to have a distinct effect on axicon-generated white light.

• Received 16 May 2012

DOI:https://doi.org/10.1103/PhysRevA.86.023808