Nonlinear frequency conversion of 3D optical bottle beams generated using a single axicon.

We demonstrate a novel experimental scheme to generate and study the nonlinear frequency conversion of a three-dimensional (3D) optical Bessel bottle beam (BBB). Using a single axicon and standard optical components and controlling the spot size and divergence of the input Gaussian beam to the axicon, we have generated stable micron-size, high-power optical BBB with tunable spatial characteristics. The BBB has a series of low-intensity regions surrounded by high intensity with diameters of ∼30µ and 17 µm, respectively, at a variable period of 2.3 to 6.4 mm along with the beam propagation. Using the single-pass second harmonic generation (SHG) of femtosecond BBB at 1064 nm in a bismuth triborate nonlinear crystal, we have generated BBB at 532 nm with output power as high as 75 mW and single-pass SHG efficiency of 1.9%. We also observed the self-healing of the BBB at both pump and SHG wavelengths. It is interesting to note that the pump beam truncation shows self-healing in the SHG beam. Such observation proves the direct transfer of the pump's spatial characteristics to the SHG beam in the nonlinear process, potentially useful for imaging even in the turbid medium in biology. This generic scheme can be used at different wavelengths and timescales (continuous-wave to ultrafast).