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Hollow Gaussian beams (HGB) are a special class of doughnut shaped beams thatdo not carry orbital angular momentum (OAM). Such beams have a wide range ofapplications in many fields including atomic optics, bio-photonics, atmosphericscience, and plasma physics. Till date, these beams have been generated usinglinear optical elements. Here, we show a new way of generating HGBs bythree-wave mixing in a nonlinear crystal. Based on nonlinear interaction ofphotons having OAM and conservation of OAM in nonlinear processes, weexperimentally generated ultrafast HGBs of order as high as 6 and power >180 mWat 355 nm. This generic concept can be extended to any wavelength, timescales(continuous-wave and ultrafast) and any orders. We show that the removal ofazimuthal phase of vortices does not produce Gaussian beam. We also propose anew and only method to characterize the order of the HGBs.

Hollow Gaussian beam generation through nonlinear interaction of photons with orbital-angular-momemtum