Frequency-conversion of vector vortex beams with space-variant polarization in single-pass geometry

Nonlinear frequency conversion processes depend on the polarization state of the interacting beams. On the other hand, vector vortex beams have space-variant polarization in the transverse beam plane. In light of these two points, is it possible to do nonlinear frequency conversion of the vector vortex beam in single-pass geometry and retain the characteristics of the beam? To address this question, here, we report an experimental scheme for single-pass second harmonic generation (SHG) of vector vortex beams. Using an ultrafast Ti:Sapphire laser of pulse width similar to 17fs and a set of spiral phase plates in a polarization based Mach-Zehnder interferometer (MZI), we have generated vector vortex beams of order as high as l(p)=12 at an average power of 860 mW. Using two contiguous bismuth borate crystals with the optic axis orthogonal to each other, we have frequency-doubled the near-IR vector vortex beam into visible vector vortex beam with order as high as l(sh)=24. The maximum output power of the vector vortex beam of order, l(sh)=2, is measured be as high as 20.5 mW at a single-pass SHG efficiency of 2.4%. Controlling the delay in MZI, we have preserved the vector vortex nature of both the pump and frequency-doubled beams. Measurement on the mode purity confirms the generation of high quality vector vortex beams at pump and SHG wavelengths. This generic scheme can be used to generate vector vortex beams across the electromagnetic spectrum in all time scales, continuous-wave to ultrafast.