Influence of polarisation states on the axial point spread function in subtractive second harmonic generation microscopy with a vortex beam

Doughnut-shaped vortex beams are widely used to enhance lateral resolution in super-resolution fluorescence microscopy and subtractive second harmonic generation microscopy. The influence of polarisation states on the axial point spread function is investigated theoretically and experimentally in subtractive second harmonic generation microscopy using a first-order Laguerre-Gaussian vortex beam. The influence of left-handed circular, right-handed circular and linear polarised states are analysed for second harmonic generation imaging and compared with results of fluorescence imaging. The results exhibit great agreement with theoretical predictions, and demonstrate the superiority of left-handed circular polarisation in achieving a complete dark central spot and an extended axial point spread function.