Third-harmonic generation microscopy with tightly focused radial polarization

Third-harmonic generation (THG) microscopy using tightly focused radially polarized light is studied numerically. Radial polarization illumination with and without an annular pupil plane mask is investigated. Our results show that a tightly focused radially polarized light with annular illumination can achieve a higher spatial resolution and an extended depth of focus. Under certain conditions, a detectable THG signal from a bulk medium can be obtained by tailoring the Gouy phase shift around the focal point.