  When focusing a light beam at high numerical aperture, the resulting electric
field profile in the focal plane depends on the transverse polarisation
profile, as interference between different parts of the beam needs to be taken
into account. It is well known that radial polarised light produces a
longitudinal polarisation component and can be focused below the conventional
diffraction limit for homogeneously polarised light, and azimuthally polarised
light that carries one unit of angular momentum can achieve even tighter focal
spots. This is of interest for example for enhancing resolution in scanning
microscopy. There are numerous ways to generate such polarisation structures,
however, setups can be expensive and usually rely on birefringent components,
hence prohibiting broadband operation. We have recently demonstrated a passive,
low-cost technique using a simple glass cone (Fresnel cone) to generate beams
with structured polarisation. We show here that the polarisation structure
generated by Fresnel cones focuses better than radial polarised light at all
numerical apertures. Furthermore, we investigate in detail the application of
polarised light structures for two-photon microscopy. Specifically we
demonstrate a method that allows us to generate the desired polarisation
structure at the back aperture of the microscope by pre-compensating any
detrimental phase shifts using a combination of waveplates.
