New features of light focusing by high numerical-aperture objective

In modern optical microscopy, a high numerical-aperture objective plays an important rule for high-resolution imaging and laser trapping. However, a high numerical-aperture objective results in a depolarisation process [1]. As a result, a linear polarised beam is depolarised into three components of the electromagnetic wave in the focal region. One of the components is along the direction of the beam propagation, i.e. along the axial direction. The contribution of the longitudinal component can be enhanced if the incident illumination over the lens aperture is centrally obstructed. As a result, the focal spot in free space is split into two peaks with a separation determined by the size of the obstruction [2] (see Fig. 1). (b) (a) Fig. 1 The focal shapes of a high numerical-aperture objective obstructed by an opaque disk. The normalized size of the obstruction is ε: (a) ε = 0; (b) ε = 0.5. Another feature caused by the longitudinal component is that the recently predicted spectral splitting phenomenon near phase singularities of focused waves [G. Gbur, T. D. Visser and E. Wolf, Phys. Rev. Lett. 88, 013901 (2002)] disappears when the numerical aperture is higher than critical values that are different between the incident polarization direction and the axial direction [3]. If the phase of the field over the lens aperture is also apodised, the focal shape of a high numerical-aperture objective becomes more complicated. For example, when the phase over the lens aperture possesses a helical structure, i.e. when the illumination on the lens has a screw optical singularity, the doughnut focal shape, which occurs for a low numericalaperture objective, does not necessarily exist when the numerical aperture is large [4]. In this presentation, the new features mentioned above will be discussed. [1] Min Gu, Advanced Optical Imaging Theory, Springer Verlag, 2000. [2] J. W. M. Chon, X. Gan, Min Gu , Splitting of the focal spot of a high numerical-aperture objective in free space , Appl. Phys. Lett. 81 (2002), 1576-1578. [3] D. Ganic, James Chon, Min Gu, Effect of numerical aperture on anomalous behaviour of spectra near phase singularities of focused waves, Appl. Phys. Lett., (2003), in press. [4] D. Ganic, X. Gan, Min Gu, Focusing of doughnut laser beams by a high numericalaperture objective in free space, submitted to Appl. Phys. Lett., (2003).