Generation of optical needle with wavelength magnitude by tight focusing of circular partially coherent beams

摘要

Based on Richards-Wolf vectorial diffraction integral, the expression of electric cross-spectral density matrix oflinearly polarized circular partially coherent (LPCPC) beams, near the focus of a high numerical aperture (NA)objective, are derived. Numerical calculations are performed to show the influence of initial coherent length and NAof the objective, on the intensity distribution and degree of coherence of the LPCPC beams in the focal region. Theresults indicated that after focusing the LPCPC beams through a high NA objective a super-length optical needle(>12λ) with wavelength beam size can be obtained. Moreover, the numerical calculations illustrate that the length ofthe optical needle can be controlled by adjusting the initial coherent length of the LPCPC beams. The influence ofNA of the objective on the length of the optical needle is much less than the initial coherent length. Setting thereference vector position on the z-axis, we can find that the coherence of the optical needle on the same wavefrontremains unchanged.