Effect of spatial coherence on propagation, tight focusing, and radiation forces of an azimuthally polarized beam

We report experimental generation of an azimuthally polarized (AP) beam with variable spatial coherence. The effect of spatial coherence on the propagation properties of an AP beam is studied both numerically and experimentally, and our experimental results agree well with the theoretical predictions. The dependence of the intensity distribution of an AP beam focused by a high numerical aperture objective lens on its initial spatial coherence is illustrated numerically, and it is found that we can shape the beam profile of a tightly focused AP beam by varying its initial spatial coherence. Furthermore, the radiation forces on Rayleigh particles induced by a tightly focused AP beam are studied, and we find that the tightly focused AP beam can be used to trap a Rayleigh particle whose refractive index is larger or smaller than that of the ambient by varying its initial spatial coherence. Our results will be useful for particle trapping and material thermal processing.

Figure 1

Experimental setup for generating a partially coherent AP beam and measuring its intensity distribution after passing through a thin lens. $L_1$, $L_2$, and $L_3$, thin lenses; RGGD, rotating ground-glass disk; GAF, Gaussian amplitude filter; LCPC, liquid crystal polarization converter; CCD, charge-coupled device; PC, personal computer.

Figure 2

Experimental results of the intensity distribution of the generated GSM beam just behind the GAF and the corresponding cross line (dotted curve). The solid curve is a result of the Gaussian fit.

Figure 3

Experimental setup for measuring the transverse coherence width ${\sigma }_0$ of the generated GSM beam. BS, 50:50 beam splitter; $D_1$ and $D_2$, single photon detectors; PC, personal computer.

Figure 4

Experimental results of the normalized fourth-order correlation function (dotted curve) and the corresponding Gaussian fit (solid curve) for three different focused beam spot sizes on the RGGD.

Figure 5

Experimental results of the intensity distribution of the focused AP beam and the corresponding cross line (dotted curve) at the focal plane for different values of ${\sigma }_0$. The solid curves are calculated by the theoretical formulas.

Figure 6

Experimental results of the focused beam profile of an AP beam with variable spatial coherence after passing through a linear polarizer for different transmission angle $\varphi$.

Figure 7

Scheme of tight focusing system.

Figure 8

Intensity distributions $I_x$, $I_y$, $I_z$, $I$ and the corresponding cross line ($y$ = 0) of the total intensity distribution $I$ of a tightly focused AP beam at the focal plane for different values of the correlation coefficients ${\sigma }_{xx},{\sigma }_{yy},{\sigma }_{xy}$.

Figure 9

Intensity distribution $I$ of a tightly focused AP beam in the $xz$ plane near focus for different values of the correlation coefficients ${\sigma }_{xx},{\sigma }_{yy},{\sigma }_{xy}$.

Figure 10

Radiation forces of a tightly focused AP beam on a Rayleigh particle with ${\varepsilon }_2=1$ (air bubble) for different values of the correlation coefficients ${\sigma }_{xx},{\sigma }_{xy},{\sigma }_{yy}$.

Figure 11

Radiation forces of a tightly focused AP beam on a Rayleigh particle with ${\varepsilon }_2=-54+5.9i$ (gold particle) for different values of the correlation coefficients ${\sigma }_{xx},{\sigma }_{xy},{\sigma }_{yy}$