X. M. Gao et al. / Natural Science 1 (2009) 229-233

SciRes Copyright © 2009 Openly accessible at http://www.scirp.org/journal/NS/

233

For C =0.0, there is two overlapping intensity rings in

focal region, as shown in Figure 3a. On increasing C.

one of these two intensity ring weakens, so that one fo-

cal ring comes into being and shifts in axial direction.

From all above focal pattern evolution, we can see that

Topological charge induces the focal shift in transverse

direction, while phase parameter leads to the focal shift

along optical axis of the focusing system. In order to

show this point, optical intensity distributions in focal

region under condition of m=7 are also calculated and

illustrated in Figure 4. Ring intensity distribution can be

used to construct a ring optical trap that is stable for

those particles in focal region whose refraction index is

bigger than that of their surrounding medium.

4. CONCLUSIONS

Focal shift of radially polarized QBG beam by phase

shifting is investigated theoretically by vector diffraction

theory in this paper. The phase shifting distribution is the

function of the radial coordinate. Simulations results

show that intensity distribution in focal region can be

altered considerably by the topological charge of QBG

beam and the phase parameter that indicates the vary

degree of the phase shifting along radial coordinate.

Dark hollow focus can be obtained in focal region of

radially polarized QBG beam, which is very desirable in

optical tweezers technique. Particularly, topological

charge induces the focal pattern evolution in transverse

direction, while phase parameter leads to the focal shift

along optical axis more significantly.

5. ACKNOWLEDGMENT

This work was supported by National Basic Research

Program of China (2005CB724304), National Natural

Science Foundation of China (60708002, 60777045,

60871088, 60778022), China Postdoctoral Science

Foundation (20080430086), and Shanghai Postdoctoral

Science Foundation of China (08R214141).

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