- About this Journal ·
- Abstracting and Indexing ·
- Aims and Scope ·
- Article Processing Charges ·
- Author Guidelines ·
- Bibliographic Information ·
- Citations to this Journal ·
- Contact Information ·
- Editorial Board ·
- Editorial Workflow ·
- Free eTOC Alerts ·
- Publication Ethics ·
- Recently Accepted Articles ·
- Reviewers Acknowledgment ·
- Submit a Manuscript ·
- Subscription Information ·
- Table of Contents

International Journal of Optics

Volume 2012 (2012), Article ID 832086, 6 pages

http://dx.doi.org/10.1155/2012/832086

## Tight Focusing of Partially Coherent Vortex Beams

Department of Engineering Science, The University of Electro-Communications, 1-5-1 Chofugaoka, Chofu-shi, Tokyo 182-8585, Japan

Received 29 May 2011; Accepted 11 July 2011

Academic Editor: Paramasivam Senthilkumaran

Copyright © 2012 Rakesh Kumar Singh. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

#### Abstract

Tight focusing of partially polarized vortex beams has been studied. Compact form of the coherence matrix has been derived for polarized vortex beams. Effects of topological charge and polarization distribution of the incident beam on intensity distribution, degree of polarization, and coherence have been investigated.

#### 1. Introduction

Polarization distribution of tightly focused optical beams has drawn considerable interest in recent years and recent advances in the field of nanophotonics have contributed much to the rapid progress of the field [1–7]. Tightly focused structure of the optical field possesses three-dimensional structure, and -, -, and -polarization components are involved in shaping the focal structure [1]. Role of longitudinal polarization component becomes significant in shaping the focused structure in contrast to low NA-focusing system where contribution of component is negligible in comparison to that of transverse polarization components [1]. Input polarization of the beam becomes a dominating factor apart from complex amplitude in shaping the focused structure. For example, optical vortex produces doughnut structure in the low NA focusing, whereas existence of the doughnut in tightly focused structure of vortex beam depends on its topological charge as well as on its polarization distribution [6, 8–12].

A vortex beam has a helical phase structure with a point of undefined phase in the heart of the surface, and this point is referred to as “phase singularity.” Accumulated phase variation around the singularity is an integral multiple of 2*π* and is referred to as “topological charge” [13]. Doughnut structure is useful in several applications ranging from optical trapping, microscopy to lithography [13–15]. Scattered vortex beam from the sample or object provides important information about the sample, and this sensing property of the optical beam is named as “singular microscopy” [16]. Helical phase structure in association with polarization distribution is also used to generate optical beams with nonuniform polarization distributions [17–19]. Optical beams with non-uniform polarization distribution are a solution of the wave equation in the cylindrical coordinates, and this family of optical beams is referred cylindrically polarized (CV) beams [19]. Azimuthally and radially polarized beams are examples of CV beams, and have been exploited in shaping the focal spot in recent years due to their unique characteristics and applications [20–23].

The azimuthally polarized nonvortex beam possesses a sharp doughnut structure and less susceptible to azimuthal aberration in comparison to circularly polarized unit-charged vortex beam [11]. However, doughnut structure with on-axis intensity null ceases to exist for the unit-charged azimuthally polarized vortex beam due to contribution from the longitudinal polarization component [11]. Focal spot may be extremely small when the incident beam possesses radial polarization distribution and such kind of beam has been used for various purposes. Therefore, the state of polarization in the focal region of an optical beam decides its shape and size. Polarization distribution of focused singular beam can play an important role in singular microscopy due to significant role of the scattered light in sensing the object.

Investigations on polarization and coherence properties of the randomly fluctuating field have also been area of interest from many years [24–28]. A fair amount of literature is available on the coherence matrix to deal with such situation. Modification has also been made in the coherence matrix to take consideration of the vectorial nature and 3D characteristics of the tightly focused beam [29–31]. Detailed study on the tight focusing of uniformly partially polarized radiation has been carried out in the past [30]. Such investigations for partially coherent vortex beams have been carried out recently [32, 33]. With increasing importance of polarized vortex beams in several applications and in physical optics, it is also important to carry out such investigations. In this paper, we have investigated effect of coherence and polarization on the intensity distribution, degree of polarization, and coherence in the focal region of a high numerical aperture systems.

#### 2. Theory

Complex field distribution of incident quasimonochromatic beam in the focal region of a high NA aplanatic lens is given [1] as
where *A *is related to the optical system parameters and *λ* is wavelength of light in the medium with refractive index in the focal region. is the focusing angle, is azimuthal angle on the incident plane, *α* is maximum angle of convergence , and the numerical aperture *. P *represents the polarization matrix distribution at the exit pupil, and is apodization factor and is equal to for an aplanatic system.

The polarization distribution matrix at the exit pupil plane is written [9] as
where *a* and *b* are the strengths of the *-,* and *-*polarized incident beams, respectively. Strength factors are position dependent in the case of nonuniformly polarized or CV beams and constant in the case of uniformly polarized beam.

The second-order coherence properties of the vector field in the focal region of a high NA system can be studied using the coherence matrix. Compact form of the coherence-polarization matrix can be written using CP matrix of the incident field, and given [30] as where is CP matrix, and element of this matrix is written as. and ,are position coordinates in the observation plane. Angle bracket and asterisk denote ensemble average and complex conjugate, respectively. is matrix and represents transpose of the matrix. Complete form of matrix depends on the input polarization distribution.

The CP matrix of the incident field takes on the formwhere and are the intensities of the and components; respectively, and and are the magnitude and phase of their complex correlation coefficients. is the CP matrix of order at the input plane, and .

Degree of coherence and degree of polarization are the important parameters in addition to intensity distribution for the evaluation of effect of partial coherence and partial polarization. These parameters at a point *r* in the focal plane of high NA system are given [30] as
where **, **, and are, respectively, intensity distribution, degree of coherence, and degree of polarization in the focal region at point .

Strength factors of *x-* and *y*-polarized component of the incident beam are independent of the position coordinate and beam referred to as uniformly or homogeneously polarized beam. Complex field distribution in the focal region for uniformly polarized beam can be obtained by substituting proper strength factor into (2) and subsequently using (1). Using trigonometric identities Matrix *M* for homogeneously or uniformly polarized vortex beam can be written [10, 11] as Here and is Bessel Function.

In the case of nonvortex beam (), (7) corresponds to the matrix for uniformly partially polarized beam [30]. In our study, we have assumed incident field as completely coherent and only focused on the partially polarized beam.

#### 3. Results

Intensity distribution, degree of polarization, and coherence in the focal region of an aplanatic system can be obtained using (5)–(7) with proper polarization matrix for different input cases for input coherence matrix given by (3). We have presented results at the focal plane () of an air aplanatic lens with . Results were evaluated for homogeneously polarized [30] and completely unpolarized [31] cases of nonvortex beam compared, and good agreement is found between both.

Results of intensity distribution and degree of polarization for polarized vortex beams are shown in Figure 2 and for two values of topological charge with and for. Figures 2(a) and 2(b) represent intensity distribution for vortex beam with , whereas intensity distribution for beam with is shown in Figures 2(c) and 2(d). Intensity possesses circular symmetry with lowest value at the center, and it decreases even to zero for fully correlated case. Size of low-intensity region increases with an increase in the topological charge. The distribution of degree of polarization as shown in second row of Figure 2 shows circular symmetry with rings with fully polarized at the center, and degree of polarization fluctuates around center. Figures 2(e)–2(h) represent change in degree of polarization due to change in topological charge and coherence. Intensity profiles of vortex beams with , and 2 in the focal plane are shown by curves *a*, *b,* and *c* in Figure 3 for and . Intensity value at the focal point decreased with an increase in the topological charge and also decreases with an increase in the correlation coefficient. Figure 4 shows distribution of degree of polarization for non-vortex () and vortex beams with and 2. Degree of polarization for vortex beams possesses unit value at the center, and its variation around the center is sharp for unit-charged beam. However non-vortex beam possesses unit value away from the center. Degree of coherence for unit-charged vortex beam with and two values of correlation coefficients are shown in Figure 5 in the focal plane along . Curves a and b in Figure 5 represent profile of and for , whereas curves c and d show results for . Results of matche with results of for both cases.

#### 4. Conclusions

Investigations on the tightly focused vortex beams have been carried out using the vectorial diffraction integral. Contribution of different polarization components in the focal region according to the incident beam polarization and vortex characteristics have direct impact on the intensity distribution, degree of polarization, and degree of coherence in the focal region.

#### Acknowledgment

The author is highly thankful to Japanese Society of promotion of Science (JSPS) for fellowship.

#### References

- B. Richards and E. Wolf, “Electromagnetic diffraction in optical systems II. Structure of the image field in an aplanatic system,”
*Proceedings of the Royal Society of London A*, vol. 253, no. 1274, pp. 358–379, 1959. View at Publisher · View at Google Scholar - L. Novotny, M. R. Beversluis, K. S. Youngworth, and T. G. Brown, “Longitudinal field modes probed by single molecules,”
*Physical Review Letters*, vol. 86, no. 23, pp. 5251–5254, 2001. View at Publisher · View at Google Scholar · View at Scopus - M. Totzeck, “Numerical simulation of high-NA quantitative polarization microscopy and corresponding near-fields,”
*Optik*, vol. 112, no. 9, pp. 399–406, 2001. View at Scopus - M. A. Lieb and A. J. Meixner, “A high numerical aperture parabolic mirror as imaging device for confocal microscopy,”
*Optics Express*, vol. 8, no. 7, pp. 458–474, 2001. View at Scopus - N. Huse, A. Schönle, and S. W. Hell, “Z-polarized confocal microscopy,”
*Journal of Biomedical Optics*, vol. 6, no. 3, pp. 273–276, 2001. View at Publisher · View at Google Scholar - N. Bokor, Y. Iketaki, T. Watanabe, and M. Fujii, “Investigation of polarization effects for high-numerical-aperture first-order Laguerre-Gaussian beams by 2D scanning with a single fluorescent microbead,”
*Optics Express*, vol. 13, no. 26, pp. 10440–10447, 2005. View at Publisher · View at Google Scholar · View at Scopus - Z. Bomzon, M. Gu, and J. Shamir, “Angular momentum and geometrical phases in tight-focused circularly polarized plane waves,”
*Applied Physics Letters*, vol. 89, no. 24, Article ID 241104, 3 pages, 2006. View at Publisher · View at Google Scholar · View at Scopus - D. Ganic, X. Gan, and M. Gu, “Focusing of doughnut laser beams by a high numerical-aperture objective in free space,”
*Optics Express*, vol. 11, no. 21, pp. 2747–2752, 2003. View at Scopus - L. E. Helseth, “Optical vortices in focal regions,”
*Optics Communications*, vol. 229, no. 1–6, pp. 85–91, 2004. View at Publisher · View at Google Scholar · View at Scopus - R. K. Singh, P. Senthilkumaran, and K. Singh, “Effect of primary spherical aberration on high-numerical-aperture focusing of a Laguerre-Gaussian beam,”
*Journal of the Optical Society of America A*, vol. 25, no. 6, pp. 1307–1318, 2008. View at Publisher · View at Google Scholar · View at Scopus - R. K. Singh, P. Senthilkumaran, and K. Singh, “Tight focusing of vortex beams in presence of primary astigmatism,”
*Journal of the Optical Society of America A*, vol. 26, no. 3, pp. 576–588, 2009. View at Publisher · View at Google Scholar · View at Scopus - Z. Zhang, J. Pu, and X. Wang, “Distribution of phase and orbital angular momentum of tightly focused vortex beams,”
*Optical Engineering*, vol. 47, no. 6, Article ID 068001, 2008. View at Publisher · View at Google Scholar - L. Allen, S. M. Barnett, and M. J. Padgett, Eds.,
*Optical Angular Momentum*, Institute of Physics, 2003. - P. Török and P. R. T. Mimro, “The use of Gauss-Laguerre vector beams in STED microscopy,”
*Optics Express*, vol. 12, no. 15, pp. 3605–3617, 2004. View at Publisher · View at Google Scholar · View at Scopus - M. D. Levenson, T. Ebihara, G. Dai, Y. Morikawa, N. Hayashi, and S. M. Tan, “Optical vortex masks for via levels,”
*Journal of Microlithography, Microfabrication and Microsystems*, vol. 3, no. 2, pp. 293–304, 2004. View at Publisher · View at Google Scholar · View at Scopus - B. Spektor, A. Normatov, and J. Shamir, “Singular beam microscopy,”
*Applied Optics*, vol. 47, no. 4, pp. A78–A87, 2008. View at Publisher · View at Google Scholar · View at Scopus - D. G. Hall, “Vector-beam solutions of Maxwell's wave equation,”
*Optics Letters*, vol. 21, no. 1, pp. 9–11, 1996. - K. S. Youngworth and T. G. Brown, “Focusing of high numerical aperture cylindrical-vector beams,”
*Optics Express*, vol. 7, no. 2, pp. 77–87, 2000. View at Scopus - Q. Zhan and J. R. Leger, “Focus shaping using cylindrical vector beams,”
*Optics Express*, vol. 10, no. 7, pp. 324–331, 2002. View at Scopus - R. Dorn, S. Quabis, and G. Leuchs, “Sharper focus for a radially polarized light beam,”
*Physical Review Letters*, vol. 91, no. 23, Article ID 233901, 4 pages, 2003. View at Scopus - C. J. R. Sheppard and A. Choudhury, “Annular pupils, radial polarization, and superresolution,”
*Applied Optics*, vol. 43, no. 22, pp. 4322–4327, 2004. View at Publisher · View at Google Scholar · View at Scopus - S. F. Pereira and A. S. Van de Nes, “Superresolution by means of polarisation, phase and amplitude pupil masks,”
*Optics Communications*, vol. 234, no. 1–6, pp. 119–124, 2004. View at Publisher · View at Google Scholar · View at Scopus - G. M. Lerman and U. Levy, “Effect of radial polarization and apodization on spot size under tight focusing conditions,”
*Optics Express*, vol. 16, no. 7, pp. 4567–4581, 2008. View at Publisher · View at Google Scholar · View at Scopus - W. Wang, A. T. Friberg, and E. Wolf, “Focusing of partially coherent light in systems of large Fresnel numbers,”
*Journal of the Optical Society of America A*, vol. 14, no. 2, pp. 491–496, 1997. View at Scopus - G. P. Agrawal and E. Wolf, “Propagation-induced polarization changes in partially coherent optical beams,”
*Journal of the Optical Society of America A*, vol. 17, no. 11, pp. 2019–2023, 2000. View at Scopus - K. Duan and B. Lü, “Partially coherent vectorial nonparaxial beams,”
*Journal of the Optical Society of America A*, vol. 21, no. 10, pp. 1924–1932, 2004. View at Scopus - T. van Dijk, G. Gbur, and T. D. Visser, “Shaping the focal intensity distribution using spatial coherence,”
*Journal of the Optical Society of America A*, vol. 25, no. 3, pp. 575–581, 2008. View at Publisher · View at Google Scholar · View at Scopus - Y. Cai, Q. Lin, H. T. Eyyuboǧlu, and Y. Baykal, “Average irradiance and polarization properties of a radially or azimuthally polarized beam in a turbulent atmosphere,”
*Optics Express*, vol. 16, no. 11, pp. 7665–7673, 2008. View at Publisher · View at Google Scholar · View at Scopus - T. Setälä, A. Shevchenko, M. Kaivola, and A. T. Friberg, “Degree of polarization for optical near fields,”
*Physical Review E*, vol. 66, no. 1, Article ID 016615, 7 pages, 2002. View at Publisher · View at Google Scholar · View at Scopus - K. Lindfors, T. Setälä, M. Kaivola, and A. T. Friberg, “Degree of polarization in tightly focused optical fields,”
*Journal of the Optical Society of America A*, vol. 22, no. 3, pp. 561–568, 2005. View at Publisher · View at Google Scholar · View at Scopus - K. Lindfors, A. Priimagi, T. Setälä, A. Shevchenko, A. T. Friberg, and M. Kaivola, “Local polarization of tightly focused unpolarized light,”
*Nature Photonics*, vol. 1, no. 4, pp. 228–231, 2007. View at Publisher · View at Google Scholar · View at Scopus - Z. Zhang, J. Pu, and X. Wang, “Focusing of partially coherent Bessel-Gaussian beams through a high-numerical-aperture, objective,”
*Optics Letters*, vol. 33, no. 1, pp. 49–51, 2008. View at Publisher · View at Google Scholar · View at Scopus - B. Chen, Z. Zhang, and J. Pu, “Tight focusing of partially coherent and circularly polarized vortex beams,”
*Journal of the Optical Society of America A*, vol. 26, no. 4, pp. 862–869, 2009. View at Publisher · View at Google Scholar · View at Scopus