Generation of equilateral-polygon-like flat-top focus by tightly focusing radially polarized beams superposed with off-axis vortex arrays

Xiaolei Wang; Bowen Zhu; Yuxin Dong; Shuai Wang; Zhuqing Zhu; Fang Bo; Xiangping Li

doi:10.1364/OE.25.026844

1. Introduction

Owing to their peculiar properties, vector beams with cylindrical symmetry in polarization such as radially polarized beams (RPB) and azimuthally polarized beams (APB) emerge as a research hot topic [1]. As one of their most pronounced properties, enriched focal field components with different strengths and characteristic spatial distributions can be generated under tightly focused condition by a high numerical aperture (NA) objective. For example, it has been shown that the focal field of a tightly focused RPB is dominant by a strong longitudinal field component with a focal area approaching 0.16λ², significantly smaller than 0.26λ² of a linearly polarized beam [2]. In addition to a sub-diffraction focal area, configuring the relative weighting factor between the longitudinal and transverse field components, a three-dimensional orientated focal polarization can even be obtained [3]. These exceptional properties by focusing vector beams have mediated a variety of advanced applications including high resolution microscopy [4], optical trapping [5, 6], electron acceleration [7] and optical data storage [8].

The fusion with beam engineering techniques including pupil plane phase or amplitude masks offers additional flexibility to further advance focal manipulation and focus shaping. Flat-top focus [9], optical needle [10], optical chain [11] and multiple focal spots [12,13] have been successfully demonstrated using a diffractive optical element with several concentric phase or amplitude modulated zones at the pupil plane. In addition, optical vortex carrying spiral phase distributions can break the rotational symmetry of polarization distribution of cylindrical polarized vector beams allowing versatile means for focus shaping. Complicated focus shaping such as optical hole [14], subwavelength hollow channel focus and multiple circularly polarized focal spots along the optical axis [15], subwavelength transversely polarized focal needle [16] can be generated by tightly focusing cylindrical vector beams concentrically superposed with an optical vortex. However, this kind of beam shaping method is restricted to only produce circular-symmetric focus patterns. Generation of non-circular-symmetric fields or even arbitrary shaped fields to meet diverse demands from different aspects such as material processing, particle manipulation and optical imaging remains elusive.

In this paper, we demonstrate the generation of non-circular-symmetric equilateral-polygon-like flat-top focus (EPFF) by tightly focusing a RPB superposed with off-axis vortex arrays. The general formula for tightly focusing RPBs superposed with off-axis vortex arrays is derived based on Richard-Wolf vector diffraction theory. The dependence of focusing characteristics on the position of off-axis vortices nested in the incident beam and NA of objective lens is theoretically studied. By judiciously adjusting the arrangement and positions of off-axis vortices in the incident beam, multiple EPFF with variant non-circular-symmetric shapes can be realized.

2. Principle

Without loss of generality, a triangle-shaped EPFF is taken as an example to illuminate the principle. The focus shaping setup is schemed in Figs. 1(a)-1(c). Figure 1(a) shows the arrangement of the three off-axis vortices. A linearly polarized Gaussian beam is first incident to the designed phase plate arranged with three off-axis vortices without changing the polarization state of the beam, and is converted to a RPB through a polarization converter, which is then tightly focused by an objective lens, as schemed in Fig. 1(c). Asymmetry energy flow in the axial direction induced by off-axis vortices leads to the intensity redistribution towards the designed EPFF at the focal plane [Fig. 1(b)].

Fig. 1 The sketch for the generation of EPFF. Without loss of generality, triangle-like flat-top focus by using three off-axis vortices is taken as an example. (a) Arrangement of three vortices (red circular) in the pupil plane. Each vortex has equal distance from the optical axis. (b) Illustration of the phase distribution of the incident beam with three off-axis vortices, off-axis vortex induced asymmetric energy flow in the axial direction, and intensity distribution of triangle-like flat-top focus at the focal plane. (c) The optical system for generating EPFF where P (ρ, ϕ) and Q (r, φ) denote points in the object space and the image space, respectively.

Download Full Size | PDF

The incident RPB can be expressed as

{\vec{E}}_{i n 0} (ρ, ϕ) = E_{ρ} {\hat{e}}_{ρ} = E_{0} e x p (- \frac{ρ^{2}}{w^{2}}) {\hat{e}}_{ρ}

where w is the waist radius of the incident Gaussian beam, (ρ, ϕ) is the polar coordinate in the object space and

{\hat{e}}_{ρ}

is the unit vector along the radial direction. ρ can be obtained by

ρ = f \sin θ

where f is the focal length of the objective lens.

Assume an off-axis vortex is centered at the point P₁ in the object space and it can be expressed as ρ₁e^j^φ¹. In this case the incident beam carrying the vortex can be expressed as [17]:

{\vec{E}}_{i n} (ρ, ϕ) = {\vec{E}}_{i n 0} (ρ, ϕ) {(ρ e^{j ϕ} - ρ_{1} e^{j ϕ_{1}})}^{m_{1}}

where m₁ is the topological charge (TC) of the vortex.

Similarly, the expression of the incident beam carrying multiple off-axis vortices, whose locations are centered at P₁, P₂, …, P_N with TC are m₁, m₂, …, m_N, respectively, can be given as

\begin{array}{l} {\vec{E}}_{i n} (ρ, ϕ) = {\vec{E}}_{i n 0} (ρ, ϕ) (^{ρ e^{j ϕ} - ρ_{1} e^{j ϕ_{1}}) m_{1}} (^{ρ e^{j ϕ} - ρ_{2} e^{j ϕ_{2}}) m_{2}} ... \times (^{ρ e^{j ϕ} - ρ_{N} e^{j ϕ_{N}}) m_{N}} \\ = {\vec{E}}_{i n 0} (ρ, ϕ) \prod_{i = 1}^{N} {(ρ e^{j ϕ} - ρ_{i} e^{j ϕ_{i}})}^{m_{i}} \end{array}

Equation (3) can be simplified as

{\vec{E}}_{i n} (ρ, ϕ) = {\vec{E}}_{i n 0} (ρ, ϕ) \sum_{i = 0}^{M} A_{n} ρ^{M - i} e^{j (M - i) ϕ}

by taking

M = \sum_{n = 1}^{N} m_{n}

where A_n denotes the coefficient determined by the product of all terms in Eq. (3).

The focal field components can be obtained according to the Richards–Wolf vector diffraction theory [18]:

\begin{array}{l} \vec{E} (r, φ, z) = \frac{- j k f}{2 π} \int_{0}^{α} d θ \int_{0}^{2 π} P (θ) (\begin{array}{l} - E_{i n} \cos θ \cos ϕ - φ) \\ - E_{i n} \cos θ \sin (ϕ - φ) \\ E_{i n} \sin θ \end{array}) \\ \times \exp [j k (z \cos θ + r \sin θ \cos (φ - ϕ))] \sin θ d ϕ \end{array}

where

P (θ) = \sqrt{\cos θ}

is the apodization function in the pupil plane,

α = \arcsin (N A / n_{0})

is the aperture angle of the objective lens and n₀ = 1 is the refraction index in the image space.

Substituting Eq. (1) and Eq. (4) into Eq. (6), we obtained the expression of tightly focused RPB superposed with multiple vortices with different off-axis locations and variant TCs as

\begin{array}{l} \vec{E} (r, φ, z) = (\begin{array}{l} E_{r} \\ E_{φ} \\ E_{z} \end{array}) = \frac{- j k f E_{0}}{2 π} \sum_{n = 0}^{M} \int_{0}^{α} d θ \int_{0}^{2 π} A_{n} ρ^{M - n} e^{j (M - n) ϕ} \sqrt{\cos θ} \sin θ (\begin{array}{l} - \cos θ \cos (ϕ - φ) \\ - \cos θ \sin (ϕ - φ) \\ \sin θ \end{array}) \\ \times e x p (- \frac{ρ^{2}}{w^{2}}) \exp [j k (z \cos θ + r \sin θ \cos (φ - ϕ))] d ϕ \end{array}

where

E_{r}

,

E_{φ}

and

E_{z}

are the radial, azimuthal and longitudinal components in the focal plane, respectively. They can be finally derived as

\begin{array}{l} E_{r} (r, φ, z) = \frac{k E_{0}}{2} \sum_{l = 0}^{M} A_{M - l} j^{l + 2} f^{l + 1} e x p (j l φ) \int_{0}^{α} {(\sin θ)}^{l + 1} {(\cos θ)}^{\frac{3}{2}} e x p (- \frac{f^{2} {(\sin θ)}^{2}}{w^{2}}) \\ \times \exp [j k z \cos θ] [J_{l + 1} (k r \sin θ) - J_{l - 1} (k r \sin θ)] d θ \end{array}

\begin{array}{l} E_{φ} (r, φ, z) = \frac{k E_{0}}{2} \sum_{l = 0}^{M} A_{M - l} j^{l + 1} f^{l + 1} e x p (j l φ) \int_{0}^{α} {(\sin θ)}^{l + 1} {(\cos θ)}^{\frac{3}{2}} e x p (- \frac{f^{2} {(\sin θ)}^{2}}{w^{2}}) \\ \times \exp [j k z \cos θ] [J_{l + 1} (k r \sin θ) + J_{l - 1} (k r \sin θ)] d θ \end{array}

\begin{array}{l} E_{z} (r, φ, z) = - k E_{0} \sum_{l = 0}^{M} A_{M - l} j^{l + 1} f^{l + 1} e x p (j l φ) \int_{0}^{α} {(\sin θ)}^{l + 2} {(\cos θ)}^{\frac{1}{2}} \\ \times e x p (- \frac{f^{2} {(\sin θ)}^{2}}{w^{2}}) \exp [j k z \cos θ] J_{l} (k r \sin θ) d θ \end{array}

where l = M-n, and the J_l (krsinθ) is the l-th order Bessel function of the first kind.

3. Focal characteristics

From Eq. (8) to Eq. (10), each field components in the focal region can be obtained. Three off-axis vortices locate equally at a distance with respect to the pupil center with an identical TC of m = 1. To unveil the influence of the location of off-axis vortices with respect to the pupil center and NA of the objective lens on the focal fields, we study the intensity distribution along the y axis in the focal plane. The Poynting vector field is also discussed to reveal the focusing energy flow under the influence of off-axis vortices.

3.1 Influence of the off-axis distance r₀ of vortices

Figure 2(a) shows the relationship between the off-axis distance r₀ of three vortices in the incident beam and the focal intensity distribution along the y axis under the condition of NA = 0.2, 0.4, 0.6 and 0.8, respectively. When NA = 0.2, the intensity distribution along y<0 experiences an attenuation and then is followed by a restoration as r₀ increases. There is a critical position at about r₀ = 0.6 where the intensity attenuation ends and the restoration begins. Around this critical position, the transverse field slightly decreases in size as r₀ increases. The focal field is ‘expelled’ outside away from the pupil center when r₀ is close to the optical axis and vice versa. It is evident in the transverse intensity distribution along the y axis at different r₀ as shown in Fig. 2(a). It is noted that the critical position decreases as the NA of the objective lens increases.

Fig. 2 (a) Variation tendency of the optical intensity along the y axis in the focal plane as the increase of off-axis distance r₀ of vortices at different NA value. The data is normalized to the maximum value of the optical intensity along the y axis in the focal plane for each r₀, respectively. (b) Transverse intensities and their constituent radial and azimuthal components obtained at variant r₀ focused by an objective lens with NA = 0.2 in Fig. 2(a).

Download Full Size | PDF

Figure 2(b) shows the transverse field at NA = 0.2 from top to bottom with r₀ = 0.2w, 0.4w, 0.6w and 0.8w, respectively. The second and third column corresponds to their radial and azimuthal components, respectively. When the three off-axis vortices are located near the center of the incident RPB, the intensity of the radial component is coupled to the azimuthal component. As vortices move away from the pupil center, the intensity of the radial component increases and that of the azimuthal component reduces. It means the coupling from the radial component to the azimuthal component gradually reduces with the increase of off-axis distance r₀. In the meantime, the intensity profile of the overall transverse field varies from a ring to three separated lobes gradually [Fig. 2(b)]. When r₀ reaches the critical position of r₀ = 0.6, the off-axis vortices induced focal intensity redistribution is pronounced. These intensity lobes in the transverse component are distinctively isolated and the intensity profile presents triangle shape, which is rotated by an angle compared with the arrangement of the off-axis vortices shown in Fig. 1(a).

3.2 Influence of numerical aperture of the objective lens

The focal intensity distribution along the y axis focused by variant NA of the objective lens is studied for three vortices with off-axis distance r₀ = 0.2w, 0.4w, 0.6w and 0.8w, respectively, as shown in Fig. 3(a). With the NA increasing, the transverse size of the focus decreases quickly at beginning and gradually saturates due to the nonlinear relationship between the aperture angle and the NA. When r₀ = 0.2w, there is also a critical position which intensifies the asymmetry of the intensity distribution for y > 0 and y < 0. It is worth noting that the critical position also moves to smaller NA as the off-axis distance r₀ increases.

Fig. 3 (a) Dependence of the intensity distribution along the y axis in the focal plane on the NA of the objective lens. The data are normalized to the maximum value of the intensity along the y axis in the focal plane for each NA respectively. (b) Transverse intensity patterns and their constituent radial and azimuthal components obtained by different NA when the off-axis distance r₀ is fixed at 0.8. The scale in Fig. 3(b) is different for each NA.

Download Full Size | PDF

The transverse intensity at r₀ = 0.8w focused by an objective lens with NA = 0.2, 0.4, 0.6 and 0.8, respectively, is shown in Fig. 3(b) from top to bottom with different scale. At this case, the critical point almost vanishes. When the NA is small and approaching the critical value, the off-axis vortices induced intensity redistribution is pronounced and the intensity profile of the transverse field follows the triangle shape. When the NA is large and away from the critical value, the influence of the off-axis vortices is reduced and the dominant radial component follows a ring shape. Figure 3(b) also shows the azimuthal component is weakened with increasing of NA for the reason that much more incident RPB is coupled into the longitudinal polarized beam leading to enhanced focusing longitudinal component.

3.3 Energy flow

Besides the analysis of the intensity distribution in the focal plane, the time-averaged Poynting vector field and the energy flow is studied here to unveil insights into the focus shaping by off-axis vortex arrays. According to Maxwell’s equations, the radial, azimuthal and longitudinal components of magnetic fields should follow

H_{r} (r, φ, z) = - \frac{i}{k} (\frac{1}{r} \frac{\partial E_{z}}{\partial φ} - \frac{\partial E_{φ}}{\partial z})

H_{φ} (r, φ, z) = - \frac{i}{k} (\frac{\partial E_{r}}{\partial z} - \frac{\partial E_{z}}{\partial r})

H_{z} (r, φ, z) = - \frac{i}{k} \frac{1}{r} (\frac{\partial (ρ E_{φ})}{\partial r} - \frac{\partial E_{r}}{\partial φ})

Hence, the time-averaged Poynting vector can be determined by

〈 \vec{S} 〉 = \frac{c}{4 π} Re (\vec{E} \times {\vec{H}}^{*})

where c is the speed of light in vacuum, the asterisk and Re (

\vec{E} \times {\vec{H}}^{*}

) denote the operation of complex conjugation and real part of (

\vec{E} \times {\vec{H}}^{*}

), respectively.

The normalized Poynting vector field and energy flow lines are obtained under the condition of NA = 0.6 and r₀ = 0.1w, 0.5w, 0.9w, respectively as shown in Fig. 4. The critical position under the condition of NA = 0.6 is approximately r₀ = 0.5w as indicated in Fig. 2(a). For a small r₀ such as 0.1w, the influence of the off-axis vortices on the axial Poynting vector field is negligible, as seen in Fig. 4(a). As r₀ approaches the critical position of r₀ = 0.5w, the Poynting vector field in y>0 almost disappeared as a consequence of deconstructive interference. In the meantime, energy flow lines change directions which is evidenced in the red frame of Fig. 4(b). When r₀ increases from 0.1w to 0.9w, the energy flow lines within the yellow frame of Fig. 4(a) change from the concave shape to the convex shape within the red frame of Fig. 4(b), and recovers to the concave shape as shown in the yellow frame of Fig. 4(c). As a consequence of the changed energy flow in the axial direction, it raises the spatial redistribution of intensity in the focal plane.

Fig. 4 Normalized Poynting vector field (color density plots) and the energy flow (white lines) along the axial direction obtained by focusing a RPB superposed with three off-axis vortices under the condition of NA = 0.6 and (a) r₀ = 0.1w (b) r₀ = 0.5w and (c) r₀ = 0.9w, respectively. The red wire frame indicates the propagation section of the phase singularity in the axial direction.

Download Full Size | PDF

4. Generation of equilateral-polygon-like flat-top focus

From the above analysis, it is clear that off-axis vortices can break the circular symmetry and shape the intensity profile of transverse field components following the arrangement of the off-axis vortices. It should be noted that the proposed approach for the flat-top focus is mainly applied to the radially polarized beam with balanced transverse and longitudinal components in the tight focus. To generate an EPFF, judicious choice of r₀ and NA is of crucial importance to tailor the focus shape and balance the transverse and longitudinal field components.

Figure 5 shows the flat-top regular-triangle-like focus generated by using the configuration of three off-axis vortices arranged in Fig. 1(a) at the condition of NA = 0.7, r₀ = 0.8w and m = 1. Figures 5(a)-5(c) depict the azimuthal, radial and longitudinal field distribution. Since the longitudinal component exhibits complementary distribution to the radial component with a comparable strength, the total intensity pattern with a flap-top area of 0.31λ² is demonstrated as shown in Fig. 5(d). Figures 5(e) and 5(f) show the intensity distribution in the x-z plane and the y-z plane, respectively. The flat-top length in Fig. 5(d) along the x axis and the y axis are 0.63λ and 0.60λ, respectively.

Fig. 5 Focal intensity distribution shaped by tightly focusing a RPB superposed with three off-axis vortices. (a), (b), (c) and (d) are the azimuthal, radial, longitudinal components and total intensity pattern in the x-y plane, respectively. The white line indicates the intensity profile along the x-axis and the y-axis. (e) and (f) are the total intensity pattern in the x-z plane and the y-z plane, respectively.

Download Full Size | PDF

By using the same method, we can obtain other EPFF as shown in Fig. 6. From left to right, they are, in turn, the arrangement of off-axis vortices in the incident beam, radial component, azimuthal component, longitudinal component and total intensity distribution in the x-y plane. From top to bottom, the number N of vortices used in tailoring the EPFF are in turn 2, 4, 5 and 6, respectively. It’s apparent that flat-top focus including bar-type-like, square-like, pentagon-like and hexagon-like can be generated.

Fig. 6 Arrangement of off-axis vortices in the incident beam and corresponding EPFF. The first column shows the location and arrangement of off-axis vortex arrays. The second to the fifth column corresponds to the radial, azimuthal, longitudinal components and the total intensity pattern in the x-y plane, respectively. From top to bottom, the number N of vortices used are in turn 2, 4, 5 and 6 corresponding to bar-type-like, square-like, pentagon-like and hexagon-like flat-top focus, respectively. The flat-top areas, from top to bottom, are 0.14λ², 0.33λ², 0.31λ² and 0.35λ², respectively.

Download Full Size | PDF

5. Conclusion

In this paper, the generation of non-circular-symmetric equilateral-polygon-like flat-top focus is demonstrated. A general formula for focal field components of a tightly focused RPB superposed with off-axis vortex arrays possessing multi-location and multi-TC has been derived. The location of off-axis vortices with respect to the pupil center is critical to introduce significant focal intensity redistributions for the subsequent focus shaping. In addition, the characteristic field distributions in the focal region are found to be dependent on the NA of the objective lens, which can influence the optimized location of the off-axis vortices. By judiciously choosing the arrangement of the off-axis vortices, off-axis distance and NA of the objective lens, flat-top foci including bar-, triangle-, square-, pentagon- and hexagon-shape have been successfully demonstrated. With non-circular symmetric shapes and sub-wavelength focal area, the proposed method may find potential applications in laser fabrication with controllable-shape and shape-dependent materials [19].

Funding

National Natural Science Foundation of China (NSFC) (No. 61275133, 61522504).

References and links

1. Q. Zhan, “Cylindrical vector beams: from mathematical concepts to applications,” Adv. Opt. Photonics 1(1), 1–57 (2009).

2. R. Dorn, S. Quabis, and G. Leuchs, “Sharper focus for a radially polarized light beam,” Phys. Rev. Lett. 91(23), 233901 (2003). [PubMed]

3. X. Li, T. H. Lan, C. H. Tien, and M. Gu, “Three-dimensional orientation-unlimited polarization encryption by a single optically configured vectorial beam,” Nat. Commun. 3(8), 998 (2012). [PubMed]

4. S. Segawa, Y. Kozawa, and S. Sato, “Resolution enhancement of confocal microscopy by subtraction method with vector beams,” Opt. Lett. 39(11), 3118–3121 (2014). [PubMed]

5. L. Huang, H. Guo, J. Li, L. Ling, B. Feng, and Z. Y. Li, “Optical trapping of gold nanoparticles by cylindrical vector beam,” Opt. Lett. 37(10), 1694–1696 (2012). [PubMed]

6. Y. Kozawa and S. Sato, “Optical trapping of micrometer-sized dielectric particles by cylindrical vector beams,” Opt. Express 18(10), 10828–10833 (2010). [PubMed]

7. J. Xu, Z. J. Yang, J. X. Li, and W. P. Zang, “Electron acceleration by a tightly focused cylindrical vector Gaussian beam,” Laser Phys. Lett. 14(2), 025301 (2017).

8. V. Parigi, V. D’Ambrosio, C. Arnold, L. Marrucci, F. Sciarrino, and J. Laurat, “Storage and retrieval of vector beams of light in a multiple-degree-of-freedom quantum memory,” Nat. Commun. 6, 7706 (2015). [PubMed]

9. T. G. Jabbour and S. M. Kuebler, “Vectorial beam shaping,” Opt. Express 16(10), 7203–7213 (2008). [PubMed]

10. H. Wang, L. Shi, and B. Lukyanchuk, “Creation of a needle of longitudinally polarized light in vacuum using binary optics,” Nat. Photonics 2(6), 501–505 (2008).

11. Y. Zhao, Q. Zhan, Y. Zhang, and Y. P. Li, “Creation of a three-dimensional optical chain for controllable particle delivery,” Opt. Lett. 30(8), 848–850 (2005). [PubMed]

12. X. Wang, L. Gong, Z. Zhu, B. Gu, and Q. Zhan, “Creation of identical multiple focal spots with three-dimensional arbitrary shifting,” Opt. Express 25(15), 17737–17745 (2017). [PubMed]

13. Y. Yu and Q. Zhan, “Creation of identical multiple focal spots with prescribed axial distribution,” Sci. Rep. 5, 14673 (2015). [PubMed]

14. L. Z. Rao, J. X. Pu, Z. Y. Chen, and P. Yei, “Focus shaping of cylindrically polarized vortex beams by a high numerical aperture lens,” Opt. Laser Technol. 41(3), 241–246 (2009).

15. L. Wei and H. P. Urbach, “Shaping the focal field of radially/azimuthally polarized phase vortex with Zernike polynomials,” J. Opt. 18(6), 25–28 (2016).

16. F. Qin, K. Huang, J. Wu, J. Jiao, X. Luo, C. Qiu, and M. Hong, “Shaping a Subwavelength Needle with Ultra-long Focal Length by Focusing Azimuthally Polarized Light,” Sci. Rep. 5, 9977 (2015). [PubMed]

17. G. Indebetouw, “Optical Vortices and Their Propagation,” J. Mod. Opt. 40(1), 73–87 (1993).

18. B. Richards and E. Wolf, “Electromagnetic Diffraction in Optical Systems. II. Structure of the Image Field in an Aplanatic,” Proc. R. Soc. Lond. A Math. Phys. Sci. 253(1274), 358–379 (1959).

19. J. K. Gansel, M. Thiel, M. S. Rill, M. Decker, K. Bade, V. Saile, G. von Freymann, S. Linden, and M. Wegener, “Gold helix photonic metamaterial as broadband circular polarizer,” Science 325(5947), 1513–1515 (2009). [PubMed]

Generation of equilateral-polygon-like flat-top focus by tightly focusing radially polarized beams superposed with off-axis vortex arrays

Abstract

1. Introduction

2. Principle

3. Focal characteristics

3.1 Influence of the off-axis distance r₀ of vortices

3.2 Influence of numerical aperture of the objective lens

3.3 Energy flow

4. Generation of equilateral-polygon-like flat-top focus

5. Conclusion

Funding

References and links

Cited By

Figures (6)

Equations (14)

Optics Express

Abstract

1. Introduction

2. Principle

3. Focal characteristics

3.1 Influence of the off-axis distance r0 of vortices

3.2 Influence of numerical aperture of the objective lens

3.3 Energy flow

4. Generation of equilateral-polygon-like flat-top focus

5. Conclusion

Funding

References and links

Cited By

Figures (6)

Equations (14)

Optics Express

3.1 Influence of the off-axis distance r₀ of vortices