1Nanophotonics Research Centre, Institute of Microscale Optoelectronics & State Key Laboratory of Radio Frequency Heterogeneous Integration, Shenzhen University, Shenzhen 518060, China
【AIGC One Sentence Reading】:We use a hybrid multi-zone filter to split optical Stokes skyrmions symmetrically/asymmetrically, enhancing optical field manipulation.
【AIGC Short Abstract】:This study introduces a hybrid multi-zone filter to achieve Hall effect-like splitting of optical Stokes skyrmions, enabling their effective separation and manipulation. By adjusting phase gradient and topological charge parameters, symmetric and asymmetric splitting is achieved, enhancing optical field manipulation and suggesting potential in high precision measurements and quantum information preservation.
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Abstract
In recent years, optical skyrmions have garnered increasing attention for their ability to introduce new degrees of freedom in manipulating optical fields. While most research has focused on creating innovative optical topological states such as merons and hopfions, there has been limited exploration into their manipulation, which hinders practical applications in this field. In this study, we utilize a hybrid multi-zone filter to induce a Hall effect-like splitting of optical Stokes skyrmions (HESSs), enabling effective separation and manipulation. By manipulating the horizontal phase gradient parameter, we independently control the separation angle of skyrmions. Additionally, we demonstrate control over the topological charge parameter to achieve symmetric and asymmetric HESSs. This effect not only enhances the manipulation capabilities of optical fields but also opens up potential applications for high precision displacement measurements and preservation quantum information.
1. INTRODUCTION
Skyrmions, initially introduced by the physicist from Britain, Tony Skyrme, are quasiparticles in condensed matter physics with topological characteristics [1]. Typically found in magnetic materials, they can be envisioned as localized, vortex-like spin textures within the material. Skyrmions have attracted considerable interest due to their potential applications in high-density magnetic information storage, transmission, and spintronic devices [2–8]. Additionally, there is a growing field of research into optical skyrmions, which are linked to light manipulation and have implications for various optical technologies. Recent advances in topological optics have spurred rapid growth in the field of optical skyrmions, providing new perspectives and techniques for achieving topological characteristics in structured light, space-time light, and light–matter interaction [9–12]. The applications encompass spin optics, imaging and metrology, optical power, structured light, topology, and quantum technologies [13–15]. However, the control of various topologies of optical skyrmions remains a nascent and evolving area of study.
The concept of optical skyrmions initially emerged in the context of electric fields of evanescent waves. In 2018, Tsesses et al. expanded this concept to electromagnetic fields through the utilization of hexagonal resonant cavity interference [16]. Du et al. demonstrated the generation of spin vector optical Neel skyrmions by harnessing the spin-orbit coupling of a tightly focused vector beam to create evanescent waves on metal surfaces [17]. Subsequent reports [18–20] have described mapping and manipulation efforts related to plasmonic skyrmions. Additionally, various techniques, including Dirac monopoles in momentum space [21], microcavities [22], pseudospins in 3D nonlinear photonic crystals [23], Raman transitions [24], and electric or magnetic field vectors in spatiotemporal optical pulses [25], have been employed to construct optical skyrmions. Analogous quasiparticles were later observed to exist in the momentum space of liquid crystal-filled microcavities [26], as well as in evanescent optical vortex lattices governed by symmetry [27], photonic crystals [28], and SPPs in an Archimedean coupling structure [29]. These quasiparticles are also known as merons or half-skyrmions. However, it should be noted that when these skyrmions propagate, they are unable to maintain topological stability.
One distinct characteristic of Stokes skyrmions is their topological stability throughout propagation. In homogeneous media, the vector texture can transition from Bloch type to Néel type and vice versa while maintaining the skyrmion number [30]. Conversely, the vector field texture remains constant throughout propagation when the texture is anti-skyrmion [31]. Specifically, adjustable Stokes skyrmions with Néel, Bloch, and anti-types textures can be created using the digital holographic approach [32]. Theoretically, it is possible to generate Stokes skyrmions from a microlaser when the skyrmion levels are controlled by the microresonator grating [22]. Optical Stokes skyrmions in the Bessel profile have the non-diffracting property [33]. The topological method is used to calculate Stokes skyrmion numbers that avoid products of polarization gradients and significantly increase precision and accuracy [34]. The direct imprint of the topologically protected polarization textures of optical Stokes skyrmions onto a material is demonstrated in the experiment [35]. Non-local quantum Stokes skyrmions are observed in two entangled photons with engineered quantum states [36]. However, the skyrmion is immobile transversely. The hydrodynamic Magnus force acting on the skyrmionic topological charge also induces a transverse skyrmion deflection, commonly known as the skyrmion Hall effect (SkHE) [5,37]. If the repulsive force produced at the edge is sufficiently strong, the skyrmion will smoothly travel along the edge. However, it can be destroyed if the current is strong enough to push the skyrmion close to the edge. Therefore, precise control over SkHE is essential before skyrmions can be extensively utilized in spintronic applications. One proposed approach to address this issue is the use of artificial antiferromagnets, or antiferromagnets, in which two linked skyrmions with opposing charges have a net topological number of zero [38,39]. However, these methods are limited to manipulating the zero topological number, making it difficult to effectively alter the SkHE. One potential method of accurately controlling the SkHE is using temperature variations to regulate the angular momentum of ferrimagnets [40]. Meanwhile, the optical skyrmion Hall effect, which can precisely modify optical SkHE and has potential applications in logic, synaptic computing, and analog memory, has received less attention. The symmetric and asymmetric manipulation of skyrmions offers an extra degree of freedom for their generation and control, thereby paving the way for the development of novel skyrmion-controlled nanophotonic devices.
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In this study, we utilized a hybrid multi-zone filter to achieve Hall effect-like splitting of optical Stokes skyrmions (HESSs), enabling effective separation and manipulation of these quasiparticles. We employed azimuthally polarized light with uniform amplitude and phase as the incident vector beam. The incident phase was shaped using two multi-zone filters and two spiral phase plates with opposite topological charges. By adjusting the horizontal parameters, we controlled the skyrmion’s position, inducing symmetric or asymmetric HESSs. Through simulation and experimental validation, we successfully demonstrated both symmetric and asymmetric skyrmion Hall-like splitting. This work highlights the emergence of complex vector field structures representing optical quasiparticles with diverse topological characteristics. These advancements bear significant implications across various fields, including modern spin optics, structured light, and topological and quantum technologies.
2. DESIGN AND THEORY
Figure 1 illustrates the generation of the Hall effect-like splitting of optical Stokes skyrmions. After modulation by multi-ring phase, azimuthally polarized light generates two split high-order skyrmions. In fact, azimuthally polarized light can be seen as a superposition of azimuthally polarized light with a topological charge of and azimuthally polarized light with a topological charge of . These two optical fields respectively generate high-order skyrmions with skyrmion numbers of and . Therefore, the incident optical field can be considered as two overlapping high-order skyrmions with opposite skyrmion numbers. The multi-ring phase is composed of two types of multi-zone filters, which carry helical phases with topological charges of and , respectively. If these two ring phases are coaxial, they will lead to the annihilation of skyrmions with opposite skyrmion numbers. To prevent the disappearance of skyrmions, we superimpose different gratings onto the two types of multi-ring phases, causing the skyrmions to be symmetrically or asymmetrically deflected to different positions. This enables the coexistence of the two types of skyrmions. Therefore, the incident optical field, which contains two overlapping skyrmions with opposite skyrmion numbers, will lead to the splitting of skyrmions after passing through the ring phase. We refer to this phenomenon as the HESS.
The tightly focused incident light field is azimuthally polarized light with uniform amplitude and phase . is based on two multi-zone filters (MZFs), two spiral phase plates with reversed topological charges, and two translation factors, which can be calculated by Eq. (1). Transmissions of MZF1, MZF2, and circular filter (CF) are calculated by Eqs. (2)–(4), respectively: where the parameters and correspond to the topological charge of the spiral phase plate and the horizontal adjusted parameter representing the multi-zone width in the MZF1 region of the translation factor, respectively. , is the horizontal coordinate, and is the focal length of the high-NA objective lens and set as 2 mm. The parameters and correspond to the topological charge of the spiral phase plate and the horizontal adjusted parameter representing the multi-zone width in the MZF2 region of the translation factor, respectively. is the radius of the phase region. represents the serial number of the transmission area in the MZF. represents the number of transparent zones in the MZF. In this paper, , , and are set as 1, , and 4, respectively. adjusts the width of the transparent zone to adjust the intensities of left and right tight focused foci. To achieve two equally intense tightly focused foci, we employ the intensity non-uniformity of the polarized light components as the evaluation criterion. Here, represents the intensity peak of the th - polarized light component, and is the average intensity of the left and right -polarized light components. Through simulations, we find that when is set to 0.568, the two horizontal polarized light components exhibit equal intensities. This indirectly confirms the balanced strength of the two tightly focused focal points. Furthermore, in this paper, we set the following parameter values: , , , and .
Based on the Richards–Wolf formulas, the calculation expression of the electric and magnetic fields in close proximity to the focus is as follows:
The variables are denoted as follows: represents the electrical or magnetic field at the plane, and shows the polar coordinates at this focusing plane; signifies the focal length; represents the wavelength; is determined by the numerical aperture of the lens (); in Eq. (5) signifies the amplitude of the input field, which is the Gaussian beam in the paper; and represent the polar angle and azimuthal angle at the incident plane, respectively; denotes the apodization function; signifies the polarization matrix for the electric and magnetic fields; and represents the wavenumber: where and represent the - and -components of the incident beam, respectively.
The incident light field is an th order azimuthally polarized vortex Gaussian beam with a topological charge ,
The , , and components , , and of the electric field in the tightly focused field are as follows:
The Stokes parameter is calculated by the difference in intensity between left and right circularly polarized electric fields and : where
In Eq. (10), , is the polar radius, is the th order Bessel function, and . When (, ), there is . Since and , for , . When (, ), there is . Since and , for , . Therefore, in these two cases, the value of is opposite at the center point when . Since the direction of a single vector in skyrmion is determined by the Stokes parameters , , and , at the center point where and are both 0, the vector direction of the skyrmion is only determined by . Therefore, the skyrmions in these two cases are antiparallel, resulting in opposite skyrmion numbers. If the centers of the two types of skyrmions coincide, they would annihilate. To generate the HESS, we achieve the coexistence of the two types of skyrmions by superposing different offset gratings to shift the centers of the skyrmions away from the focal plane center to different positions.
3. RESULTS AND DISCUSSION
We will demonstrate the symmetric HESS. and are assigned values of 9 and , respectively, to generate the symmetric skyrmions. The experimental setup is presented in Appendix A. Figures 2(a1)–2(e1) present the related information of the left sections depicted in Figs. 5(a1)–5(f1) (see Appendix B). It should be noted that Appendix C offers magnified views of the intensity profiles depicted in Appendix B. Figures 2(a1)–2(c1) show the numerical Stokes parameters , , and , respectively. The reconstruction of the optical skyrmions was achieved through Stokes polarimetry, more specifically, through the reconstruction of the Stokes parameters , , , and , which were computed from a set of intensity measurements. A CMOS camera allowed measurement of the required intensities, from which the Stokes parameters were reconstructed using the relations , , and . Figure 2(d1) presents the 3D vector field, and Fig. 2(e1) presents the 2D top view. All arrows are represented with the color-labeled component. The topological properties of the skyrmion texture can be characterized by the skyrmion number , with where describes the normalized Stokes vector field, and the integral is taken over the region where it is confined. For a skyrmion texture, is an integer that describes the number of times the vector wraps around a unit sphere. It can be found that when the radius is , reaches the maximum value of 1.99267 close to 2. This matches the same radius of the dashed circles in Figs. 2(a1)–2(c1) and 2(e1). Thus, the left skyrmion is designated as the higher-order skyrmion with the positive skyrmion number. Figures 2(a2)–2(e2) present the related information of the left sections depicted in Figs. 5(a2)–5(f2). Figures 2(a2)–2(c2) show the corresponding experimental Stokes parameters , , and , respectively. Figure 2(d2) presents the 3D vector field of the experimental Stokes skyrmion, and Fig. 2(e2) presents the corresponding 2D top view. It can be found that when the radius is μ, the measured reaches the maximum value of 1.91905 close to 2. This matches the same radius of the dashed circles in Figs. 2(a2)–2(c2) and 2(e2). Thus, we prove in the experiment that the left skyrmion is designated as the higher-order skyrmion with the positive skyrmion number. Figures 2(a3)–2(e3) present the related information of the right sections depicted in Figs. 5(a1)–5(f1). Figures 2(a3)–2(c3) show the corresponding numerical Stokes parameters , , and , respectively. Figure 2(d3) presents the 3D vector field of the numerical Stokes skyrmion, and Fig. 2(e3) presents the corresponding 2D top view. When the radius is , the measured reaches the minimum value of close to . This matches the same radius of the dashed circles in Figs. 2(a3)–2(c3) and 2(e3). Thus, we prove in the simulation that the right skyrmion is designated as the higher-order skyrmion with the negative skyrmion number. Figures 2(a4)–2(e4) present the related information of the right sections depicted in Figs. 5(a2)–5(f2). Figures 2(a4)–2(c4) show the corresponding experimental Stokes parameters , , and , respectively. Figure 2(d4) presents the 3D vector field of the experimental Stokes skyrmion, and Fig. 2(e4) presents the corresponding 2D top view. It can be found that when the radius is μ, the measured reaches the minimum value of close to . This matches the same radius of the dashed circles in Figs. 2(a4)–2(c4) and 2(e4). Thus, we prove in the experiment that the right skyrmion is designated as the higher-order skyrmion with the negative skyrmion number. Therefore, both our experiments and simulations have shown that the left and right symmetric skyrmions have opposite skyrmion numbers, thereby forming the symmetric HESS.
Figure 2.Symmetric HESS. (a1)–(c1) Numerical distributions of , , and , respectively, for the left sections depicted in Figs. 5(a1)–5(f1). (d1) 3D vector field. (e1) 2D top view of the skyrmion. (a2)–(e2) Corresponding experimental results. (a3)–(e3) Numerical results for the right sections depicted in Figs. 5(a1)–5(f1). (a4)–(e4) Corresponding experimental results.
Figure 3.Asymmetric HESS. (a1)–(c1) Numerical distributions of , , and , respectively, for the left sections depicted in Figs. 6(a1)–6(f1). (d1) 3D vector field. (e1) 2D top view of the skyrmion. (a2)–(e2) Corresponding experimental results. (a3)–(e3) Numerical results for the right sections depicted in Figs. 6(a1)–6(f1). (a4)–(e4) Corresponding experimental results.
Figure 5.Symmetric splitting of intensity distributions. (a1)–(f1) Numerical and (a2)–(f2) captured profiles of the IXP, IYP, IL+45 P, IL-45 P, IRCP, and ILCP, respectively.
Next, we will demonstrate the asymmetric HESS. and are assigned values of 9 and , respectively, to generate the asymmetric skyrmions. Figures 3(a1)–3(e1) present the related information of the left sections depicted in Figs. 6(a1)–6(f1) (see Appendix B). Figures 3(a1)–3(c1) show the numerical Stokes parameters , , and , respectively. The reconstruction of the optical skyrmions was achieved through Stokes polarimetry, more specifically, through the reconstruction of the Stokes parameters , , , and , which were computed from a set of intensity measurements. Figure 3(d1) presents the 3D vector field, and Fig. 3(e1) presents the 2D top view. All arrows are represented with the color-labeled component. It can be found that when the radius is , reaches the maximum value of 1.99334 close to 2. This matches the same radius of the dashed circles in Figs. 3(a1)–3(c1) and 3(e1). Thus, the left skyrmion is designated as the higher-order skyrmion with the positive skyrmion number. Figures 3(a2)–3(e2) present the related information of the left sections depicted in Figs. 6(a2)–6(f2). Figures 3(a2)–3(c2) show the corresponding experimental Stokes parameters , , and , respectively. Figure 3(d2) presents the 3D vector field of the experimental Stokes skyrmion, and Fig. 3(e2) presents the corresponding 2D top view. It can be found that when the radius is μ, the measured reaches the maximum value of 1.92691 close to 2. This matches the same radius of the dashed circles in Figs. 3(a2)–3(c2) and 3(e2). Thus, we prove in the experiment that the left skyrmion is designated as the higher-order skyrmion with the positive skyrmion number. Figures 3(a3)–3(e3) present the related information of the right sections depicted in Figs. 6(a1)–6(f1). Figures 3(a3)–3(c3) show the corresponding numerical Stokes parameters , , and , respectively. Figure 3(d3) presents the 3D vector field of the numerical Stokes skyrmion, and Fig. 3(e3) presents the corresponding 2D top view. When the radius is , the measured reaches the minimum value of close to . This matches the same radius of the dashed circles in Figs. 3(a3)–3(c3) and 3(e3). Thus, we prove in the simulation that the right skyrmion is designated as the higher-order skyrmion with the negative skyrmion number. Figures 3(a4)–3(e4) present the related information of the right sections depicted in Figs. 6(a2)–6(f2). Figures 3(a4)–3(c4) show the corresponding experimental Stokes parameters , , and , respectively. Figure 3(d4) presents the 3D vector field of the experimental Stokes skyrmion, and Fig. 6(e4) presents the corresponding 2D top view. It can be found that when the radius is μ, the measured reaches the minimum value of close to . This matches the same radius of the dashed circles in Figs. 3(a4)–3(c4) and 3(e4). Thus, we prove in the experiment that the right skyrmion is designated as the higher-order skyrmion with the negative skyrmion number. Therefore, both our experiments and simulations have shown that the left and right asymmetric skyrmions have opposite skyrmion numbers, thereby forming the asymmetric HESS.
Figure 6.Asymmetric splitting of intensity distributions. (a1)–(f1) Numerical and (a2)–(f2) captured profiles of the IXP, IYP, IL+45 P, IL-45 P, IRCP, and ILCP, respectively.
Appendix D contains information about the polarization and Stokes vector distributions before and after splitting. Appendix E presents the polarization ellipses distribution for each beam spot shown in Figs. 2 and 3. Two tilted phase profiles with and create two distinct separation channels for the incident beam. The topological charges of the spiral phases in the two channels are and , respectively. Conventional azimuthally polarized light can be considered as a superposition of azimuthally polarized light with topological charges of and . Hence, azimuthally polarized light with topological charges of and can generate high-order skyrmions with skyrmion numbers of and , respectively. For the separation channel with a spiral phase of topological charge , when azimuthally polarized vortex beams carrying topological charges of and pass through this channel, the topological charge of the azimuthally polarized vortex beam with charge will be neutralized to 0. As a result, after passing through the spiral phase with a topological charge of , the azimuthally polarized vortex beams with topological charges of and are transformed into azimuthally polarized vortex beams with topological charges of and 0. Although the azimuthally polarized vortex beam with a topological charge of 0 can be seen as a superposition of two high-order skyrmions with opposite skyrmion numbers, when these two skyrmions are not separated, the skyrmions with opposite skyrmion numbers will cancel each other out, resulting in no visible skyrmion structure overall. Therefore, the azimuthally polarized vortex beam with topological charges of and 0 will only produce high-order skyrmions with skyrmion numbers of . In summary, high-order skyrmions with skyrmion numbers of and , after passing through the spiral phase with a topological charge of , will yield high-order skyrmions with skyrmion numbers of . This means that the spiral phase with a topological charge of will filter out high-order skyrmions with skyrmion numbers of . Similarly, the spiral phase with a topological charge of will filter out high-order skyrmions with skyrmion numbers of . Multi-zone filters are used to adjust the intensity of the Stokes parameters that construct the two types of high-order skyrmions.
4. CONCLUSIONS
In conclusion, we have introduced a technique for Hall effect-like splitting of optical Stokes skyrmions within a tightly focused system. The vector beam utilized in our approach consists of azimuthally polarized light. By integrating two multi-zone filters and two spiral phase plates, each with reversed topological charges, we successfully created a hybrid multi-zone filter capable of realizing Hall effect-like splitting of two optical Stokes skyrmions with opposite skyrmion numbers. Adjusting the horizontal parameter enabled precise manipulation of the skyrmion positions. Our simulations and experiments confirmed both symmetric and asymmetric manifestations of Hall effect-like splitting of optical Stokes skyrmions. Unlike previous studies on optical skyrmions, we are the first, to the best of our knowledge, to discover that azimuthally polarized light can carry both positive and negative skyrmions. Furthermore, we have achieved the symmetric and asymmetric separation of these skyrmions using multi-zone filters for the first time. Asymmetric separation, in particular, enables the arbitrary manipulation of positive and negative skyrmions, offering an additional degree of freedom for their generation and control. This advancement opens the door to the development of novel skyrmion-controlled nanophotonic devices. The emergence of complex vector field structures, characterizing optical quasiparticles with diverse topological characteristics, underscores the significance of our approach across various applications including modern spin optics, imaging, structured light, and topological and quantum technologies.
APPENDIX A: EXPERIMENTAL SETUP
A laser beam with a wavelength of 633 nm is expanded and collimated through a spatial filter comprising an objective lens, a pinhole, and a lens (Fig. 4). This setup is designed to enhance the beam size and produce a pure Gaussian beam. The resultant Gaussian beam is directed through a beam splitter (BS) onto a spatial light modulator (SLM), specifically the UPOLabs-HDSLM80R Pro model with a resolution of and a pixel pitch of 8 μm. On the SLM, a phase map of a multi-zone filter is loaded. The light, after being modulated by the SLM, is reflected back by the SLM and then redirected by the BS. Since the SLM only modulates the phase of linearly polarized light in a specific direction, a polarizer is employed to filter out the phase-modulated linearly polarized light in that particular direction. This filtered light then passes through a -plate with a value of , converting it into azimuthally polarized light. The azimuthally polarized light is reflected by a mirror and subsequently passes through a beam-reducing system consisting of two lenses and a filter, which removes the first-order diffracted light field. The refined light field is then tightly focused using a high numerical aperture objective to generate the desired light field at the focal plane. To capture the light field at the focal plane, an imaging objective is utilized. The polarization state of the light field is adjusted using a quarter-wave plate (QWP) and a polarizer. Finally, the adjusted polarized light field is captured by a CMOS camera, specifically the FLIR GS3-U3-123S6C-C model with a resolution of and a pixel pitch of 3.45 μm.
APPENDIX B: INTENSITY PROFILES IN THE SIMULATIONS AND EXPERIMENTS UNDER DIFFERENT POLARIZATION STATES FOR THE SYMMETRIC AND ASYMMETRIC HESSs
In this paper, and are assigned values of 9 and , respectively, to generate the symmetric skyrmions. In Fig. 5, , , , , , and represent the intensities of the , , linearly , linearly , and right circularly and left circularly polarized light components. To experimentally measure , , , and we passed the optical field through a linear polarizer set at 0°, 90°, 45°, and , respectively, while the intensity of the or polarization components was acquired by passing it simultaneously through a QWP at 45° and a linear polarizer at 0° or 90°, respectively. Figures 5(a1)–5(f1) show the numerical intensity distributions of the above polarized light intensities, respectively. Figures 5(a2)–5(f2) present the corresponding captured intensity profiles. It can be found that the captured and numerical intensity distributions are the same approximately. Upon careful comparison with the actual numerical image, we have found that the experimentally captured image is magnified by approximately 427 times. Unfortunately, the selected areas in the experimental images shown in Figs. 5 and 6 are somewhat limited in size, causing the experimental intensity images to appear marginally larger than the simulated intensity images.
In this paper, and are assigned values of 9 and , respectively, to generate the asymmetric skyrmions. Figures 6(a1)–6(f1) show the numerical intensity distributions of , , , , , and , respectively. Figures 6(a2)–6(f2) present the corresponding captured intensity profiles. It can be found that the captured and numerical intensity distributions are the same approximately.
APPENDIX C: MAGNIFIED VIEWS OF INTENSITY PROFILES IN APPENDIX B
Figures 7(a1)–7(f1) offer magnified views of the left sections depicted in Figs. 5(a1)–5(f1), respectively. Figures 7(a2)–7(f2) present magnified views of the left sections depicted in Figs. 5(a2)–5(f2), respectively. Figures 7(a3)–7(f3) offer magnified views of the right sections depicted in Figs. 5(a1)–5(f1), respectively. Figures 7(a4)–7(f4) present magnified views of the right sections depicted in Figs. 5(a2)–5(f2), respectively.
Figure 7.Magnified images of symmetric splitting in intensity distributions. (a1)–(f1) Numerical profiles of the left sections depicted in Figs. 5(a1)–5(f1), respectively. (a2)–(f2) Corresponding experimental results. (a3)–(f3) Numerical results for the right sections depicted in Figs. 5(a1)–5(f1). (a4)–(f4) Corresponding experimental results.
Figure 8.Magnified images of asymmetric splitting in intensity distributions. (a1)–(f1) Numerical profiles of the left sections depicted in Figs. 6(a1)–6(f1), respectively. (a2)–(f2) Corresponding experimental results. (a3)–(f3) Numerical results for the right sections depicted in Figs. 6(a1)–6(f1). (a4)–(f4) Corresponding experimental results.
APPENDIX D: POLARIZATION AND STOKES VECTOR DISTRIBUTIONS BEFORE AND AFTER SPLITTING
Figures 9(a1)–9(a4) and Figs. 9(b1)–9(b4) illustrate the Stokes vectors and polarization distributions of azimuthally polarized light with topological charges of and , respectively, after propagating for 200 mm before splitting. Figures 9(a3) and 9(b3) illustrate the distribution of for azimuthally polarized light with topological charges of and , respectively. When these two distributions are superimposed, the result is zero, which reflects the distribution of azimuthally polarized light. Figures 9(c1)–9(c4) and Figs. 9(d1)–9(d4) depict the Stokes vectors and polarization distributions following symmetric and asymmetric splitting. The explanation of azimuthally polarized (AP) light as a superposition of two azimuthally polarized lights with opposite spiral phases (SPs) could indeed be clearer. Upon examining Eq. (D1), it becomes evident that azimuthally polarized light can be interpreted as the superposition of phases in phase space for azimuthally polarized waves possessing topological charges of and :
Figure 9.(a1)–(a4) and (b1)–(b4) Stokes vectors and polarization distribution of azimuthally polarized light with a topological charge of and after propagating 200 mm before splitting, respectively. (c1)–(c4) and (d1)–(d4) Stokes vectors and polarization distribution after symmetric and asymmetric splitting, respectively.
In Eq. (D2), the radial and azimuthal index values for the LG modes and are (0, 2) and (0, 0), respectively. In Eq. (D3), the radial and azimuthal index values for the LG modes and are (0, 0) and (0, ), respectively. As demonstrated in Eq. (D2), azimuthally polarized light with a topological charge of can be expressed as a superposition of a left-handed circularly polarized vortex beam with a topological charge of 2 and a right-handed circularly polarized plane wave with a topological charge of 0. Notably, the amplitude functions for all azimuthally polarized light discussed in this study are Gaussian functions. Upon diffraction, the left-handed circularly polarized vortex Gaussian beam with a topological charge of 2 is transformed into a second-order Laguerre-Gaussian mode, while the right-handed circularly polarized Gaussian beam with a topological charge of 0 becomes a zeroth-order Laguerre–Gaussian mode. Consequently, azimuthally polarized light with a topological charge of can be regarded as the superposition of the left-handed second-order Laguerre–Gaussian mode and the right-handed zeroth-order Laguerre–Gaussian mode. The combination of these two circularly polarized components produces a radially varying polarization distribution that transitions from left-handed circular polarization (LCP) or right-handed circular polarization (RCP) to linear polarization (LP) and back to RCP or LCP. The corresponding Stokes vector exhibits an orientation that is directed upward or downward at the center and gradually shifts to downward or upward at the boundary, resulting in a Stokes skyrmion [32]. Since the Stokes vector undergoes azimuthal variations with a periodicity of , the corresponding skyrmion number is consequently either or 2. As shown in Eq. (D3), the case is analogous for azimuthally polarized light with a topological charge of . When compared to the paraxial approximation, the non-paraxial approximation introduces longitudinal components. However, it is important to note that the Stokes vector is solely dependent on the transverse components and remains unaffected by the longitudinal components. Consequently, under both the non-paraxial and paraxial approximations, there is no significant difference observed in the Stokes vector. As such, even under the non-paraxial approximation, an azimuthally polarized vortex beam has the capability to generate a high-order Stokes skyrmion. Our simulations have revealed an intriguing result: when the absolute value of the topological charge of the vortex beam is equivalent to the order of the azimuthally polarized beam, the resulting skyrmion number is precisely twice the topological charge. Radially distributed rings after splitting are unrelated to the LG mode; instead, they are caused by diffraction from the alternating boundary rings in the multi-zone filters. Therefore, these rings are not the radial rings produced by the LG mode. The resulting optical topological structure within the given boundary is a skyrmion, not a skyrmionium.
APPENDIX E: POLARIZATION ELLIPSES DISTRIBUTION FOR EACH BEAM SPOT IN FIGS.?2 AND 3
Figures 10(a) and 10(b) show the numerical and experimental polarization ellipses distribution calculated based on the Stokes parameters in Figs. 2(a1)–2(c1) and Figs. 2(a2)–2(c2), respectively. Figures 10(c) and 10(d) show the numerical and experimental polarization ellipses distribution calculated based on the Stokes parameters in Figs. 2(a3)–2(c3) and Figs. 2(a4)–2(c4), respectively. Figures 11(a) and 11(b) show the numerical and experimental polarization ellipses distribution calculated based on the Stokes parameters in Figs. 3(a1)–3(c1) and Figs. 3(a2)–3(c2), respectively. Figures 11(c) and 11(d) show the numerical and experimental polarization ellipses distribution calculated based on the Stokes parameters in Figs. 3(a3)–3(c3) and 3(a4)–3(c4), respectively. In Figs. 10 and 11, outside the single skyrmion polarization evolution region (denoted by the white dashed circle), we observe that the opposite-handed circular polarization transitions to elliptical polarization. Notably, the location of the opposite-handed circular polarization corresponds closely with the dashed circular region in the Stokes parameter distributions shown in Figs. 2 and 3. Therefore, we calculate the skyrmion number within the dashed circular region outlined in Figs. 2 and 3. The calculated skyrmion number within the defined boundary aligns well with the simulation results.
Figure 10.(a)–(d) Polarization ellipses distribution calculated based on the Stokes parameters in Figs. 2(a1)–2(c1), 2(a2)–2(c2), 2(a3)–2(c3), and 2(a4)–2(c4), respectively.
Figure 11.(a)–(d) Polarization ellipses distribution calculated based on the Stokes parameters in Figs. 3(a1)–3(c1), 3(a2)–3(c2), 3(a3)–3(c3), and 3(a4)–3(c4), respectively.