Abstract
In this paper, the polycyclic tornado circular swallowtail beam (PTCSB) with autofocusing and self-healing properties is generated numerically and experimentally and their properties are investigated. Compared with the circular swallowtail beam (CSB), the optical distribution of the PTCSB presents a tornado pattern during the propagation. The number of spiral stripes, as well as the orientation of the rotation, can be adjusted by the number and the sign of the topological charge. The Poynting vectors and the orbital angular momentum are employed to investigate the physical mechanism of beam-rotating. In addition, we also introduce a sector-shaped opaque obstacle to investigate the self-healing property of the PTCSB, passing through it with different center angles and discuss the influence of the scaling factor along the propagation direction. Our results may expand the potential applications in the optical spanner and material processing.
© 2022 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
Caustic beams as the concrete manifestation of the catastrophe theory in light phenomenon are the envelopes of families of rays, which tend to a focus spot, curves or surfaces [1]. The most fundamental optical catastrophe [2] is the fold catastrophe characterized by a cubic power in the spectral phase, which can form the phase space by choosing the state and control parameters. Its caustic field was defined by the Airy function, and it was experimentally demonstrated in 2007 [3]. Subsequent to the Airy beams, the Pearcey beam represented by the cusp catastrophe was observed in 2012 [4], which is generated by a quartic power in phase space, and corresponding to the caustic field is described by the Pearcey function. In 2017, $Zannotti$ et al. [5,6] systematically investigated the swallowtail and butterfly catastrophe corresponding to the swallowtail and butterfly beams in the light field. Clearly, each order of the catastrophe forms a specific stable diffraction structure. With the increasing dimensionality of the control parameters, their diffraction structures are given by fold, cusp, swallowtail, hyperbolic umbilic, elliptic umbilic, butterfly, and parabolic umbilic catastrophes [7].
On the other hand, there are some remarkable properties among the great variety of caustic beams. The well-known properties of these caustic beams are autofocusing and self-healing, which have been studied theoretically and demonstrated experimentally [8–11]. In addition, the vortex beams have been drawn considerable attention in recent decades due to their many applications in various fields [12–14]. The optical vortices (OVs) embedded in a light beam are associated with doughnut-like intensity patterns and have phase singularity in the core [15]. They not only have a homogeneous polarized distribution but also carry the orbital angular momentum (OAM) determined by the topological charge [16]. Very recently, people have investigated the propagation dynamics of the vortices imprinted in various background beams. For example, Airy vortex beams [17], Pearcey vortex Gaussian beams [18], Laguerre Gaussian beams [19], partially coherent beams [20], and Bessel vortex beams [21]. The structured beams with OAM have further paved the way toward novel opportunities for scientific research and advanced applications. For example, optical communication [22], high-resolution microscopy [23], and particle manipulation [24]. However, up to now, most of these researches only focus on the properties of the lower-order catastrophe such as Airy and Pearcy beams, which are the representatives of one-dimensional(1D) and two-dimensional (2D), respectively. To the best of our knowledge, the properties of the high-order catastrophe vortex beams have not been explored yet.
The rotation of the structured light has recently attracted lots of attention with the various beams. Zamboni-Rached [25] proposed a new class of nondiffracting beams, namely Frozen beams, which are suitable superpositions of co-propagating Bessel beams, and experimentally demonstrated in 2012 [26]. The evolution of the twisted light fields of Frozen waves has the possibility to enhance the capacity in communication systems [22]. They also used the Frozen waves to trap microparticles through weakly dispersive media over relatively small distances [27]. In contradistinction to Frozen waves, the caustic beams with phase manipulation can increase intensity at focal plane and the distance of the optical trapping of microparticles. The other cases of rotating field, including higher-order Bessel beams [28,29], Laguerre-Gaussian beams [30] and ring Airy beams [31], are performed by superposition of two same co-propagation beams. Besides, Wu $et$ $al$. [32] only numerically calculated the ring-Airy beam with the spiral phase, but they do not experimentally demonstrate them and investigate the self-healing of the beams.
In this paper, we develop a simple and convenient method to generate the chiral optical field of the high-order catastrophe. We discuss the propagation dynamics of the polycyclic tornado circular swallowtail beam (PTCSB) in detail. We numerically and experimentally demonstrate the autofocusing and self-healing of the PTCSB from different perspectives, such as the intensity transverse profile, the peak intensity, the Poynting vector, and the OAM. Last, we obtain a conclusion, simply summarizing the content and listing some potential applications.
2. Theory
The caustic field $C_{n}(a)$ can be defined by catastrophe theory in terms of the standard diffraction integral, which can be defined [33,34]
where $p_{n}(a, s)$ can completely determine the properties of the caustic field as the canonical potential function, as the control parameter can change the potential completely, and it can be defined by where the control parameters $a=\left (a_{1}, a_{2}, \ldots, a_{j}\right )$ include all dimensionless control parameters $a_{j}$ with $j=1,2, \ldots, n-2$. When we change the dimensionality of the control parameters, the caustic field possibly becomes a point, curved line, curved surface, or hypersurface.For the high-order swallowtail catastrophe $C_{5}(a_{1}, a_{2}, a_{3})$, according to the swallowtail catastrophe integral, the swallowtail beam ${Sw}(a_{1}, a_{2}, a_{3})$ can be represented [6]
Under the paraxial optical system, the spatial beam propagating along the $z$-axis can be represented by (2+1) dimensional potential-free Schrodinger equation in free space, which can be described in the form [35]
Figure 1 illustrates the generation process of the PTCSB phase. The ASZP is formed by superposing two-layer chiral subphases (SP1 and SP2). It is clear that the subphases are constituted by three parts consisting of the radial phase, the equiphase and the spiral phase, respectively. The case of superposing the annular spiral phase is similar to produce the interference between vortex beam and plane wave [38]. In addition, by changing the radial shift factors $\alpha$ and $\beta$ to offset radially the optical vortex, the interference with the plane wave can be extended [39]. Finally, the phase patterns of the PTCSB are generated by superposing ASZP and the phase of CSB, as illustrated in Figs. 1(b1)–1(b3).
Now we discuss the propagation properties of the PTCSB by substituting Eq.(4) into the diffraction integral formula assuming paraxial propagation [40]
3. Results and Discussions
The experimental realization of the PTCSB is implemented through a computer-generated hologram reproduced by the reflective spatial light modulator (rSLM, 1900 1200 pixels ), as schematically presented in Fig. 2. A linearly polarized Gaussian beam at 532 nm is emitted from the He-Ne laser. When the beam passes through the beam expander, the beam is collimated and expanded, and then reflected by a mirror. Then, by imprinting a phase mask containing the formation of the proposed PTCSB on a computer-controlled rSLM, the expanded beam can be constructed that utilizes interference between the plane wave and the swallowtail beam. For the purpose of selecting the desired beams, a 4f system consisting of two thin lenses ($L_{1}$ with its focal length $f_{1}$=300 mm and $L_{2}$ with its focal length $f_{2}$=500 mm) with a circular aperture (CA) is established. Besides, the function that is used to produce the pattern of interference fringes on the rSLM can be adjusted by the number and the width of the interference fringes to make the modulated beam clearly. The positive first-order fringes from the rSLM can be regarded as our desired PTCSBs. This method can efficiently and feasibly reproduce various optical beams compared with conversational methods [42]. Last, we employ the beam quality analyzer (BQA) to measure the intensity distributions at different propagation distances.
Fristly, we display a typical simulation results in Fig. 5. In our simulation, the ASZP with topological charge $\ell _{1}=0$ is superposed on CSB, and multioptical bottles marked in the white oval in Fig. 5(a) are formed. The PTCSB inherits the autofocusing property from the CSB, but it does not exhibit the rotating characteristic during the propagation. We define the intensity contrast values to clearly claim the intensity change, which are described by $I_{m}/I_{0}$, where $I_{m}$ is the maximum intensity at an arbitrary plane during the propagation and $I_{0}$ is the maximum intensity of the PTCSB at the initial plane. We can find that the intensity contrast values remain almost constant before reaching the autofocusing plane, then abruptly increase when the PTCSB reaches the autofocusing plane. Finally, the intensity contrast values decrease with a slightly oscillation because of the interference between the main ring and the sub-ring.
Figure 4 depicts the propagation characteristics of the PTCSB in free space. Obviously, the PTCSB has a circularly symmetric input profile that consists of a dark disk and a series of concentric intensity rings with decreased oscillating intensity, which is similar to the input profile of the CSB. From Figs. 4(c1)–4(c2), it can be clearly found that the optical field can rotate continuously and becomes a tornado pattern automatically. As the propagation distance increases, the tornado pattern becomes firstly smaller, and then becomes larger. In addition, we find that the rotation direction of the optical field occurs to change after the autofocusing plane, and the rotation direction of the optical field is clockwise in Fig. 4(c2) marked by the white curved arrows, which is clearly displayed the direction-rotating. However, the optical field begins to rotate counterclockwise after this position, shown in Figs. 4(c3)–4(c4). This phenomenon is similar to that reported in Ref. [4] because of the inversion of the swallowtail function at the autofocusing plane.
On the other hand, the PTCSB forms a hollow core surrounded by the tornado shape with a helical wave-front. In this case, this light field with a helical phase structure and the phase gradient can generate an azimuthal component. The property of the PTCSB is very promising application in biomedical treatment field [43] and optical tweezers micromanipulations [27], which can avoid damaging the surrounding cells before reaching the particular target. Additionally, the intensity that is sharply increased also enhances the optical trapping force to obtain greater stability. The corresponding experimental results at planes $1-4$ marked in Fig. 4(a) display in Figs. 4(d1)–4(d4), respectively. The numerical intensity distributions agree with those of the experimental results. Therefore, the PTCSB exhibits visibly autofocusing property numerically and experimentally.
To investigate the physical insight of the rotating characteristic of the PTCSB, we show the transverse component of the Poynting vector and the OAM of the PTCSB at the various planes along the propagation direction. The Poynting vector for a linear polarized field can be defined by [44]
Different from the Poynting vector, the handedness of the OAM is more uniform and orderly. The OAM is written by [45]
Figure 6 presents numerical results associated with four different cases of the PTCSB, where the tolopogical charge takes the values $\ell _{1}=4, \ell _{1}=8, \ell _{1}=\ell _{2}=8,$ and $\ell _{1}=-8, \ell _{2}=8$, respectively. Figures 6(a1)–6(a4) show the normalized transverse intensity distribution at $z=105\,z_{R}$ plane with the different cases. Figures 6(b1)–6(b4) are the corresponding intensity contrast values. When $\ell _{1}=4$, we can observe that there are 4 spiral stripes, while there are 8 spiral stripes when $\ell _{1}=8$. With the increase of the topological charge, the number of the spiral stripes also increases. Since the number of the spiral stripes of the transverse intensities is the same as the value of the topological charge, we can utilize the method to measure the topological charge of optical vortices. Additionally, we learn that the rotation direction of the inside lobe is opposite to that of the outside lobe compared with Fig. 6(a3) and Fig. 6(a4), which shows that the signs of the topological charge are determined the rotation direction of the PTCSB. Therefore, the chirality properties of the PTCSB for each layer can be controlled by adjusting the signs of the topological charge.
We also demonstrate the self-healing ability of the PTCSB through numerical simulation and experiment. Figures 7(c1)–7(c4) show numerical results of the intensity distribution of the PTCSB by passing through a sector-shaped opaque obstacle associated with the center angle $\theta =\pi / 6$ at planes $1-4$ marked in Fig. 7(a), respectively. It can be described that the sector-shaped block is placed at the initial plane $z =0$, which causes its first few rings to be partially blocked. We can find that the region of the unblocked PTCSB rotates into the defect region during propagation, which is gradually reconstructed the blocked region, as shown in Figs. 7(c1)–7(c2). However, the focus position of the unblocked PTCSB is larger than that of the blocked PTCSB compared with Fig. 4(c4), and then the spiral shape of the intensity distribution is recovered before the focus position. Our experimental results agree reasonably well with the numerical results, as shown in Figs. 7(d1)–7(d4).
Fig. 8 displays the normalized intensity of the PTCSB passing through a sector-shaped opaque obstacle with different center angles and $b$. We learn that when the PTCSB propagates through the sector-shaped opaque obstacle with the center angle $\theta =\pi / 6$, the blocked PTCSB with $b=0.3$ mm begins to finish the autofocus at $z=118z_{R}$ plane, while that with $b=0.2$ mm disappears at $z=71.25z_{R}$ plane, which means that the waist width has obvious effect on the focal distance of the PTCSB and the focal position is larger with the increase of $b$. Additionally, as the center angle increases to $\pi /3$, the depth of the autofocus, which is defined as the distance corresponding to the light intensity being 0.705 times the maximum light intensity, is almost a constant.
4. Conclusion
In summary, we numerically and experimentally investigate the autofocusing and self-healing properties of the PTCSB. Unlike the properties of the CSB, the PTCSB generated through the appropriate phase modulation presents a tornado wave during the propagation. We further study the evolution process of the self-healing after passing through a sector-shaped opaque obstacle. Our numerical anticipations have been supported by the experimental results well. We discuss the Poynting vectors and OAM to illustrate the rotating mechanism of the beam clearly. Moreover, when the scale factor $b$ is larger, the focal point is larger, and the intensity of the focal point will be affected by the center angle of the sector-shaped opaque, but the focusing depth will be almost unchanged. This new type of the beam may be useful in some applications of optical spanners and optical communication.
Funding
National Natural Science Foundation of China (11374108, 11775083, 12174122); Guangzhou Municipal Science and Technology Project (2019050001); Natural Science Foundation of Guangdong Province (2018A030313480).
Disclosures
The authors declare no conflicts of interest.
Data availability
Data underlying the results presented in this paper are not publicly available at this time but may be obtained from the authors upon reasonable request.
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