Vortex beam (VB) has attracted great attention due to its unique optical properties including a helical wavefront, phase singularity, and ability to carry orbital angular momentum (OAM) of l? per photon, where l is the topological charge, and ? is the reduced Planck constant. VBs have been widely used in super-resolution imaging, laser microfabrication, optical manipulation, and ultra-large capacity optical communication. Especially, OAM can twist molten material and can be used for chiral nanostructure microfabrication. In recent years, chiral structured light fields with twisted intensity distribution and OAM have attracted great research interest due to their advantages in optical microfabrication. Alonzo et al. constructed a spiral cone phase using the product of the spiral phase and cone phase and generated a spiral cone light field with chiral intensity distribution. Subsequently, Li et al. proposed a spiral light field with an automatic focusing effect by a power-exponential phase. However, these structures of chiral optical fields are simple. Therefore, generating a flexible and tunable chiral structured light field becomes important. However, the generation of flexible chiral light fields remains challenging. In this paper, we proposed a simple and efficient approach for generating tunable chiral structured beams (TCSB), which exhibited flexible adjustability and multi-ring chiral structures. Such light fields would be beneficial to flexible chiral structure micromachining, optical manipulation, and optical communications.

We proposed and generated a TCSB by constructing an annular phase (AP) which consisted of multiple annular sub-phases (ASPs). Specifically, every sub-phase was constructed by introducing an equiphase and radial phase based on a classical spiral phase, and then a monocyclic TCSB was generated by imposing such ASP on an incident Gaussian beam. The number and direction of the twisted intensity lobes were flexibly and individually controlled by manipulating the topological charge, equiphase, and radial phase. Moreover, we used multiple ASPs to generate multi-ring chiral optical fields, which could be more flexible in practical applications. Experimentally, chiral light fields could be generated by phase modulation and observed via the CCD, as described in Fig. 5.

The structures of the tunable chiral beams can be flexibly manipulated by controlling the topological charge (Fig. 6). The number and direction of the twisted intensity lobes are determined by the number and sign of the topological charge. By controlling the equal phase, the twisted lobe direction can be arbitrarily controlled (Fig. 9). More complex chiral structured beams with three-ring and four-ring structures are constructed, and this validates the effectiveness of our proposed approach. Additionally, the equal phase gradient is employed to control dynamically the rotation of the light fields (Video 1). The advantage of this rotation also makes this chiral beam beneficial for twisting transiently molten matter, machining complex chiral nanostructures, and sorting multiple particles.

In summary, we have developed an effective method to generate TCSBs by multiple ASPs. The properties of the twisting lobes, including the twisting directions, lobe orientations, and lobe number can be freely manipulated by controlling the topological charge sign, magnitude, and equiphase, respectively. Our findings offer a novel promising technology to manufacture chiral microstructures. Moreover, the flexible TCSBs also provide an innovative method for optical manipulation and optical communications.