Recently, the conversion between different angular momentum has attracted widespread attention, which can be divided into spin angular momentum (SAM) and orbital angular momentum (OAM). When the circularly polarized light carrying SAM interacts with the medium, SAM of light will change and therefore trigger the optical rotation effect, which is universally applied in the manipulation of particles and biological cells. OAM has been proved to owe a widely applications in both classical and quantum areas, such as optical microfabrication, micromanipulation, OAM entanglement and super-resolution optical imaging. More compelling and attractive, the optical communication capacity could be increased to another level by using infinite eigenstates of vortex beam as information carriers. Of note, the conversion between SAM and OAM breaks through the limitation in traditional optics that different angular momentum can only be adjusted individually. The coupling between different angular momentums has been proved to be available in the optical control of multi-state magnetization, optical nanoprobing, multi-state information encoding for quantum computing and optical communication. This kind of angular momentum conversion processes are associated with the Pancharatnam–Berry phase induced by inhomogenous transformation of the polarization direction. Therefore, inhomogeneous anisotropic medium becomes a reliable technique to achieve angular momentum conversion, such as q-plate and metasurface. However, these elements achieve angular momentum conversion with restricted direction and extremely rely on precisely designed geometric structures, which limit their applications especially considering many complex environments, such as under water, cloud and mist, sand and dust weather, etc. Therefore it is of fundamental relevance to realize the coherent control of spin and orbital angular momentum conversion of light by scattering system considering a variety of practical applications.

In this article, we experimentally realize the active control of spin and orbital angular momentum conversion of light by strongly scattering medium with the help of the feedback-based wavefront shaping (FBWS) method. Considering the coupling conversion control can be applied to identical degrees of freedom or between distinct degrees of freedom, we experimentally implement four classifications of angular momentum conversion by scattering system, including the conversion of OAM-OAM, OAM-SAM, SAM-SAM and SAM-OAM. The objective function in each conversion process obtains a significant increase and approaches a stable value with the operating of genetic evolution algorithm. These repeatable and stable experimental results verify that the angular momentum conversion process in scattering medium is flexible and tunable. Figure 1 is the experimental setup for OAM conversion. Figure 1(a) is the preparation of OAM state. Figures 1(b) and 1(c) are the modulation and detection system for OAM-OAM and OAM-SAM conversion, respectively. We use liquid crystal space light modulator (SLM) to modulate the light beam and the spiral phase plate (SPP) to generate and detect OAM. Figure 2 is the experimental results for OAM conversion, where red circles indicate the target region. Figures 2 (a) - (h) shows the light intensity distribution diagram captured by CCD. Figures 2 (i) and 2(j) are the intensity curves of the target region varying with generations. The light intensity of the target area keeps growing and eventually reaches convergence as the generation increases. Figure 3 is the experimental setup for SAM conversion. Figure 3 (a) is the preparation of SAM state. Figures 3 (b) and (c) are the modulation and detection system for SAM-SAM and SAM-OAM conversion, respectively. Figure 4 shows the experimental results of the SAM conversion. This work proves that the scattering system has the potential to become a new angular momentum conversion element which can effectively break through the limitations of previous elements including restricted conversion functions and highly demanding geometric structures. It also paves the way for the classical and quantum optical communication especially under complex environments where the scattering effect commonly destroys the optical information. Moreover, we realize the coupling control of not only one single degree of freedom but also different degrees of freedom in scattering medium. This kind of heterogeneous degree of freedom coupling can directly establish high-dimensional freedom optical channel conversion in complex environment.

FIG. 1. (a) The preparation of input light possessing different OAM states. (b) The modulation and detection system for OAM-OAM conversion. (c) The modulation and detection system for OAM-SAM conversion.

FIG. 2. (a)-(d) Results of different topological charges l=4,6,8,10, respectively. The left image shows OAM state, the middle image shows intensity distribution of light after scattering. The right image shows intensity distribution of light after 200 generations optimization. (i) The curves of intensity varying with generations for different topological charges. (e) Intensity distribution of vortex beam with l=2. (f) Intensity distribution of light captured by CCD after scattering. (g) Intensity distribution of light after 200 generations optimization. (h) Intensity distribution after deflecting the optical axis of the polarizer to the vertical direction corresponding to the right circularly polarized light. (j) The curve of intensity varying with generations in OAM-SAM conversion.

FIG. 3. (a) The preparation of input SAM state with SAM =+ℏ. (b) The modulation and detection system for SAM-SAM conversion. (c) The modulation and detection system for SAM-OAM conversion.

FIG. 4. Experimental results for conversion from SAM to SAM or OAM.

This research is published by “Zhengyang Mao, Haigang Liu, and Xianfeng Chen, Active Control of Interconversion of Spin and Orbital Angular Momentum of Light by a Scattering System, Phys. Rev. Applied 18, 024061(2022)”。

Link: https://doi.org/10.1103/PhysRevApplied.18.024061