The team led by Gao Chunqing and Fu Shiyao from Beijing University of Technology has made significant breakthroughs in the study of photon angular momentum regulation

Recently, a team led by Gao Chunqing and Fu Shiyao from the School of Optoelectronics at Beijing University of Technology combined optical spatial coordinate transformation with photon spin Hall effect to construct a photon angular momentum filter for the first time internationally, achieving on-demand regulation of photon spin angular momentum and orbital angular momentum.

The related achievements were published in the top optical journal Advanced Photonics (SCI Zone 1, IF: 17.3) under the title of "Photon total angular momentum manipulation". This work has received funding from the National Key R&D Program, the National Natural Science Foundation of China, the Beijing Natural Science Foundation, and the Postdoctoral Innovation Talent Support Program. Li Lang, a doctoral student from the School of Optoelectronics at Beijing University of Technology in 2022, is the first author of this paper, and Special Researcher Fu Shiyao is the corresponding author of this paper.

Related studies have shown that rotating objects carry angular momentum, which exists not only in macroscopic objects but also in microscopic particles such as photons. There are two different forms of angular momentum contained in photons: spin angular momentum (SAM) and orbital angular momentum (OAM). The total angular momentum of photons (TAM) is the sum of photon SAM and OAM under the paraxial approximation. It provides two degrees of freedom and has broad application prospects in cutting-edge fields such as lidar, laser processing, optical communication, optical computing, quantum information, etc. The effective recognition and on-demand control of photon TAM states are important foundations for their applications. However, existing methods for recognizing photon TAM states still have problems such as limited dynamic range, low recognition accuracy, and inability to adjust filtering on demand, which restricts their application and development.

In response to the above issues, in order to achieve large-scale and high-precision photon TAM pattern recognition and on-demand regulation, the team introduced wavefront replication into optical spatial coordinate transformation, combined with photon spin Hall effect, designed and prepared a high-precision photon TAM state separation device, as shown in Figure 1 (a~f). We have achieved high-precision separation of up to 42 photon TAM states. The photons of different TAM states are distributed in specific regions of the separation plane, which can be recognized and measured through image processing and other means, as shown in Figure 1 (g).

Figure 1. The mode separation device proposed and prepared by the team and the distribution of TAM states on the separation plane

On the basis of separating the devices, the team designed an inverter conversion device that transforms from the separation mode to the original light field, and then achieved on-demand filtering control of photon TAM through cascaded separation devices and inverter conversion devices in the form of 4-f filtering, as shown in Figure 2.

Figure 2. Structure of photon angular momentum filter

The research team conducted a large amount of experimental verification work on the system, taking the incident light field of four TAM superposition states as an example, as shown in Figure 3 (a). When the separation plane is not subjected to spatial filtering, the output light field remains consistent with the input light field, which is a petal shaped scalar vortex light field; The output light field after spatial filtering is filtered out by two single TAM modes and converted into a classical unclassifiable Bell state, manifested as a circular cylindrical vector light field. The total angular momentum spectrum changes of the input and output system beams are shown in Figure 3 (b).

Figure 3. Verification results of four TAM superposition states incident experiments

This system achieves the separation of 42 photon TAM modes and the forward and backward transformation of the beam in the spatial and separation domains. It has been experimentally proven that the cascaded separation and inverse transformation components can achieve on-demand regulation of the total photon angular momentum of the input light field. This work achieves simultaneous filtering of photon OAM and SAM states, making it possible to suppress photon angular momentum edge modes in the OAM domain, providing a new approach for high-fidelity photon computation and quantum radar signal processing.

Source: Beijing University of Technology