Research Report: Electrically Controlled Engineering of Synthetic Magnetic Fields in Polarized Photons

Academic Background and Research Purpose

In recent years, synthetic gauge theory has shown its potential in controlling the propagation of light and its state evolution in non-magnetic photonic systems. However, the synthetic magnetic fields generated by different mechanisms have not achieved significant effects in controlling polarized photons. Moreover, the magnetic fields reported in the past are usually synthesized in a fixed geometric configuration, which is difficult to adjust. Therefore, engineering synthetic magnetic fields for photons remains a challenging topic. This paper proposes a universal spin-¹⁄₂ theoretical framework and synthesizes a magnetization vector in anisotropic mediums engineered to control different polarized photons.

Source of the Paper

This paper was authored by Guohua Liu, Zepei Zeng, Haolin Lin, Yanwen Hu, Zhen Li, Zhenqiang Chen, and Shenhe Fu from Jinan University’s College of Physics & Optoelectronic Engineering, Guangdong Provincial Key Laboratory of Optical Fiber Sensing and Communications, and Guangdong Provincial Engineering Research Center of Crystal and Laser Technology. It was published on July 11, 2024, in the 11th volume, 7th issue of Optica. The DOI of the paper is https://doi.org/10.1364/optica.527811.

Research Process

Theoretical Model

The research starts from the transport phenomenon of electron spins in spatially varying magnetic fields, by introducing a framework similar to the Pauli equation, based on the coherent coupling of different polarization states of photons in synthetic photonic mediums. Using Maxwell’s Theory, the two-component motion equation of the spin wave function is derived, where the synthetic magnetic fields are realized through electrically controlled anisotropic mediums. We define a phase mismatch quantity, which contributes a synthetic magnetic field through its spatial dependence, achieving electrical control over the photon’s magnetic moment.

Experimental System Design

The experimental system is designed based on an electrically controlled LiNbO3 crystal, where lateral modulation is achieved by applying a spatially varying external electric field. By using the electro-optic effect, the spatial distribution of different principal refractive indices is adjusted, thereby controlling the gradient of the synthetic magnetic field. We align the incident light beam with the optical axis of the crystal, allowing photons to propagate along the optical axis, and the initial spin state is prepared via three wave plates (half-wave plate, quarter-wave plate, and vortex wave plate).

Experimental Steps

1. Photon Deflection and Splitting Experiment: Under the application of voltage, by observing the deflection and splitting of photons, the Lorentz force of the synthetic magnetic field on polarized photons is verified.
2. Experiment on Separation of Complex Spin Textures: By engineering spin textures with non-trivial topological structures, the higher-order Stern-Gerlach effect is verified.

Experimental Results and Data Analysis

The experiments successfully observed the separation effect of photon spin under different voltages. The experimental results indicate that photons indeed experience an external lateral force similar to that brought by weak magnetic fields, such as the Earth’s magnetic field. This is analogous to the separation distance and trajectory in the quantum spin Stern-Gerlach effect. The authenticity of the experimental results was validated through simulation experiments.

Additionally, we achieved a higher-order Stern-Gerlach effect through experiments, observing the splitting of photons with non-trivial topological wavefronts under the influence of synthetic magnetic fields.

Research Conclusions and Significance

By constructing a universal spin-¹⁄₂ theoretical framework and implementing synthetic magnetic fields in engineered photonic mediums, this paper proposes a new method for the manipulation of photonic magnetic moments. Synthetic magnetic fields can electrically control anisotropic mediums, resulting in photonic magnetic effects and thus enabling a variety of applications including polarization selection and conversion. This study not only helps to explore quantum spin transport phenomena but also provides new ideas for the design of optical devices centered on polarized photons.

Research Highlights

1. Successfully constructed a theoretical model for equivalent spin-¹⁄₂ photons.
2. Experimentally verified the Lorentz force effect on polarized photons in a synthetic magnetic field and achieved the higher-order Stern-Gerlach effect for the first time.
3. Proposed an engineering platform capable of electrically controlling photon’s magnetic moments, which is helpful for future applications in fields like polarization selection and conversion.

Further Research Prospects

The synthetic two-level systems exhibited in this paper are not only equivalent to quantum two-level systems but can also be analogous to various systems in nonlinear optics (as shown in Table 1). These analogies provide numerous possibilities for exploring other spin transport phenomena using polarized photons. Future research may consider designing appropriate synthetic magnetic fields to realize topological (pseudo-)spin Hall effects, or determining crystal geometries to produce radially symmetric pseudo-Lorentz forces, thereby controlling pseudo-spin. These explorations will further advance the use of synthetic magnetic fields in practical applications.

This paper, through both theoretical and experimental research, reveals the transport phenomena of photon spins in synthetic magnetic fields, providing new opportunities to explore the application of polarized photons in classical and quantum information processing.