Structured light beams with controllable polarization along arbitrary trajectories

A structured light refers to a light field that is “tailored” in both space and time, characterized by its unique distribution of amplitude, phase, and state of polarization in both space and time.

Polarization plays a crucial role in structured light beams. In addition to manipulating the polarization in a single transverse plane, controlling the polarization along the propagation direction in longitudinally structured light beams is also an important dimension.

Current research mainly focuses on beam polarization control along the optical axis. However, this greatly limits the spatial freedom of polarization for longitudinally structured light beams.

It is worth exploring whether it is possible to break free from the constraints of the optical axis and manipulate the polarization along any transmission trajectory in space. This can lead to the generation of structured light beams with polarization varying along arbitrary trajectories, opening up new possibilities for creating customized spatially structured lights.

In a study published in Opto-Electronic Scienceresearchers used three-layer metallic metasurfaces to generate structured light beams with polarization variations along arbitrary spatial trajectories.

To achieve these customized spatially structured lights, the work first designed a phase modulation function in the input plane, consisting of a series of expanding circular rings with moving centers. The curve of the propagation trajectory is formed by the convergence of the conical beams emitted by these moving circular rings in the plane of entry.

Second, by controlling the amplitude and phase difference of the orthogonally polarized beam components corresponding to the circular moving rings, polarization control can be achieved at any point of the predetermined transmission trajectory.

Following the predetermined helical transmission trajectory, they projected continuous variations of structured light polarized from 15° to 75° along the equator on the Poincare sphere, as well as continuous variations of structured light polarized from 15° in polarization circular and then at 75° along the equator of the northern hemisphere.

A terahertz focal plane imaging system was used to obtain the intensity, amplitude, and phase shift of the electric field components E_x and E_y at different transmission distances Mr. Under the modulation of the metasurface, a high-brightness main lobe surrounded by several side lobes appeared, and the overall intensity distribution of the light field resembled a zero-order Bessel function.

By observing the intensity distribution of the light field at different transmission distances z, they found that the main lobe of the beam rotated counterclockwise in a continuous range from 5 mm to 15 mm, with I_x gradually decreasing and me_y gradually increasing.

The difference is that the phase difference of E_x and E_y between two structured lights in the range of 5 mm to 15 mm gradually changes from 0 to -π, respectively, corresponding to the synthesized polarization states, which are consistent with theoretical simulations.

The proposal for structured light beams with arbitrary trajectories with variable longitudinal polarization provides a practical method for continuously tuning the characteristics of spatially structured light beams with non-axial transmission and opens up new opportunities for creating structured light beams personalized space. This technique has potential uses in optical encryption, particle manipulation, and biomedical imaging.