Minimalist metasurfaces realize angular momentum holography

Optical angular momentum can be divided into spin angular momentum, which usually appears as circularly polarized polarization, with each photon having a quantized value ( ± ?) , and orbital angular momentum , which carries a spiral phase and each photon having an infinite value ( l−). Optical angular momentum has facilitated many applications in classical and quantum optics, including optical communications, optical tweezers, the spin Hall effect, and quantum microscopy.

Metasurface holography can effectively manipulate multiple physical dimensions of light, showing great potential in high-capacity information technology. In recent years, researchers have realized multi-channel multiplexed holographic technology by using the two eigendimensions of orbital angular momentum and spin angular momentum as independent information carriers. However, the two intrinsic dimensions of arbitrary comodulation within the subwavelength scale have not been realized so far.

In addition, the current structural design methods such as segmentation and interlacing of metasurfaces to achieve multifunctional multiplexing violate the goal of obtaining compact and high-efficiency devices in principle, which fundamentally limit the efficiency of the device and inevitably introduce additional crosstalk.

Recently, Professor Duan Huigao’s research group of Hunan University and Professor Yuan Xiaocong’s research group of Shenzhen University fully cooperated with spin and orbital angular momentum as information reuse carriers based on the minimalist metasurface structure to achieve angular momentum holography. This achievement overcomes the efficiency and crosstalk limitations of conventional multifunction multiplexed devices. The realization of angular momentum holography stems from a new general wavefront modulation design strategy, which relies on independently controlling the two spin eigenstates of photons and arbitrarily superimposing them in each working channel, so as to achieve arbitrary modulation of the spatial wavefront.

The results were published in Light: Science & Applications under the title “Angular Momentum Holography via a Minimalist Metasurface for Optical Nested Encryption.”

The first author of the paper is Dr. Yang Hui of Hunan University, and the co-corresponding authors are Associate Professor Hu Yueqiang, Professor Duan Huigao of Hunan University and Associate Professor Xie Zhenwei of Shenzhen University. This work has been supported by the National Natural Science Foundation of China, Hunan Science and Technology Innovation Leading Talent Program, Hunan Natural Science Foundation and other projects.

The realization of angular momentum holography is based on the co-regulation of spin angular momentum and orbital angular momentum. Among them, the simultaneous regulation of spin and orbital angular momentum can realize spin-orbit locked holograms, which can only reconstruct the target holographic image under a specific vortex circularly polarized light incidence. The regulation of spin angular momentum can realize the holography of spin superposition, that is, fully polarized vector holography. The implementation principle of angular momentum holography (see Figure 1).

Figure 1: Schematic diagram of angular momentum holography on a minimalist metasurface.

Based on the above principles, the research team theoretically and experimentally demonstrated angular momentum holography with 16 independent channels (see Figure 2). As a proof of principle, spin-orbit-locked holograms and spin-superimposed holograms with 8 independent channels are shown here, both of which theoretically have unlimited multiplexed channel numbers.

Figure 2: Spin-orbit locked holograms and spin overlay holographic function display.

Using the dual-function angular momentum holography implemented, the research team also demonstrated a novel optical nested encryption scheme that can realize ultra-high capacity and secure multi-user information transmission (see Figure 3). Optical nested encryption can break the limitations of most existing optical encryption platforms (limited information capacity or low security) while catering to the increasing need for parallel high-security information transmission.

Figure 3: Schematic diagram of optical nested encryption based on angular momentum holography.

Summary and outlook

This work proposes a generalized wavefront shaping design strategy based on minimalist metasurfaces, which can overcome the limitations of existing metasurface structure reuse design strategies. The hypersurface angular momentum holography that can fully cooperate with the dimensions of spin angular momentum and orbital angular momentum is displayed, and a novel optical nesting encryption scheme is developed by using it.

The research results open up a new path for manipulating angular momentum in the sub-wavelength scale, and have broad application prospects in the fields of optical communication, information security and quantum science. (Source: LightScience Applications WeChat public account)

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