Longitudinally Encoding and Decoding Information in Light Field Arrays Based on Metasurface

An optical frozen wave was used to realize the longitudinal modulation of the OAM modes, and a tetratomic macro -pixel dielectric metasurface was designed to experimentally generate such a light field array with encoded information. The tetratomic macro -pixel is shown in Fig. 2(a), which considers the intrinsic relationship and interaction between the optical responses of rectangular meta -atoms and can achieve complex amplitude modulation because of polarization -dependent interference. According to this principle, the transmission phases co0A and co0B and rotation angles theta A and theta B of the meta -atoms A and B in a tetratomic pixel can be obtained by combining equations (3) and (8). The transmission fields of rectangular nanopillars with different lengths and widths were scanned under the conditions of height H0= 590 nm and period P0=400 nm to find 17 different geometries, of which the transmitted phases co0 linearly increased as shown in Fig. 2(b). Results and Discussions As an example, the message  Northwestern Polytechnical University  is used to demonstrate the longitudinally encoding capability of this specially structured light field. According to the ASCII hexadecimal code elements, the letters and spaces in the message are transformed into 74 superposition states, which are then divided into a 5x5 beam array. The longitudinal variation of each beam channel contains three segments, which are located at propagation distances of z=0-0.4 mm, 0.40.8 mm, and 0.8-1.2 mm, respectively. Because the superposition state in each beam channel can change three times along the longitudinal dimension, the total mode capacity of the beam array is expanded to 163. The simulated and measured intensity distributions of beam arrays in z1=0.1 mm, z2=0.5 mm, and z3=0.9 mm planes are shown in Fig. 3. The experimental measurements are consistent with that of the simulations. Therefore, the correct message can be obtained by decoding two hexadecimal digits in a Z -shaped sequence starting from the first line in z1. It should be noted that this method requires the measurement of the light field modes in three different planes for decoding. In this study, a longitudinal scanning method is used. However, to improve the decoding efficiency, a split -plane imaging approach can also be used to obtain the light -field distribution in three longitudinal planes simultaneously by splitting the light twice and then imaging different longitudinal planes. In addition, according to the propagation characteristics of light waves, if the complex amplitude information of the light field is measured in a single plane, the complex amplitude distributions of other planes can be obtained by numerical calculations, and the light field patterns of multiple longitudinal planes can then be obtained. Conclusions In this study, a dielectric metasurface that can independently control the amplitude and phase of two light fields is proposed. Flexible modulation of the OAM modes of the beam array in the longitudinal dimension is achieved using the spectral modulation principle of frozen waves. Using these longitudinally modulated light fields, exponential expansion of the mode capacity is experimentally realized to enhance the mode capacity of free -space optical communication. The longitudinal dimensional coding and decoding functionality verified in this study is expected to be a breakthrough in the capacity improvement of free -space optical communication.