Optical Information Multiplexing With Nonlinear Coding Metasurfaces

Metasurfaces,as the two-dimensional planar structures composed of the meta-atoms with sub-wavelength scale,have provided excellent performances for manipulating the wave fronts of lights.Previous researches only focused on linear metasurfaces,however,most of the metasurfaces have only a single operating frequency and small information capacity,which are not enough to meet the increasing demand for highly integrated and compact multifunctional devices.In this case,the research and exploitation for the nonlinear optical responses of metasurfaces become very necessary because it can introduce multidimensional information storage channels originating from light-frequency conversion processes,which are very beneficial to realizing optical information multiplexing and enlarging the information capacity.More recently,as an alternative research approach,coding metasurfaces have caught much attention due to the convenient digital signal processing.In this paper,by combining the coding and nonlinear optical processes,we proposed the concept of nonlinear coding metasurfaces and thoroughly studied its applications in multiplexing.The main contents are illustrated as follows:Firstly,we proposed the Pancharatnam-Berry-phase-based nonlinear coding metasurfaces,in which the convenient digital signal processing was employed to design the multidimensional information channels in nonlinear optical processes.By selecting eight split-ring resonators with predesigned orientation angles as coding elements,we realized 1-bit,2-bit and 3-bit coding at the channels of third harmonic generation,fundamental frequency,and second harmonic generation in nonlinear coding metasurfaces,respectively.Secondly,by applying the method of nonlinear coding,we designed an ultrathin multifocal metalens with different focal lengths under different channels.From the numerically calculated and experiment results of metalens,we can find that the wave front of the transmitted beam for each channel can be converged simultaneously.In addition,the sample with smaller radius will be expected to achieve more excellent focusing performances for the channels of fundamental frequency and second harmonic generation.The nonlinear coding metasurfaces provide a flexible and convenient avenue for the application of multifocal metalens in the field of real space encoding imaging,optical data storage and optical communication.Finally,by utilizing the extra degree of freedom introduced by nonlinear coding,we further constructed the nonlinear coding multifunctional metasurface,in which the function of focusing,operational symbols ‘+' and ‘-' are encoded into the channels of third harmonic generation,fundamental frequency and second harmonic generation,respectively.Then we numerically calculated and experimentally measured the far-field distributions under the three channels,which further verified its different function under different channels.Besides,we also theoretically analyzed the applications of nonlinear coding metasurfaces in multi-dimensional optical data storage and optical information encryption,which shows its great potential in the fields of optical encryption,multi-channel optical data storage and real space encoding imaging.