Metamaterials are artificial materials which can have electromagnetic response characteristics that ordinary materials in nature do not have by selecting different constituent materials and geometric parameters of structural units in metamaterials. However, the development of metamaterials is very limited due to the difficulty of three-dimensional processing processes. Metasurfaces are the two-dimensional counterparts of metamaterials. Compared with metamaterials, metasurfaces have the advantages of simple preparation process, high integration and powerful functions, and have broad application prospects in the fields of classical optics and quantum optics. In the field of classical optics, metasurfaces are mainly used to develop optical components with higher integration and more innovative functions, which is of great significance for the research of integrated optical devices. In the field of quantum optics, metasurfaces can not only reduce the complexity of quantum optics experimental devices, improve their stability and scalability, but also provide a new research platform for quantum optics research. Therefore, exploring the application value of metasurfaces in degrees of freedom such as wavelength, polarization, and orbital angular momentum is crucial to the fields of classical optics and quantum optics.
The state of light passing through metasurfaces depends on the degrees of freedom of passing photons. The degrees of freedom of light include wavelength, incident or output direction, and orbital angular momentum and polarization, etc. Specifically, the phases for any set of orthogonal polarizations can be arbitrarily and independently encoded using the metasurface's propagating phase and geometric phase in combination. Furthermore, by utilizing the subwavelength-scale periodicity of the metasurface and the principle of interference, the complete decoupling of near-field and far-field information encoding for any set of orthogonal polarizations is feasible. Furthermore, arbitrary and independent encoding of far-field information at two wavelengths has also been demonstrated to be feasible. However, no work has yet demonstrated that a complete decoupling of dual-wavelength near-field and far-field information encoding can be achieved.
The research group led by Prof. Wang Shuming from the Nanjing University recently has theoretically proved that the super-structured surface can achieve complete decoupling of the near-field and far-field functions of the same polarization at two working wavelengths. When encoding intensity patterns in the near field, the far field functions can be holography, focusing and beam deflection. Relevant research results were published in Chinese Optics Letters, Volume 21, Issue 2（Jun Liu, et al., Metasurfaces enabled dual-wavelength decoupling of near-field and far-field encoding. ）.
In this work, firstly, we designed the pillars whose propagation phase difference at two working wavelengths covers , which is made of amorphous silicon. Then, using the non-dispersive properties of the geometric phase, the two-wavelength phase can be adjusted arbitrarily and independently only by the rotation angle. Next, the design process to realize the decoupling of the near-field intensity distribution and the far-field function is introduced in detail in combination with the interference principle, and the dual-wavelength decoupling of the near-field pattern and the far-field holographic pattern is successfully simulated. There is crosstalk between the two-wavelength near-field patterns, but this crosstalk is not transferred to the intensity distribution of the far-field holographic patterns. In addition, the far-field function can be designed as a focusing function. According to our theoretical analysis, when the NA of the metalens is small, the final output focal length is 4 times of the initial input focal length. The simulation results are in good agreement with the theoretical analysis.
Figure 1 Schematic diagram of dual-wavelength near-field patterning and far-field holographic decoupling on a metasurface
This method can adjust the focal length of the two working wavelengths arbitrarily, and has great design flexibility for different application scenarios. The work opens a new way to control electromagnetic waves in multi-wavelength application scenarios, which can not only improve the information density and security of metasurfaces, but also can be well applied to applications such as virtual reality and stimulated emission depletion microscopy(STED) and other multi-working wavelength application scenarios.