Design and research of long-infrared dual-wavelength confocal metalens

Multi-wavelength confocal lens is an indispensable part of optical system, the traditional optical confocal system is often added by a certain number of optical lenses, or uses a different combination of optical lenses of different materials to implement multi-wavelength co-focusing,making the system possess a larger volume and weight, which, however, is difficult to meet the requirements for high integration and miniaturization of the system. As an optical element composed of two-dimensional planar subwavelength micro-element structure, the metalens has the advantages of flatness, ultra-thinness and regulating light waves, and has great potential applications in highly integrated and miniaturized optical confocal systems. According to relevant research reports, it is known that the existing research schemes of multi-wavelength confocal metalens have some shortcomings, such as relatively complex structure and relatively low focusing efficiency. In this work, a kind of metalens composed of simple micro-element structure is proposed and designed, which can simultaneously realize the long infrared dual wavelength confocal function. Based on the generalized Snell's law and the transmission phase modulation mechanism, a scientific evaluation function is established to select the optimal array of micro-elements structure to form a metalens. With the elliptical nano silicon column in a simple micro-element structure, the wavefront phase of the long infrared dual wavelength in the orthogonal linear polarization state can be adjusted independently and efficiently , while reducing the wavelength crosstalk and improving the focusing efficiency. The design results show that the proposed metalens achieves dual wavelength co-focused with a wavelength of 10.6 and 9.3 μm, and has a high focusing efficiency, The focusing spot is close to the diffraction limit. The quantitative analysis of the redundancy of the structural parameters of the metalens micro-element structure is made, and the trend of its influence on the focusing efficiency and the allowable deviation range of the micro-element structure parameters are obtained, which provides a theoretical basis for further precisely controlling the device fabrication. The matalens designed in this work is expected to meet the requirements for integration and miniaturization of long infrared optical confocal system, and has important applications in laser surgery, industrial cutting and welding and other fields.