Mid-infrared Metasurfaces Based On Phase-change Physics And Applications Of Dynamically Tunable Materials

Metasurfaces are an artificial structure that can operate under subwavelength or non-periodic arrangements.Compared to traditional bulk optical lenses,metasurfaces have many advantages such as smaller volume,lighter weight,higher integration,higher efficiency,and lower cost.However,the functionality of traditional metasurfaces cannot be changed once they are fabricated,which is contradictory to practical application requirements.Therefore,more and more researchers have begun to focus on tunable metasurfaces.Currently,there are mainly five methods for tunable metasurfaces:mechanical,optical,electrical,thermal,and chemical.Among these methods,tunable metasurfaces based on phase-change materials have developed rapidly because they have advantages such as fast phase transition rate（up to nanosecond level）,repeatable phase transition,continuous tuning,and easy implementation.Classic sulfur-based phase-change material Ge₂Sb₂Te₅（GST）has the characteristic of a large difference in the real part of refractive index before and after the phase transition in the near-infrared waveband.However,the effect of the imaginary part of GST cannot be ignored,which can lead to energy and efficiency loss during light propagation.Therefore,researchers have begun to study a new sulfur-based phase-change material Ge₂Sb₂Se₄Te₁（GSST）doped with GST.GSST has a significantly reduced imaginary part of refractive index in the near-infrared waveband and can achieve an imaginary part close to 0 in the mid-infrared waveband,while the real part has a large difference before and after the phase transition.In the existing work on wavefront tuning metasurfaces based on GSST,the tuning of phase is achieved by covering different states of phase with the same structural pillar and adopting a discrete step-like phase distribution.This seriously affects the coverage of phase in two states,and phase control is limited to x-polarized incident light.This article aims to address these issues through research as follows:（1）To address the increasing demand for food fingerprinting spectroscopy,aldehyde carbonyl groups have been identified as sensitive indicators of lipid oxidation with a molecular absorption peak at 5.8μm.However,their low abundance makes detection challenging.In this study,a tunable metasurface was designed to enhance radiation energy and improve detection flexibility.By combining GSST with natural materials with two-state refractive indices at 5.8μm,the effective structure of the metasurface was changed through phase transition,breaking the traditional approach of covering phase with fixed structural dimensions.The metasurface achieved continuous phase modulation and was no longer polarization sensitive.Based on this,a polarization-insensitive tunable metasurface was designed,with the focal points before and after the phase transition located at f_a=140μm and f_c=180μm,respectively.The switching contrast ratio CR reached 20.96 d B before and after the phase transition.Additionally,a polarization-insensitive polarization angle-switchable metasurface was designed using the generalized Snell’s law,achieving a beam deflection of-16.86°before phase transition and-35.45°after phase transition.（2）Polarization detection technology has been widely used in many fields for measuring and analyzing the polarization state of light,including remote sensing,astronomy,microscopy,and telecommunications.In this paper,based on this demand,the degree of freedom of the wavefront-controllable metasurface in polarization was explored,and variable focus metasurfaces for polarization multiplexing and polarization conversion were implemented under both linear and circular bases.In the polarization-multiplexing variable-focus metasurface based on GSST,before the phase transition,x-polarized light was focused at f_a-x=140μm,and y-polarized light was focused at f_a-y=180μm.After the phase transition,x-polarized light was focused at f_c-x=160μm（left1/3 of the metasurface）,and y-polarized light was focused at f_c-y=180μm（right 1/3 of the metasurface）.The polarization contrast ratios before and after the phase transition reached CR_a=16.83d B and CR_c=35.85d B,respectively.In the polarization-conversion variable-focus metasurface based on GSST,left circularly polarized light was focused at f_a=120μm before the phase transition,while left and right circularly polarized light were both focused at f_c=180μm after the phase transition.The switching contrast ratio before and after the phase transition reached CR=17.03d B.In addition,this paper also designed a polarization-rotating angle-switchable metasurface based on polarization multiplexing,which realized that the x-polarized incident light was deflected by-16.86°in the y direction before the GSST phase transition,and the y-polarized light was deflected by-35.45°in the x direction.After the GSST phase transition,the x-polarized incident light was deflected by 16.86°in the y direction,and the y-polarized light was deflected by35.45°in the x direction.