Study On The Electromagnetic Characteristics Of Active Modulation Metasurfaces Based On Phase Change Material GST

Metasurfaces are artificially constructed arrays of nanostructured elements with sub-wavelength thickness arranged in a periodic or quasi-periodic form.By controlling the interaction with incident waves,metasurfaces generate unique optical field manipulation capabilities.In recent years,plasmonic metasurfaces based on metal and other plasmonic materials have been developed rapidly,forming multi-structured and multi-funtional metadevices,which have important applications in wave absorption,polarization conversion,wavefront manipulation,etc.With the continuous enrichment of plasmonic metasurface structures,researchers put forward demanding application requirements,for example the original passive metasurfaces with fixed functions can become tunable or reconfigurable in real time by introducing functional materials with tunable electrical or optical properties in order to realize active modulation of metasurfaces.With the advent of active metadevices,the application scope of metasurfaces has been extended to optical switch modulation,optical sensing,tunable metalens,etc.Phase change material Ge₂Sb₂Te₅?GST?is an ideal functional material to realize active modulation of metasurfaces.It has stable amorphous state and crystalline state.Its phase transition process is ultrafast,non-volatile and repeatable,which makes it an important research topic to achieve active modulation of metasurfaces by means of GST in recent years.This thesis focuses on the active modulation behaviors in two typical composite plasmonic metasurfaces based on GST as functional materials,namely metal-dielectric-metal?MDM?configuration metasurfaces and electromagnetically induced transparency?EIT?metasurfaces,studying the dynamic modulation mechanisms and exploring new applications.The MDM configuration metasurfaces have the ability to control both of the amplitude and reflection phase of the incident electromagnetic waves.Through the design of dynamically modulated metasurfaces,two different functional metadevices with tunable resonant amplitude and reflection phase can be formed.Firstly,application in amplitude modulation.Using GST as the dielectric layer,a tunable near-infrared absorber has been realized,in which the absorption wavelength can be tuned through the change of refractive index during the GST phase transition.Because of the outstanding wavelength tunability,the metasurface has potential applications in optical modulation and switching devices.In addition,the effect of dielectric material on the resonant wavelength of absorber metasurface has been analyzed quantitatively,which provides a novel method to increase the resonance frequency of metasurface without shrinking down the structure sizes,thus reducing the difficulty of fabricating high-frequency metadevices.Secondly,based on the characteristics of the MDM configuration metasurface in manipulating the reflection phase of electromagnetic waves,we proposed another reflection phase tunable metasurface with GST as a dielectric layer.In addition to the obvious difference between refractive index before and after the phase transition,GST can also generate various stable intermediate states by controlling the phase change conditions,which enables the tunable metasurface to realize continuous and conspicuous modulation of the reflection phase.Through the precise design of the structure size,the reflectivity can be kept stable while the reflection phase is continuously adjusted,which leads to potential applications in high-efficiency wavefront shaping.In addition,the active modulation behavior of composite plasmonic metasurface,namely EIT metasurface has been studied.Different from previous methods of directly utilizing GST as the dielectric layer,GST has been designed into space-separated strip structures combined with bright and dark meta-atoms,which constitute the EIT metasurface,respectively.In this way,two different modulation effects on the EIT-like transparent window can be obtained.This new modulation method not only helps us to comprehensively study the active modulation mechanism of EIT metasurface,but also improves the flexibility of active metadevices,which makes the metasurface has practical value in tunable slow-light devices and multi-terminal-input logic devices.