Study On Polarization Manipulation And Dynamically Tunable Characteristics In Bilayer Metasurfaces

As one of the most important characteristics of the optical field,polarization plays an important role in the interactions between optics and matters.Polarization manipulation has a wide application in the area of data store,sensing and communications and so on.Metasurfaces,composed of subwavelength artificial astoms,offer superior abilities in manipulating polarization than natural materials.Common passive metasurfaces,once manufactured,have fixed functionalities and the limited performance will hinder actual applications.Herein,the dynamic metasurfaces is highly desirable.We investigate polarization characteristics and its tunability characteristics in bilayer chiral metasurface and gradient phase metasurface.The brief content of this dissertation is as follows:（1）Asymmetric transmission and dual-band circular dichroism are demonstrated in twisted ring apertures structure and asymmetrically split rings structure by employing the coupling between the two metamaterial layers,respectively.The linear asymmetric transmission is realized in the bilayer ring apertures structure,and the asymmetric transmission parameter reaches 0.47.A weak circular dichroism is obtained in the bilayer apertures structure.To enhance the circular dichroism effect,a left-handed circular polarization chiral mirror is designed by breaking the n-fold rotational symmetry and mirror symmetery.Circular dichroism reaches-0.88 in reflection mode.Coupled mode theory is employed to analyze left-handed circular polarizations chiral mirror.The theoretical results match well with the simulation results.A dual-band circular dichroism effect in reflection mode is obtained by constructing the concentric asymmetrically split ring structure with different sizes.Selective polarization manipulation for right-and left-handed circular polarizations is obtained in the low and high frequency band,respectively.Circular dichroism reaches-0.6 and 0.79.（2）Dynamic asymmetric transmission and circular dichroism are demonstrated by interpreting the Ge₂Sb₂Te₅（GST-225）layer to bilayer asymmetrically split rings,concentric asymmetrically split rings and Z-shaped structures.A large frequency tuning range of both resonant asymmetric transmission and circular dichroism up to 38.7%in asymmetrically split ring metamaterial.Additionally,the hybrid metamaterial can be employed to explore phase transition process via detecting polarization conversion or circular dichroism since the resonant signal is quite sensitive to proportions of amorphous and crystalline states.In contrast to the above-mentioned dynamic manipulation in single band.A concentric asymmetric split ring structure is proposed,it shows dual-band dynamic circular dichroism effect when changes the amorphous GST-225 to crystalline GST-225.Bilayer Z-shaped structure is proposed to demonstrate three tunable chiroptical responses.The maximum linear asymmetric transmission parameter reaches 0.6 with amorphous state of GST-225 layer.Through the whole phase transitions of GST-225 layer,it also maintains a relatively high value 0.47.Tunable circular dichroism and circular asymmetric transmission are also demonstrated and all of the chiroptical responses achieve an extraordinary modulation depth of near 100%in the resonance frequency.（3）Bilayer gradient phase metasurface based on complementary structure exhibit anomalous asymmetric optical phenomena of circular polarizations,and then a dynamic anomalous reflection manipulation is demonstrated by using GST-225.In the proposed gradient phase metasurface,anomalous reflection efficiency and the extrinction ratio reach up to 41.3%and 30 d B,respectively.By optimizing the metallic layer thickness,the anomalous reflection efficiency achieves a maximum value of 75%.Treating the anamolous angle as the siginal,the structure is employed to analyze the changes of refractive index of the surrounding medium.A dynamic anomalous reflection is studied by integrating GST-225 layer to the bilayer complentry gradient phase metasurface.The anomalous reflection efficiency reaches 86%and the modulation depth achieves 95.3%.