Voltage Tunable Mid-wave Infrared Reflective Varifocal Metalens Via An Optomechanic Cavity

The metasurface is an artificial optical material formed by an array of subwavelength-scale optical antennas whose response regarding the amplitude,phase,and polarization of light can be specifically designed to achieve an ultrathin planar lens.Most of the early studies on metalenses were on static metasurfaces.However,in fields such as optical imaging and modulation,the lens’s focal length is often required to be adjustable within a certain range.In recent years,many scholars have used different methods to realize zoom metalens,but they suffer from various problems such as low response speed,huge driving elements,polarization dependence,and complicated fabrication processes.In addition,they operate mostly in the visible,near-infrared,and terahertz bands,ignoring the mid-infrared band with important applications such as gas detection and thermal imaging.To address these problems,this thesis investigates a mid-infrared electrically tunable reflective zoom metasurface lens based on an optical-mechanical cavity and discusses two modes.This thesis firstly introduces the principle of metalens and electrostatic microelectromechanical system(MEMS),followed by the design scheme and principle of the zoom metalens.After that,the phase and reflectivity characteristics of the metalens unit are simulated using FDTD Solutions software;the variation of the cavity reflector curvature with voltage is simulated using COMSOL Multiphysics software;then the cavity curvature data are imported into FDTD Solutions,and the varifocal effect is simulated in conjunction with the metalens.As a result,with the initial focal length of 80.35 μm,the maximum focal length variation of mode 1 is 20.3 μm,and the maximum variation of mode 2 is 7.25 μm.Then,the dispersion characteristics and angular characteristics of the two modes devices were investigated in this thesis.The results show that the device is insensitive to wavelength and can achieve the varifocal function at other wavelengths;the device can work normally at 15° oblique incidence,and the operating mode is destroyed at 30° oblique incidence,where the device performance is significantly degraded.Finally,this thesis presents an application scenario for the dynamic coupling of mid-infrared quantum cascade lasers to mid-infrared fibers using the studied devices.The results show that the maximum dynamic range of coupling efficiency is 9%to 38%.This thesis proposes a practical research method of introducing mechanical structure changes together with optical structure into the simulation and explores the effect of mechanical structure curvature changes on the optical field.It expands the working wavelength of the varifocal metalens,filling the gap in the mid-infrared field,and extends the practical applications of the varifocal metalens.