Study Of Silicon-based Mid-infrared Subwavelength Planar Optical Lens

Silicon is the most critical material in modern semiconductor technology,and also an important optical material because of its high refractive index and high transmission,which can find wide applications in various fields such as silicon photonics,mid/far-infrared imaging.Silicon based planar optics gains its popularity with the interaction between mature semiconductor processing technology and micro-optics,various micro optical components were developed such as planar waveguide,photonic crystal,micro-ring resonator...,etc.Silicon microlens is an important branch of planar optics,which exhibits advantages over traditional bulk devices in terms of CMOS compatible mass production and suitable for light-weight,arrayed,large scale integrated configuration,some of the devices have been widely used in fields such as detectors,display,photovoltaic.The basic understanding of these devices,however,was not extensively studied and most of these models were semi-empirical based,and also traditional planar optics has its intrinsic drawbacks and therefore new devices need to be developed in order to meets the trends of miniaturization of next generation optical devices.In this thesis,a systematical study of binary microlens arrays including simulation,design,fabrication and measurement was carried out,moreover,new concepts of silicon based planar optical devices were explored including overgrowth microlens and metasurface based dielectric metalens.The main work is as follows:1)Binary microlens arrays analysis and design by rigorous vector model:traditionally most of the analysis of binary optics was based on scalar theory,which fails to predict the optical properties and sometimes give misleading results with the miniaturization of devices with the size comparable with working wavelength.Rigorous vector electromagnetic method(FDTD)was applied to study the performance of binary microlens arrays in terms of fabrication error,f number,and incident angle.A novel integrated design algorithm employing rigorous FDTD model and simulated annealing optimization algorithm was proposed and used to design the structure of the binary microlens arrays,diffraction efficiency was dramatically improved compared with the original structure.2)Fabrication,characterization,and focal properties analysis of binary microlens arrays:fabrication processes were systematically optimized and microlens arrays was obtained with high precision and uniformity,optical measurement system was setup and optical properties such as focusing efficiency,points spread function,focal depth of silicon microlens was characterized.It is found that the focal shift of mid-infrared microlens arrays was larger than expected compared with traditional model,and it was studied using rigorous FDTD model and measurement analysis,which provide reference value for the integration of mid-infrared micro-optical system.3)Convex micro/nano silicon microlens fabrication by overgrowth using chemical vapour deposition:traditional binary micro-optics has its intrinsic drawbacks such as multi-alignment lithography,low diffraction efficiency.Novel fabrication method by a-Si:H chemical vapour overgrowth deposition was proposed,the PECVD deposition recipe was optimized in order to get film with high transmission,good surface quality,low film stress.High quality convex micro/nano lens array was obtained and the relation between lens structure and deposition parameters was deduced.The lens arrays was characterized at telecommunication and mid-infrared wavelength.4)All dielectric based mid-infrared metalens:all dielectric metasurface was studied in mid-infrared,rigorous coupled wave algorithm(RCWA)was employed to study the transmission and phase delay caused by silicon scatters,and the metalens structure was designed using RCWA.a-Si:H on MgF2 structure was used in order to increase the transmission efficiency.Various fabrication process including EBL,ICP,Liftoff was optimized and dielectric pillars was abtained with high aspect ratio and uniformity.Measurement results show that the metalens can lead diffraction limited focus and has wavelength-scale imaging with the resolution comparable with commercial counterparts,the performance can be further improved by optimizing design strategy and minimizing fabrication error.Note that mid-infrared wavelength is almost ten times longer than visual band and MIR optics is far less developed than that of visual band,which means that MIR metalens have advantages over visual counterparts in terms of mass producible production,the author anticipates that MIR metalens will firstly find its real word application and commercialized in the near future.