Investigation of Optical Effects of Chalcogenide Glass in Precision Glass Molding and Applications on Infrared Micro Optical Manufacturing Dissertation Presented in Partial Fulfillment of the Requirements for the Degree Doctor of Philosophy in the Graduate School of The Ohio State University By Lin ZHANG, M.S. Graduate Program in Industrial and Systems Engineering The Ohio State University 2019 Dissertation Committee Professor Allen Y. Yi, Advisor Professor Jose M. Castro Professor Jerald Ralph Brevick 1 Copyrighted by Lin Zhang 2019 2 Abstract Precision glass molding (PGM) is being considered as an alternative to traditional methods of manufacturing large-volume, high-quality and low-cost optical components. In this process, glass optics is fabricated by replicating optical features from precision molds to glass at elevated temperature. Chalcogenide glasses are emerging as alternative infrared materials for their wide range infrared transmission, high refractive index and low phonon energy. In addition, chalcogenide glasses can be readily molded into precision optics at elevated temperature, slightly above its glass transition temperature (Tg), which in general is much lower compared to oxide glasses. The primary goal of this research is to evaluate the thermoforming mechanism of chalcogenide glass around Tg and investigate its refractive index change and residual stresses in molded lens in and post PGM. Firstly, a constitutive model is introduced to precisely predict the material behavior in PGM by integrating subroutines into a commercial finite element method (FEM) software. This modeling approach utilizes the Williams-Landel-Ferry (WLF) equation and Tool- Narayanaswamy-Moynihan (TNM) model to describe (shear) stress relaxation and structural relaxation behaviors, respectively. It is predicted that ‘index drop’ occurred inside the molded prism due to rapid thermal cycling and the cooling rate above Tg can introduce large geometry deviations to the molded optical lens. Secondly, the refractive index variations inside molded lenses are further ii evaluated by measuring deviation angle through a prism & wavefront changes through molded lens using a Shack-Hartmann wavefront sensor (SHS), while the residual stresses trapped inside the molded lenses are obtained by using a birefringence method. Measured results of the molded infrared lenses combining numerical simulation provide an opportunity for optical manufacturers to achieve a better understanding of the mechanism and optical performance variation of chalcogenide glasses in and post PGM. Upon completion of the aforementioned research, two typical micro IR optics are designed, fabricated and tested, an infrared SHS and a large field-of-view (FOV) microlens array, as demonstrations. A novel fabrication method combining virtual spindle based high-speed single- point diamond milling and PGM process is adopted to fabricate infrared microlens array. The uniqueness of the virtual spindle based single-point diamond milling is that the surface features can be constructed sequentially by spacing the virtual spindle axis at an arbitrary position based on a combination of rotational and transitional motions of the machine tool. After the mold insert is machined, it is employed to replicate the optical profile onto chalcogenide glass. On the other hand, an infrared compound-eye system consisting of 3×3 channels for a FOV of 48°×48° is developed. The freeform microlens array on a flat surface is utilized to steer and focus the incident light from all three dimensions (3D) to a two-dimension (2D) infrared imager. Using raytracing, the profiles of the freeform microlenses of each channel are optimized to obtain the best imaging performance. To avoid crosstalk among adjacent channels, a micro aperture array fabricated by 3D printing is mounted between the microlens array and IR imager. The imaging tests of the infrared compound- eye imaging system show that the asymmetrical freeform lenslets are capable of steering and forming legible images within the design FOV. Compared to a conventional infrared camera, this iii novel microlens array can achieve a considerably larger FOV while maintaining low manufacturing cost without sacrificing image quality. Finally, two rapid heating processes are explored and demonstrated by using graphene-coated silicon as an effective and high-performance mold material for precision glass molding. One process is based on induction heating and the other one is based on mid-infrared radiation. Since the graphene coating is very thin (~45 nm), a high heating rate of 5~20 C/s can be achieved. The contact surface of the Si mold and the polymer substrate can be heated above the Tg within 20 s and subsequently cooled down to room temperature within tens of seconds after molding. The feasibility of this process is validated by fabrication of optical gratings, micropillar matrices, and microlens arrays on polymethylmethacrylate (PMMA) substrate with high precision. The uniformity and surface geometries of the replicated optical elements are evaluated using an optical profilometer. Compared with conventional bulk heating molding process, this novel rapid localized heating process could improve replication efficiency with better geometrical fidelity. iv Dedication This document is dedicated to my family. v Acknowledgments The present research work was carried out at Department of Integrated Systems Engineering at the Ohio State University and with the support of II-IV foundation block-gift research program since 2015. It has been a great honor to have such an opportunity. I would like first to express my sincere gratitude to my advisor, Professor Allen Y. Yi, who provides me the opportunity to work in the field of precision optical manufacturing. I appreciate his guidance, encouragement and support throughout my Ph.D. study, and also for his careful reviews of my publications and dissertation. I would like to thank Professor Jose M. Castro and Professor Jerald Brevick for their guidance, suggestions and service on my doctoral committee. I want to thank previous senior lab fellows, Dr. Lei Li, Dr. Peng He, Dr. Xin Zhao and Dr. Jian Zhou, who shared the knowledge of glass forming science and simulation related skills without reservation. Special thanks goes to Dr. Hui Li, who collaborated with me and provided great help on chalcogenide glass molding project. Without the help of those senior lab fellows, my research project would not have a good start. I want to thank my officemate, Wenchen Zhou, who collaborated with me on various projects in the past thousands of days and nights. I am also grateful to work with other excellent labmates, Dr. Yufeng Yan, Dr. Xiaohua Liu, Dr. Neil Naples, Dr. Dan Zhang, Dr. Min Wu, Tiantong Chen, vi Kaiyu Cai, Junjie Pan and many other members in Professor Jose M. Castro and Professor James L. Lee's groups. I want to thank the assistance from the machine shop supervisors, Joshua Hassenzahl and William Tullos, in Department of Integrated Systems Engineering. They taught me and allowed me to use most of the machine tools in the workshop and provided me valuable suggestions on machining. I want to thank Derek A. Ditmer and Dave Hollingshead at OSU Nanotech West for their help on photolithography and other related processes. Finally, I would like to express my sincerest appreciation to my wife, Xue Ji, for her faith and love to me. The same moral and mental understanding support us to pursuit excellence. I am also indebted to my parents, brothers, sisters, grandparents and parents-in-law for their endless and unflagging support in my life. vii Vita August, 1988 ..................................................Born, China July, 2011 .......................................................B.S., College of Mechanical Science & Engineering, Jilin University, Changchun, Jilin, China July, 2014 .......................................................M.S., College of Mechanical Science & Engineering, Jilin University, Changchun, Jilin, China August, 2015 to present .................................Graduate Research Associate, Department of Industrial and Systems Engineering, The Ohio State University, Columbus, Ohio, USA viii Publications Journal Publications: 1. L. Zhang, N. J. Naples, W. C. Zhou, & A. Y. Yi “Fabrication of infrared hexagonal microlens array by novel diamond turning method and precision glass molding”, Journal of Micromechanics and Microengineering 29, 065004 (2019). 2. L. Zhang, & H. Huang “Micro machining of bulk metallic glasses: a review”, The International Journal of Advanced Manufacturing Technology 100, 637–661 (2019). 3. L. Zhang, W. C. Zhou, & A. Y. Yi “Investigation of thermoforming mechanism and optical properties’ change of chalcogenide glass in precision glass molding”, Applied Optics 57, 6358- 6368 (2018). 4. L. Zhang, W. C. Zhou, N. J. Naples, & A. Y. Yi “Investigation of index change in compression molding of As40Se50S10 chalcogenide glass”, Applied Optics 57, 4245-4252 (2018). 5. L. Zhang, W. C. Zhou, N. J. Naples, & A. Y. Yi “Fabrication of an infrared Shack–Hartmann sensor by combining high-speed single-point diamond milling and precision compression molding processes”, Applied Optics 57, 3598-3605 (2018). 6. L. Zhang, W. C. Zhou, & A. Y. Yi “Rapid localized heating of graphene coating on a silicon mold by induction for precision molding of polymer optics”, Optics Letters 42, 1369-1372 (2017). ix 7. M. Wu, L. Zhang, E. D. Cabrera, J. J. Pan, H. Yang, D. Zhang, Z. G. Yang, J. F. Yu, J. M. Castro, H. X. Huang,