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Optical and Structural Characterization of Natural Nanostructures

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Optical and Structural Characterization of Natural Nanostructures Link¨opingStudies in Science and Technology Dissertation No. 1795 Optical and Structural Characterization of Natural Nanostructures L´ıaFern´andezdel R´ıo Department of Physics, Chemistry and Biology (IFM) Link¨opingUniversity, SE-581 83 Link¨oping,Sweden Link¨oping2016 The picture on the front page shows the scarab beetle Chrysina gloriosa and the background is an optical microscopy image form its exoskeleton. During the course of research underlying this thesis, L´ıaFern´andezdel R´ıo was enrolled in Agora Materiae and Forum Scientium, multidisciplinary doctoral programs at Link¨opingUniversity, Sweden. c L´ıaFern´andezdel R´ıo ISBN 978{91{7685{670{3 ISSN 0345{7524 Printed by LiU-Tryck, Link¨oping2016 A mi familia COLORS are the smile of Nature Leigh Hunt iv Abstract The spectacular biodiversity of our planet is the result of millions of years of evo- lution. Over this time animals and plants have evolved and adapted to different environments, developing specific behavioral and physical adaptations to increase their chances of survival. During the last centuries human's curiosity has pushed us to study and understand the phenomena and mechanisms of the nature that surrounds us. This understanding has even led to the fields of biomimetics where we seek solutions to human challenges by emulating nature. Scarab beetles (from the insect family Scarabaeidae) have fascinated humans for centuries due to the brilliant metallic shine of their chitin-rich exoskeletons and more recently for their ability to polarize reflected light. This doctoral thesis fo- cuses on the optical characterization of the polarized reflected light from beetles in the Chrysina genus, although beetles from other genera also have been inves- tigated. All the Chrysina beetles studied here share one characteristic, they all reflect left-handed near-circular polarized light. In some cases we also detect right- handed polarized light. We have observed two different main behaviors among the studied Chrysina bee- tles. Those which are green-colored scatter the reflected polarized light, whereas those with metallic appearance are broadband specular reflectors. We present a detailed analysis of the optical properties with Mueller-matrix spectroscopic ellip- sometry combined with optical- and electron-microscopy studies of the exoskele- tons. This allow us to create a model that reproduces the optical properties of these structures. The model consists of a chiral (helicoidal) multilayer structure with a gradual change of the pitch and a constant rotation of the optic axis of the layers. Beetles are not alone to have polarizing structures in nature and it is known that many birds and insects have the ability to detect linearly polarized light. This raises the question of whether the polarization properties of the beetles are the direct or indirect results of evolution or just pure coincidence. In order to get a better understanding of the possible reasons of this particular ability, we present a simulation study of different possible scenarios in nature where incoming light could be polarized or unpolarized, and where we consider detectors (eyes) sensitive to different states of polarized light. If the beetles are able to use this character- istic for camouflage, to confuse predators or for intraspecific communication is, however, still unknown and requires further investigation. My research results provide deeper understanding of the properties of light re- flected on the beetle's exoskeleton and the nanostructures responsible for the polarization of the reflected light. The developed model could be used as bio- inspiration for the fabrication of novel nano-optical devices. My results can also complement biological behavioral experiments aiming to understand the purposes of this specific optical characteristics in nature. v vi Popul¨arvetenskaplig sammanfattning Under de senaste ˚arhundradena har m¨anniskans nyfikenhet drivit oss att studera och f¨orst˚abiologiska fenomen och mekanismer. Detta har ¨aven lett till utvecklingen av s˚akallad biomimetik d¨arvi s¨oker l¨osningar p˚am¨anskligaproblem genom att efterlikna naturen. Skalbaggar fr˚anfamiljen bladhorningar (Scarabaeidae) har fascinerat m¨anniskor i m˚anga˚arp˚agrund av de ofta har metallgl¨ansandeskal och redan i forntida Egypten anv¨andesde som smycken och amuletter. Numera ¨arderas f¨orm˚agaatt reflektera polariserat ljus en orsak till att m˚angaforskare visar ett stort intresse, framf¨oralltp˚agrund av att dessa optiska effekter inte ¨ars˚avanliga i naturen. Att ljuset ¨arpolariserat betyder att ljusv˚agorr¨orsig i samma plan. Polariserat ljus kan bli linj¨art,elliptiskt eller cirkul¨art. A˚ andra sidan, i opolariserat ljus, som solljus, ¨arljusv˚agorspridda och r¨orsig i m˚angaplan. Polariserat ljus anv¨andsi, exempelvis, LCD-sk¨armar,3D filmer och polariserade solglas¨ogon. Mina forskningsresultat ger en djupare f¨orst˚aelseav egenskaperna av ljuset som skalbaggarna reflekterar. Dessutom ges en detaljerad beskrivning av den un- derliggande nanostruktur som ansvarar f¨orpolarisationsfenomenen. Mina resultat kan ocks˚akomplettera biologisk beteendeforskning med syfte att f¨orst˚ade specifika optiska egenskapernas roll i naturen. Min forskning fokuserar p˚aoptisk karakterisering av det reflekterade ljuset fr˚an skalbaggar av sl¨aktet Chrysina fr˚anCentralamerika. Det reflekterade ljuset kan f¨or vissa infallsvinklar och ljusv˚agl¨angder bli i det n¨armastecirkul¨arpolariserat. Oftast ¨ardetta ljus v¨ansterpolariserat. Skalbaggar ¨arinte de enda djur son har polariserande strukturer i naturen. Fj¨arilarreflekterar till exempel linj¨ar-polariserat ljus. Dessutom har m˚angaf˚aglar och insekter ocks˚af¨orm˚aganatt detektera linj¨artpolariserat ljus. Detta v¨acker fr˚aganom skalbaggarnas polarisationsegenskaper ¨arett resultat av evolution eller bara en ren tillf¨allighet.F¨oratt f˚aen b¨attref¨orst˚aelsef¨orde m¨ojligaorsakerna gjorde vi d¨af¨oren simuleringsstudie av olika m¨ojligascenarier i naturen, d¨ardet inkommande ljuset var polariserat eller opolariserat, och detektorerna (¨ogonen) var k¨ansligaf¨orolika polarisationstillst˚and. I mina studier observerar jag tv˚atyper av reflektion hos de skalbaggar som studerats. Skalbaggar som ¨argr¨onf¨argadesprider det reflekterade ljuset medan skalbaggar med metalliskt utseende ¨ar mer spegelreflekterande. I forskningen anv¨andsMueller-matrisellipsometri, en avancerad teknik som ger en detaljerad analys av skalets optiska egenskaper. Denna teknik har kombinerats med mikroskopi. Detta till˚ateross att skapa en modell som ˚atergerde optiska egenskaperna hos strukturerna. Modellen best˚arav en vriden flerskiktsstruktur med en gradvis f¨or¨andradstigning. Studierna kan d¨armedanv¨andassom biomimetisk inspiration, t. ex. f¨ortillverkning av nya nano-optiska komponenter. vii Acknowledgements It is very difficult to summarize in a few lines, how grateful I am to all the people that has been by my side during all this time, I will try to do my best. To begin with, I would like to explain that this thesis is part of my doctoral stud- ies and my Swedish adventure. It all started seven years ago, when I arrived to Sweden to study for one year. I fell in love with this country and its people so I decided to stay an extra year. During the second year I resolve to finish my degree in Sweden and that would not have been possible without Prof. Kenneth J¨arren- dahl's support. He not only helped me with all the administrative paperwork but he also gave me the opportunity to write my Bachelor diploma work with him at Applied Optics. And this was only the beginning. After that diploma work I wrote my Master's and then I was given the opportunity to continue with in this PhD. For all of this, for his constant support, supervision and comprehension I will always be thankful to my supervisor, Kenneth J¨arrendahl. I would also like to express my admiration for my co-supervisor Prof. Hans Arwin. Thanks for always being there to help me under any circumstance, for sharing your knowledge and for introducing Karin to me. Karin, you a very special person in my life, you know it. I am grateful to my second co-supervisor, Arturo Mendoza- Galv´an, for challenging me and guiding me even from the distance, although com- munication become a bit tricky when I end up in his spam list. Jan Landin, thank you for your invaluable help with the beetles, for sharing your knowledge and sav- ing our little collection from being eaten by the terrible "museum beetles". To all my coworkers, Torun, Roger, Christina, Anna Maria, Irina, Sergiy and Lars, thanks for your help in countless fields (science, administration, computers, Swedish homework,...) and interesting fikor! I also want to thank Maria for her help with the TEM images, it has been a pleasure working with you. During this time I have met a lot of people that have made all of this more en- joyable. Thanks to all of them, specially to my very good friends F´atima,Cecilia, Lida, Abeni, Elaine and Zhafira for all lunches, dinners, fikor, discussions and laughs. To Rafa, thanks for everything, it was fun! But all of this would have not been possible without the constant support from my family and to thank them, I am sorry, I should continue in Spanish. Mam´a,Pap´a vosotros s´ıque val´eismucho. Yaya, Elisa, Sandra, Sergio, Elena, Alba, J. Carlos, Kiki, Joel gracias por estar tan cerca a pesar de la distancia. A mi familia adop- tiva, especialmente a Blanca, gracias. Y sobretodo gracias a la persona que me convenci´ode quedarme en Suecia, gracias por retarme a superarme cada d´ıay por estar siempre ah´ıpara ayudarme, gracias Alberto. Lía Fernández del Río Linköping, November 2016 x Contents I Introduction 1 1 Background 3 2 Theory 5 2.1 Optical parameters . 5 2.2 Polarized light . 7 2.3 The polarization ellipse . 9 2.4 The Stokes vector . 10 2.5 The Mueller matrix . 11 3 Reflection and transmission characteristics 13 3.1 Basic reflection and transmission characteristics . 13 3.1.1 Reflection at a plane isotropic surface .
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