\title{Structural Color from Colloidal Glasses}
\author{Sofia Magkiriadou}
\degreemonth{November} % month final submission occurs.
\degreeyear{2014}
\degree{Doctor of Philosophy}
\field{Physics}
\department{The Department of Physics}
\advisor{Vinothan N. Manoharan} % 

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\begin{abstract}
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% Also limited to 350 words. I claim $\mu \sim \omega^4$ is a single word.

When a material has inhomogeneities at a lengthscale comparable to the wavelength of light, interference can give rise to structural colors: colors that originate from the interaction of the material's microstructure with light and do not require absorbing dyes. In this thesis we study a class of these materials, called \textit{photonic glasses}, where the inhomogeneities form a dense and random arrangement. Photonic glasses have angle-independent structural colors that look like those of conventional dyes. However, when this work started, there was only a handful of colors accessible with photonic glasses, mostly hues of blue.

We use various types of colloidal particles to make photonic glasses, and we study, both theoretically and experimentally, how the optical properties of these glasses relate to their structure and constituent particles. Based on our observations from glasses of conventional particles, we construct a theoretical model that explains the scarcity of yellow, orange, and red photonic glasses. Guided by this model, we develop novel colloidal systems that allow a higher degree of control over structural color. We assemble glasses of soft, core-shell particles with scattering cores and transparent shells, where the resonant wavelength can be tuned independently of the reflectivity. We then encapsulate glasses of these core-shell particles into emulsion droplets of tunable size; in this system, we observe, for the first time, angle-independent structural colors that cover the entire visible spectrum. To enhance color saturation, we begin experimenting with \textit{inverse} glasses, where the refractive index of the particles is lower than the refractive index of the medium, with promising results. Finally, based on our theoretical model for scattering from colloidal glasses, we begin an exploration of the color gamut that could be achieved with this technique, and we find that photonic glasses are a promising approach to a new type of long-lasting, non-toxic, and tunable pigment.

\end{abstract}

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\noindent This thesis is largely based on the following publications:

\begin{itemize}

 \item Chapter 2: Sofia Magkiriadou, Jin-Gyu Park, Young-Seok Kim, and Vinothan N. Manoharan, ``Disordered packings of core-shell particles with angle-independent structural colors.'' \textit{Optical Materials Express} \textbf{2}(10), 1343--1352 (2012). 

 \item Chapter 4: Jin-Gyu Park, Shin-Hyun Kim, Sofia Magkiriadou, Tae Min Choi, Young-Seok Kim, and Vinothan N. Manoharan, ``Full-spectrum photonic pigments with non-iridescent structural colors through colloidal assembly.'' \textit{Angewandte Chemie} \textbf{126}(11), 2943--2947 (2014).

 \item Chapter 3: Sofia Magkiriadou, Jin-Gyu Park, Young-Seok Kim, and Vinothan N. Manoharan, ``Absence of red structural color in photonic glasses, bird feathers, and certain beetles.'' \textit{Physical Review E} \textbf{90}, 062302 (2014). 

\end{itemize}
%\noindent Some of the work presented in this thesis has resulted in the following patent:

%\begin{itemize}

% \item Vinothan N. Manoharan, Sofia Magkiriadou, and Jin-Gyu Park, ``Photonic balls containing a microstructure of core-shell particles exhibiting angularly-independent structural color.'' US20140254017 (2014).

%\end{itemize}
\noindent The author has also contributed to:

\begin{itemize}

 \item W. Benjamin Rogers, Madeleine Corbett, Sofia Magkiriadou, Philippe Guarillof, and Vinothan N. Manoharan, ``Breaking trade-offs between translucency and diffusion in particle-doped films.'' \textit{Optical Materials Express} \textbf{4}(12), 2621--2631 (2014). 

\end{itemize}

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\begin{acknowledgments}

Where to begin! Graduate school has been a learning experience in science and beyond, and I am lucky to have encountered many people along the way who taught me not only with their knowledge, but also with their wisdom.

First and foremost, I am thankful to my research advisor, Vinny Manoharan. I think one of the most important things I have learned while working with Vinny is to loose fear of doing things that I do not know how to do. Moreover, I have learned from his methodical ways and from his high standards of writing papers and giving presentations. 
 
I cannot imagine how any of this work would have been done without Jin-Gyu Park. Jin-Gyu is not only an excellent chemist that can produce a variety of colloidal particles upon request, but also an insightful conversationalist with questions that make me think hard. 
 
My first experiments on structural colors were based on the work of Jason Forster and Eric Dufresne. They both welcomed me warmly into the field and shared all that they knew; I am grateful for all the support that I received from them. Also a Yalie, Rick Prum, who first understood that the feathers of blue birds are structurally colored, has always been excited to exchange biology for physics facts; it has been very interesting to learn about structural colors from his point of view.

When I joined the Manoharan group, I had the luck to be surrounded by the experimental genius of Dave Kaz. Not only did I thoroughly enjoy our conversations about machining, but also I learned that it is possible to make a high-quality working apparatus from unlikely parts. Of the same generation was Ryan McGorthy, who helped me gradually feel at home in the wet lab - starting with teaching me how to pipette. For my initiation to microscopy, I have to thank Guangnan Meng. Guangnan was also my deskmate for many months, during which he developed the ability to sense when I was confused without even looking at me; this prompted many good conversations, on science and on life.

On my other side sat for many months Becca Perry, who always has an ear available for trouble-shooting and who has actively shown her support from the day I joined the group. Becca and Tom Dimiduk have been my invaluable resources for questions related to programming. Ben Rogers has taught me a lot with his clear way of thinking and infallible sense of perspective. Nick Schade has been my default resource for all the basic physics I should already know; a great consultant for preparing talks; and a reliable source of good spirits.  

Beautiful colloidal particles and great suggestions for experiments came from Tom Kodger and Rodrigo Guerra, both of the Weitz group. Young-Seok Kim, from the Korea Electronics Technology Institute; Prof. Gi-Ra Yi, from Sungkyunkwan University; and Shin-Hyun Kim, now a professor at the Korea Advanced Institute of Science and Technology, have also contributed significantly to this work, with materials and with ideas. 

Nicolas Vogel, now a professor at the University of Erlangen-N{\"u}rnberg, was integral in our experiments on inverse glasses and an enthusiastic lab companion. Mathias Kolle offered a lot of advice on spectrometry techniques.

It was very fun and instructive to teach electronics lab with Tom Hayes, and an honor to know Stan Cotreau and have taken his machine shop class.

Melissa Franklin, Jacob Barandes, and Jim McArthur were generous with their wise words throughout graduate school. 

I am grateful to Elise Novitski for her constant support during this time, and for proof-reading this thesis.

Finally, I would probably be somewhere else, thousands of miles away from here, if it were not for my family's invincible and contageous optimism. 
 
\end{acknowledgments}

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