Design and realization of flat optical filters for novel applications

FACULTY OF ENGINEERING

ANDARCHTECTURE

Design and Realization of Flat Optical Filters for Novel Applications

Boxuan Gao

Doctoral dissertation submitted to obtain the academic degree of Doctor of Photonics Engineering

Supervisors

Prof . Jeroen Beeckman , PhD - Prof Kristiaan Neyts , PhD

Department of Electronics and information ^Systems

Faculty of Engineering and Architecture , Ghent University

December 2020

GHENT

UNIVERSITY

Design and Realization of Flat Optical Filters for Novel Applications

Boxuan Gao

Doctoral dissertation submitted to obtain the academic degree of Doctor of Photonics Engineering

Prof. Jeroen Beeckman, PhD - Prof. Kristiaan Neyts, PhD

Department of Electronics and Information Systems Faculty of Engineering and Architecture, Ghent University Supervisors

December 2020

llh FACULTY OF ENGINEERING AND ARCHITECTURE

lllllll

GHENT

UNIVERSITY

Wettelijk depot: D/2020/10.500/125 NUR 926, 959

Members of the Examination Board

Chair

Prof. Em. Daniël De Zutter, PhD, Ghent University

Other members entitled to vote

Ties De Jong, PhD, Merck Window Technologies B.V., the Netherlands Prof. Herbert De Smet, PhD, Ghent University

Prof. Youri Meuret, PhD, KU Leuven Prof. Philippe Smet, PhD, Ghent University

Prof. Guofu Zhou, PhD, South China Normal University, China

Supervisors

Prof. Jeroen Beeckman, PhD, Ghent University Prof. Kristiaan Neyts, PhD, Ghent University

Contents

Acknowledgements v

Samenvatting vii

Summary xi

List of Figures xv

List of Tables xxv

List of Abbreviations xxvii

List of Publications xxix

1 Introduction 1

1.1 Motivation . . . 2

1.1.1 Projectors . . . 2

1.1.2 Smart Windows . . . 5

1.2 Thesis Objectives . . . 6

1.3 Thesis Structure . . . 8

2 Design and Realization of a DBR Filter 9 2.1 Introduction . . . 9

2.2 Theoretical Investigation . . . 11

2.2.1 Layer Numbers and Refractive Index Contrast . . . 12

2.2.2 Layer Thickness Investigation . . . 12

2.2.3 Inclination of Incidence . . . 16

2.3 Application Objectives and DBR Design . . . 17 i

2.3.1 Application Overview . . . 17

2.3.2 Simulation Model Development . . . 19

2.3.3 Optimization Method . . . 24

2.3.4 Optimized Design . . . 26

2.4 Device Fabrication . . . 28

2.4.1 Material Investigation . . . 28

2.4.2 Conventional DBR Fabrication . . . 34

2.4.3 Designed DBR Fabrication . . . 36

2.5 Device Characterization and Discussion . . . 38

2.6 Summary . . . 40

3 Compact Beam Combiner Based on LC Grating 43 3.1 Introduction . . . 43

3.2 Theoretical Description . . . 45

3.2.1 Liquid Crystal . . . 45

3.2.2 Alignment Methods . . . 46

3.3 Design and Experiments . . . 48

3.3.1 Geometric Phase Grating Design . . . 48

3.3.2 Tests and Experiments . . . 51

3.4 Diffractive Laser Beam Combiner . . . 66

3.5 Summary . . . 74

4 Smart Windows Investigation 75 4.1 Background . . . 75

4.2 Chemical Scattering Windows . . . 77

4.2.1 Introduction . . . 77

4.2.2 Cell Preparation . . . 79

4.2.3 Optical Measurements . . . 79

4.2.4 Result and Discussion . . . 83

4.2.5 Discussion and further work . . . 87

4.3 Liquid Crystal Smart Window . . . 87

4.3.1 Introduction . . . 87

4.3.2 Theory and Method . . . 89

4.3.3 Sinusoidal Frequency Domain Numerical Simulation and Experiment . . . 93

CONTENTS iii

4.3.4 Time Domain Simulation and Experiment . . . 103

4.3.5 Conclusion . . . 104

4.4 Summary . . . 106

5 Conclusions and Future Work 107 5.1 DBR filter for the collimation of Lambertian emitted light . . . 107

5.2 LC thin film for laser beam combining . . . 108

5.3 Smart windows for light modulation . . . 109

5.3.1 Hydrogel based scattering windows . . . 109

5.3.2 Electrically driven LC window . . . 110

Bibliography 111

Acknowledgements

On the completion of my PhD, there comes the realization that so much I have experienced and learned, the way I view on my study and my life has also been refreshed. All of these are attributed to the abundant experience I received during my almost six years here. I would like to express my deepest gratitude to all those whose kindness and advice that have made this happen.

First, I am very thankful to my promoters Professor Jeroen Beeckman and Professor Kristiaan Neyts for giving me the opportunity to be able to complete my PhD at the Liquid Crystal and Photonics Group, I have learned a lot from this experience and it is a valuable period of time for me. Your advice have always provided me with new ways of thinking and help me make improvement in my studying. I am also greatly indebted to my colleagues. I would like to thank Varsenik and Samira, who have helped me a lot to begin my PhD smoothly not only about the lab introduction but also in living here. I should also thank John, a responsible colleague who has spent a lot of time on helping me fabricating devices and giving me multitudes of valuable instructions on the process, I really enjoyed our discussion, from which I have benefited a lot. I am also grateful to Chun-Ta, although he has left the group years ago, he still discusses with me about the scientific work related to liquid crystals from time to time. These are really helpful for me to try to find ways to tackle with the difficulties. I also need to mention Migle and Ingrid, you two brought so many happy hours in the office and it really helps to get through the PhD. For Brecht and Xiangyu, my liquid crystal colleagues, I really enjoy the time we spent in the office as well as in the lab. I couldn’t express my gratitude to all of you one by one and word has failed me in this very moment, but you know what I want to say, Sheng, Frederik, Dang, Bavo, Yerzhan, Rogier, Lucas, Gilles, Inge, Filip, Johan... Thank you all for being my friendly LCP colleagues during these years and I would really miss all of you.

Moreover, I shall also convey my sincere thanks to all the my Chinese friends with whom I spent so many weekends and holidays, and shared so many mo- ments in Gent or around Europe. I hope we could still get together after our graduation!

Last my thanks would go to my beloved family and friends in China. During my six years’ studying in Gent, they have always been so supportive for me no matter where they are. I can only get over all the difficulties and finally

v

complete my study with their encouragement. Thank you for being there for me all the time.

I have spent such a long time in Gent and have encountered so many lovely people and moments. This small but beautiful city becomes an important part of my life. I would never forget the things happen here and wish one day I could have the luck to come back once more! Thank you, Gent!

Boxuan Gao Gent, November 29, 2020

Samenvatting

Vlakke optische filters worden gebruikt in heel wat toepassingen. Met een aangepast ontwerp kunnen specifieke optische functies gerealiseerd worden. In dit werk worden vlakke optische filters ontworpen en gefabriceerd voor twee soorten toepassingen: optische projectoren en slimme ramen. De grootte van de filters is een belangrijke factor want deze varieert van enkele millimeter tot meerdere meter. De filters die werden ontwikkeld en getest in dit doctoraat worden schematisch voorgesteld in de onderstaande figuur.

Figuur met gerealiseerde cellen (a) Een DBR filter om de fotoluminescente emissie in een ruimtehoek te verhogen. Het groene licht wordt doorgelaten voor kleine invalshoeken en wordt gereflecteerd voor grote invalshoeken. (b) Diffractief filter op basis van vloeibaar kristal met foto-alignering. Deze optische roosters kunnen gebruikt worden om laserbundels dichter bij elkaar te brengen. (c) Temperatuur-gestuurd hydrogel-gebaseerd slim raam dat licht verstrooit. (d) Commercieel vloeibaar-kristal-gebaseerd slim raam dat

elektrisch kan schakelen tussen een transparante toestand en een donkere niet-verstrooiende toestand (figuur van Merck [1]).

Projectiebeeldschermen worden vaak gebruikt in vergaderzalen en cinema’s.

Hoe het licht in zo’n projector wordt gegenereerd en hoe dit licht verder wordt verwerkt is belangrijk voor de beeldkwaliteit en de kijkervaring. Vaste-stoflichtbronnen (zoals LEDs of lasers) worden momenteel verkozen als lichtbron in projectiesys-

vii

(b)

emission

reflection layer

%

temen. Verdere verbeteringen zijn nog nodig om deze lichtbronnen optimaal in te zetten in projectoren. Een belangrijke parameter die behouden blijft tijdens projectie is ´etendue, een maat voor de geometrische dimensie van de lichtbun- del, die de diameter en de openingshoek van de bundel bevat. Het optische systeem van de projector heeft een maximale ´etendue en alleen de emissie van de lichtbron binnen deze ´etendue wordt gebruikt door de projector. In dit werk wordt gewerkt aan twee verschillende aspecten om de ´etendue van de lichtbron te verminderen. Reductie van de ´etendue biedt de mogelijkheid om een compactere optische projector te bouwen met een lagere kost.

Om de emissiehoek van een lichtgevende fotoluminescente laag te vermin- deren heb ik een optische component ontworpen die bestaat uit een opeenstapel- ing van dunne lagen, en Distributed Bragg Reflector (DBR, gedistribueerde Bragg reflector) wordt genoemd. Het is een multi-lagenstructuur opgebouwd uit alternerende lagen van materialen met lage en hoge brekingsindex. Als de dikte van de lagen in de DBR gelijk is aan een kwart golflengte, dan zal licht onder loodrechte inval met de juiste golflengte grotendeels gereflecteerd worden dankzij het fenomeen van constructieve interferentie aan de verschil- lende oppervlakken. Reflectie met hoge effici¨entie kan bekomen worden met een vrij dunne DBR en de verliezen kunnen laag zijn, waardoor zulke structuren vaak gebruikt worden in optische filters, micro-caviteiten, verticale-caviteit oppervlakte-emitterende lasers, fotonische kristallen, organische licht emitterende diodes en om lichtemissie te verbeteren. Hier wordt de DBR gebruikt om de ruimtehoek van de emissie te verminderen, namelijk door de lamberti- aanse emissie te vernauwen tot een kleinere ruimtehoek. Om dit te kunnen verwezenlijken laat de DBR licht binnen een bepaalde ruimtehoek door, ter- wijl licht buiten deze ruimtehoek gereflecteerd wordt. Het gereflecteerde licht komt terecht op het fosformateriaal, dat nu dienst doet als lambertiaanse ver- strooier. Ideaal gezien zal het licht, na enkele keren heen- en weer te kaat- sen, terechtkomen in de gewenste emissiehoek. In tegenstelling tot traditionele DBRs waarin hoge reflectie wordt bereikt voor loodrechte inval, wordt hier schuin invallend licht gereflecteerd. Bovendien is ook de transmissie van de DBR in dit werk zeer belangrijk. Een vlakke band met hoge transmissie is noodzakelijk voor licht onder een kleine ruimtehoek, zowel voor blauw als voor groen licht. Het ontwerp van deze aangepaste DBR is gebeurd met Matlab sim- ulaties. Een kwaliteitsfactor werd ingevoerd die het effect van de verschillende parameters combineert tot ´e´en getal. Honderden iteraties zijn typisch nodig om een geconvergeerde oplossing te vinden. Om een hoge transmissie te bekomen werd de depositie en nabehandeling van de dunne lagen geoptimaliseerd. De lagen van de gekozen materialen silicium dioxide (SiO2) en titantium dioxide (TiO2) werden uitvoerig optisch gekarakteriseerd. Een nagloeiprocedure werd ontwikkeld om de transparantie van de lagen te verbeteren. Metingen van de gefabriceerde filters tonen aan dat ze functioneren zoals verwacht. Naast het limiteren van de ´etendue van lambertiaanse emitters geeft dit onderzoek aan dat er andere mogelijkheden zijn om DBRs te gebruiken in optische toepassin- gen.

ix Een andere methode om de ´etendue te reduceren is door de oppervlakte van de bron te verminderen. In een projectiesysteem met laserbronnen zijn een groot aantal laserbronnen nodig om het vereiste vermogen te bereiken, aangezien het vermogen van een enkele bron gelimiteerd is. Onvermijdelijk leidt dit tot een verhoging van de totale oppervlakte van de gecombineerde lichtbron en van de totale ´etendue. Conventionele manieren om rijen van licht- bronnen samen te brengen zijn gebaseerd op spiegels of prisma’s die de afstand tussen de lichtbundels verkleinen. Dit resulteert typisch in zware en grote sys- temen. De fysieke afmetingen van de behuizing van de lasers limiteert hoe dicht de laserbundels bij elkaar kunnen gebracht worden en het veranderen van de richting is niet evident. Ik onderzocht daarom een compact alternatief met hoge effic¨entie om bundels samen te voegen. Vloeibaar kristal (Liquid Crys- tal, LC) is een materiaal dat gebruikt wordt in beeldschermen. Dit materiaal heeft anisotropie in de brekingsindex, en kan gebruikt worden om een half- golflengteplaatje te maken. Door de richting van de alignering te veranderen, kan de fase van circulair gepolariseerd licht dat door de laag gaat, gemoduleerd worden. Om de orientatie van de snelle as continu te veranderen wordt fo- toalignering gebruikt, hetgeen aanleiding geeft tot een geometrische fase ofwel Pancharathnam-Berry fase rooster. Fotoalignering is een methode om de richt- ing van LC te bepalen. Het principe van deze aligneringstechniek is dat de vloeibaarkristalmoleculen loodrecht aligneren op de lineaire polarisatierichting van de lichtbundel waarmee het materiaal beschenen wordt. In vergelijking met de klassieke techniek van mechanisch wrijven levert deze techniek veel meer flex- ibiliteit. In deze thesis wordt fotoalignering gerealiseerd door twee orthogonaal circulair gepolariseerde ultra-violet (UV) laser bundels te laten interfereren.

Het interferentiepatroon heeft een roterende lineaire polarisatierichting en dit patroon levert dan een vergelijkbare alignering van het LC. Een dunne laag LC met dit aligneringspatroon resulteert in een geometrisch faserooster dat zowel compact als effici¨ent is. Voor het combineren van laserbundels zijn ge- ometrische faseroosters nodig. Afhankelijk van de specifieke situatie kunnen deze roosters met verschillende parameters ontworpen worden. In deze thesis wordt aangetoond dat twee parallelle laserbundels met succes dichter bij ekaar kunnen gebracht worden.

Slimme ramen zijn schakelbare optische filters, maar typisch met veel grotere afmetingen, in de orde van ´e´en meter of meer. In deze thesis heb ik slimme ramen onderzocht met zowel hydrogels als met vloeibare kristallen als actief materiaal, waarbij enkele specifieke aspecten werden bekeken. De ontwikkeling van slimme ramen zit sinds enkele jaren in een stroomversnelling aangezien ze een belangrijke rol kunnen spelen in het terugdringen van het energiever- bruik in gebouwen. De twee types slimme ramen die werden onderzocht in deze thesis veranderen de transmissie van zonlicht door te schakelen tussen twee toestanden onder invloed van een externe stimulus. Zoals reeds vermeld in de vorige paragraaf is LC een anisotroop materiaal. Afhankelijk van de aligneringsrichting ondervindt het licht dat door de LC laag gaat een andere brekingsindex. In LC slimme ramen zit de actieve LC laag tussen twee glas- platen met een coating van een transparante geleidender. Het LC is normaal gezien parallel gealigneerd aan het oppervlak van het glas en het schakelen

tussen verschillende toestanden wordt bekomen door een elektrisch veld tussen de conductieve lagen aan te leggen. Het LC zal aligneren langs het elektrisch veld en dus loodrecht komen te staan op het glasoppervlak. Het aansturen met sinuso¨ıdaal vari¨erende spanning (Alternating Current, AC) is nodig omwille van kleine hoeveelheden ionen in het LC. Door de AC aansturing kunnen bepaalde fenomenen optreden omwille van de beperkte geleidbaarheid van de conductieve laag en de capaciteit van de LC laag. Voor componenten met afmetingen van enkele centimeter is dit effect bijna niet waarneembaar, maar voor componen- ten met grote afmetingen wordt het wel belangrijk. Om het gedrag van het LC te kunnen voorspellen in grote componenten werd een simulatieprogramma ge- bruikt, waarin twee gekoppelde niet-lineaire differentiaalvergelijkingen opgelost worden, rekening houdend met de complexe geometrie van het venster, door middel van de eindige elementenmethode. Een slim venster werd gemaakt en uitgemeten en de accuraatheid van het simulatiemodel werd geverifieerd.

De slimme ramen gebaseerd op hydrogel hebben een ander soort schakelmech- anisme. In deze thesis werden thermosensitieve hydrogels gebruikt. In vergeli- jking met de LC gebaseerde slimme ramen is dit een passieve component, in de zin dat er geen elektrisch signaal nodig is om het venster te doen schakelen of het het te houden in ´e´en van de toestanden. De hydrogels kunnen gemakkelijk opgelost worden in water en na oplossing is de vloeistof transparant. Als de omgevingstemperatuur (en de temperatuur van de vloeistof) stijgt boven een bepaalde temperatuur, dan ondergaat de hydrogel een fasetransitie en klontert het samen tot clusters, wat lichtverstrooiing tot gevolg heeft. Deze eigenschap kan gebruikt worden om zonlicht te weren in de zomer. Om inzicht te krijgen in de thermo-optische eigenschappen en om de meest geschikte oplossing te vin- den voor de toepassing van slimme ramen werden kleine prototypes gemaakt met verschillende polymeeroplossingen. Een optische opstelling werd gebouwd om lichtverstrooiing te karakteriseren. Met behulp van een integrerende bol werd de lichtverstrooiing in verschillende richtingen gecollecteerd en opgeme- ten. Het resultaat is dat vrijwel al het inkomende licht wordt verstrooid en een deel van het verstrooide licht wordt gevangen in de substraten waarna het uit de component lekt langs de zijkant. Als een opvolgproject werd de gese- lecteerde polymeeroplossing gebruikt voor een groter raam dat werd getest in een modelhuisje. Een xenon lamp werd hierbij gebruikt als artifici¨ele zon. Het slim raam bleek inderdaad een deel van het zonlicht buiten te houden.

Summary

Planar optical filters are used in various applications. With particular de- signs, various optical functionalities can be realized. In this thesis, planar optical filters are designed and fabricated for two types of applications: optical projection systems and smart windows. The size of the studied devices ranges from a few mm to several meter. The devices that have been developed and tested in this work are illustrated in the figure below.

Figure with studied devices. (a) DBR filter to increase photoluminescent emission within a cone angle. Green light is transmitted for small angles and reflected for larger angles. (b) Diffractive layer obtained by photoaligned LC.

The gratings can be used to combine two laser beams. (c) Hydrogel-based temperature-driven smart window that scatters light. (d) Commercial liquid crystal smart window, that can switch electrically between a transparent state and a dark non-scattering state (figure from Merck [1]).

Projection displays are widely used in meeting rooms and cinema’s. The way light is generated and processed in such a device is crucial for the image quality and the viewing experience. Solid state light (from LEDs or lasers) has emerged in recent years as the preferred light source in projection systems.

Improvements in the optical design are required to fully exploit the advantages of these new light sources. An important physical quantity that is conserved during projection is the ´etendue, a measure for the geometric dimension of the

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beam, including the area and solid angle. The optical system of the projector has a maximum ´etendue and therefore only the emission of the light source within this ´etendue can be used by the projector. In this work I present two approaches to limit the ´etendue of the light source. The reduction of the

´etendue of the light sources results in a compacter optical projection system, with a lower cost.

For limiting the solid angle of the emission from a phosphor, I have designed a stack of thin films that form a Distributed Bragg Reflector (DBR). It is a multilayer structure consisting of alternating layers of high and low refractive index materials. When the thickness of the layers equals a quarter-wavelength, normal incident light with the corresponding wavelength will be reflected due to constructive interference of all partial reflections. Reflection with high efficiency can be reached for relatively thin DBRs with low loss, and this principle has been widely used in various areas such as optical filters, micro-cavities, vertical- cavity surface-emitting lasers, photonic crystals, organic light emitting diodes and enhancement of emission efficiency. Here the DBR is used to decrease the solid angle of the emission, i.e. by collimating the originally lambertian emission into a smaller cone angles. To realize this, the emission that is within the required cone angle can be transmitted, while the light emitted outside the cone angle should be reflected back towards the light source. For the emission that is reflected back from the DBR, the light source acts in good approximation as a lambertian scatterer. Ideally, after a few bounces back and forth between the DBR and the light source, all the light should be emitted within the desired emission cone. In contrast to conventional DBRs where high reflectivity is obtained at normal incidence, here the DBR has to reflect oblique light. Furthermore, a high transmission band is required for incident light with small angles, for both the exciting blue light and the emitted green light. The design for this particular DBR has been made using Matlab simulations. A figure of merit is introduced in the simulation containing all the requirements.

Several hundreds of iterations are conducted to find an optimized solution.

To guarantee a high transmission, the chosen materials, titania (TiO2) and silicon dioxide (SiO2) have been selected because of their optical parameters. A post-annealing procedure has been developed to drastically enhance the layer transparency. The fabricated devices function well as can be seen from the measurements. In addition to reducing the ´etendue of a lambertian light source, this research can be seen as an approach to use DBRs in more complex optical applications.

Another approach for the reduction of the ´etendue is based on minimizing the area of a light source. In a projection system that uses laser light, it may be necessary to use a number of lasers to achieve a certain optical power, because the maximum optical power of an individual source is limited. Because each laser has a certain dimension, the total area of the combined light source is much larger than for a single laser beam. This increases the ´etendue of the light source and would require a projection system with a very large ´etendue.

A conventional way to interlace arrays of light sources is based on multiple reflections on mirrors or prisms. This approach results in bulky systems, the

xiii tolerances on the mechanical assembly limit how close the beams can be, and the choice of the beam redirection is not flexible. Therefore I investigated an- other approach for a high-efficiency beam combiner. Liquid crystal (LC) is an optical material that has been widely used in displays. It has an anisotropy in the refractive index and can be used to make a half-waveplate. By realizing a continuous rotation of the alignment direction over the surface, the phase of circularly polarized light that passes through the layer can be modulated con- tinuously. To vary the orientation of the fast axis, a photoalignment method is used to align the liquid crystal with a continuous rotation, leading to a ge- ometric or Pancharathnam-Berry phase grating. Photoalignment means that LC molecules align perpendicular to the linear polarization direction of the illumination of a UV laser beam. Compared with the conventional mechanical rubbing alignment, it provides more flexibility in spatial control. In this thesis, periodic photoalignment is realized by interference of two orthogonally circu- larly polarized ultra-violet (UV) laser beams. The interference pattern has a rotating linearly polarized direction. The resulting geometric phase grating is compact and efficient. For the beam combiner application, multiple of such geometric phase gratings are used to realize this functionality. Gratings can be designed with different parameters, according to the desired diffraction angle.

In this thesis, I have shown that two parallel red laser beams can be successfully brought closer to each othe.

Smart windows are switchable optical filters with a much larger size, in the order of a meter. In this thesis, I have investigated smart windows using both hydrogels and liquid crystal as active materials, focusing on different aspects.

In recent years, smart windows have been widely developed for the reduction of energy consumption in energy efficient buildings. This type of windows can change the transmission properties for solar radiation by switching between different states, in response to external stimuli. As mentioned in the previous paragraph, LC is an anisotropic material. According to the alignment direction, the light passing through the LC layer experiences different refractive indices.

In liquid crystal smart windows, the liquid crystal layer is held between two glass plates, which are coated with a transparent conductive layer. The LCs are normally pre-aligned parallel with the window surface, and the switching is realized by applying an electric field between the conductive layers on both surfaces. The LC will then align along the electric field, which is perpendicular to the window surface. Driving with Alternating Current (AC) is required due to the presence of small amounts of ions in LCs. However this also leads to an interplay between the resistance of the transparent electrodes and the capacitance of the liquid crystal layer. While the resistance causes no problems on centimeter-sized devices, the RC-effect becomes important large windows.

To predict the LC behavior in the large-sized windows, we have implemented two sets of differential equations in a simulation, and the complex geometry of the window is taken into account by means of a finite-element method. A corresponding smart window device has been fabricated to verify the accuracy of the simulation model.

The hydrogel-based smart windows, on the other hand, have another switch- ing mechanism. In this thesis, thermoresponsive hydrogels are investigated for smart windows. Compared with LC-based smart windows, this material can realize a passive smart window device, which means that no active driving is needed for switching between different states (or maintaining states). These hydrogels are dissolved in water and the solution is transparent under normal conditions. When the temperature increases beyond the cloud point, the ma- terial experiences a phase transition and aggregates into clusters, resulting in a scattering state. This property can be used in buildings to block strong sunlight during the summer. To have an insight in the thermo-optic properties and to select the appropriate candidates for smart window applications, small smart window prototypes have been fabricated with various polymer solutions and an optical set-up has been built to measure the scattering properties. With the help of an integrating sphere, scattering into different directions are collected and measured. The result shows that most light is scattered and that a small amount is trapped inside the cell and leaking out at the side of the cell. As a follow-up of the project, the selected polymer solution has been used in larger smart windows that are implemented in a model house. A xenon arc lamp is used as an artificial sun. The smart windows prove to be able to efficiently block sunlight.

List of Figures

1.1 Schematic overview of how an optical filter can process an inci- dent light beam. The light can be transmitted, diffracted, scat- tered or reflected, all with a certain wavelength dependency. . . 2 1.2 Inside view of a laser projector (image taken from https://ep

son.com/laser-projectors). The image shows two arrays of blue laser diodes as the light source. The blue laser light is sent to the rotating yellow phosphor ring to generate green and red light. The resulting white beam is split up in three parts, modulated, combined into a white beam again, and finally sent through the complex projector lens system. . . 3 1.3 An example of a phosphor wheel. Image is from (https://mate

rion.com/resource-center/newsletters/newsletter-archi ves/optical-innovation-news-2015-to-2017/next-generat ion-phosphor-wheel). . . 4 1.4 An example of a blue laser bank and its emission profile [2]. . . 4 2.1 Reflection mechanism of a Bragg reflector, taking into account

optical path length and phase change due to reflection. . . 10 2.2 Reflection curve simulated for an ideal DBR . . . 11 2.3 Reflectivity of the DBR versus wavelength with different num-

ber of pairs. The larger the number of pairs, the higher the reflectivity. . . 13 2.4 Reflectivity of DBR versus wavelength with different refractive

index contrasts. Higher contrast gives higher reflectivity and a larger bandwidth. . . 13 2.5 Reflectivity of a DBR versus wavelength for different thicknesses

of the two layers. With thicker layers the reflection band shifts to longer wavelengths. . . 14 2.6 Simulated DBR spectra with layer thickness equal to (a) a quarter-

wavelength and (b) three-quarter of a wavelength. . . 15 2.7 Simulated DBR spectra (a)without (b)with a FP layer. . . 15

xv

2.8 Illustration for a DBR with an inserted FP layer. . . 16 2.9 Optical path of the inclined incident light. . . 16 2.10 2D plots of the transmission of the DBR, as a function of both

angle of incidence and wavelength. (a) TE polarization, (b) TM polarization and (c) for unpolarized light. . . 17 2.11 (a) A schematic graph for ´etendue. (b) ´Etendue conserved in an

optical system. . . 18 2.12 Wavelength conversion setup: (a) conventional Lambertian dis-

tribution of the green emitting material. (b) Increased intensity in a 45^◦ cone angle by using a DBR to concentrate light within the cone. . . 19 2.13 Two-dimensional schematic representation of the design require-

ments for both incident angle and wavelength, the requirement for each region is indicated. . . 20 2.14 Transmission spectrum of a simple DBR with five pairs of layers. 21 2.15 The adjusted transmission spectrum for an eleven layer DBR

with high transmission in the required ranges. . . 22 2.16 (a) The 2D transmission spectrum for original quarter-wave DBR

(see table 2.1). (b) The 2D transmission spectrum for the altered structure with the thickness of the first and last layer replaced by eighth-wave thickness (27.48 nm). . . 23 2.17 (a) The 2D transmission spectrum for original quarter-wave DBR

(table 2.1). (b) The 2D transmission spectrum for the altered structure with the thickness of the first and last layer replaced by three-eighth-wave thickness (82.43 nm). . . 26 2.18 Transmission of the designed eleven-layer DBR. (a) Plot of the

transmission as a function of wavelength for different incident angles; (b) 2D plot of the transmission as a function of both wavelength and incident angle. The color represents the trans- mission value, with indication of the three targeted regions. (c) and (d) are the 2D plot for the performance TE and TM respec- tively. . . 27 2.19 A schematic for electron beam evaporation. . . 29 2.20 A schematic for the ellipsometer. . . 30 2.21 Light interaction with the thin film resulting in reflection and

refraction. . . 30 2.22 Both glass and silicon substrates are placed on the holder for

e-beam evaporation. . . 31 2.23 The fitting graph of Ψ and ∆ between the measurement and

build-in model for SiO2. . . 32

LIST OF FIGURES xvii 2.24 The fitting graph of Ψ and ∆ between the measurement and

build-in model for TiOx. . . 32 2.25 The extracted wavelength dependent refractive indices for (a)SiO2

(b)TiOx. . . 33 2.26 The spectrometer used to measure transmission spectrum (a)

Perkin Elmer 35 UV/VIS spectrometer; (b) a simplified schematic diagram for sample measurement. . . 33 2.27 Transmission spectra of TiOx under different annealing condi-

tions. . . 34 2.28 Transmission properties of the annealed TiO layer. (a)The com-

parison of the evaporated TiO layer before and after annealing (650^◦C in oxygen environment); (b)the measured transmission spectrum from the spectrometer. . . 34 2.29 The corresponding wavelength dependent refractive indices for

the two materials extracted from ellipsometer measurements. . 35 2.30 A schematic diagram for a conventional simple DBR. The thick-

ness of each layer stays the same respectively for the high and low refractive index layer. . . 35 2.31 Photographs of the seven-layer DBR trial. (a)Fabricated DBR

by e-beam evaporation. (b)DBR after annealing. . . 35 2.32 Transmission spectra of the fabricated device. Comparisons are

taken after one, three, five and seven layer depositions. . . 36 2.33 Evaporated devices after (a) one (b) three (c) five and (d) seven

layers have been deposited on the DBR. . . 37 2.34 Transmission spectra of the fabricated device. Comparisons with

simulations are taken after three, five, seven and eleven layer depositions of the DBR. . . 37 2.35 Cross-sectional SEM image of the fabricated eleven layer DBR

and measured thicknesses for the top two layers. The measured thicknesses in this image have limited accuracy due to thickness variations and the oblique side view of the SEM. . . 38 2.36 Special holder to measure the transmission with oblique incident,

the second glass is used to compensate for the lateral displace- ment. . . 38 2.37 Experimental and simulated transmission spectra (400 nm to

700 nm) of the fabricated DBR for different angles of incidence (a) normal incidence (b)θ=20^◦ (c)θ=40^◦ (d)θ=60^◦. The re- duction in the measured transmission under large incident angle is caused by the reflection of the compensating glass substrate in the holder. . . 39

2.38 Images of the fabricated sample observed under different angles with the city of Ghent in the background (a) normal view with green transmission; (b) 20^◦ inclination with green transmission;

(c) 40^◦ inclination with blue transmission; (d) 60^◦ inclination with purple transmission. . . 39 3.1 Photoalignment patterns on the substrate to realize a beam com-

biner. . . 45 3.2 The set-up to realize the beam combiner function. The two

gratings on the sample have the same LC rotating direction. . . 45 3.3 Photoalignment pattern including a cylindrical lens in the set-

up, the blue lines indicate the positions in the grating with the same director orientation. . . 45 3.4 (a) A schematic illustration of the LC state. (b) Definition of

azimuthal and zenithal angle. . . 46 3.5 LC molecules aligned planarly by mechanical rubbing. . . 47 3.6 Photoalignment by a linearly polarized UV beam. . . 48 3.7 Grating pattern written by the interference of two orthogonally

circularly polarized UV beams. . . 49 3.8 The structure of a LC geometric phase grating. Λ is the grating

period. . . 49 3.9 Schematic illustration of the working principle of the geometric

phase LC grating. . . 51 3.10 (a) UV ozone treatment machine. (b) Spin-coater. . . 52 3.11 The relative absorption spectrum for BY [3]. . . 53 3.12 (a) The wavelength-dependent extraordinary and ordinary re-

fractive indices of E7. (b) The corresponding E7 layer thickness required for 100% first order diffraction as a function of the wave- length. . . 54 3.13 (a) Glue dispenser in cleanroom. (b) The fabricated cell. Green

part indicates the glue pattern with two openings. . . 54 3.14 The transmission spectrum of an empty cell measured by the

spectrometer. The thickness of the cell is estimated as 3.17µm. 55 3.15 Microscope setup used to observe the sample with crossed ana-

lyzer and polarizer. . . 56 3.16 Polarization microscope image of a linearly aligned LC sample

between two crossed polarizers. (a) The LC director is parallel to one of the polarizers. (b) The LC director is rotated over 45^◦. 56

LIST OF FIGURES xix 3.17 Schematic illustration of the set-up for photoalignment of a ge-

ometric phase grating, based on two orthogonally circularly po- larized UV laser beams. . . 57 3.18 Illustration of the procedure to fabricate a cell with a LC geo-

metric phase grating. . . 58 3.19 Fabricated geometric phase grating observed by a polarization

microscope with crossed polarizers. The images are recorded for different angles of the grating. . . 58 3.20 Transmission spectrum of the LC geometric phase grating (zero

order). (a) Spectrum measured with the spectrometer. (b) Com- parison between the measured spectrum and the simulated spec- trum, based on a thickness ofd= 3.76µm. . . 59 3.21 The set-up for observing the diffraction phenomenon of the fab-

ricated cell with Helium-neon laser. . . 59 3.22 (a) Diffraction pattern of the grating with period 6.51µm for il-

lumination with circularly polarized HeNe laser light. (b) Diffrac- tion pattern of the grating with period 12.89µm for illumination with circularly polarized HeNe laser light. . . 60 3.23 (a) Relation between the UV angle of incidence and the cor-

responding theoretical diffraction angle for 633 nm light. (b) The relation between the theoretical diffraction angle and the experimentally measured diffraction angle. . . 60 3.24 (a) Transmission spectra for the two samples measured with the

spectrometer (zero order). (b) Polarization microscope image of the grating and diffraction pattern obtained with the 532 nm green laser. . . 61 3.25 Relation between the fabricated cell thickness and the wave-

length for which the zero order diffraction is minimal, when three different types of spacers are used. . . 62 3.26 Nylon coated and rubbed cell of 1 inch filled with the RM mix-

ture, observed between two crossed polarizers. (a) Alignment direction at 45^◦ to the polarizer and analyzer. (b) Alignment direction is parallel to one of the polarizers. . . 63 3.27 Photoaligned BY sample filled with RM LC, forming a grating

as observed under crossed polarizers by PLM for different orien- tations. . . 63 3.28 Spin-coated RM grating, observed under PLM with crossed po-

larizers and different orientations. . . 64 3.29 Set-up to test the stability of the polymerized layer, showing the

laser source, the tested sample and the beam dump. . . 64

3.30 The microscope images of the RM samples after exposure to the blue laser. (a) The planar aligned LC cell with the LC director parallel to one of the polarizers and 45^◦ to the polarizers; (b) Photo-aligned LC grating cell. . . 65 3.31 (a) First order diffraction efficiency for red (633 nm) as a func-

tion of the cell thickness. (b) First-order and zero-order relative diffraction powers for cell thickness 1.5µm as a function of wave- length. . . 66 3.32 (a) Cell thicknesses estimated from the spectrometer transmis-

sion for empty cells with spacer diameter 1.6 µm. (b) Wave- lengths for minimal zero-order transmission with corresponding measured cell thickness of different devices. . . 66 3.33 LC films with thickness 0.73 µm and 2.2 µm both have high

diffraction efficiency at wavelength 414 nm. . . 67 3.34 Set-up for writing the photoalignment pattern. With (1)(2): a

pair of lenses to expand the UV beam; (3): polarizating beam splitter; (4)(5): set of mirrors to reflect the split beams for in- terference; (6)(7): quarter-waveplates to convert the linearly po- larized beam to circular polarization; (8): sample position. A schematic drawing is shown on the bottom. . . 68 3.35 Microscope images of the fabricated grating with different ob-

servation angles, observed by PLM. . . 69 3.36 Two fabricated samples designed to function as beam combiner.

On the left side the center of the two circular gratings are at a distance of∼1.2 mm, on the right side, the two gratings are next to each other. . . 69 3.37 Transmission spectrum measured for the two gratings on the

sample where the distance between the gratings is 1.2 mm. . . 70 3.38 Set-up for the characterization of the beam combiner. Top: pho-

tograph of the setup; bottom: schematic drawing of the set-up. 70 3.39 Experimental set-up and schematic diagram for photoalignment

for a diffractive component that combines lensing with a diffrac- tion grating. . . 72 3.40 (a) Picture of the setup to expand the laser beam. (b) Pattern

of the diverged beam on the observer’s plane. . . 73 3.41 (a) Setup to test the combined lens/grating component. (b)

Obtained beam image on the observer’s plane, due to collimation in one direction and horizontal diffraction to another position. 73 4.1 Switching sequence of an electrochromic laminated glass window

[4]. . . 75

LIST OF FIGURES xxi 4.2 Liquid crystal smart window mechanisms. (a) Smart window

switches from an opaque state to a transparent state in response to the applied voltage. (b) Smart window switches from a low- transmission to a high-transmission state in response to the ap- plied voltage. . . 76 4.3 LC smart window that switches between an opaque and a trans-

parent state [5]. . . 77 4.4 (a) Example of the structure of a polymer that can be dissolved

in water, to form a thermo-responsive hydrogel. (b) Demonstra- tion of the application of a chemical smart window with thermo- optic property. . . 78 4.5 (a) Glue pattern used for the test cells. (b) UV lamp for curing

the glue. . . 79 4.6 Cells filled with the four different polymer solutions, picture

taken at room temperature with a fluorescent lamp in the back- ground. From left to right: 40B, 100A, 100B, 200A. Pictures are taken eight days after the cells are made. . . 80 4.7 Pictures of the heated samples. From left to right: 40B, 100A,

100B, 200A. Now the fluorescent light is only partly visible. . 80 4.8 (a) Top: set-up built for the optical measurement. The compo-

nents used are indicated by numbers. (1): red laser; (2): sample attached to a hotplate; (3): integrating sphere and (4): hotplate control panel. Bottom: detailed picture of position (2), showing the detector and the hotplate. (b) Schematic diagram of the set-up. . . 81 4.9 Schematic diagram illustrating the measurement of different light

contributions. (a) The total amount of light. (b) The forward scattering. (c) The forward scattering and the direct transmis- sion. (d) The backward scattering and the specular reflection.

. . . 82 4.10 Light fractions after incidence onto the sample filled with poly-

mer solution 40B. Compositions are indicated with different col- ors in the chart. Transmission (T): red; forward scattering (FS):

blue; backward scattering and specular reflection (SR+BS): green;

loss (L): purple. . . 83 4.11 Light fractions after incidence onto the sample filled with poly-

mer solution (a)100A and (b)100B. The two solutions have the same polymer repeating unit number but different concentration. 84 4.12 Light fractions after incidence onto the sample filled with poly-

mer solution 200A. . . 84

4.13 Temperature dependency of the light fractions for sample 100B.

(a) The measured light fractions: transmission (T), forward scat- tering (FS), backward scattering and specular reflection (BS+SR).

(b) Fraction of the light with forward scattering and transmis- sion subtracted from the total amount: (BS+SR+A). . . 85 4.14 Teflon and silicone spacer sheets used to make thicker samples. 85 4.15 Variation of the different fraction of light as a function of temper-

ature for polymer solution 40B for two thicker cells with different thicknesses. . . 86 4.16 Image showing the light scattered from the edges of the window

for three different temperatures. . . 86 4.17 Temperature dependency of the fractions of light, measured for

polymer solution 100A and 100B in a 1mm thick cell. . . 86 4.18 (a) Inclination angle of the LC orientation along the direction

perpendicular to the cell surface for different voltages. (b) Ef- fective permittivity of the liquid crystal, as a function of ap- plied voltage. (c) Phase retardation experienced by light pass- ing through a LC layer with thickness 25 µm, as a function of applied voltage. (d) Transmission spectra for three different ap- plied voltages. . . 91 4.19 (a) Geometry of the up-scaled LC cell used in this work. The

top figure is the top view with colored dashed lines showing different regions. The bottom one is the side view, the magenta part shows the glue. (b) and (c) show single contact and dashed line contact busbar formats respectively. A, B and C indicate the experimental measured points. The insets of these three figures indicate the dimensions and formats of the busbar. (d) Schematic view of the direction of the two crossed polarizers (P:

polarizer. A: analyzer) and the liquid crystal fast axis. There is an angle of 45^◦between the fast axis and each polarizer. . . . 92 4.20 Geometry generated by Matlab PDE solver for the single contact

format. (a) The boundaries and surfaces. (b) The generated meshes. . . 94 4.21 Voltage distribution calculated for applied voltage 4 V and fre-

quency 60 Hz. (a) Top conductive layer connected with 4 V. (b) Bottom conductive layer connected with 0 V. . . 95 4.22 Simulation results for applied voltage 4 V. (a) Voltage distribu-

tion with applied frequency 60 Hz. (b) Voltage distribution with applied frequency 600 Hz. . . 95 4.23 Simulation results for applied voltage 24 V. (a) Voltage distri-

bution with applied frequency 60 Hz. (b) Voltage distribution with applied frequency 600 Hz. . . 96

LIST OF FIGURES xxiii 4.24 The relation between the simulated effective voltage and the

applied voltage, the simulated effective voltage is taken from two positions. . . 97 4.25 Transmission through the guest-host cell as a function of voltage. 97 4.26 Simulation for transmission through the guest-host cell. (a) Ap-

plied voltage is 4 V, 60 HZ. (b) Applied voltage 4 V, 600 Hz. (c) Applied voltage 24 V, 60 Hz. (d) Applied voltage 24 V, 600 Hz.

The transmission for zero applied voltage is: 34.44%. . . 98 4.27 (a)Transmission spectra of position A, B and C, when there is

no voltage applied. (b)Transmission spectra of position A, for both experiment and simulation. . . 100 4.28 Simulation results for applied voltage 4 V. (a)Voltage distribu-

tion with applied frequency 60 Hz. (b)Voltage distribution with applied frequency 600 Hz. . . 101 4.29 Simulation results for applied voltage 24 V. (a)Voltage distribu-

tion with applied frequency 60 Hz. (b)Voltage distribution with applied frequency 600 Hz. . . 101 4.30 Simulated voltage distribution for the complete line contact for-

mat, with applied voltage (a)4 V and 60 Hz. (b)4 V and 600 Hz.

(c)24 V and 60 Hz. (d)24 V and 600 Hz. . . 102 4.31 Simulated voltage distribution in the time domain with an ap-

plied square wave. (a) 4 V and 60 Hz. (b) 4 V and 600 Hz. (c) 24 V and 60 Hz. (d) 24 V and 600 Hz. . . 104 4.32 Comparison of voltage signals when a square wave voltage is

applied. (a) & (c) Simulation; (b) & (d) Experiment. (a) & (b) 4 V and 60 Hz. (c) & (d) 4 V and 600 Hz. . . 105 4.33 Comparison of voltage signals when a square wave voltage is

applied. (a) & (c) Simulation; (b) & (d) Experiment. (a) & (b) 24 V and 60 Hz. (c) & (d) 24 V and 600 Hz. . . 105

List of Tables

2.1 Layer thicknesses of the DBR for the original quarter wave thick- ness and for the calculated spectrum in figure 2.15 . All thick- nesses are in nm. . . 22 2.2 Design parameters of the DBR after the first iteration (’starting

point’) and after the optimization ofG (’final optimized’). All thicknesses are in nm. . . 26 2.3 Two cases of starting point for extra optimization tests. All

thicknesses are in nm. . . 27 2.4 The optimized design for the two cases in table 2.3. All thick-

nesses are in nm. . . 27 2.5 Anneal tests for the deposited thin film. . . 31 3.1 Light power measured for each diffraction order . . . 59 3.2 Grating properties . . . 60 3.3 Mixture for polymerizable LCs . . . 62 3.4 Composition for the spin-coating solution of polymerizable LCs

mixture . . . 64 3.5 Diffraction efficiency for 4 diffraction orders of the gratings on

two different samples . . . 71 3.6 Relative powers measured for beam path A . . . 71 3.7 Relative powers measured for beam path B . . . 71 4.1 Information about the polymer solutions that have been tested. 78 4.2 Power consumption simulated for single contact format . . . 98 4.3 Comparison of the voltage between simulation and experiment

for single contact format . . . 100 4.4 Power consumption simulated for dashed line contact format . 102

xxv

4.5 Comparison of the voltage between simulation and experiment for dashed line contact format . . . 103

List of Abbreviations

AOI Angles of Incidence BS Beam Splitter BY Brilliant Yellow

DBR Distributed Bragg Reflector DCM Dichloromethane

DMF Dimethylformamide FPI Fabry–P´erot Interferometer FWHM Full Width Half Maximum HWP Half Wave Plate

ITO Indium Tin Oxide LC Liquid Crystal

LCD Liquid Crystal Display LCP Left Circular Polarization LED Light Emitting Diode MOE Micro-Optical Elements MWIR Mid-Wave Infrared OPD Optical Path Difference PB Pancharatnam-Berry

PDE Partial Differential Equation PDLC Polymer Dispersed Liquid Crystal PLM Polarized Light Microscopy

QWP Quater Wave Plate

xxvii

RCP Right Circular Polarization RMS Root Mean Square

SEM Scanning Electron Microscope TE Transverse Electric

TM Transverse Magnetic UV Ultra-Violet

List of Publications

[1] Gao, B., Puthenparampil George, J., Beeckman, J., & Neyts, K. (2020).

Design, fabrication and characterization of a distributed Bragg reflector for reducing the ´etendue of a wavelength converting system. OPTICS EXPRESS, 28(9), 12837–12846.

[2] Gao, B., De Jong, T., Nie, C., Osterodt, J., Neyts, K., & Beeckman, J.

(2020). Driving issues of large area liquid crystal devices. LIQUID CRYSTALS.

[3]Gao, B., Neyts, K., & Beeckman, J. (2018). Modeling upscaled voltage- driven liquid crystal smart windows. In 22nd Conference on Liquid Crystals:

Chemistry, Physics and Applications (CLC’2018), Abstracts. Jastrzebia G´ora, Poland.

[4]Xue, X., Gao, B., Beeckman, J., & Neyts, K. (2019). Thin film optical axis gratings by photo-alignment and spin-coating of liquid crystal. In 15th European Conference on Liquid Crystals: posters. Wroclaw.

[5]Neyts, K., Nys, I., Gao, B., Stebryte, M., Berteloot, B., Xue, X., . . . Beeckman, J. (2019). Liquid Crystal TV and OLED TV: issues and opportu- nities. In 15th European Conference on Liquid Crystals, Abstracts. Wroclaw, Poland.

xxix

Chapter 1

Introduction

An optical filter is a device that can transmit, reflect, diffract or scatter light, depending on the angle of incidence, wavelength or polarization of the incident light. In many cases only transmission and reflection of light under perpendicular incidence is considered. Depending on the mechanism of oper- ation, absorptive or interference filters are considered. Absorptive filter works by absorbing certain wavelengths of the incident light, while letting the other wavelength pass through. Interference filters consist of a stack of optical coat- ings and selectively reflect certain wavelengths. According to the wavelength range that can pass through the optical filter, there are shortpass, longpass and bandpass filters. Normally the filters are passive devices, and the wavelength characteristics are fixed. If multiple wavelengths are to be filtered, different filters can be combined. Light that is passing through or that is reflected by a filter at the same angle as the incident beam is referred to as the ’zero order’.

In this thesis, we will consider optical filters is considered in a broader way:

• Instead of focusing only on perpendicular transmission and reflection, other aspects are considered: reflection for oblique incidence, diffraction of light and scattering (see Fig.1.1. This allows to use optical filters in specific applications.

• Some optical filters in this thesis are dynamic, in the sense that the properties of the filter can be changed in time by external stimuli.

In this thesis the design, optimization, realization and characterization of planar optical filters are presented for a number of specific applications. The fil- ters are designed to fulfill some complex sets of requirements for two different type of applications, each of which has two sub-topics. First, for a projec- tion system, a distributed Bragg reflector (DBR) is optimized with the aim to increase the light collection efficiency for a photoluminescent phosphor light source. Additionally, a compact and efficient diffractive beam combiner based on photoalignment of liquid crystals is designed and optimized. The second application is smart windows. An active transmission filter based on liquid

1

crystals has been developed and evaluated for the application of large-scale absorptive smart windows. In addition, a temperature-switchable scattering hydrogel has been investigated as smart window material.

Figure 1.1: Schematic overview of how an optical filter can process an incident light beam. The light can be transmitted, diffracted, scattered or reflected, all with a certain wavelength dependency.

1.1 Motivation

1.1.1 Projectors

Projectors are widely used in meeting rooms, home entertainment and cine- mas. A projector is an optical device that projects an image (or moving images) onto a projection screen. Incandescent light bulbs or halogen lamps are widely used as a light source. Typically, images are split in a red, blue and green part and each of these colors are impinging on a microdisplay, based either on Liquid Crystal on Silicon (LCOS) technology or on micro-electro-mechanical technology (often referred to as Digital Light Processing or DLP technology).

Recently, solid state light is gradually developing as preferred light source for projection systems, for its higher brightness, better stability, possibility of in- creased color gamut and efficiency. The solid state light source can be a Light Emiting Diode (LED) in the case of cheap, handheld projectors. Lasers as light sources are for high-end projectors in cinemas that use three lasers for red, green and blue light. Such systems offer the broadest possible color gamut, but the laser sources (mainly for red and green) are very expensive. Other laser-based projectors use a blue laser diode source and a luminescent material to gener- ate green and red. In an optical system with a luminescent light converter, the amount of photoluminescent light collected by the projection system is of crucial importance. For example in the projection system described above, the collection efficiency determines the total flux of light, which is important for the image quality and the viewing experience. Solid state phosphor materials have various benefits such as a long life time and low energy dissipation [6],

incident beam

'^N

reflection

scattering

optical filter

diffraction transmission

1.1. MOTIVATION 3

Figure 1.2: Inside view of a laser projector (image taken from

https://epson.com/laser-projectors). The image shows two arrays of blue laser diodes as the light source. The blue laser light is sent to the rotating yellow phosphor ring to generate green and red light. The resulting white beam is split up in three parts, modulated, combined into a white beam again, and finally sent through the complex projector lens system.

making it a promising candidate as light source for projection systems [7, 8, 9].

The phosphor layer is exited by a blue laser and the photoluminescent light is collected by a lens. Figure 1.2 shows an example of the inside view of a laser projector, where the solid light source is an array of blue laser diodes. The blue light is transferred to green or red light by a rotating phosphor wheel (as shown in Fig.1.3). By using reflections on (dichroic) mirrors and a prism, the three colors are combined in a white beam. After the modulation by digital mirror devices, and projection, the image is formed on the screen.

To take full advantage of the solid state light sources, it is important to optimize the total brightness. In this thesis, it is realized by increasing the efficiency through reducing the ´etendue of the light source. The ´etendue of the projection system is limited by the numerical aperture of the projection lens and the area of the optical modulators, warranting the need of limiting the ´etendue of the light source. As the fluorescent solid state light source has normally a Lambertian emission, not all the emitting light can be collected by the lens. The ´etendue can be limited by a reduction of the emission angle of the solid light source, trying to keep the same flux within the narrower solid angle.

The ´etendue can also be reduced by decreasing the light source area. Since the laser sources have limited power, an array of solid state lasers is required for high power applications, as for example the blue laser array shown in Fig.1.4.

Due to the lateral dimension of the individual laser sources, the parallel beams

Figure 1.3: An example of a phosphor wheel. Image is from (https://mate rion.com/resource-center/newsletters/newsletter-archives/optica l-innovation-news-2015-to-2017/next-generation-phosphor-wheel).

can not be placed very close to each other. To reduce the distance between the parallel beams (and to reduce the ´etendue), it is important to use a beam combiner. Usually this is done by a series of mirrors that have to be carefully positioned and inclined to shift the beams laterally, that is perpendicularly to the propagation direction.

Figure 1.4: An example of a blue laser bank and its emission profile [2].

* ® ® ^{# #} 7

1.1. MOTIVATION 5

1.1.2 Smart Windows

In recent years, smart windows have been investigated as a promising com- ponent for energy efficient buildings [10, 11, 12], in which the adjustable trans- mission is utilized to block strong sunlight. Conventionally, curtains and blinds are used for this goal, but they have to be operated manually or with electro- mechanical systems that are not always reliable. Smart windows have shown to be able to block the transmittance of sunlight up to 68% [10]. Smart win- dows can change the transmission for solar radiation by switching between different states, in response to external stimuli such as an applied electric field, heat or light. Thermotropic or photochromic smart windows that respond to heat and light, are passive (for example photochromic technology can be used in sunglasses), they can not be manually controlled and need no electricity.

Electrically-controlled smart windows can be switched to different states by the applied voltage, and this requires some energy consumption. There also exist applications for smart windows in Augmented Reality (AR) [13], where the window is electrically switched between a clear and a scattering state.

Smart windows can be classified into privacy windows and energy saving win- dows [14, 15, 16] depending on the properties of the switching states. There are various technologies to realize smart windows, ranging from chromic ma- terials [17, 18, 19] (such as electrochromic devices [20]), liquid crystals [21] to electrophoretic or suspended-particle devices [10, 22]. There are several factors that characterize smart windows, for example the range over which the trans- mittance can be modulated, the lifetime, the switching time and the energy consumption. It is important for future smart window applications that these factors are thoroughly studied.

In this thesis, two types of smart windows based on different materials are investigated. For the privacy window, different hydrogels are tested as the material. This type of window can switch between a transparent and an opaque state under influence of the temperature. The sun light is not absorbed, but partially reflected due to a large amount of back scattering.

For LC-based energy saving window, the transmission is changed due to the reorientation of LC doped with dye molecules. When the transmission is high, light and heat can enter the building. In the case of strong direct sunlight it is preferred to have a lower transmission. However, as there is still some transmission through the window, the view through the window remains preferably clear. This can be realized by using absorbing dye molecules, of which the direction can be switched following the reorientation of LC molecules [23]. While the electrical driving of LC devices does not pose many problems when working on centimeter sized devices, different types of driving issues arise when up-scaling such devices to areas of several square meters, due to the fact that the thin-film transparent conductors used in these device exhibit a finite sheet conductivity.

1.2 Thesis Objectives

Challenge #1: Design and realization of a wavelength and angle dependent DBR.

The ´etendue of a light source with given area can be limited by restraining the emission angle of the emitted light. The solid light source considered here is a phosphor layer, that is stimulated by a blue laser and emits light as a Lam- bertian emitter, which means that the light goes out in all directions. With one mirror attached to the backside of the phosphor layer, the emitted light is transmitted forward however still over an entire hemisphere. The DBR is a multilayered structure of two materials with different refractive index that can reflect light for certain wavelengths with efficiency close to 100%. The aim of the DBR here is to collimate the Lambertian emission into a smaller solid an- gle, thus reducing the ´etendue of the light source. To realize this function, the DBR should be able to reflect the wide wavelength range of the emission light under large incident angles (θ >45^◦), thus redirecting it back to the phosphor for a second chance to be emitted with a small angle; while maintaining high transmission for the same wavelength under small incident angles, which is not the case for a common DBR consisting of a stack of quarter wavelength layers.

There is an additional requirement on high transmission for the exciting blue laser beam under small incident angles (θ <22^◦). The specific requirements will be discussed later in more detail. However, it is clear that the design and optimization of the DBR filter, is quite novel and challenging. To fulfil the specific requirements, a simulation model and method needs to be developed to find an optimized design. Since the design aims at a thin structure, the properties of the DBR should be reached within a relatively low number of layers (9 or 11 as an example), therefore materials with high refractive index contrast are preferred. The substrate and the thin films need to be able to resist the intense irradiation with the blue exciting light and an associated high temperature of operation. There are different types of materials for DBR fabrication, such as organic polymers [24], semiconductors [25] or metal deriva- tives [26, 27] and various fabrication methods exist, for example chemical vapor deposition [28, 29], sol-gel [30, 31], sputtering [27, 25] and electron-beam evap- oration [32, 33, 34]. Depending on the chosen materials, thoroughly tests on the corresponding fabrication procedures are requested for accurate thickness control, robust structure, and high reflectivity as well as high transmission.

Challenge #2: Design and realization of a compact high efficiency beam combiner.

In projection systems, large numbers of blue laser diode light sources need to be combined to obtain the desired brightness. Still, as mentioned above, the optical ´etendue of the laser array should be small enough to match the optical ´etendue of the projection system. To achieve this purpose, it is impor- tant that the different laser beams are brought together on a small area while

1.2. THESIS OBJECTIVES 7 remaining parallel. So far this has been realized with conventional optical components such as prisms and mirrors, which are rather bulky and not highly accurate. In this thesis, the developed component relies on thin film liquid crystal diffraction gratings patterned by photoalignment. To achieve this goal, components consisting of different zones of the LC gratings with different peri- ods and diffraction directions are required. The transition regions should also be narrow and sharp in order to keep the design compact. Photoalignment is the technology that allows to control the LC direction at the surface in a rather flexible and accurate way, as the alignment of the LC only depends on the di- rection of the linear polarization of the illuminating UV light. Different types of patterns can be fabricated through this method[35, 36, 37]. One photosensitive layer is needed to record the alignment pattern, of which the material should be able to follow the designed pattern and align the liquid crystals accurately, while staying stable to maintain the pattern and not interacting with the LCs.

Since the phase change of the light incident on the photoaligned LC grating is continuous, the diffraction efficiency can go up to nearly 100%. The difficulty is that to obtain the desired efficiency, the fabricated grating is required to be clear, uniform, and should have a certain thickness that depends on the birefringence of the LC material. Investigations and adjustments have to be done on the optical photoalignment set-up for creating the desired pattern, on testing the photoalignment material and the liquid crystals. The LC gratings should also be stable and have a long life time under the high power of the laser beams that need to be diffracted, for example in projection systems. In this case polymerizable liquid crystals come into use since the polymerization provides more stable structures. For the two types of LC material, different fabrication procedures and tests are required.

Challenge #3: Scattering evaluation for hydrogel-based smart windows.

For another application of thin optical films, smart windows with different materials are investigated in this research. The hydrogel-based smart windows can switch between different states in response to external heat, as a result changing the transmission properties for the incident light. Various types of smart window exist and different factors are used to analyze the window be- havior. For the privacy window that switches between transparent and opaque due to scattering of the light, it is important to evaluate its ability to block the light and to which direction the light is scattered. Here several thermo- responsive hydrogels are tested for selecting the appropriate candidate for the temperature-dependent privacy window. The polymer solutions with different concentrations and different repeating units are prepared. Different from pre- vious literature where mostly only transmission is considered [38, 39, 40], here the scattering at different directions needs to be collected and measured for evaluation. A stable temperature-control method is also required to study the transition behavior of the polymer window.

Challenge #4: Driving issues of electrically controllable liquid crystal smart windows.

The LC-based smart window that switches between transparent and translu- cent state, as there are always small amounts of ions in the liquid crystals that would screen the electric field in case of steady state voltages. While the elec- trical driving of LC devices does not pose many problems when working on centimeter sized devices, different types of driving issues arise when up-scaling such devices to areas of several square meters, due to the fact that the thin-film transparent conductors used in these device exhibit a finite sheet conductivity.

The interplay between the resistance of the transparent electrodes and the ca- pacitance of the liquid crystal layer influences the switching of the window and the homogeneity over the surface. This non-uniform distribution depends on different parameters, for example the geometry of the device, the contacting scheme, the LC layer thickness, the LC permittivity and its anisotropy. Quanti- fying this effect analytically is possible for a number of simplified geometries, as demonstrated in [41], but for more complex geometries, numerical simulations are necessary to evaluate the voltage and transmission distribution. A spe- cific simulation model should be built that includes the parameters mentioned above, and the parameters should be free to adjust for different requirements.

Driving signals with different wave forms (sinusoidal or square), different am- plitudes and frequencies need to be implemented as input parameters. A smart window prototype should be fabricated for experimental measurements to an- alyze the accuracy of the simulation model.

1.3 Thesis Structure

The thesis contains mainly three parts to cover the different challenges men- tioned above. With each chapter there is an introduction section to introduce the background, the basic principles and the research objectives; then the work is explained in detail with designs, procedures and results; the last section deals with the conclusions and perspectives. The content is organized as follows:

Chapter two is about the design and realization of the distributed bragg reflector to reduce the ´etendue of the light source.

Chapter three focuses on the compact, high efficient liquid crystal diffraction grating, which is used to realize a laser beam combiner.

Chapter four discusses two types of smart windows: polymer scattering smart window and LC energy saving smart window. For each different aspects are targeted.

Finally, in chapter five, we give the whole thesis work a brief conclusion, discuss the contributions from this work and tasks for future research.