Crystals and periodicity usually go hand in hand. However, some of the most intriguing classical materials, including quasicrystals and high-entropy alloys, occur when crystals break this rule. This research project uses new developments in machine learning to create a new way to simulate these systems, and uses this method to explain how and why these aperiodic crystals form.

Light-emitting diodes (LEDs) vervangen andere typen kunstmatige verlichting in rap tempo, omdat ze zuiniger en robuuster zijn. LEDs vormen dan ook een alsmaar groeiende markt van vele tientallen miljarden. De meest voorkomende technologie maakt gebruik van InGaN om blauw licht te maken onder elektrische aandrijving. “Fosforen” zetten vervolgens een deel van dit blauwe licht om in de andere kleuren van de regenboog. Helaas werken bestaande fosforen vooral goed in toepassingen waarbij lage lichtintensiteit voldoende is. Bij hogere lichtintensiteit treedt “verzadiging” op: de efficiëntie van kleuromzetting wordt minder. Dit leidt tot energieverliezen. Daarnaast kan verzadiging de kleurbeleving van een LED-lamp ongewenst blauwig oftewel “koel” maken, aangezien vooral “warme” rode fosforen last hebben van dit probleem. De onderzoekers willen innovatieve fosformaterialen ontwerpen die efficiënt blijven ook bij hoge lichtintensiteit. Ze gaan samengestelde nanomaterialen maken met twee componenten, waarbij blauw licht wordt geabsorbeerd door de ene component en rood licht uitgezonden door de andere. Via het ontwerp van de samengestelde fosfor kan de snelheid van energieoverdracht van de ene naar de andere component worden gecontroleerd. Berekeningen wijzen uit dat slim gebruik van energieoverdracht verzadiging van de kleuromzetting kan verminderen. Dit project zal deze berekeningen toetsen en de praktische mogelijkheden verkennen om dit concept te gebruiken. Het kan daarmee de basis leggen voor vervolgonderzoek waar de beste ontwerpen verder worden ontwikkeld tot heldere rode fosforen. Deze zijn nodig voor de realisatie van zuinigere verlichting met een prettigere kleurbeleving voor de consument.

The system we study is a suspension of colloids and polymers. Such systems have been extensively studied in the past, primarily to understand the mechanism behind the depletion interaction. This effective interaction between the colloids results as the polymers push the colloids closer to maximize the available free volume. Recently Mahynski et al [1]., studied the role of polymers on the phase behavior of colloidal hard sphere crystals. Hard spheres exhibit a fluid-solid transition at a packing fraction on 0.494, and the stable solid phase is the FCC phase. Hard spheres can also form the HCP crystal phase, however it differs in free energy from the FCC phase of the same density, by a margin of 0.001 kT. The HCP and FCC polymorphs differ in the stacking sequence of close packed hexagonal planes. As the difference in free energy is very less between the two polymorphs, experimentally a RHCP or random hexagonally close packed crystal is obtained. Here random refers to the stacking sequence of the hexagonal planes. Mahynski et al [1] found that upon adding hard chains (polymers) to the crystal phase, the HCP phase becomes the globally stable phase. The reason is the following. At a given density, the total void space in both polymorphs is the same, however the way they are distributed is different. Both phases exhibit two tetrahedral voids and a octahedral void per colloid. For FCC, the OV voids are capped on all sides by a tetrahedral void, whereas in the HCP phases the OV voids share faces. The octahedral void has six times the volume of a tetrahedral void, and polymers prefer to occupy this void. If the size of the polymer exceeds the typical size of OV void, they would prefer the void arrangement of the HCP phase, as they can share the space of multiple OV voids. Mahynski et. al., [1] measured the free energy penalty associated with inserting a polymer of a certain length M, in both the HCP and FCC polymorphs both a at fixed density. They find that beyond a certain value of the chain length M, the free energy penalty incurred by the polymer is much lesser in the HCP phase, thereby making it the globally stable phase. Our plan is to estimate the phase behavior of such a colloid polymer system. The phase behavior of the system for chain length M = 1, has been known for some time [2]. In the limit of size ratio of the colloid to monomer is ~ O(10^-1) or lesser, the system is known to exhibit (metastable) fluid-fluid, fluid-solid, and Solid ? Solid coexistence.

Combining the successful fields of nanoplasmonics and ultracold atoms offers opportunities to study a wide range of novel optical phenomena. In the field of nanoplasmonics, subwavelength sized structures in metal films are used to excite and manipulate surface plasmons. Because it is so tightly confined, the surface plasmon is very suitable for integration on photonic devices and can be used to enhance many optical detection processes. In the field of ultracold atoms, lasers and magnetic fields are used to cool, trap and probe atomic gases. The high degree of control makes ultracold atoms a powerful tool to study phenomena in fields ranging from condensed matter physics to quantum information processing and cavity quantum electrodynamics. In this proposal we will combine the fields of ultracold atoms and nanoplasmonics by trapping atoms in the near-field of plasmonic nanostructures, to study and control the coupling between atoms and light close to plasmonic nanostructures. For instance, we will exploit the large absorption cross-section of the atom to inhibit the transmission of a metal nanohole array. By changing the internal state of the atom, we will even switch this transmission. Furthermore, we will study how the atoms couple to plasmonic modes that exist on the nanostructured surface and use this coupling to probe the fundamental nature of these modes. Because of the strong resonant polarizability of the atoms, we can even change the nature of plasmonic excitations on the surface. My expertise in both fields put me in a unique position to study how trapped atoms change the optical properties of plasmonic nanostructures and how these structures influence the optical properties of the atoms, in order to create a novel platform for plasmonic quantum optics. These are extremely relevant issues, given the large scientific and technological drive in developing active photonic and plasmonic devices.

Liquid crystals are phases that combine properties from both a solid and a liquid. In the nematic phase, the positions of the molecules are random, but the molecules are preferentially aligned along one direction, the nematic director. Recently, two truly fascinating novel liquid crystals, the twist-bend (NTB) and splay nematic (NS) phase (see Fig. 2), have been discovered for achiral banana- and pear-shaped molecules. In the NTB phase banana-shaped molecules spontaneously twist and bend, and form a helical structure. In the NS phase, pear-shaped particles splay and form a periodic structure of alternating splay. Furthermore, a splay-bend nematic (NSB) with alternating domains of splay and bend was predicted 4 decades ago, but its observation remains elusive to date both in experiments and simulations. Nematic liquid crystals exhibit peculiar rheological, optical, and electrical properties, and are frequently used in optoelectronic applications. From a fundamental and technological point of view, there is an urgent need to better understand the mechanism behind the spatial modulations in these exotic nematic phases. Predicting the macroscopic properties (periodicity, helical pitch) of these spatially modulated nematic phases based on the microscopic particle details and thermodynamic state point is still an unresolved problem. Here, we propose to attack this problem by developing a classical density functional theory to predict the macroscopic properties from the microscopic particle details and by performing state-of-the-art simulations. Our investigations will be focused on colloidal banana- and pear-shaped particles that were recently synthesized. We will combine simulations with machine learning to classify liquid crystal phases and to determine the order parameters that enable us to study the phase transformation kinetics and to develop a Landau theory. Questions that will be addressed are: Is it possible to stabilize a NTB, NSB, and NS phase by entropy alone? How do these phases nucleate and grow? How does an interface between a modulated nematic with an isotropic phase look like? Can we determine the relevant order parameters and unveil the fluctuation modes and couplings thereof? This project builds strongly upon a theoretical framework that we recently developed to predict the macroscopic pitch and handedness of chiral nematic phases. We are in a unique position to extend this framework to these novel spatially modulated nematic phases, and to use these theoretical predictions to guide simulations on banana- and pear-shaped particles. The fundamental understanding developed here will be important for the synthesis of novel banana- and pear-shaped particles as well as of bent-core mesogens and for the design of liquid crystals with potential optoelectronic applications.