My main research interest is the role that geometry and symmetry play in nature, especially how they affect the physical properties of soft condensed matter materials, photonic materials, and biological units. In particular my future research will focus on experimental investigations and numerical simulations to study the effects of local geometry of building blocks and overall structural symmetry on the properties of new artificial materials and use my results to tailor new materials with desired properties.

*    Bandgap Study of Photonic Crystals and Quasicrystals

*    Photonic Crystal integrated Waveguide for High Efficiency Thermal Photovoltaic

*    Film formation and cracking during suspension drying

*    Toward the dream of “designer” materials

 

Bandgap Study of Photonic Crystals and Quasicrystals

 

Similar to semiconductors for electrons, photonic crystals are structured electromagnetic media, possessing bandgaps for photons. Photonic crystals can thus confine light of certain frequencies within wavelength-scale cavities, around sharp bends, and along well-controlled optical paths. Photonic crystals can also exhibit negative refraction and are thus useful for sub-wavelength focusing. Quasicrystals have quasi periodic order and higher point group symmetries, which can be favorable for photonic bandgaps or negative refraction in all directions. We previously designed and constructed the world’s first 3-dimensional photonic quasicrystal, and obtained the first-ever visualization of Brillouin zone of a quasicrystal, by measuring its electromagnetic wave transmission. We proved that the nearly spherical Brillouin zones of icosahedral quasicrystals make these quasicrystals good candidates for complete photonic bandgaps. The study of 3-D photonic quasicrystals is a rapidly emerging field with many unanswered fundamental physics questions.

We are now fabricating more photonic quasicrystals of different structures, composition and materials, to systematically study how dielectric contrast, building block geometry design, and beam polarization affect quasicrystals’ photonic properties. This work will contribute to the understanding of all structures with long-range quasiperiodic order, shed new light on potential applications of photonic quasicrystals, and have a major impact on the development of photonics-related technologies.

 

Related publication:

• W. Man, M. Megens, P. Steinhardt, P. M. Chaikin, NATURE 436 993 (2005)

Measurements of Photonic Gaps in Icosahedral Quasicrystals

 

Photonic Crystal integrated Waveguide for High Efficiency Thermal Photovoltaic

 

         Due to the fossil-fuel crisis and the potential impact of green house gases on the climate, our world desperately needs to develop technologies for creating high-efficiency renewable clean energy sources, for example, solar energy. Solar energy can be converted to heat and thermal radiation with greater than 95% efficiency; however, converting thermal radiation or solar energy to electricity with high efficiency remains the biggest challenge in this field. The long-term goal of this project is to design, fabricate, and integrate thermal photovoltaic devices for converting solar energy into electricity with high efficiency. The new approach of this project is to use integrated photonic crystals to limit the thermal radiation into a narrow wavelength range, which matches the absorption wavelength of photovoltaic materials.

         Collaborating with Prof. Glenn Alers in Physics at UCSC and Prof. Jerry Moloney in Math at University of Arizona, we will investigate the use of photonic crystals made with an optically selective materials embedded in a SiC waveguide to focus thermal emission to a narrow band tuned to the efficiency maximum of a photovoltaic material. The objective of this project is to demonstrate that a photonic crystal absorber can improve the efficiency of collecting solar energy and converting it to electricity. A new type of two-stage photonic crystal waveguide will be used to both absorb concentrated solar light and focus it into a narrow band of thermal emission that could be coupled to the quantum efficiency peak of a photovoltaic cell. The narrow band thermal radiation would permit the device to exceed the 31% power efficiency limit predicted by Shockley-Queisser (SQ) for un-concentrated single gap solar cells.

Film formation and cracking during suspension drying

Thin films with specific thickness, strength, optical, and chemical properties are often achieved by drying colloidal suspensions. Unfortunately, drying often promotes cracks if the particles fail to deform enough under the capillary pressure to form a void-free continuous film. The complicated lateral flow and substantial variations across the films during uncontrolled drying had compromised previous attempts to measure the critical capillary pressure at which cracks start.

We recently constructed a theory, by combining Griffith’s energy criterion with a non-linear stress-strain relation based on Hertzian contacts between elastic spheres, to predict the critical capillary pressure for cracking and the critical thickness below which films do not crack. We fabricated a high-pressure ultrafiltration chamber to measure directly the capillary pressure at which films of different thickness, formed from latex or silica dispersions of different sizes, crack. We also tracked the additional cracks at higher pressures to study the spacing and flaw distributions. We confirmed that cracking of thin films of colloidal particles is controlled by the recovery of elastic energy as predicted in our theory, and we identified the important role of nucleation sites (flaws) for cracking.

One lesson we learned from our study of granular material packing inspired by m&m candy is that geometry does matter. Ellipsoids can pack randomly more densely than spheres because of their extra degree of freedom associated with their rotational axes. We will extend the cracking study to non-spherical colloids. For drying of colloidal suspension, the primary structural issues are packing, i.e. contacting neighbors, and pore size, as the former scales the elastic response and the latter limits the maximum capillary pressure. Generally speaking, extension of our theory and experiments to more realistic and non-spherical colloid suspension will further improve our ability to anticipate the capillary pressure at which the first crack occurs and the thickness below which colloidal films do not crack, which can be useful for coating, printing, nano-fabrication and other thin-film techniques.

 

Related publication:

• W. Man and W. B. Russel, PHYS. REV. LETT. 100 198302 (2008)

Direct measurements of critical stresses and cracking in thin films of colloid dispersions

 

• W. B. Russel, N. Wu, and W. Man, LANGMUIR 24 (5) 1721 (2008)

A generalized Hertzian model for the deformation and cracking of close-packed colloidal arrays saturated with liquid

Toward the dream of “designer” materials

Our approaches will involve research in three levels. First, we will use existing and simple building blocks to construct new materials for studying the impact of different global structural symmetries. Second, to investigate building blocks themselves with various geometries and try to optimize their geometry design for special interests. Finally, to use these results to achieve guided self-assembly of particular building blocks to make “designer materials” with desired structures and symmetries for different applications. Once we can control the geometry shape and physical properties of various kinds of building blocks and have figured out the desired overall structural symmetries for applications, one of the ultimate goals will be to achieve self-assembly for “designer materials” with fantastic new properties. To do that, we need to understand the interactions between building blocks and have powerful control on the processing to guide self-assembly.

Some of my other studies on processing control, including thin film drying and suspensions shearing will bring us closer to the dream of achieving controlled and guided self-assembly. We will extend our study in controlled drying and controlled shearing of experiments to non-spherical colloidal suspensions, and also to explore other methods for guided self-assembly, including but not limit to the usage of surface modification and external fields.