Current work:

Optical bound state in the continuum 

The propagation of waves can be easily understood from the wave equation, but the localization of waves (i.e. creation of bound states) is more complex. Localization of a wave typically requires separating the discrete bound state from the continuum of propagating states. This is why we were taught in QM classes that bound states are found at negative energy, separated from the positive-energy scattering states. However, this is not always the case. Under certain circumstances, an optical bound state that coexists with the continuum of propagating states is possible.

We discovered the existence of such an "optical bound state in the continuum" in some systems. One example is on the surface of a semi-infinite photonic crystals (a photonic crystal is a structure whose refractive index varies periodically in space). Another example is on photonic-crystal slabs (i.e., slabs that has periodic index variation). After numerical and analytical studies on these systems, we also fabricated such a structure, and experimentally demonstrated the existence of such an unusual bound state.

This new way to trap light can enable opportunities for both basic science and applications, and we are looking into a few such directions.

Publications:
1. C. W. Hsu, B. Zhen, S.-L. Chua, S. G. Johnson, J. D. Joannopoulos, and M. Soljacic, "Block surface eigenstates within the radiation continuum", Light: Science & Applications (in press).
2. C. W. Hsu, B. Zhen, J. Lee, S.-L. Chua, S. G. Johnson, J. D. Joannopoulos, and M. Soljacic, "Observation of trapped light within the radiation continuum", submitted.

Optical properties of nanoparticles

Light scattering from small air molecules gives rise to the blue sky, and light scattering from larger (micron-sized) particles gives rise to rainbows and white paint. When the particle is in between the small and the large size scales, dramatically different behaviors are possible.

For example, the resonant scattering and absorption of nanosized metallic particles can exhibit localized surface plasmon resonances that give strongly wavelength-dependent response. We are taking advantage of this wavelength selectivity to implement a new approach of making transparent screens for laser projectors.

We are also experimenting on nanoparticles with active optical properties. One thing we currently work on is the infrared-to-visible frequency conversion when the nanoparticles are doped with active ions.

Past Projects:

Electron transfer dynamics. We use time-dependent density functional theory (TDDFT) to study the dynamics of electron transfer in a molecular heterojunction of Zinc phthalocyanine and fullerene, which is a widely used donor-acceptor pair in organic thin-film solar cells.

Coarse-grained DNA model. Using density functional theory (DFT), we derive the effective interaction between DNA nucleotides, and construct an efficient coarse-grained model of double-stranded DNA.
Publication: C.W. Hsu, M. Fyta, G. Lakatos, S. Melchionna, and E. Kaxiras, "Ab initio determination of coarse-grained interactions in double-stranded DNA," J Chem Phys 137, 105102 (2012).

Cell deformation in microchannel. With a combination of the lattice Boltzmann method and Brownian Dynamics, we model the migration and deformation of cells flowing in a microfluidic channel.
Publication: C.W. Hsu and Y.-L. Chen, "Migration and Fractionation of Deformable Particles in Microchannel," J Chem Phys 133, 034906 (2010).

Self-assembly of DNA-linked nanoparticles. By applying theories in polymer physics, we provide theoretical descriptions for the self-assembly kinetics and the equilibrium distribution of DNA-linked nanoparticles.
Publication: C.W. Hsu, F. Sciortino, and F.W. Starr, "Theoretical Description of a DNA-Linked Nanoparticle Self-Assembly," Phys Rev Lett 105, 055502 (2010).

Phase diagram of DNA-linked nanoparticles. From numerical simulations, we discover that nanoparticles tethered with DNA strands can exhibit rich phase diagrams with multiple amorphous (i.e., liquid) phases or multiple crystalline phases.
Publications:
1. C.W. Hsu, J. Largo, F. Sciortino, and F.W. Starr, "Hierarchies of Networked Phases Induced by Multiple Liquid-liquid Critical Points," PNAS 105, 13711 (2008). (on Wesleyan newsletter)
2. W. Dai, C.W. Hsu, F. Sciortino, and F.W. Starr, "Valency Dependence of Polymorphism and Polyamorphism in DNA-Functionalized Nanoparticles," Langmuir 26, 3601 (2010).

Liquid-liquid phase transition in lattice models. We use simple lattice models to show that interpenetrating bond networks can lead to a multitude of liquid-to-liquid phase transitions.
Publication: C.W. Hsu and F.W. Starr, "Interpenetration as a Mechanism for Liquid-Liquid Phase Transitions," Phys Rev E 79, 041502 (2009).