W a h T u n g L a u ' s H o m e p a g e
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W a h T u n g L a u ' s H o m e p a g e
Ph.D. in Electrical Engineering
Ph.D. Minor in Physics
Stanford University
wahtung at gmail dot com
Postdoctoral researcher in the Department of Physics, University of Toronto, with Professor Sajeev John (Starting February 1, 2010).
Doctoral advisor: Professor Shanhui Fan
Thesis topic: "Manipulation of Thermal Electromagnetic Fields in Nano-photonic Structures".
Research Interests
Theoretical and computational nano-photonics, with emphasis in photonic thermal transfer and thermal emission.
Applications of dynamical systems theory to thermal and photonic nanostructures.
Research Projects
"Universal Features of Coherent Photonic Thermal Conductance in Multi-layer Photonic Band Gap Structures" (pdf)
Featured in the October 9, 2009 issue of Physical Review Focus, and Physical Review B Editors' Suggestions in October 2009).
How good can photonic crystals be used as thermal insulators ? We show that, for given material indices, there is a lower limit in normalized thermal conductance of a multi-layer photonic crystal, with respect to the vacuum conductance, and this limit is independent of the thicknesses of the layers. This result is related to the fact that, the distribution of photonic bands over the entire frequency domain is ergodic. This is the first comprehensive mathematical treatment of higher-order photonic bands.
"Tuning Coherent Radiative Thermal Conductance in Multilayer Photonic Crystals" (pdf / html) (a brief introduction)
Using dielectric-vacuum multi-layer photonic crystal as an exemplary system, we show that the thermal conductance of the crystal can be well below vacuum when the temperature, or lattice constant of the crystal, is large. This is due to the existence of partial photonic band gaps over the entire frequency domain. Moreover, at very small temperature, or small lattice constant, photons concentrate at a small frequency range where the material is described by an effective index well above 1, and the conductance can be much larger than that of vacuum. This allows the thermal conductance of the crystal to be arbitrarily tuned above or below the vacuum conductance.
"Spatial Coherence of the Thermal Electromagnetic Fields in the Vicinity of a Dielectric Slab" (pdf)
At the far-field range, coherence length of emitted radiative fields of any thermal objects is approximately half the wavelength. At the near-field range, however, the coherence length exhibits much richer features. We show that for a dielectric slab, the coherence length of the emitted fields can be much longer than half the wavelength at the near-field range, due to the guided modes that restrict dominant field propagation in two dimensions (and thus further enhances lateral delocalization of emitted fields as compared to the far-field case). Furthermore, at extremely close distance from the slab surface, the coherence length will drastically reduce to subwavelength range due to thermally fluctuating localized charges. Such spatial variation of coherence length can be tuned by changing the size of the slab.
"Anomalous Modal Structure in a Waveguide with a Photonic Crystal Core" (pdf)
For a typical waveguide with a high-indexed dielectric core, there is only one single-mode regime is where only the fundamental mode exists, and this mode is always of even symmetry. We show that, by punctuating periodic array of holes along the transverse and propagating directions, the waveguide can have multiple "single-mode regimes", where only one of the higher-order mode remains in each regime. Also, we can arbitrarily alter the parities of the single-guided modes by simply changing the number of columns of the hole array in the transverse direction.
"Creating Large Bandwidth Line Defect by Embedding High-Index Dielectric Waveguides into Photonic Crystal Slabs" (pdf / html)
It is well known that a line defect in a slab photonic crystal can guide waves due to the presence of photonic band gaps. However, strong bragg scattering at the interface of the crystal and the defect severely limits the guiding bandwidth to be far less than the size of the band gap. We propose that by inserting air trenches between the defect and both sides of the bulk crystal, scattering on the guided wave can be significantly reduced and the guiding bandwidth can cover most of the band gap.
Links
SLAC National Accelerator Laboratory (about my former workplace)
Department of Electrical and Electronic Engineering, the University of Hong Kong (about my undergraduate college)
World Peace Buddhists (about my buddhist practice), and some selected Buddhist teachings