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
Postdoctoral fellow in the Department of Physics, University of Toronto
Advisor: Professor Sajeev John
Ph.D. in Electrical Engineering, Ph.D. Minor in Physics, Stanford University
Doctoral advisor: Professor Shanhui Fan
Contact: wahtung at gmail dot com
Research Interests
Theoretical and computational nanophotonics, with emphasis in:
- Photonic thermal phenomena.
- Quantum optics.
- Theory of dynamical systems in nanophotonics.
- Nanophotonics for biosensing.
Presentation
"Treating Heat as Waves in Photonic Microstructures"
Research Projects
"Exponential Suppression of Thermal Conductance Using Coherent Transport and Heterostructures" (pdf)
Can the flow of heat be completely suppressed? A typical thermal insulator can only reduce heat flux algebraically to the inverse of its size. With multilayer photonic crystal heterostructures operated in the coherent transport regime, we realize a new mechanism of thermal transport where heat flux can be suppressed exponentially. This would lead to ultimate thermal insulation.
"Thermal Rectification Through Vacuum" (pdf)
We show that, given the same temperature gradient, thermal radiation flowing through a micron-sized vacuum gap can be made to be pronouncedly larger in one direction than in the reversed direction. This is achieved with the vacuum gap sandwiched between two different blocks of silicon carbide with different crystal structures. Thus a thermal diode through vacuum is realized.
"Universal Features of Coherent Photonic Thermal Conductance in Multilayer Photonic Band Gap Structures" (pdf)
(Featured in the Physical Review Focus 24, st. 14 (October 9, 2009), Physical Review B Editors' Suggestions in October 2009, and Physorg.com on December 10, 2009).
In our earlier work, we showed that below-vacuum thermal conductance is possible using photonic band gap materials. Here we derive analytically the lower limit of thermal conductance achievable by such structures. Counter-intuitively, this limit is independent of the geometric details of the crystals. This reveals the fact that, the total size of photonic band gaps over the entire frequency space is a conserved quantity, due to ergodic nature of their distribution. 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)
Vacuum is commonly thought to be the best thermal insulator. Here we show for the first time that below vacuum thermal conductance is possible. The idea is to suppress the transmission of thermal radiation in vacuum with photonic band gaps. Multilayer photonic crystal is the simplest microstructure for this purpose. In addition, the thermal conducting capability of such structure is highly tunable.
"Spatial Coherence of the Thermal Electromagnetic Fields in the Vicinity of a Dielectric Slab" (pdf)
Any object at finite temperature losses heat by emitting thermal radiation to the free space. Normally, such radiation is incoherent and distributes rather evenly in all directions. We show that a thin slab of dielectric with thickness comparable to the radiation wavelength, is capable of emitting thermal field coherently with long range of order. Potentially, this may allow thermal radiation of each wavelength component to be emitted directionally, thus forming a thermal antenna. Furthermore, such directionality can be controlled by merely changing the thickness 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 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 changing the number of columns of the hole array.
"Creating Large Bandwidth Line Defect by Embedding High-Index Dielectric Waveguides into Photonic Crystal Slabs" (pdf / html)
An important goal of nanophotonics is to achieve light guiding through micro-scale of space, thus replacing electric current in metal wires for signal transmission in next generations of computers and microelectronic devices. A line defect in a slab photonic crystal can guide light 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 below the size of the band gap. We propose that by inserting air trenches between the defect and the bulk crystal, scattering can be reduced markedly and the guiding bandwidth can be extended to almost the entire band gap, thus allowing more information to be transmitted.
Links
SLAC National Accelerator Laboratory (about my former workplace)
Department of Electrical and Electronic Engineering, the University of Hong Kong (about my undergraduate college)
SGI at UofT and World Peace Buddhists (about my buddhist practice), and some selected Buddhist teachings