Manipulation
of Thermal Electromagnetic Fields Using Nanophotonic
Structures
Speaker:
Wah Tung Lau
Research
Advisor: Professor Shanhui Fan
Other
examiners:
Professor Mark
Brongersma (Chair), Professor
David Miller, Professor
Peter Peumans
Department
of Electrical Engineering, Stanford
University
Thursday
July 23, 2009 at 3:15pm (Refreshments served at 3:00pm)
Room
200, Applied
Physics Building, Stanford University
Abstract
Heat
can be carried by phonons,
electrons, convectional
currents, or thermal
electromagnetic fields (thermal radiation). In this talk, we will examine how
the transport, and
emission of thermal electromagnetic fields can be engineered
using nanophotonic structures.
First,
we will consider a multi-layer photonic crystal as the thermal-conducting medium.
The crystal is composed of alternate layers of lossless dielectric slabs and
vacuum, such that heat transport is coherent and is only due to photons. The
thermal-conducting behavior of the crystal is determined by two opposing
mechanisms: the enhancing effect from evanescent tunneling of photons, and the
suppressing effect due to the photonic band
gaps. By
tuning the thicknesses of
the layers, we can control the relative dominance of the two effects, and the
medium can be tuned from being thermally more conducting, to thermally more
insulating than vacuum at a fixed temperature.
[1]
Since
vacuum is commonly thought to be the best thermal
insulator, the ability for the multi-layer photonic crystals to suppress thermal conductance
below that of vacuum is
especially significant. We will see that there exists a lower limit for such
conductance suppression with respect to vacuum, and
this limit is generally
independent of the thicknesses of the layers. Furthermore, such independence
reveals the fact that distribution of photonic bands in frequency space is
ergodic.
[2]
Lastly,
we will look at the properties of thermal electromagnetic fields emitted from
lossy dielectric
slabs. At near-field distance from the slab surface, the
coherence length of the emitted fields in free space can be drastically enhanced from that of
the blackbody, due to the excitation of
waveguide modes. Such coherence
enhancement can potentially be exploited to make thermal
antennas, in which heat
can be emitted
directionally. [3]
Related
Publications
http://www.physics.utoronto.ca/~wlau/publications.html
[1]
Tuning Coherent Radiative Thermal Conductance in Multilayer Photonic Crystals
W.
T. Lau, J. -T. Shen, G. Veronis, S. Fan, and P.V. Braun, Applied
Physics Letters 92 103106 (pdf
/ html / a
brief introduction), (2008).
[2]
Universal Features of Coherent Photonic Thermal Conductance in Multi-layer
Photonic Crystals
W.
T. Lau, J. –T. Shen, and S. Fan, (submitted to Physical Review B).
[3]
Spatial Coherence of the Thermal Electromagnetic Fields in the Vicinity of a
Dielectric Slab
W.
T. Lau, J. -T. Shen, G. Veronis, and S. Fan, Physical
Review E76 016601 (pdf),
(2007).
