Radiative
properties of negative index materials
A negative index material (NIM), which possesses
simultaneously a negative permeability (m) and a negative permittivity (e),
is an emerging material that has caught the attention of many
scientists and engineers after it was first demonstrated in
Science 292, 77 (2001). Many of the unique features
associated with NIMs were summarized in Veselago’s original paper
published in Sov. Phys. Usp. 10, 509 (1968), including
negative phase velocity, reversed Doppler effect, and the prediction
of a planar lens. The lack of simultaneously occurrence of negative
e and
m in natural materials
hindered any further study on negative index materials (NIMs) for
some 30 years. Micro/nanotechnology enabled the fabrication of
structures with effective negative
e and m in the same
region. Recent studies have shown that NIMs have other remarkable
properties such as the amplification of evanescent waves and hold
enormous promise for applications such as perfect lens and optical
communications.
We have studied the unique
radiative properties of NIMs. Photon tunneling, which relies
on evanescent waves to transfer radiation energy, has important
applications for radiative transfer in thin-film structures,
microscale thermophotovoltaic devices, and scanning thermal
microscopy [1]. By introducing NIM layers into multilayer thin-film
structures, we show that photon tunneling can be greatly enhanced,
due to the excitation of surface or bulk polaritons [2-4]. Energy
transmission by photon tunneling between two semi-infinite
dielectrics is studied using the structure shown in Fig. 1 (left).
An NIM layer is assumed to insert into the vacuum gap that separate
the two dielectrics. For the purpose of convenience, the modified
matrix formulation is employed to calculate the transmittance of the
multilayer structure. Different arrangements and number of NIM
layers have been investigated. When an ideal NIM layer of e=m=-1 is employed, the
transmittance of this layered structure is plotted in Fig. 1
(right). The transmittance becomes unity at d3=d2
regardless of the angle of incidence , wavelength , and polarization. The hemispherical
transmittance is also unity since both the propagating waves and
evanescent waves will transmit. Therefore, the NIM layer can
significantly enhance the transmittance.
|
|
Figure 1. Transmittance enhancement by photon tunneling with
a NIM [2]. |
The group velocity upon
reflection by a NIM has been one of the controversies in
understanding the optical properties of NIMs. We have investigated
the direction of group velocity upon reflection by a dispersive
medium regardless of whether it is a NIM or not. It was found that
the group velocity is not always perpendicular to the group front
[5]. We have also investigated the impact of surface and bulk
polaritons on the optical properties, especially the reflectance, of
a negative index metamaterial (NIM) sandwiched between different
dielectric layers. Regime maps are developed to describe the
polariton dispersion relations and to help understand the effect of
the NIM layer thickness on the polariton resonance frequencies [6].
The reflectance from a dielectric material to another becomes zero
at the Brewster angle for p-polarization. The reflectance
from NIM can be zero for any polarization, that is, the Brewster
angle may occur for either polarization. The criteria for the
Brewster angle are determined analytically and presented in a regime
map. The underlying mechanisms are explained based on the framework
of the Ewald-Oseen extinction theorem [7].
We further propose a new kind of coherent thermal
emission source that pairs a negative permittivity (but positive
permeability) layer with a negative permeability (but positive
permittivity) layer without any gratings. Coherent thermal emission
can occur not only for p polarization but also for s
polarization, see Fig. 2 below. Moreover, the emission frequency and
emission angle can be controlled by adjusting the film thicknesses
[8,9]. The findings on the spectacular radiative properties of NIMs
may help develop advanced energy conversion devices.
|
|
Figure 2. Coherent emission of the proposed structure
[8.9]. |
There exist promising opportunities
for the application of NIMs to energy conversion devices and thermal
radiation control. Experimentally verification of the predicted
unique radiative properties is very important. Challenges exist in
realizing NIMs or single negative materials in the near-infrared and
visible regions.
This research has been supported by the National Science
Foundation.
Publications
1.
Zhang, Z.M., Fu, C.J., and Zhu, Q.Z., 2003, "Optical and
Thermal Radiative Properties of Semiconductors Related to
Micro/Nanotechnology," Advances in Heat Transfer, 37,
pp. 179-296.
2.
Zhang, Z.M., and Fu, C.J., 2002, “Unusual Photon Tunneling in
the Presence of a Layer with a Negative Refractive Index,”
Applied Physics Letters, 80, pp.1097-1099.
3.
Fu, C.J., and Zhang, Z.M., 2003, “Transmission Enhancement
Using a Negative-Refraction Layer,” Microscale Thermophysical
Engineering, 7, pp. 221-234.
4.
Fu, C.J., Zhang, Z.M., and Tanner, D.B., 2005, “Energy
Transmission by Photon Tunneling in Multilayer Structures including
Negative Index Materials,” Journal of Heat Transfer (in
press).
5.
Zhang, Z.M., and Park, K., 2004, “Group Front and Group
Velocity in a Dispersive Medium upon Refraction from a Nondispersive
Medium,” Journal of Heat Transfer, 126, pp. 244-249.
6.
Park, K., Lee, B.J., Fu, C.J., and Zhang, Z.M., 2005, “Study
of the Surface and Bulk Polaritons with a Negative Index
Metamaterial,” Journal of the Optical Society of America B,
22, pp. 1016-1023.
7.
Fu, C.J., Zhang, Z.M., and First, P.N., 2005, “The Brewster
Angle with a Negative Index Material,” Applied Optics, 44,
pp. 3716-3724.
8.
Fu, C.J., Zhang, Z.M., and Tanner, D.B., 2005, “Planar
Heterogeneous Structures for Coherent Emission of Radiation,”
Optics Letters, 30, pp. 1873-1875.
9.
Fu, C.J., and Zhang, Z.M., 2005, “Further Investigation of
Coherent Thermal Emission from Single Negative Materials,” submitted
to Microscale Thermophysical Engineering.
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