8 Controlling Thermal Radiation with Surface Waves
299
tures can be found in Ref. [128]. Several groups have reported absorbers based on this
idea [18, 20, 53, 74, 75]. In these systems, the absorption takes place predominantly
in the upper metallic layer. A slightly different system has been developed for thermal emitters in the infrared [101]. Here, the insulator is silica and the device works
in the IR by taking advantage of dielectric losses in silica. It is important to stress
that although these systems are usually periodic arrays of absorbers, the periodicity
does not play a significant role. Instead, the systems can be viewed as a collection of
independent absorbers. The small interaction between scatterers can be understood
by noting that the velocity mismatch between a surface plasmon propagating along
the bare surface and the guided mode propagating in the structure is very large. A
similar system has been reported recently. It consists in a periodic array of graphene
disks deposited on a dielectric coated metal [119].
Based on the same general principle of absorption by subwavelength resonant
structures, it is possible to use the absorption resonance of thin slits in a bulk metal. By
controlling the slit width, it is possible to modify the effective index. The resonance
frequency is therefore fixed by the depth and the width of the slit. Again, these slits
are resonant absorbers. They can lead to total absorption. The resonances of these
systems have been studied by a number of authors [70, 93]. As these slits can be
periodically arranged with periods smaller than half a wavelength, these structures
do not diffract and behave as metamaterials.
Another type of periodic structures has been designed to generate a controlled
absorption. It can be described as a periodic array of rectangular slits in a thin
metallic film. The structure can be considered to be analogous to a LC resonant cavity
as discussed in Refs. [71, 124]. A similar structure with negative index properties
has been also studied for resonant absorption [6]. Other schemes based on patterning
metallic structures can also be used to control thermal emission [82, 111].
8.1.8 Multilayers and Photonic Crystals for Controlled Emission
In this section, we review tailoring emissivity using interferences. Although it is not
based on surface waves, it is useful to briefly quote these works. We start with simple
planar systems and we continue with photonic crystals. A simple approach is based
on using multilayered systems, the simplest possible structure being an absorbing
dielectric slab. The interferences lead to a non trivial angular and spectral dependence
of the emission for a silicon slab as discussed in Ref. [63]. For thermophotovoltaics
application, it is necessary to have a quasimonochromatic source in the near infrared.
Rephaeli and Fan [104] have proposed to use a tungsten slab covered by a stack of
Si/SiO 2 layers that act as an interferential filter. This system has an emissivity with
low dependence on the angle and displays a peak at the desired frequency. As in
this spectral range, Si and SiO 2 are transparent, emission is due to the tungsten and
the stack plays the role of a filter. Drevillon et al. were able to design a layer stack
emitting at a well-defined frequency over all angles [39] by using an optimization
299
tures can be found in Ref. [128]. Several groups have reported absorbers based on this
idea [18, 20, 53, 74, 75]. In these systems, the absorption takes place predominantly
in the upper metallic layer. A slightly different system has been developed for thermal emitters in the infrared [101]. Here, the insulator is silica and the device works
in the IR by taking advantage of dielectric losses in silica. It is important to stress
that although these systems are usually periodic arrays of absorbers, the periodicity
does not play a significant role. Instead, the systems can be viewed as a collection of
independent absorbers. The small interaction between scatterers can be understood
by noting that the velocity mismatch between a surface plasmon propagating along
the bare surface and the guided mode propagating in the structure is very large. A
similar system has been reported recently. It consists in a periodic array of graphene
disks deposited on a dielectric coated metal [119].
Based on the same general principle of absorption by subwavelength resonant
structures, it is possible to use the absorption resonance of thin slits in a bulk metal. By
controlling the slit width, it is possible to modify the effective index. The resonance
frequency is therefore fixed by the depth and the width of the slit. Again, these slits
are resonant absorbers. They can lead to total absorption. The resonances of these
systems have been studied by a number of authors [70, 93]. As these slits can be
periodically arranged with periods smaller than half a wavelength, these structures
do not diffract and behave as metamaterials.
Another type of periodic structures has been designed to generate a controlled
absorption. It can be described as a periodic array of rectangular slits in a thin
metallic film. The structure can be considered to be analogous to a LC resonant cavity
as discussed in Refs. [71, 124]. A similar structure with negative index properties
has been also studied for resonant absorption [6]. Other schemes based on patterning
metallic structures can also be used to control thermal emission [82, 111].
8.1.8 Multilayers and Photonic Crystals for Controlled Emission
In this section, we review tailoring emissivity using interferences. Although it is not
based on surface waves, it is useful to briefly quote these works. We start with simple
planar systems and we continue with photonic crystals. A simple approach is based
on using multilayered systems, the simplest possible structure being an absorbing
dielectric slab. The interferences lead to a non trivial angular and spectral dependence
of the emission for a silicon slab as discussed in Ref. [63]. For thermophotovoltaics
application, it is necessary to have a quasimonochromatic source in the near infrared.
Rephaeli and Fan [104] have proposed to use a tungsten slab covered by a stack of
Si/SiO 2 layers that act as an interferential filter. This system has an emissivity with
low dependence on the angle and displays a peak at the desired frequency. As in
this spectral range, Si and SiO 2 are transparent, emission is due to the tungsten and
the stack plays the role of a filter. Drevillon et al. were able to design a layer stack
emitting at a well-defined frequency over all angles [39] by using an optimization