8 Controlling Thermal Radiation with Surface Waves
287
are delta correlated as shown by the fluctuation-dissipation theorem. However, when
these currents excite extended modes of the emitter such as a surface wave, a spatial
correlation can be built in the source. It follows that the source can become directional
as it was first demonstrated in Ref. [45].
Another basic property of thermal sources is the impossibility of modulating the
emitted flux at high frequencies. Indeed, the standard approach relies in modulating
the temperature. As cooling a source is a slow process, high speed modulation cannot
be obtained. Typical modulation frequencies for available IR sources are on the order
of tens of Hz. To go beyond this limitation, it has been proposed recently to modulate
the emissivity instead of modulating the temperature [46]. This modulation can be
performed without modifying the emitter temperature by using e.g. a phase material
change or modulating the doping of an active absorber. These approaches are no
longer limited by the cooling dynamics.
In summary, it is seen that many limitations of usual thermal sources such as
a poor directivity, a broad spectrum and a low intensity frequency modulation can
be overcome by using advanced concepts of nanophotonics. This paves the way
towards the design of smart IR incandescent sources. This is all the more important
as it is very difficult to produce efficient LEDs in the IR. This is due to a fundamental
limit based on the spontaneous photon emission rate that scales as the cube of the
frequency. When moving the wavelength from 1 to 10 µm, the rate of spontaneous
emission is thus decreased by a factor of 1000. In what follows, we review several
experiments showing how emissivity can be modified by taking advantage of the
interplay between surface waves and surface microstructures.
8.1.1.5 Thermal Sources and Partial Coherence
A thermal source is often considered to be an incoherent source. The aim of this
section is to briefly review the basic concepts of coherence in order to clarify how
it is possible to design a partially coherent thermal source. Let us first remind what
is a blackbody source. We start by reminding that the term blackbody is sometimes
used to refer to the radiation at thermal equilibrium, sometimes used to refer to an
isothermal cavity that can be used to experimentally produce radiation approaching
the radiation field at thermodynamic equilibrium, and sometimes used to refer to a
perfectly absorbing surface which can absorb incident radiation at any wavelength
and from any direction and polarization. Only the last will be called blackbody
hereafter. In other words, we use the term blackbody to refer to a material with an
emissivity which is equal to 1 for any wavelength and angle.
The coherence properties of the electromagnetic field at thermodynamic
equilibrium are well known [43]. They are characterized by the correlation function ∪E n (r, t)E m (r ≈ , t ≈ )∼. Field correlations are often called second-order coherence.
Intensity correlations which are widely studied in quantum optics will not be considered here. It is customary to introduce spatial coherence and temporal coherence of
a given electromagnetic field. The spatial coherence is characterized by the field correlation function at a given time ∪E n (r, t)E m (r ≈ , t)∼ whereas the temporal coherence
287
are delta correlated as shown by the fluctuation-dissipation theorem. However, when
these currents excite extended modes of the emitter such as a surface wave, a spatial
correlation can be built in the source. It follows that the source can become directional
as it was first demonstrated in Ref. [45].
Another basic property of thermal sources is the impossibility of modulating the
emitted flux at high frequencies. Indeed, the standard approach relies in modulating
the temperature. As cooling a source is a slow process, high speed modulation cannot
be obtained. Typical modulation frequencies for available IR sources are on the order
of tens of Hz. To go beyond this limitation, it has been proposed recently to modulate
the emissivity instead of modulating the temperature [46]. This modulation can be
performed without modifying the emitter temperature by using e.g. a phase material
change or modulating the doping of an active absorber. These approaches are no
longer limited by the cooling dynamics.
In summary, it is seen that many limitations of usual thermal sources such as
a poor directivity, a broad spectrum and a low intensity frequency modulation can
be overcome by using advanced concepts of nanophotonics. This paves the way
towards the design of smart IR incandescent sources. This is all the more important
as it is very difficult to produce efficient LEDs in the IR. This is due to a fundamental
limit based on the spontaneous photon emission rate that scales as the cube of the
frequency. When moving the wavelength from 1 to 10 µm, the rate of spontaneous
emission is thus decreased by a factor of 1000. In what follows, we review several
experiments showing how emissivity can be modified by taking advantage of the
interplay between surface waves and surface microstructures.
8.1.1.5 Thermal Sources and Partial Coherence
A thermal source is often considered to be an incoherent source. The aim of this
section is to briefly review the basic concepts of coherence in order to clarify how
it is possible to design a partially coherent thermal source. Let us first remind what
is a blackbody source. We start by reminding that the term blackbody is sometimes
used to refer to the radiation at thermal equilibrium, sometimes used to refer to an
isothermal cavity that can be used to experimentally produce radiation approaching
the radiation field at thermodynamic equilibrium, and sometimes used to refer to a
perfectly absorbing surface which can absorb incident radiation at any wavelength
and from any direction and polarization. Only the last will be called blackbody
hereafter. In other words, we use the term blackbody to refer to a material with an
emissivity which is equal to 1 for any wavelength and angle.
The coherence properties of the electromagnetic field at thermodynamic
equilibrium are well known [43]. They are characterized by the correlation function ∪E n (r, t)E m (r ≈ , t ≈ )∼. Field correlations are often called second-order coherence.
Intensity correlations which are widely studied in quantum optics will not be considered here. It is customary to introduce spatial coherence and temporal coherence of
a given electromagnetic field. The spatial coherence is characterized by the field correlation function at a given time ∪E n (r, t)E m (r ≈ , t)∼ whereas the temporal coherence