Lithography-free directional control of thermal emission. Mitradeep Sarkar1Maxime Giteau1Michael Enders1and Georgia T. Papadakis1 1ICFO-Institut de Ciencies Fotoniques The Barcelona Institute of Science and Technology

2025-05-02 0 0 2.49MB 10 页 10玖币
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Lithography-free directional control of thermal emission.
Mitradeep Sarkar,
1
Maxime Giteau,
1
Michael Enders,
1
and Georgia T. Papadakis
1, ∗
1
ICFO-Institut de Ciencies Fotoniques, The Barcelona Institute of Science and Technology,
08860 Castelldefels (Barcelona), Spain
Blackbody thermal emission is spatially diffuse. Achieving highly directional thermal emission
typically requires nanostructuring the surface of the thermally emissive medium. The most common
configuration is a subwavelength grating that scatters surface polaritonic modes from the near-field to
the far-field and produces antenna-like lobes of thermal emission. This concept, however, is typically
limited to a particular linear polarization, and requires sophisticated lithography. Here, we revisit
the simple motif of a planar Salisbury screen. We show analytically how the interplay between the
real and imaginary part of the dielectric permittivity of the emitter layer defines the directional
characteristics of emission, which can range from diffuse to highly directional. We propose a realistic
configuration and show that hexagonal Boron Nitride thin films can enable grating-like thermal
emission lobes in a lithography-free platform.
Nanophotonics: Thermal Emission, Mid-Infrared , Phonon Polaritons
INTRODUCTION
Mid-infrared (IR) thermal emission is ubiquitous in nature. Gaining control over its spatial and spectral characteristics
is central to various applications. Examples include light and energy harvesting [
11
,
29
], as in thermophotovoltaic
systems [
9
,
25
,
28
,
43
,
55
], radiative cooling [
31
], IR sources [
17
,
23
], thermal camouflage [
30
] and molecular sensing
[
4
,
36
]. Spectrally narrow thermal emission is required in various applications, such as in sensing [
13
,
32
,
33
,
49
], IR
sources [7, 51], as well as for maximizing light-harvesting conversion efficiency [37, 40]. In these applications, control
over the spatial characteristics of thermal emission is key.
Since blackbody thermal emission is spectrally broad and spatially diffuse, photonic design can be employed to
narrow its spectral range [
10
,
53
] and to control its directionality [
3
], or both [
14
,
49
]. A plethora of sophisticated
nanostructures have been considered for tailoring thermal emission, such as mid-IR antennas [
1
,
48
], multi-layered
films and one-dimensional photonic crystals [
7
,
41
,
50
], complex three-dimensional resonators [
8
,
34
,
35
], and gratings
[
20
,
23
]. In fact, the principle of operation of many of these motifs and other sophisticated designs [
16
,
19
,
21
,
52
] lies
within the physics of a photonic grating. As Greffet et al. experimentally demonstrated in [
14
], one can thermally
excite near-field surface phonon polariton modes [
6
] on a silicon carbide (SiC) surface. Using a subwavelength grating,
it was shown that these modes can diffract into propagating modes at specific angles, thus enabling an ultra-high
degree of directional control in thermal emission. Such surface phonon polaritons occur strictly within the Reststrahlen
band of SiC, which is the frequency range where its dielectric function is negative [
26
]. The effect can be generalized
to any material with a polar resonance in the mid-IR [
15
], as well as to plasmonic media for frequencies below their
plasma frequency, where they support surface plasmon polaritons [45, 47].
In both plasmonic and polar media, however, surface polaritons occur solely for p-polarization [
38
,
39
], thus the
aforementioned concept is also constrained to p-polarized radiation. Furthermore, the Reststrahlen band where surface
phonon polaritons occur is spectrally narrow [
6
], thus limiting the frequency range of operation of a grating and related
motifs for thermal emission control. Importantly, nanoscale patterning requires complex nanolithography or synthesis.
In contrast to previous attempts to control the directionality of a thermally emitted beam via a nanostructure, here,
we revisit the concept of a Salisbury screen [
44
]. A Salisbury screen is a three-layer planar structure that operates
based on constructive interference between the layers, and yields near-unity thermal emissivity on resonance [
12
]. It is
constructed out of a thin thermal emitter on a thick dielectric spacer with a back-side reflector, as shown in Fig.1c.
We study the directionality of thermal emission from a Salisbury screen in a systematic and analytical way, with
respect to the optical properties of the emitting layer. By considering two regimes of thermal emission: spatially
diffuse and directional, we present simple nanophotonic strategies to achieve both, in a lithography-free configuration.
We show that a key characteristic for highly directional thermal emission is a high-absolute value of the dielectric
permittivity (
) of the emitter layer, which can be found in various polar media [
6
]. As a practical example, we leverage
the high-dielectric permittivity of hexagonal Boron Nitride (hBN) near 7
µm
to demonstrate that grating-like thermal
emission lobes that are highly directional, without any lithography, for both linear polarizations.
We note that recent works have reported directional thermal emission in similarly planar configurations [
2
,
21
,
22
,
27
].
However, in [
27
], the photonic band gap of photonic crystals was used to achieve strong directionality, which requires a
very large number of layers. In addition, it was shown that Salisbury-like heterostructures can yield directionality,
however the concept was limited to metal-spacer-metal structures, hence much weaker performance. In [
2
], by employing
arXiv:2210.01026v1 [physics.optics] 3 Oct 2022
2
FIG. 1. a) Black-body radiation can be classified as diffuse and the polar plot of a typical black-body emissivity is shown. b)
Periodic nano-structures (gratings) have directional emission at well defined narrow spectral ranges. c) The schematic of the
meta-structure comprising of a lossy emitter (permittivity
e
) and a lossless spacer (permittivity
m
) in the Salisbury screen
configuration. Light in the spacer will acquire a phase of
π
for reflection from the PEC and Ψon reflection from the lossy
emitter. When the phase matching condition (constructive interference in the structure) is satisfied, the structure has high
emissivity. Directional thermal emission at a particular angle
θ
with angular spread ∆
θ
is achieved when specific interference
conditions are satisfied.
strong optical anisotropies, it was shown that one can use the Brewster’s condition to achieve directionality, however
this concept pertains to near-grazing angles of incidence. In [21], broadband directional emission was experimentally
demonstrated, whereas in [
22
], it was theoretically shown that epsilon-near-zero films on a reflector can direct a
thermally emitted beam for p-polarized light. Nevertheless, in all the above, the reported directionality remains
significantly inferior to that of a grating [14].
DIRECTIONAL CONTROL WITH PLANAR META-STRUCTURE
The directionality of a thermally emitted beam can be described by the dependence of the thermal emissivity,
E
(
θ
),
on the zenith angle of emission,
θ
. From Kirchhoff’s law of thermal radiation [
24
], the emissivity is equal to the
absorptivity,
E
=
A
. For opaque emitters, therefore,
E
= 1
−R
, where
R
is the reflectivity of the emitter. In order to
evaluate the performance of various geometries as directional thermal emitters at a particular wavelength and linear
polarization (namely s and p, pertaining to transverse electric and transverse magnetic waves, respectively), with
respect to a polar diagram such as those in Figs. 1(a)-(c), we consider two relevant properties: (i) the angular spread
of the emissivity, ∆
θ
, which is defined here as the full width half maximum (FWHM) of the lobe, and (ii) the contrast
between maximum and minimum emissivity at this wavelength, defined as
C
=
Rmin(θ)−Rmax(θ)
. The ratio
C/
∆
θ
expresses the degree of directionality of a thermal beam. Since we can write ∆
θ≈ |dR/dθ|−1
, we define a figure of
merit of the directionality of a thermal beam (FOM) as
FOM = C×
dR
dθ
.(1)
By the definition of Eq. 1, a low
FOM
indicates diffuse (unidirectional) thermal emission, such as a the spatial
characteristic of a thermal emission from a blackbody, whereas a large
FOM
suggests highly directional emission, such
as that achieved by a the grating [
14
]. These are shown in Figs. 1 (a), (b), respectively. In the following section,
we will discuss a strategy to achieve both narrowband diffuse and narroband directional thermal emission with a
lithography-free planar configuration, the Salisbury screen [
44
]. In this configuration, a dielectric spacer layer is
sandwiched between a thin lossy emitter and a perfect electric conductor (PEC) as shown schematically in Fig.1 (c).
摘要：

Lithography-freedirectionalcontrolofthermalemission.MitradeepSarkar,1MaximeGiteau,1MichaelEnders,1andGeorgiaT.Papadakis1,1ICFO-InstitutdeCienciesFotoniques,TheBarcelonaInstituteofScienceandTechnology,08860Castelldefels(Barcelona),SpainBlackbodythermalemissionisspatiallydiuse.Achievinghighlydirecti...

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