Multiresonant Grating to Replace Transparent Conductive Oxide Electrode for Bias Selected Filtering of Infrared Photoresponse

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Multiresonant Grating to replace Transparent Conductive Oxide Electrode for bias selected
filtering of infrared photoresponse
Tung Huu Dang1,2, Mariarosa Cavallo1, Adrien Khalili1, Corentin Dabard1, Erwan Bossavit1,
Huichen Zhang1, Nicolas Ledos1, Yoann Prado1, Xavier Lafosse3, Claire Abadie1, Djamal
Gacemi2, Sandrine Ithurria4, Grégory Vincent5, Yanko Todorov2, Carlo Sirtori2, Angela Vasanelli2*,
Emmanuel Lhuillier1*
1 Sorbonne Université, CNRS, Institut des NanoSciences de Paris, 4 place Jussieu, 75005 Paris,
France.
2 Laboratoire de physique de l’Ecole Normale Supérieure, ENS, Université PSL, CNRS, Sorbonne
Université, Université Paris Cité, 24 Rue Lhomond, 75005 Paris, France
3Centre de Nanosciences et de Nanotechnologies, CNRS, Univ. Paris-Sud, Université Paris-
Saclay, 10 Boulevard Thomas Gobert, 91120 Palaiseau, France
4 Laboratoire de Physique et d’Etude des Matériaux, ESPCI, PSL Research University, Sorbonne
Université, CNRS UMR 8213, 10 rue Vauquelin, 75005 Paris, France.
5 DOTA, ONERA, Université Paris Saclay, 6 Chem. de la Vauve aux Granges, 91120 Palaiseau,
France.
Abstract: Optoelectronic devices rely on conductive layers as electrodes, but they usually introduce
optical losses that are detrimental for the device performances. While the use of transparent
conductive oxides is established in the visible, these materials show high losses at longer
wavelengths. Here, we demonstrate a photodiode based on metallic grating acting as an electrode.
The grating generates a multi-resonant photonic structure over the diode stack and allows strong
broadband absorption. The obtained device achieves the highest performances reported so far for
a mid-wave infrared nanocrystal-based detector, with external quantum efficiency above 90 %,
detectivity of 7x1011 jones at 80 K at 5 µm, and sub 100 ns time response. Furthermore, we
demonstrate that combining different gratings with a single diode stack can generate bias
reconfigurable response and develop new functionalities such as band rejection.
Keywords: light-matter coupling, active photonics, reconfigurable response, infrared, nanocrystals,
photoresponse.
*To whom correspondence should be sent: angela.vasanelli@ens.fr, el@insp.upmc.fr

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Optoelectronic devices rely on semi-transparent electrodes that allow light to be absorbed or emitted
from the optically active materials. In the visible range, transparent conductive oxides (TCO), such
as ITO (tin-doped indium oxide), AZO (aluminum-doped zinc oxide), or FTO (fluorine-doped tin
oxide), play this role by combining a high transmission all over the visible range with high
conductivity. When infrared wavelengths are targeted,1,2 the transmission of the TCO becomes a
major concern. In the near-infrared, a trade-off between good conductivity and low optical losses
can easily be found by lowering the electrode thickness or carefully tuning the TCO properties.3
However, both methods come at the price of reduced conductivity and, thus, an increased contact
resistance for the device. This may prevent the injection of large currents in light-emitting diodes
(LED)4,5 or result in poor impedance matching for detectors, preventing the fastest device operation.
The problem becomes even more dramatic in the mid-wave infrared (MWIR) with low TCO and
substrate transmission. In the context of mid-infrared nanocrystal (NC)-based sensing,6–8 the issue
has mostly been swept under the rug. Indeed, until recently, developing diode stacks has been the
focus of most of the effort, leaving optical loss in the contacts as a second-order concern. However,
the performances of the mid-wave infrared diodes have been significantly improved9 over the recent
years10 with reported near-unity efficiency, with electroluminescence matching the
photoluminescence efficiency in LED,4 or even for detection. Future improvement will require
addressing the problem of contact absorption in the infrared.11
Beyond reducing the TCO layer thickness, the most basic strategy to design a partly transparent
electrode is to use a metallic grid.12–16 In this case, the electrode transmission is mainly set by the
metal filling factor. On the one hand, the filling factor has to be low to render the electrode
transmission as high as possible. On the other hand, a low filling factor not only presents a higher
sheet resistance but also lengthens the collection pathways of the photogenerated charges, which
is strongly detrimental in the case of low-mobility materials such as NCs. A combination of a metallic
grid/pad and thin TCO layer17,18 has also been tested as an approach to combine transparency and
conductivity or as a way to tune the work function of the TCO layer.19 However, the problem has not
been adequately solved. Meanwhile, the Ebessen’s group20 has brought evidence that under certain
conditions of resonance, the transmission of a subwavelength aperture can significantly exceed the
value set by its filling factor. This pioneer work has later been expanded toward the field of
extraordinary optical transmission21–23 and plasmonic nanohole array.24,25 Here, we explore the
design of a resonant metallic grid26–28 offering an alternative to TCO as a transparent electrode in
the infrared. The grating is designed so that multiple resonances arise, hence allowing a broadband
enhancement of the infrared NC absorption. Furthermore, we carefully optimize the geometrical
parameters of the diode to maximize the absorption beyond the simple filling factor of the metallic
grid. We demonstrate that, by doing so, we are able to obtain the highest performing mid-wave
infrared NC-based photodiode reported so far with responsivity close to 3 A.W-1, detectivity reaching
7x1011 jones at 80 K, and a sub-100 ns time response.
In the second part of the paper, we demonstrate the use of these gratings as a strategy to achieve
bias reconfigurable photoresponse. So far, the concept of multicolor detectors has been explored
independently of bias reconfigurability.29–31 Bi32–34 and multicolor35 response is mainly obtained by
associating devices presenting different band edge energies. In particular, bicolor detectors have
been obtained by stacking two diodes33,36 on top of each other, similar to what is done for multi-
junction solar cells. However, colloidal fabrication with a large number of steps can be highly
challenging. Here, we show that dual-band reconfigurable response can be obtained from a single
diode stack. By multiplexing back-to-back diodes with different gratings on a single chip, various
operating modes can be obtained, enabling band selection and new functionalities such as narrow-
band rejection, which so far remains unreported for nanocrystal-based devices.
Current best mid-wave infrared NC-based diodes rely on the HgTe/Ag2Te stack, introduced by
Ackerman et al.37 In such diodes (Figure 1b), the mid-wave infrared absorption relies on HgTe NCs.
Here we grow them using the procedure developed by Keuleyan et al.38 The final targeted cut-off

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wavelength is 5 µm (i.e., 2000 cm-1), with the grown particles displaying an exciton peak at around
3 µm, as shown in Figure 1a. The cut-off wavelength will later be redshifted upon ligand exchange
procedure and cooling (see Figure S1 and S14). Initial long-capping ligands are replaced by a
mixture of HgCl2 and mercaptoethanol39,40 to achieve a stronger interparticle coupling as well as to
maintain good surface passivation.41 HgTe with such band edge tends to have an ambipolar
character, and Hg or chalcogenides excess on the surface can induce either stronger n or p-type
behavior.10 The absorbing layer was initially surrounded by a sapphire substrate, and a thin ITO
layer was used as an electron extractor. Sapphire was used instead of glass because the latter has
a large absorption above 4 µm, whereas sapphire maintains a high transmission up to 6 µm while
remaining cost-effective. The ITO thickness (50 nm, Figure S3) was reduced compared to the typical
one used in the visible (180 nm range). At cryogenic temperature (80 K), the stack indeed behaves
as a diode presenting a strongly rectifying behavior for the IV curve, see Figure 1c and Figure S7
for further characterization. The successful character of this diode has been confirmed by its reuse
within the following papers of the same group,33,42 but also by others,9,43,44 and can be considered
the most efficient platform for NC-based mid-IR sensing to date.
Figure 1 HgTe NC-based photodiode using a transparent conductive oxide as the electrode.
a. Absorption spectrum of HgTe NCs used as MWIR absorber, as they are by the end of synthesis
and at room temperature. b. Schematic of a photodiode based on sapphire/ITO/HgTe/Ag2Te/Au
stack. c. I-V curves under dark condition and under illumination (λ ≈ 4.4 µm – the power in the
caption is the nominal laser power) measured at 80 K for a diode stack such as the one depicted in
b. d. Transmission spectra for a pristine sapphire substrate and for a substrate coated with ITO
layer. e. Absorption map (at 3 µm) for the diode stack depicted in b. The scale bar is 350 nm long.
f. Simulated absorption spectra within the NC and the ITO for the diode stack depicted in e.
In spite of this success, the use of ITO remains a clear issue, see Figure 1d-f. The transmission of
50 nm ITO at 5 µm is around 40 %. As a result, the targeted device absorption in the range of 3-5

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µm (i.e., the atmospheric transparency window that enables long-distance imaging) is not dominated
by HgTe but rather by the ITO layer (Figure 1e-f), which ends up generating thermal losses rather
than photocurrent. In the next part of this paper, we demonstrate that the careful design of a metallic
grating offers an interesting alternative to ITO that not only reduces the optical loss in the metal but
also shapes the spectral response. Our metallic structure is depicted in Figure 2a, S2-4, in which a
1D gold grating replaces the ITO layer. The diode stack remains unchanged (i.e., same material,
same order), and only the geometrical parameters will be optimized. The grating size and periodicity
are tuned to generate 3 resonances30. On top of the gold stripes, metal-dielectric-metal cavities are
formed with a first resonance driven by the stripe width through the relation λ=2.neff.s, where λ is the
resonant wavelength, neff is the effective modal index, and s is the grating stripe width. This
resonance is called TM cavity mode and results from the coupling of the incoming light to the TM0
mode of the microcavities.45 We found that s=350 nm induces a resonance at 4.1 µm in TM
polarization, see Figure 2d. Apart from the cavity mode, the second resonance under TM
polarization is a spoof surface plasmon (SP) whose wavelength is determined by the grating period.
The existence of surface plasmon polaritons at the interface between the top gold layer and the
nanocrystal film can be derived from Maxwell’s equations for TM polarization. 46 In the absence of
the grating, the surface plasmon polariton cannot be optically excited by free-space photons due to
momentum mismatch. However, in our structure, diffracted light from the grating stripes acquires
extra momentum and thus can couple with the surface plasmon polaritons. As a result, the period
can be set at 1800 nm to match the SP with the exciton and broaden the band edge feature of the
NCs, although other values have also been explored, see Figure 2b. In this case, we generate the
SP resonance at 3.9 µm (Figure 2e), and the photocurrent spectrum presents a strong peak
corresponding to the cavity and plasmon-enhanced band edge. When shorter periods are chosen,
the SP resonance occurs at shorter wavelengths, and the cavity mode becomes visible at the band
edge, leading to two observable features. More detailed information on the design procedure is
given in Figure S5 and S8-S12.
Under TE polarization, the effective optical index discontinuity between the HgTe on sapphire and
the HgTe NC film on top of the metal allows for reflection to occur, forming a Fabry Perot cavity with
resonance wavelength determined by the distance between the stripes. This resonance does not
match the band edge and appears blue-shifted, see Figure 2c and f, but contributes to the
broadening and enhancing of the absorption signal.

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Figure 2 Resonant 1D grating used as a transparent conductive electrode. a. Schematic of a
photodiode based on sapphire/1D Au grating/HgTe/Ag2Te/Au stack. b. (resp c.) Photocurrent
spectra for the diode depicted in a. for various values of the grating period in TM (resp TE)
polarization. Broad features at 5370, 3830 and 2890 cm-1 are due to water absorption, while the
CO2 is leading to a sharp feature at 2350 cm-1. d. Absorption map in TM polarization at 4.1 µm. e.
Absorption map in TM polarization at 3.9 µm. f. Absorption map in TE polarization at 3 µm. Parts d-
f have been performed while the grating period is 1800 nm, the gold stripe width is 350 nm, and the
NC film thickness is 350 nm. The common scale bar for parts d -f is 400 nm.
Regarding the diode stack thickness, hopping transport in NC films limits the carrier mobilities in
HgTe and thus a thick NC film (≥500 𝑛𝑚) is not favorable for the photocurrent collection. On the
other hand, too thin NC film could result in poor light absorption and increased ohmic losses in the
metal. We thus optimize the diode stack thickness to maximize the overall NC device absorption,
considering that a reasonable film thickness for efficient charge transport is usually below 500 nm,
see Figure S11-12. The basic design guideline for ITO-based diodes is: the thicker the diode, the
better. On the contrary, we identify an optimum thickness for the absorbing layer coupled to the
grating corresponding to 350 nm. It is worth mentioning that the choice of the top gold layer thickness
is also important. Usually, such metallic back reflector thickness is generally designed to match the
material skin depth. Here, we have observed that a top metallic layer with a thickness below 80 nm
can be optically leaky and therefore reduce the absorption within the NC layer.
Application to mid-infrared sensing
Now that the concept of the metallic grating is established, we switch to a 2D grid with a square
lattice to maximize the resonance absorption in both polarizations, see Figure 3a. Note that, far
from the resonance, the overall transmission of the grid is reduced with respect to that of the grating,
as the structure is equivalent to two crossed polarizers. The benefit of such a 2D grid over the 1D
grating and the ITO layer is depicted in Figure 3b-d: the absorption is selectively increased close to
the exciton peak, while it is reduced at shorter wavelengths, resulting in an increased responsivity
We observe on the photocurrent spectra (Figure 3b) a dramatically enhanced signal over the MWIR

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region thanks to the designed resonances, while the rectifying behaviour is mostly maintained, see
Figure S13-15. When comparing the grid to the ITO device, we can see that the photocurrent signal
is also increased away from the resonances thanks to the improved transmission and reduced
contact resistance.
Figure 3 Resonant 2D grid for mid-wave photodiodes. a. Schematic of a photodiode based on
sapphire/2D Au grid/HgTe/Ag2Te/Au stack. On the right panel, we show an SEM image of a
fabricated 2D gold grid. The scale bar is 1 μm. b. Photocurrent spectra for a diode in which the
bottom electrode is made of ITO film, 1D grating, and 2D grid. For both grating and grid electrodes,
the Au stripe width and periodicity are s = 350 nm and p = 1800 nm, respectively. c. Responsivity
(in front of a 600°C blackbody source) as a function of the temperature for a diode in which the
bottom electrode is made of ITO film, 1D grating, and 2D grid. d. Specific detectivity (at 1 kHz) as a
function of the temperature for a diode in which the bottom electrode is made of ITO film, 1D grating,
and 2D grid.
Responsivity is very close to 3 A.W-1, whereas a 100 % external efficiency would correspond to 3.2
A.W-1 for a device with a 4 µm peak. Note that the polarization independence introduced by the grid
should have increased the responsivity by a factor of two. However, one should take into account
the increased reflectivity of the device and metal losses due to the higher filling factor of the 2D grid.
On the one hand, the reflection for out-of-resonance wavelengths increases due to higher metal
coverage. On the other hand, NC absorption resulting from the surface plasmon resonance in 2D
grid is not exactly double of that of the 1D grating because the additional metal stripes limit the
spatial overlap of the mode with the NCs. At cryogenic temperature, white noise is prevailing, see
Figure S13. The specific detectivity at 80 K is 7x1011 jones, while the device presents a 5 µm cut-
off wavelength. This is currently the highest value reported for a mid-wave infrared photodiode based

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on NCs, see Table 1 for detection figures of merit of state-of-the-art mid-wave infrared-operated NC-
based devices.
The device’s time response (T90-10%) to a pulse of light from a quantum cascade laser resonant with
the particle band edge is around 80 ns (Figure S15) for a device size of 100x100 µm2, close to the
laser rise time. Thus, we also have used a complementary approach to determine the device
bandwidth by applying a microwave rectification technique (i.e., only dark current). In this case, the
-3 dB bandwidth is found to be around 20 MHz, corresponding to a rise time of trise=0.35/f3dB≈18 ns.
This suggests that the device’s dynamics are mainly determined by the capacitance of the diode.
Table 1 Figures of merit relative to light sensors operating in the MWIR and based on NCs. (PC:
photoconductive, PT: phototransistor, PD: photodiode, * set-up limited)
Absorbing
material/Cut-
off λ (µm)
Operating
mode
Responsivity
(A.W-1)
Response
time
Detectivity
(jones)
Operating
temperature
(K)
Specific
feature
Ref.
Intraband MWIR device
HgSe
PC
5 x 10-4
8.5 x 108
80
intraband
47
HgSe
PC
0.7
8.75 ms
108
300
intraband
48
HgSe
PD
5 x 10-3
200 ns
2 x 109
80
Intraband/mixed
materials
12
HgSe
PC
77 x 10-3
1 μs
1.7 x 109
80
intraband
49
HgSe
PC
0.145
2.1 s
-
300
intraband
50
Ag2Se
PD
19 x 10-3
-
7.8 x 106
300
intraband
barrier device
51
Ag2Se
PC
13.3 x 10-3
-
3 x 105
300
intraband
barrier device
52
Ag2Se
PC
8 x 10-6
~10s
-
300
intraband
53
Ag2Se
PC
350 x 10-6
-
-
90
intraband
54
Inter band MWIR device
HgTe
PC
0.25
-
2 x 109
130
Agregated
material
55
HgTe
PD
1.3
1 μs
3.3 x 1011
85
Fisrt diode
with Ag2Te as
HTL
37
HgTe
PD
1.62
-
4 x 1011
85
With
resonator
18
HgTe
PD
0.4
2.5 μs*
3 x 1010
85
Stack diode
33

8
HgTe
PD
2.7
-
2.7 x 1011
80
Homo p-i-n
junction
10
HgTe
PC
0.7
11 μs*
2 x 1010
80
Multiresonant
pattern
30
HgTe
PD
2.65
80 ns
7 x 1011
80
This work
In the first part of the paper, we demonstrated that metallic grids and gratings could advantageously
replace TCO. In the last part of the paper, we show that the resonant electrode opens up a new
degree of freedom and can be used to obtain novel functions, such as a bias-reconfigurable
response. For this purpose, multiple gratings with different periods have been fabricated on the
same chip. As mentioned, the grating period determines the diode spectral response, as shown in
Figure 2b and c. Now, instead of operating the diodes in a solely vertical geometry, we apply bias
between two electrodes, each connected to a grating, see Figure 4a. This method enables the
formation of back-to-back diodes from a single diode stack (see Figure 4b), which considerably
eases the fabrication compared to the approach where multiple diodes are built on top of each
other.33,36,56
Here, by only using HgTe NCs with a 5 µm cut-off wavelength and properly coupling two gratings of
similar polarization, it is possible to obtain a bias switch of the response from MWIR to SWIR only,
see Figure 4c and Figure S16. Under 0 V operation, current sign is directly connected to the spectral
range where the grating presents a resonance, enabling the use of photocurrent sign as spectral
information, see Figure S16b.
This approach also enables generating new strategies to reshape the spectral response. When the
currents flowing in the two diodes are equal in intensity, since they are opposite in sign, the device’s
overall response becomes null. This effect leads to a spectral band rejection. In Figure 4d, we show
that the spectral response at 3 µm can be turned off while MWIR band remains strong (the green
part). This property can be particularly interesting to prevent optical countermeasures. Indeed, a
typical countermeasure strategy to prevent infrared imaging is blurring the sensor using a high-
power laser. In our device, thanks to the capacity to turn off the response over a selected band,
possible saturation of the detector can be prevented, such that it is possible to continue imaging
even if the detector has been spotted.

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Figure 4 Band filtering properties of back-to-back diodes. a. Schematic of an array of back-to-
back diodes in which the periods of the two bottom electrodes are varied. In vertical diode
configuration, the grating period determines the resonance wavelength for each diode. A back-to-
back diode configuration can be created by applying a bias voltage through the bottom gratings. b.
Equivalent circuit for the back-to-back diode. Depending on the bias sign, the diode is either
connected in forward or reverse mode. In this example, only the diode associated with grating 1 is
activated. c. Photocurrent (absolute value) spectra under +/- 40 mV in the case where 1200 and
1600 nm gratings are selected. With bias, the MWIR response can be turned ON and OFF. d.
Photocurrent (absolute value) spectra, under positive and negative bias, in the case where 1400
and 1800 nm gratings are selected. With bias, the response of a narrow band can be turned ON
and OFF.
To summarize, the concept of transparent conductive electrodes for infrared offers no
straightforward strategy. Here, we demonstrate that a careful design of metallic grating can minimize
the optical loss while enhancing the effective absorption in the active semiconductor layer. Here, we
propose a strategy combining three resonances from a single grating. The obtained device presents
outstanding performances with responsivity close to 3 A.W-1 corresponding to EQE above 90% for
a device with 5 µm cut-off wavelength and operation at 80 K. Detectivity at cryogenic temperature
reaches 7x1011 jones, while the diode response time under mid-wave infrared excitation is below
100 ns. We also demonstrate that different metallic gratings can be combined with a single diode
stack to form multicolor and reconfigurable photodiodes. Compared to existing strategies, our
approach is far easier to fabricate while offering new functionalities, such as band rejection, which
can be used against countermeasure applications. Now that MWIR NC-based sensors achieve high
performances, it becomes clear that the next challenge will be raising the operating temperature to
reach hot operation10,57 (i.e., above 130 K).

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DATA AVAILABILITY
The data that support the findings of this study are available from the corresponding author upon
request.
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ACKNOWLEDEGMENTS
The project is supported by ERC starting grant blackQD (grant n° 756225) and AQDtive (grant
n°101086358). We acknowledge the use of clean-room facilities from the “Centrale de Proximité
Paris-Centre” and the french RENATECH network. This work was supported by French state funds
managed by the ANR more specifically within the grants Copin (ANR-19-CE24-0022), Frontal (ANR-
19-CE09-0017), Graskop (ANR-19-CE09-0026), NITQuantum (ANR-20-ASTR-0008-01), Bright
(ANR-21-CE24-0012-02), MixDferro (ANR-21-CE09-0029) and Quicktera (ANR-22-CE09-0018).

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Author contributions
A.V. and E.L. designed the project and made it funded. Y.P. and A.K. synthesized the nanoparticles
under the supervision of S.I. T.H.D., A.V., and G.V. with support of Y.T. and C.S. performed the
electromagnetic simulation. T.H.D. fabricated the device with clean room support of X.L., M.C., C.D.,
E.B., H.Z., N.L., and C.A. T.H.D. and D.G. conducted experimental characterization. T.H.D., A.V.,
and E.L. wrote the manuscript with input from all the authors. All authors have proofread the paper’s
content.
COMPETING INTEREST
The authors declare no competing interests.
Supporting Information
Supporting Information include (i) supplementary methods, (ii) HgTe NC used as infrared absorbing
layer, (iii) Fabrication process of the diode, (iv) discussion of gold electrode motivation and
resistance, (v) Performance of diode using ITO as transparent electrode, (vi) Electromagnetic design
of the bottom electrode, (vi) Optimization of the diode geometrical factor, (vii) Performances of the
diode with 2D multi resonant grid, (viii) Bias dependence of the back-to- back diode operation.

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