# Enhancing the Infrared Emission from Silver Chalcogenide Quantum Dots through Microcavity Coupling

Lina Makke, Erwan Bossavit, Dario Mastrippolito, Andrei Shcherbakov, Mariarosa Cavallo, Huichen Zhang, Tommaso Gemo, Albin Colle, Adrien Khalili, Muchuan Hua, et al.

► To cite this version:

Lina Makke, Erwan Bossavit, Dario Mastrippolito, Andrei Shcherbakov, Mariarosa Cavallo, et al. "Enhancing the Infrared Emission from Silver Chalcogenide Quantum Dots through Microcavity Coupling." Advanced Optical Materials, 2025, 10.1002/adom.202402747. hal-04894981

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Distributed under a Creative Commons Attribution 4.0 International License
Enhancing the Infrared Emission from Silver Chalcogenide Quantum Dots
through Microcavity Coupling

Lina Makke<fnref n="1" />$, Erwan Bossavit<fnref n="2" />,<fnref n="3" />$, Dario Mastrippolito<fnref n="2" />, Andrei Shcherbakov<fnref n="2" />, Mariarosa Cavallo<fnref n="2" />,
Huichen Zhang<fnref n="2" />, Tommaso Gemo<fnref n="2" />, Albin Colle<fnref n="2" />, Adrien Khalili<fnref n="2" />, Muchuan Hua<fnref n="4" />, Xavier Lafosse<fnref n="5" />,
Xiang Zhen Xu<fnref n="1" />, Mathieu G. Silly<fnref n="3" />, Debora Pierucci<fnref n="2" />, Emmanuel Lhuillier<fnref n="2" />, Benjamin T. Diroll<fnref n="4" />,
Aloyse Degiron<fnref n="6" />, Sandrine Ithurria<fnref n="1" />*

**Abstract:** Nanocrystals (NCs) appear as a promising platform for cost effective infrared optoelectronics, while offering a simplified coupling to the CMOS platform. However, this perspective is slowed down by toxicity concerns, the most effective materials being based on Pb and Hg. There is currently a large effort to bring forth new platforms that are active in the infrared with a reduced heavy metal content. Here, we focus on silver chalcogenides, which, thanks to a combination of inter- and intraband transitions, appear quite suited to cover both short and mid wave infrared ranges. However, this material being less mature, the achieved photoluminescence (PL) appears quite broad which is problematic for device integration. Here, we demonstrate that a strong control over the PL spectrum can be obtained through integration into a dielectric cavity which magnifies the electric field by a factor 20 and narrows the PL full width at half maximum down to 15 nm for an emission at telecom wavelength. The PL is also highly directional to avoid most waveguiding effects which is of utmost interest for enhancing the efficiency of NC based LEDs.

**KEYWORDS** : silver chalcogenides, infrared luminescence, microcavity.

<footnote n="1">¹Laboratoire de Physique et d'Etude des Matériaux, ESPCI, PSL Research University, Sorbonne Université, CNRS, 10 rue Vauquelin 75005 Paris, France</footnote>

<footnote n="2">² Sorbonne Université, CNRS, Institut des NanoSciences de Paris, INSP, F-75005 Paris, France.</footnote>

<footnote n="3">³Synchrotron SOLEIL, L'Orme des Merisiers, Départementale 128, 91190 Saint-Aubin, France.</footnote>

<footnote n="4">⁴Center for Nanoscale Materials, Argonne National Laboratory, 9700 S. Cass Avenue, Lemont, Illinois 60439, United States.</footnote>

<footnote n="5">⁵Centre de Nanosciences et de Nanotechnologies, CNRS, Université Paris-Saclay, C2N, Palaiseau 2110, France.</footnote>

<footnote n="6">⁶Université Paris Cité, CNRS, Laboratoire Matériaux et Phénomènes Quantiques, 75013 Paris, France.</footnote>

<footnote>$^*$ those authors have equal contributions</footnote>

<footnote>*To whom correspondence should be sent: sandrine.ithurria@espci.fr</footnote>
# 1. INTRODUCTION

II-VI semiconductor nanocrystals (NCs) are the most mature semiconductor colloidal materials, enabling a unique degree of complexity. This includes atomic scale size control[1,2] and heterostructure formation to design the NCs' absorption and emission spectra[3], shape their wavefunction distribution[4] and even engineer their scattering rates[5-7]. Though highly mature, the heavy metal content of the most developed materials has often been seen as a bottleneck toward their commercialization. This is why a large effort has been focused on the synthesis of InP NCs as an alternative to CdSe for the design of visible light downconverter used in display. In the infrared[8], most of the efforts rely on Pb and Hg[9] based materials, asking for a similar effort to develop alternative heavy metal-free materials. In the search for infrared-active colloidal nanomaterials, the initial challenges that arise are, first, the need for a narrow band gap and, second, the availability of a well-defined method to synthesize the material in nanoparticle form. While several candidates can be screened, their integration into photonic or optoelectronic devices requires some degree of stability. Two material platforms appear promising: III-V materials and silver chalcogenides, both benefiting from a surface chemistry close to the one of existing II-IV and IV-VI semiconductors, which are already well investigated in the infrared range. While InAs and InSb benefit from all the developments made for InP in the visible, they also present a dramatic instability to air exposure which enforces the growth of a shell to gain chemical stability. Furthermore, they remain to date limited below 2 µm[10-13].

Silver chalcogenides[14-17] also present a narrow interband gap (down to 0.2 eV for bulk Ag₂Te[18]) and can be further doped to enable intraband transitions[19-24] in the mid and long wave infrared. This covers a significant part of the infrared range with a single material. This being said, the current level of integration into infrared devices for silver chalcogenides lags far behind what has been achieved with Pb and Hg based materials. Previous efforts have been focused on light sensing[18,25-28] and photoluminescence (PL) emission in the near IR range[29,30] where water absorption is minimized (thus enabling deep bio imaging). Meanwhile, the integration of silver chalcogenides into photonic structures appears to need further development. Indeed, silver chalcogenides NC-based light emitting diodes (LEDs) with efficiency close to 20 % have been reported[31]. This value reflects a high electrical efficiency with electron to photon conversion close to 100 %. Thus, any further increase of the external quantum efficiency requires optimized light out-coupling from the LED. With no light management strategy, 4 out of 5 photons are trapped by total internal reflection in the substrate before being reabsorbed into the material and generating heat losses that detrimentally impact the luminescence efficiency.

It then becomes critical to develop strategies that favor an anisotropic emission from the quantum dots and maximize the luminescence signal thanks to maximization of the pump power. Here, we demonstrate the integration of silver chalcogenide core-shell NCs with emission around telecom wavelength into a dielectric microcavity. From this method, we show that emission becomes highly directional while the linewidth of the emission is reduced by a factor of 20.

# 2. RESULTS AND DISCUSSION

We start by growing and characterizing Ag₂Se/ZnSe [32] core-shell nanoparticles. The cores are grown using the procedure proposed by Ouyang et al.[33] with slight modifications, including the use of a selenium precursor instead of tellurium, adjustments in the proportions of the precursors, and changes in the reaction time, which is crucial to reach longer wavelengths, see Figure S1-S3. The ZnSe shell was grown by adding Zn(OAc)₂ powder directly into the QDs reaction medium without changing the temperature, through a modified version of the Yarema procedure[34]. The obtained nanoparticles have a slightly faceted aspect, with an average size of 3 nm and a shell with a thickness of 0.3 nm, according to transmission electron microscopy images, see Figure 1b and S4.
The absorption edge of the obtained core-shell heterostructures appears at 1250 nm, while the luminescence peak is around 1500-1600 nm depending on the exact core size, see **Figure 1a**. It is important to note that the luminescence is quite broad, with a full width at half maximum above 200 nm and possibly even larger[30]. This is a clear limitation when compared to the PbS and HgTe counterparts that can present much narrower linewidths down to below 100 nm[35]. It is likely that some progress in this regard can be achieved by narrowing the size distribution of the NCs. However, the exact ratio of homogeneous to inhomogeneous broadening remains unclear due to the lack of single particle measurements with these emergent infrared materials. It follows that the extent of the benefit of a narrower size distribution is still unknown, which later motivates our choice to focus on a complementary method where a microcavity is used to shape the photoluminescence (PL) spectrum.

The dynamics of the optical properties are then investigated. Time-resolved PL (Figure 1c) measurements on the core-shell particles present a quasi mono-exponential decay with a characteristic lifetime of 6 ns. The PL decay of the core-only particles clearly displays a multiexponential behavior (Figure 1c), which suggests a drastic reduction of the non-radiative channels with the addition of a shell. Transient absorption measurements (Figure S5) conducted at low fluence further confirm the absence of a faster decay component at the early stages which would be evidence of carrier trapping, indicating that the dynamics are mostly driven by the ns decay observed in PL. The shell, therefore, provides a clear advantage in enhancing the PL lifetime of the NCs.

<figure><img src="image_2.png" /><figcaption>**Figure 1** Optical and electronic properties of Ag₂Se/ZnSe core-shell NCs. a. Absorption and PL spectra. b. Transmission electron microscopy image. The inset is a high-resolution image on a single particle highlighting the core-shell structure c. Time resolved PL for Ag₂Se and Ag₂Se/ZnSe NCs. Red lines are exponential fit of the data. Single exponential is used for core-shell, while bi</figcaption></figure>
exponential has been used for core only. Core level spectra, acquired at 700 eV of Ag 3d (d.), Zn 3d (e.) and Se 3d (f.).

X-ray photoemission spectroscopy is then used to unveil the chemical environment of each atom. The full survey spectrum is available in Figure S6. In addition to silver, selenium and zinc which we expect, additional contributions from carbon and sulfur are present, which we attribute both to the leftover of native ligands (mainly thiol) and to the ethanedithiol introduced to make the film more conductive for the purpose of limiting the charging effect during the photoemission measurement. Ag 3d states display a main contribution that we attribute to silver coupled to selenide, with a 368 eV binding energy (BE), see **Figure 1d**. At higher energy, we observe a second contribution with a reduced relative weight. A higher B.E. suggests an environment with a decreased electronic density. This can be due to either the presence of sulfur on the surface from the ligand exchange treatment or the diffusion of the atoms from the core to the shell, a process similar to the cation exchange. However, silver is also one of the few elements for which the binding energy of the metallic state is larger than the binding energy of oxidized states[36], so the presence of metallic islands cannot be fully excluded.

On the survey spectrum acquired with a high photon energy (1150 eV), close to the one from a laboratory setup (1486 eV, Al Kα source), the Zn 2p state (binding energy at around 1022 eV) appears with an extremely small magnitude, Figure S6. The ZnSe shell being relatively thin, the presence of Zn may be poorly probed using a high energy X-ray source. Therefore, we choose a lower photon energy (700 eV obtained from TEMPO beamline of synchrotron SOLEIL) to reduce the photoelectron escape depth and acquire measurements with higher surface sensitivity. Under such condition, the Zn 3d state appears (**Figure 1e**) with a single contribution, that we attribute to Zn bonded to Se. The Se 3d state clearly presents two features (**Figure 1f**) that we attribute to Se bonded to Ag in the core and to Zn in the shell.

A study of the valence band and of the secondary electrons cut-off has been conducted using even lower photon energy (150 eV) to maximize the signal. From figure S6 we determine the work function to be 4.8 eV, while the valence band maximum is estimated to be 0.63 eV below the Fermi level. Given a band gap energy around 0.8 eV, this locates the fermi level closer to the conduction band, suggesting a non-degenerate *n*-type character for this material.

In order to shape the emission of the NCs, we propose to integrate them into a dielectric microcavity. The concept has already been investigated for various nano-emitters such as 2D materials[37], emissive defects in silicon[38], epitaxially grown quantum dots[39] or visible NCs[40–42], but far less so for infrared NCs[43,44]. Here the dielectric mirror is deposited on glass that maintains a high level of transparency in the short-wave infrared. The dielectric mirror is made of 15 layers, alternating SiO₂ as the low index medium (n=1.45) and Ti₃O₅ as the high index medium (n=2.46), see the scanning electron microscopy image in **Figure 2a**. In a conventional Bragg mirror design, each layer has a thickness of λ₀/4n where λ₀ is the center wavelength of the mirror's stop-band and *n* the refractive index of the layer under consideration. In such a mirror the index contrast drives the linewidth of the mirror stop band while the number of layers tunes the reflectivity. However, this kind of design offers no control over the transmission outside of the stopband, which can result in quite low values at the pump wavelength (transmission is at 70 % for a periodic Bragg at 642 nm, see Figure S7). Here, we choose to deviate from the periodic stack, so that we generate an additional anti-reflection band (with transmission reaching 93 %) at the optical excitation wavelength around 633 nm, in order to maximize the optical excitation of the NCs. The stop band is centered around 1550 nm which matches the PL of our core-shell NCs, see **Figure 2d**. In the visible, the coating provides the glass slide a reddish aspect, see **Figure 2b**. When the mirror is imaged in the infrared using an InGaAs camera operating from 900 to 1650 nm, we clearly observe its reflective behavior, see **Figure 2c**.
<figure><img src="image_3.png" /><figcaption>**Figure 2** Characteristics of the aperiodic dielectric mirror. a. Scanning electron microscopy image of the dielectric mirror made of SiO₂ and Ti₃O₅ stack. b. (resp. c.) Visible image (resp. infrared image, acquired with an InGaAs sensor) of a simple glass slide (right) and of a glass slide coated with dielectric mirror (DM) (left). d. Transmission spectrum of the DM mirror (black), overlapped with the PL from the core-shell (red).</figcaption></figure>

The cavity is made by depositing a highly concentrated solution of NCs (capped with native ligands) on top of a first mirror, then placing a second mirror face down onto the NCs. The structure is held flat and maintained together by applying mechanical pressure. A schematic of the obtained structure is provided in **Figure 3a**.

Electromagnetic simulations are then conducted to unveil the potential of the cavity to magnify an incident electromagnetic field, see **Figure 3b-c**. The enhancement in the squared norm of the electric field can reach a maximum factor of 160, but the average magnification within the NC layer is only around 20. This is due to the poor overlap between the electric field and the nanocrystal film which can be seen in **Figure 3b**, and which is attributed to the aperiodic character of the dielectric mirrors used here. Indeed, the cavity length necessary to obtain a resonance at a given wavelength $\lambda_{res}$ depends both on the phase shift in reflection $\phi$ of the dielectric mirror and on the mode order m (an integer) according to the following equation [45]:

$$L_{cav} = \frac{\lambda_{res}*(m+\frac{\phi}{\pi})}{2n_{NCs}}$$

In the ideal case of a cavity formed by two periodic mirrors where each layer has a thickness of $\lambda_0/4n$ the phase shifts upon reflection from each interface add up to $\pi$. The previous equation then becomes:

$$L_{cav} = \frac{\lambda_{res}*(m+1)}{2n_{NCs}}$$

In this case, the lowest order mode achievable (where $L_{cav}$ is the smallest positive value, thus where losses are minimized and enhancement is maximized) is $m=0$ (i.e., $L_{cav} = \frac{\lambda_{res}}{2n_{NCs}}$), which corresponds to a field distribution where the maximum of enhancement resides in the middle of the cavity, see Figure S8a. In the case of the aperiodic stack used here, the light travels different distances in each layer because of the different thicknesses, leading to an overall shift $\phi \ll \pi$, and the previous equation becomes:
$$L_{cav} \approx \frac{\lambda_{res}*m}{2n_{NCs}}$$

Due to this, the 0th order of the cavity can only be reached with extremely small values of $L_{cav}$ which are not possible to fabricate. The lowest order mode achievable is then m=1, leading to a different field distribution for a resonance at the same wavelength. Here the maxima of enhancement are outside the nanocrystal film, see Figure S8b. Figure S8 shows the field distributions and the overlaps between electric field and nanocrystal film for the two cases discussed here. Even though the maximum values of enhancement are similar for both cases, it appears quite clearly that the average enhancement inside the NC film is reduced when using an aperiodic mirror compared to a periodic one, with values 3 to 4 times lower depending on the resonance wavelength. Hence, there appears to be a tradeoff between the field magnification at the emission wavelength and the maximization of the transmission of the mirror at the pump wavelength.

<figure><img src="image_4.png" /><figcaption>**Figure 3** Field enhancement within the cavity. a. Schematic of the cavity made of two dielectric mirrors deposited on glass and sandwiching a Ag₂Se/ZnSe NC film. b squared norm of the electric field map inside the cavity. c. Zoom on the central part of map b.</figcaption></figure>

To reveal the emission of the material coupled to the cavity we then use Fourier plane imaging, and to access the dispersion diagram of the emission (PL signal vs energy and emission angle). The PL from the bare NCs (i.e., a film not coupled to a cavity) displays a broad signal that has a non-dispersive behavior. Once in the cavity, the dispersion appears dramatically affected in several ways, see **Figure 4a**, Figure S9 for a direct comparison between coupled and uncoupled nanocrystals, and Figure S10 for the polarized signal. A first striking effect is the narrowing of the linewidth of the PL spectrum, which is better highlighted by looking at the spectrum in a given direction (i.e., for a given value of $k_x/k_0$), see **Figure 4b**. The FWHM can be as narrow as 15 nm (or 8 meV) which is much narrower than the PL linewidth obtained from the most monodisperse short wave infrared NCs[46] (around 100 meV). The narrowing of the PL induced by the cavity corresponds
to a factor of 20. The second benefit of the cavity is to induce a high directivity of the emission. Indeed, the dispersion is no longer flat, showing instead a parabolic profile. This leads to the radiation diagram (i.e., 2D angular dependence at a given energy) appearing with a ring shape, the radius of which depends on the probed wavelength. In a typical LED stack made on a glass substrate, any light emitted with an angle larger than the critical angle will be waveguided within the substrate. For glass with an optical index ≈1.45 the critical angle is arcsin(1/1.45)≈43°. The numerical aperture of the Fourier imaging setup has been chosen to roughly match this value, $k_x/k_0=0.65$ corresponds to a 41° angle. In other word, if some PL emission is collected by our setup, it will be directed within the escape cone of a typical LED substrate. Here directionality is maintained for emission all over the stop band of the mirror, as pointed in **Figure 4c-e**.

<figure><img src="image_5.png" /><figcaption>**Figure 4 Shaping PL with cavity.** a. Dispersion diagram (energy vs in-plane wavevector) for Ag₂Se/ZnSe core-shell NCs within the cavity. b. PL spectra along three different directions. Radiation diagram at 1.5 µm (c.), 1.45 µm (d.) and 1.4 µm (e.). The white circle is a guide for the eyes that has a radius corresponding to the NA of the microscope objective (NA = 0.65).</figcaption></figure>

### 3. Conclusion

To summarize, we have synthesized a heavy metal-free core-shell nanostructure made of Ag₂Se/ZnSe. The material displays a broad luminescence with mono-exponential dynamics and a characteristic lifetime of around 6 ns. We then coupled this emitting layer to a dielectric microcavity. We showed that in this cavity, the field enhancement reaches a factor 20 compared to the isolated material. The PL from the material inside the cavity can be as narrow as 15 nm for an emission around telecom wavelength, a value typically 20 times smaller than the one from the bare film. Moreover, the emission is made directional enough to avoid waveguiding effects in the substrate, which should ensure a better out-coupling of the emitted light into the far field when coupled to
optoelectronic devices. The next challenge is now to integrate such a cavity within an LED stack to
demonstrate narrow non-coherent light sources in the short and mid-wave infrared.

**4. Experimental section**

**Chemicals:** Silver Acetate (Ag(OAc)₂, Sigma-Aldrich, 99,99%), 1-Octadecene (ODE, Thermo Scientific, 90%), Dodecanethiol (DDT, Alfa Aesar, 98%), Tri-n-butyl Phosphine (TBP, Cytec, 99%), Selenium pellets (Se, Sigma-Aldrich, 99,999%), Zinc Acetate dihydrate (Zn(OAc)₂, 2H₂O, VWR, >98%), Methanol (MeOH, VWR, >99,8%), Oleylamine (OLA, Sigma-Aldrich, 70%), Toluene (VWR, >98%), Chlorobenzene (PhCl, VWR, >99%), Octylamine (Sigma-Aldrich, 99%), Tetrachloroethylene (TCE, Uvasol, Supelco), Chloroform (CHCl₃, VWR, 99-99.6%), n-Hexane (VWR, 99%)

**1M TBPSe Precursor** In a glovebox, 1.58 g of Se powder was dissolved in 20 mL of TBP. The mixture was stirred overnight and stored in the glovebox for further use.

**Ag₂Se core** In a 50 mL three neck flask, 601.2 mg Ag(OAc)₂ (3.6mmol), 4.8 mL DDT and 25.2 mL ODE were degassed at 70°C for 15 min. The solution was then placed under argon atmosphere and heated to 140°C. Once at the set temperature, 6 mL TBPSe 1M were swiftly injected and the particles were allowed to grow for 1 to 4 hours depending on the desired size. After that time the solution was allowed to cool down. At 90°C, 2 mL Octylamine was added. CHCl₃ was added at 80°C and the solution was separated in four test tubes of 50 mL. The volume was completed to 20 mL with Chloroform and the solution was sonicated for 1 min then centrifuged for 1 min at 6000 rpm to remove byproducts. The supernatant was taken and the QDs were precipitated by adding 20 mL MeOH to the reaction solution. The QDs were redispersed in Toluene and a second washing step was done by adding an equal amount of Methanol and centrifuging. The precipitate was resuspended in PhCl on which a few drops of DDT and a few drops of Methanol were added and centrifuged for a last washing step. The QDs were dried, weighted, and redispersed in PhCl to have a concentration of 200 g/L.

**ZnSe shell growth** Ag₂Se core was synthesized by putting under vacuum at 70°C: 600 mg Ag(OAc), 25.2 mL ODE, 4.8 mL DDT for 15 min. Under argon atmosphere, the temperature was elevated to 140°C, at which 4.8 mL TBPSe was injected. After four hours, 1.8 g dried Zn(OAc)₂ was introduced and the shell was left to grow for 30 min. The product was cooled down to room temperature. During cooling down, at 90°C 2 mL OLA was added and approximately 10mL Toluene. The solution was centrifuged once, then MeOH was added on the supernatant, and it was centrifuged again. The precipitate was sonicated in toluene with a few drops of OLA and it was precipitated one last time with methanol. The final product was suspended in PhCl.

**TEM** A drop of diluted QDs solution in hexane was drop-casted on to a copper grid covered with an amorphous carbon film. The grid was degassed overnight under a secondary vacuum. Imaging was conducted using a JEOL 2010 transmission electron microscope (TEM), operated at 200 kV. A ThermoFisher Spectra 300, operated at 300 kV, was also used for the high-resolution image.

**EDX Spectroscopy** Drops of concentrated QDs solution were drop-casted onto a Si wafer with a (100) crystallographic orientation. The wafer was attached to a conductive carbon tape. The Energy Dispersive X-Ray spectroscopy was performed on an FEI Magellan scanning electron microscope, operated at 20 kV and 1.6 nA. The X-Ray analysis was made with an Oxford Probe.
**Absorption Spectroscopy** Visible-NIR absorption spectra were obtained by a Cary5000 spectrometer. For measurements, 20-40 µL of QDs solution was diluted in 3 mL TCE and filled in a HELMA QS 100-10-40 quartz cuvette.

**Infrared absorption and luminescence Spectroscopy** Fourier transform infrared spectroscopy (FTIR) spectra are recorded on a Thermo IS50R using white light as source a CaF₂ beamsplitter and DTGS detector in the 12000-4000 cm⁻¹ range. The PL was measured with the same FTIR operated in step scan mode. The sample is excited with a 642 nm laser modulated at 10 kHz with 140 mW of peak power. The signal from the FTIR detector is then fed into a Stanford instrument lock-in amplifier and finally refed to the electronics of the FTIR.

**Time-resolved photoluminescence measurements** of samples in solution (hexanes), or in solid films, were performed using a ≈35 fs 640 nm pump (≈1 µJ·cm⁻²) generated using an optical parametric amplifier from a Ti:sapphire fundamental. Photoluminescence was collected by an optical fiber and directed through a monochromator to a superconducting nanowire array. To collect multiple spots from the samples, they were translated within the same plane of the excitation/collection.

**Transient Absorption.** The fundamental of the same Ti: sapphire laser was split into pump and probe branches, with the pump directed into an OPA and converted to 640 nm light, and chopped to 1kHz. The probe was focused through a sapphire plate to generate a NIR supercontinuum. The pump and probe were spatially overlapped at the sample, with temporal offset controlled using an optical delay stage on the pump. Pump-on and pump-off spectra were collected to obtain transient absorption signals.

**X-ray photoemission measurements (XPS):** For photoemission spectroscopy, we used the TEMPO beamline of synchrotron SOLEIL. Samples are handled in glovebox. A ligand exchange solution is prepared by dissolving ethanedithiol at 0.01% in acetonitrile. The concentration of the Ag₂Se(/ZnSe) NCs solution is increased by a factor of three, thanks to a precipitation and redispersion in a reduced volume of toluene. In total, two layers are drop-cast onto a gold coated silicon substrate. For each layer, the sample is dipped in the ligand exchange solution for 15 seconds and then rinsed in acetonitrile for 10 seconds. The sample is kept under a nitrogen atmosphere before its introduction in the ultra-high vacuum photoemission setup. Samples were introduced in the preparation chamber and degassed until a vacuum below 10⁻⁹ mbar was reached. Then samples were introduced into the analysis chamber. The signal was acquired using an MBS A-1 photoelectron analyzer. The acquisitions were done at constant pass energy (50 eV) within the detector. A photon energy of 150 eV was used for the acquisition of the valence band and secondary electron cutoff, while 700 eV photon energy was used for the analysis of the core levels. The zero binding energy (BE) reference (i.e., the Fermi level) was taken at the leading edge of a clean Au film. The acquired data were processed by subtracting a /linear background and using pseudo-Voight function (20% weighted Lorentzian contribution) for peak fitting.

**Electromagnetic simulation:** The optical properties of the structures are simulated using the finite element method (FEM) software COMSOL Multiphysics, *2D Frequency Domain Interface*. A Floquet periodicity is set as the boundary conditions for the edges on the left and right of the model. Two periodic ports are placed away from the structure on either side, separated from the main structure by 20 µm of either substrate or air to avoid nonphysical interferences. One of the ports turns on the wave excitation while the other turns it off. The direction of the incoming wave is then determined by which port is "on". The top-most and bottom-most regions are defined as perfectly matched layers to absorb outgoing waves and minimize possible nonphysical reflections due to the limited model size.
The total thickness of air and substrate was chosen to be 30 µm to avoid nonphysical interferences. The thickness of the different oxide layers for the dielectric mirror was obtained from optical ellipsometry and confirmed by SEM observation. The thickness of the NC layer was left as an adjustable parameter. The relevant optical property of the materials, the complex optical index, is obtained: from COMSOL's built-in database for glass and air; from a Drude model for gold; from optical ellipsometry for the NC layer and the different oxide layers that make up the dielectric mirror.

A physics-controlled mesh is enabled with extremely fine element size. The reflectance (resp. the transmittance) of the structure is determined by integrating the absolute value of the time-averaged power outflow over a boundary placed behind the "on" port (resp. behind the "off" port) and dividing by the input power defined in the port. The dissipated power per unit volume in the metals and nanocrystals can be calculated using the formula $P = -0.5\omega|F|^2Im(\varepsilon)$, where $F$ is the electric field, $\omega$ is the angular frequency of the incident wave, and $Im(\varepsilon)$ is the imaginary part of the material permittivity. The absorption of one material is calculated by integrating the dissipated power over the volume of the material, and then by dividing by the incident power defined in the port. Finally, the angular dispersion of the photoluminescence can be simulated using the reciprocity principle between near-field excitation and far-field emission³: the far-field emission from the structure with integrated incoherent emitters is reciprocal to the intensity of the near-field excited in the structure by the incident external light averaged over the position of the emitters. Thus:

$$P(\theta, \phi; \lambda) \propto P_i(\lambda) \times \iiint_V |E(\theta, \phi; \lambda; r)|^2 d^3r$$

where $P(\theta,\phi;\lambda)$ is the power emitted from the structure in the direction $(\theta,\phi)$, at wavelength $\lambda$, $P_i(\lambda)$ is the power emitted at wavelength $\lambda$ by a single emitter (obtained from the measurement of the PL of NCs in solution), $V$ is the volume of NCs, and $E(\theta,\phi;\lambda; r)$ the local electric field in position $r$ excited by a plane wave of wavelength $\lambda$, and incident along the direction $(\theta,\phi)$ (obtained from solving Maxwell's equations using COMSOL). Since all studied structures are axisymmetric along the direction normal to the surface, it is assumed $P(\theta,\phi)$ is independent of $\phi$ and $P(\theta)$ is calculated for a single value of $\phi$.

**Dielectric mirror deposition:** The top side of the glass is coated with an optimized interferential filter made of 15 alternating titanium pentoxide ($Ti_3O_5$, Neyco,99.99%) and silicon dioxide ($SiO_2$, Neyco, 99.99%) thin layers starting with $Ti_3O_5$, with a total thickness of 2.998 µm. These layers have been deposited by ion beam assisted e-beam evaporation (IBAD MEB 800, Plassys). Ion assistance is provided by an End Hall ion gun (eH1000 source, Kaufman & Robinson KRI). The design, synthesis, and refinement of the custom coating have been performed theoretically with a commercial software (Essential MacLeod, Thin Film Center Inc.).

$SiO_2$ and $Ti_3O_5$ (denoted as L and H for low and high refractive indices materials respectively) have been chosen because of their transparency in the considered spectral ranges but also for their refractive index contrast ($n_L= 1.460 \pm 0.012$ at $\lambda=642$ nm and $n_L= 1.453 \pm 0.018$ at $\lambda=1550$ nm whereas $n_H=2.464 \pm 0.053$ at $\lambda=642$ nm and $n_H=2.309 \pm 0.080$ at $\lambda=1550$ nm).

Conventional Bragg reflectors consisting of alternate quarter wavelength optical thicknesses (QWOT) whose reference wavelength is 1550 nm, provides a poor anti-reflective behavior ($R \approx 25\%$) at shorter wavelengths in the visible range (630-650 nm). The optical thicknesses have been thus optimized to match the desired specifications ($R_{max}$ around $\lambda=1550$ nm and $R_{min}$ around $\lambda=642$ nm) leading to a stack denoted as $H(LH)^7$.
Both Ti₃O₅ and SiO₂ were evaporated at a rate of 0.25 nm·s⁻¹. Starting materials are 1 to 3 mm pieces of Ti₃O₅ and SiO₂ put in molybdenum liners. The background pressure in the chamber was 1.0 × 10⁻⁷ mbar, and the working pressure was ≈ 3.0 × 10⁻⁴ mbar with flows of 10 standard cubic centimeters per minute (sccm) of Ar and 10 sccm of O₂ for the ion source, and 10 sccm of Ar for the keeper (plasma bridge neutralizer). Discharge currents were set to 1.6 A and 2 A during Ti₃O₅ and SiO₂ deposition steps respectively. Under those conditions the ion source discharge voltage was 150 V and 75 V during Ti₃O₅ and SiO₂ deposition steps respectively. The optical thicknesses were followed in real time using both a quartz microbalance and an in-situ spectroscopic ellipsometer (M2000, J.A. Woolam) at 60.5° of incidence.

The actual deposited stack for the quasi-Bragg mirror for each layer starting from glass, was: 153.4 nm (Ti₃O₅)/258.0 nm (SiO₂)/225.0 nm (Ti₃O₅)/232.3 nm (SiO₂)/183.4 nm (Ti₃O₅)/233.4 nm (SiO₂)/211.5 nm (Ti₃O₅)/207.4 nm (SiO₂)/163.7 nm (Ti₃O₅)/190.0 nm (SiO₂)/220.4 nm (Ti₃O₅)/193.8 nm (SiO₂)/266.3 nm (Ti₃O₅)/134.1 nm (SiO₂)/125.4 nm (Ti₃O₅).

**Cavity integration:** The pristine solution with chlorobenzene solvent was slowly evaporated with air flow until a thick paste was obtained. This concentrated solution was deposited on a Dielectric Mirror. A second Dielectric Mirror was put on top facing the first and the stack was pressed down first manually, then, with the help of switchable magnets and weights. The cavity was left to dry overnight while being pressed.

**Infrared microscopy:** The images of the spatial distribution of the photoluminescence in the Fourier space were obtained with a BX51WI upright microscope from Evident/Olympus coupled to an Acton SP-2356 spectrometer (Princeton Instruments) in imaging mode and an InGaAs camera cooled at -60°C (model NIRvana 640ST from Teledyne Princeton Instruments) via a Bertrand lens (focal length 30 mm) and a Telan lens (focal length 20 mm). Bandpass filters were inserted in the optical path to image the photoluminescence in the Fourier plane at selected wavelengths, as indicated in the figures. The microscope objective used in these experiments was a 50X LMPLN50XIR objective from Evident/Olympus (NA =0.65). The samples were excited through the same objective with the light originating from a 633 nm HeNe laser from Thorlabs. A dichroic mirror (Thorlabs DMLP950R) was used to separate the pump from the photoluminescence signal. The dispersion relations were obtained by inserting a slit at the entrance of the spectrometer and by dispersing the light with an 85-groove/mm grating within the spectrometer. The photoluminescence spectra were acquired with the same technique, but without the Bertrand lens.

## SUPPORTING INFORMATION

Supporting Information includes details about (i) growth of core and core-shell NCs, (ii) short time scale dynamics probed by transient absorption (iii) electronic structure probed by photoemission, and (iv) polarized dispersion diagrams.

## ACKNOWLEDGEMENTS

The project is supported by ERC grants Ne2dem (grant 853049), Forward (grant n° 771688) and AQDtive (grant n°101086358). We acknowledge the use of clean-room facilities from the "Centrale de Proximité Paris-Centre" and Renatech network. This work was supported by French state funds managed by the Agence Nationale de la Recherche through the grant Bright (ANR-21-CE24-0012), MixDFerro (ANR-21-CE09-0029), Quicktera (ANR-22-CE09-0018), E-map (ANR-23-CE50-0025), DIRAC (ANR-24-ASM1-0001), camIR (ANR-24-CE42-2757), Piquant (ANR-24-CE09-0786) and NanoSaclay, (ANR-10-LABX-0035). Work performed at the Center for Nanoscale Materials, a U.S.
Department of Energy Office of Science User Facility, was supported by the U.S. DOE, Office of
Basic Energy Sciences, under Contract No. DE-AC02-06CH11357.

CONFLICT OF INTEREST

The authors declare no competing financial interest.

Data availability

The data that support the findings of this study are available from the corresponding author upon
reasonable request.

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