# Grayscale evaporation of organic thin-film microstructures

Stéphane Calvez, David Bourrier, Justine Bonneau, Ludovic Salvagnac, Isabelle Séguy, Olivier Gauthier-Lafaye, Henri Camon

► To cite this version:

Stéphane Calvez, David Bourrier, Justine Bonneau, Ludovic Salvagnac, Isabelle Séguy, et al. "Grayscale evaporation of organic thin-film microstructures." Optical Materials Express, 2025, 15 (4), pp. 604–617.

HAL Id: hal-04975166

https://laas.hal.science/hal-04975166v1

Submitted on 4 Mar 2025

HAL is a multi-disciplinary open access
archive for the deposit and dissemination of sci-
entific research documents, whether they are pub-
lished or not. The documents may come from
teaching and research institutions in France or
abroad, or from public or private research centers.

L'archive ouverte pluridisciplinaire HAL, est
destinée au dépôt et à la diffusion de documents
scientifiques de niveau recherche, publiés ou non,
émanant des établissements d'enseignement et de
recherche français ou étrangers, des laboratoires
publics ou privés.

<figure><img src="image_1.png" /></figure>

Distributed under a Creative Commons Attribution 4.0 International License
<figure><img src="image_2.png" /></figure>

# Grayscale evaporation of organic thin-film microstructures

STEPHANE CALVEZ,\* DAVID BOURRIER, JUSTINE BONNEAU, LUDOVIC SALVAGNAC, ISABELLE SÉGUY, OLIVIER GAUTHIER-LAFAYE, AND HENRI CAMON

LAAS-CNRS, Université de Toulouse, CNRS, 7 avenue du Colonel Roche, F-31400 Toulouse, France
\*scalvez@laas.fr

**Abstract:** We demonstrate that the deposition of organic thin films whose thickness is spatially shaped with sub-100-µm lateral resolution can be achieved by evaporation through a reusable flux-controlling micrometric-hole sieve that is separated from the to-be-coated substrate. This approach offers a practical way to grayscale-pattern organic materials without exposing them to detrimental ambient air or chemical solutions. As such, it facilitates the fabrication of long-lasting organic microstructured devices such as the reported spatially-varying absorptive color filters based here on the organic semiconductor (4,4'-Bis[4-(di-p-tolylamino)-styryl]biphenyl (DPAVBi)). Additionally, the ability to readily make multiple thickness-coated samples offers a material-sparing yet convenient way to measure the properties of the evaporated material, a feature exploited hereafter to extract the optical dielectric constants of DPAVBi over the visible and near-infrared range. This work and its extensions to inorganic compounds and freeform shaping pave the way for a new generation of devices that should find applications in light-matter coupling studies, hyperspectral imaging, structural coloring, or optical systems exploiting freeform optics.

Published by Optica Publishing Group under the terms of the [Creative Commons Attribution 4.0 License](https://creativecommons.org/licenses/by/4.0/). Further distribution of this work must maintain attribution to the author(s) and the published article's title, journal citation, and DOI.

## 1. Introduction

Over the past three decades, the ability to make (optical) freeform surfaces, i.e. to define the surface contours of a particular layer or substrate in three dimensions, has become of prime importance [1,2]. Indeed, this processing step is central in the fabrication of many micro-optical elements including gratings [3], micro-lenses [4], spatially-graded color filters [5,6], or even waveguide tapers [7].

Typically, the desired three-dimensional (3D) shapes are essentially obtained by patterning a resist layer using grayscale lithography, eventually followed by a transfer into the resist-subjacent material by dry etching. The former grayscale lithography is commonly performed on a layer-by-layer basis using a sequence of exposures through optical masks [hard/soft masks] or on a point-by-point basis using dose-modulated e-beam [3] or laser [8] lithography. As such, this is often a time-consuming process. However, this restriction could readily be alleviated through the use of a full-surface single ultraviolet-light exposure step whose illumination profile is tailored by transmission through a halftone mask [9], should the longer (e-beam) fabrication of the sub-diffractive-feature patterned mask be tolerable. Similarly, Li et al. [10] have recently proposed a bottom-up approach to create 3D structures in a single step. The technique, referred to as grayscale evaporation, relies on the evaporation through a via-hole mask positioned at a distance from the to-be-coated substrates. Given the directional nature of this type of deposition, the deposited thickness profile is effectively controlled by the local density of holes in the mask, in a similar way to the image grayscale coding resulting from halftone printing. Their demonstration
was however limited to centimeter-scale structures whose profile was controlled with ~1 mm lateral resolution.

Here, we adapt grayscale evaporation to be in a position to create sub-millimeter-scale 3D patterns with sub-100-µm lateral resolution. Furthermore, we deliberately chose to apply this technique to organic compounds so as to circumvent the impairments often encountered with top-down processes that are related to the sensitivity or the degradation of organic materials upon exposure to chemical (solvent, acid, basic) solutions or to standard atmospheric conditions. In contrast, we exploit the ability to selectively remove the evaporated compounds with respect to the mask material (here nickel) to obtain re-usable masks, thereby gearing towards more cost-effective and sustainable manufacturing. Last but not least, this grayscale evaporation permits to drastically reduce the number of samples to prepare by spatially multiplexing thin films of different thickness, a helpful feature when determining the material properties. Here, we illustrate this specific advantage by extracting the optical dielectric constants of the chosen representative organic semiconductor (OSC), (4,4'-Bis[4-(di-p-tolylamino)-styryl]biphenyl (DPAVBi)) [11], from normal-incidence reflectivity/transmission measurements.

The paper itself is organized in two sections. The first part addresses the design, fabrication and characterization of the evaporation mask while the second establishes the grayscale evaporation properties and above-mentioned uses through the thorough characterization of a DPAVBi-coated sample.

## 2. Evaporation mask fabrication and characterization

As mentioned above, the grayscale evaporation technique relies on the local restriction of the evaporated material flux by a halftone shadow mask sieve that is set at a fixed position away from the target deposition substrate. This section describes the fabrication of this shadow mask and its characterization prior and after use.

### 2.1. Fabrication

The evaporation masks are made of an approximately 2 µm-thick sieve membrane supported by a structure whose thickness is about 50 µm. Their fabrication is schematically described in Fig. 1. In short, it starts from a 4-inch-diameter Si wafer that is coated with a bilayer constituted of a 200 nm-thick copper layer on a 50 nm-thick titanium layer. A first optical contact lithography step using a 2.3 µm-thick positive photoresist (ECI3012) with Bottom Anti-Reflection Coating (BARC) treatment is used to define a set of pillars whose pattern corresponds to the sieve hole pattern. This step is followed by the growth of the nickel membrane at 0.25 µm/min in the resist opened areas by electroplating and removal of the remaining photoresist and BARC layer (using a 5 min-long O₂-plasma dry etching (run with a 50 sccm flow, at 7.5 mTorr chamber pressure, and using 400W/25W source/RF powers). A subsequent sequence of lithography (into a 100 µm-thick laminated dry film (Dupont WBR2000 series)) and electroplating at 2.5 µm/min is performed to define the supporting frame that allows the manual handling and the screw mounting of the ~2×3 cm² masks on the evaporator sample holding plate. The overall structure is then freed by etching away the silicon wafer and the Ti layer (at rate of ~100 µm/h) using a 40% diluted KOH solution held at a temperature of 90°C, and by a (30 s-duration) removal of the copper layer using a standard copper wet etchant solution (from MicroChemicals GmbH).

The chosen sieve pattern is a matrix of 7×7 rectangular (300×350 µm²) patches, each of which consisting of a 10 µm-pitch hexagonal array of set-diameter holes. The coded hole diameter varies from patch to patch covering the 2 to 8 µm range with a 0.125 µm step, a stepsize commensurate with the mask making machine resolution. This diameter range was selected based on the ease of fabrication by direct contact lithography techniques but also to avoid the commonly-observed clogging of nanometric holes during evaporation [12]. This choice however means that the grayscale evaporation can not be controlled with a spatial resolution smaller than
<figure><img src="image_3.png" /><figcaption>**Fig. 1.** Schematic (not-to-scale) description of the evaporation mask fabrication process. PR (resp. BARC) stands for photoresist (resp. Back Anti-Reflection Coating layer).</figcaption></figure>

a few times the selected pitch (i.e., here, a few tens of micrometers). The latter consideration was taken into account as an indication on the minimum size of the patches to select to systematically obtain uniform area of deposited material.

## 2.2. Characterization

A characterization of the fabricated evaporation masks was carried out since the locally-deposited thickness is essentially proportional to the facing opened surface in the sieve mask [13]. Prior to the silicon wafer removal (i.e. at step #12 on Fig. 1), Scanning Electron Microscopy (SEM) images (see Fig. 2) of each of the test matrix 49 patches were recorded and the corresponding effective hole diameter was evaluated (using standard image post-processing (edge detection and Hough transform)).

<figure><img src="image_4.png" /><figcaption>**Fig. 2.** SEM images of a sub-section of the sieve mask where the coded hole diameter is (a) 2 µm and (b) 8 µm respectively.</figcaption></figure>

As it can be seen on Fig. 3, the extracted dimensions of the holes are observed to follow the design ones but for a slight (<~0.25 µm) undersize that could be either pre-compensated or corrected (by reducing the BARC etch time). We note that this size discrepancy worsens as the sieve pattern gets closer to the edge of the wafer as result of the somewhat imperfect uniformity in the first photoresist/BARC opening (see Fig. 1).
<figure><img src="image_5.png" /><figcaption>**Fig. 3.** Mean diameter extracted for each of the 49 patches for 3 different evaporation masks (red, cyan and green marks corresponding respectively to an evaporation mask located at the center, mid-way and the edge of the processed wafer). The fit values stated in the legend for the hole diameters, d, are in micrometers.</figcaption></figure>

<figure><img src="image_6.png" /><figcaption>**Fig. 4.** Ellipsometric confirmation of evaporation mask re-usability (measurements taken for 59.4 degrees of incidence). Note that the deposition led to a 87 nm of DPAVBi deposit.</figcaption></figure>

Furthermore, prior to any deposition, a variable-angle spectroscopic ellipsometric measurement (using a Horiba Jobin Yvon uvisel instrument) of an unstructured edge part of the evaporation mask membrane was performed. This measurement was reproduced after deposition and once the mask has been cleaned to evaluate the ability to re-use the mask. Figure 4 shows typical examples of the obtained variable angle spectroscopic ellipsometry (delta, psi) parameters. The results show that the recorded ellipsometric parameters after mask cleaning (10 minutes dip in acetone followed by 1 minute in iso-propanol before drying it using a nitrogen gun) are within the experimental error (less than 2° off) of the ones before coating. This observation suggests the
successful recovery of the membrane initial state after the cleaning treatment, and thereby the
ability to re-use the mask.

**3. Grayscale evaporation and characterization**

The above-fabricated masks were then used to obtain grayscale evaporation on a fused silica
substrate of a representative organic compound, DPAVBi. The prepared sample subsequently
permitted the extraction of the material dielectric constants.

**3.1. Sample preparation and preliminary characterization**

As mentioned before, the grayscale evaporation is based on the local restriction of the evaporated
material flux by a shadow mask sieve and is carried out according to the schematic process
flow of Fig. 5. However, to avoid obtaining a coated copy of the shadow mask pattern on the
substrate, the distance between the shadow mask and sample, dMS, needs to be sufficient to allow
hole-to-hole flux homogenization. Here, this separation was chosen to be ~1% of sample to
evaporation crucible distance, DCS (see Fig. 5).

<figure><img src="image_7.png" /><figcaption>**Fig. 5.** Schematic process of the evaporation cycle used to coat the samples using the above-fabricated sieve masks, the out-of-scale purple drop symbolizing the evaporated material. The green arrows show the additional steps undertaken to re-use the masks.</figcaption></figure>

Thermal evaporation at a crucible temperature of 240 °C, at a pressure of 3.6 × 10⁻⁷ mbar and with 0.05 nm/s deposition rate of the organic compound was performed in a Plassys MEB550B tool into which the samples and shadow masks were mounted on a custom holder.

Figure 6 shows a stitched optical microscope image of the above-described matrix taken on
one of the produced samples. The observed homogeneous areas with gradually changing color
(from blue to yellow) suggest that the grayscale deposition was successful.

To complement this observation, stylus profilometry of the various patches was performed.
The recorded (vertical) cross-sections the 4 topmost patches of the C = 7 column (see Fig. 6)
are presented on Fig. 7(a). They confirm that each patch has a ~200 µm-width uniform plateau
with on either side a ~100 µm transition zone, the latter being considered as the achieved spatial
resolution (for the chosen deposition conditions). Additionally, as shown on Fig. 7(b) and as
expected, the deposited height is confirmed to vary linearly with the mask opened surface (ruled
by the coded-hole-diameter, φ(P) = 1.875 µm + 0.125 µm, N, where N is the patch number that
can be related to the line, L, and column, C, through N = C + 7·L-7). The maximal deposited
thickness of 55 nm was chosen on the basis of the high absorption strength of OSCs.

The reflectivity and transmission characterization of the sample whose back surface was left
uncoated were carried out with a 1 nm-spectral-resolution optical spectrum analyzer (Ocean
<figure><img src="image_8.png" /><figcaption>**Fig. 6.** Stitched optical microscope image of the DPAVBi-coated sample measured using a Sensofar S-NEOX microscope equipped with a 20x objective. The chosen line and column indexing is also provided for reference.</figcaption></figure>

<figure><img src="image_9.png" /><figcaption>**Fig. 7.** Stylus profilometry (TENCOR P17) measurements of the grayscale-evaporated DPAVBi sample under study. (a) : measurement in the vertical direction for the patches of the 7th column, lines 4 to 7 (or equivalently coded hole diameter of 5.125, 6.25, 7.125 and 8 µm). (b) : evolution of the deposited height with coded hole area.</figcaption></figure>

Optics Flame UV-VIS-ES) and a fiber-coupled white light source (Thorlabs SLS201 L/M) that is optically-relayed to a form ~100 µm-diameter spot size probing beam. The measurements were referenced with respect to sample-free transmission and a silver-coated mirror reflectivity. The alignment on the various test patches was achieved with the help of a monitoring camera.

The recorded reflectivity/transmission curves of the patches of the 7th column (see Fig. 6) are shown on Fig. 8 and the full evolution of these characteristics acquired over the full matrix are provided as the spectral maps in Fig. S1 of the Supplement 1. In all cases, no scattering was detected on the sample position monitoring camera, a consistent feature with the sub-3 nm roughness shown in Fig. 7(a). As expected from the blue absorbing/emitting nature of the DPAVBi compound, a drastic reduction of the transmission in the 400-450 nm spectral band can be observed. This proof-of-principle demonstration warrants, in turn, that the implemented grayscale evaporation provides an effective way to fabricate stepped or graded attenuation color filters with sup-100 µm-scale in-plane dimensions whereby the varied color is set by the choice of the absorbing organic material(s).
<figure><img src="image_10.png" /><figcaption>Fig. 8. Measured reflectivity (a) and transmission (b) curves from the patches of the 7th column, lines 1 to 7 (and their respective coded aperture diameter). Note that the vertical scales are distinct to improve readability.</figcaption></figure>

## 3.2. Material characterization

As another example of application of the grayscale deposition, the dielectric constants of the material under study were extracted from the above-presented surface-normal reflectivity/transmission dataset measured on this single-deposition sample. The method builds upon the redundant information resulting from the fact that the fabricated sample combines patches of different yet calibrated thickness, in the very same way as performed in in-situ growth monitoring [14].

The thickness-dependent reflectivity and transmission data shown in Fig. 8 and in Fig. S1 were fitted, using the complex refractive index as parameters, on a wavelength by wavelength basis using transfer matrix thin film calculations [15] taking into account the incoherent back-surface reflection and an unpolarized NA = 0.22 illumination. Effectively, the model of this single layer coated sample involves three planar partial mirrors forming a cascade of two Fabry-Pérot interferometers. As such, the numerical fitting practically implements the analytical formula provided in [16] compounded by an averaging in both polarization and incidence angle.

Examples of the fitting results obtained at selected wavelengths across the visible range are given in Fig. 9 and show good agreement with the measured data. The resulting reflectivity and transmission maps are also provided in the Supplement 1 Fig. S2 to further visually confirm the accuracy of the fits over the full recorded spectral coverage.

Variable-angle spectroscopic ellipsometry was also carried out on a silicon wafer that was coated at the same time as the grayscale evaporated sample (with 87 nm of DPAVBi). In Fig. 10 are presented the inferred dielectric constants assuming a single oscillator model [17] whose expressions are given by:

$$n(E) = n_{\infty} + \frac{f_{osc}/osc \cdot (\frac{2}{osc} - (E_{osc} - E_g)^2) \cdot (E - E_{osc}) + 2 \cdot f_{osc} \cdot osc(E_{osc} - E_g)}{(E - E_{osc})^2 + \frac{2}{osc}}$$

$$k(E) = \begin{cases} \frac{f_{osc} \cdot (E - E_g)^2}{(E - E_{osc})^2 + \frac{2}{osc}} & \text{if } E > E_g \\ 0 & \text{if } E \le E_g \end{cases}$$

where $E_{osc}$ is the oscillator energy, $f_{osc}$ its strength, $\Gamma_{osc}$ the damping factor, $E_g$ the bandgap energy and $n_{\infty}$ the achromatic refractive index contribution.
<figure><img src="image_11.png" /><figcaption>**Fig. 9.** Fitting results of the thickness-dependence of the reflectivity (a) and transmission (b) at selected wavelengths on the grayscale evaporated DPAVBi sample. Note that the vertical scales are distinct to improve readability.</figcaption></figure>

<figure><img src="image_12.png" /><figcaption>**Fig. 10.** DPAVBi dielectric constants inferred from reflectivity and transmission (R&T) measurement of the grayscale evaporated sample and from ellipsometric characterization of coated silicon wafer samples using a single oscillator model (with an energy of $E_{osc} = 2.929$ eV, a strength of $f_{osc} = 0.0072$, and a damping factor of $\Gamma_{osc} = 0.192$, arising from a bandgap energy of $E_g = 1.0$ eV over a $n_\infty=1.674$ contribution).</figcaption></figure>
Comparison to the complex refractive index extracted using the above-described analysis shows good agreement over the transparent region and maximum deviation in the absorptive spectral band, taken to originate from the standard yet somewhat too simple dispersion model used for the ellipsometric analysis.

**4. Conclusions**

We have reported grayscale evaporation of organic compounds with sub-100-µm lateral resolution using re-usable nickel-based micrometric-hole sieve masks made by conventional contact lithography and electrolytic deposition steps. As such, this approach offers a practical way to (grayscale)-pattern any evaporable organic materials without deteriorating them through exposure to the air or chemical solutions. It can also readily be extended to evaporable inorganics (metallic or dielectric) compounds, bearing in mind that the above-stated re-Usability of the mask depends on the availability of a highly selective remover of the deposited material with respect to the mask material. Since this condition can easily be obtained for most organics (using acetone) and for some metals (Al, Cu, ... ) against nickel, the established approach is deemed to have a broad applicability and to offer a potential route towards more sustainable grayscale thin-film microprocessing through the reduction of mask fabrication runs.

In addition and more specifically, the technique was proven to allow the fabrication in a single deposition step of samples whose coated thickness spatially varies according to the sieve open-area pattern. Selecting the blue-absorbing DPAVBi compound as the grayscale evaporated material, a spatially-varying blue/yellow color filter was created. By extension, the graded color could be modified by selecting one or several evaporable organic semiconductor (OSC) compounds with the appropriate absorption profile(s). This choice will certainly take advantage of the catalog of materials developed for Organic Light Emitting Diodes (OLEDs), by using for instance the most stable and popular OSCs such as the green emitting compounds, 8-Hydroxyquinoline aluminum (Alq3) [18] or Tris(2-phenylpyridinato)iridium(III) (Ir(ppy)3) [19], or the red emitting material, Tris(1-phenylisoquinoline)iridium(III) (Ir(piq)3) [20]. Furthermore, the insertion of such graded-thickness organic spacer layer into metallic-mirror Fabry-Pérot interferometers should provide a convenient way to study light matter interactions [21], and in particular to probe strong coupling effects with evaporable spin crossover materials such as the tri-azolyl borate iron complex [22].

Furthermore, the ability to readily obtain such multiple-thickness coated samples is considered to be an asset as it offers a material-sparing way to deduce the (potentially-thickness-dependent) material properties. The benefits of this approach were illustrated here with the extraction of the optical dielectric constant of DPAVBi over the visible and near-infrared range.

Future work that builds upon this demonstration will certainly include extensions to spatially shape thin films of inorganic compounds [10,23] with more complex yet purposely-designed patterns. These advances should, in turn, facilitate prospective implementations of compact graded optical filters for hyperspectral imaging [24], of structural coloring elements [25,26] or even of millimeter-diameter freeform lenses for ophthalmologic applications [27].

**Funding.** Agence Nationale de la Recherche (ANR-22-CE09-0019); Région Occitanie Pyrénées-Méditerranée (2018-003267); "OPALE", the joint laboratory between LAAS-CNRS and Essilor-Luxottica.

**Acknowledgments.** This work was supported in part by RENATECH, the French network of cleanroom facilities.

**Disclosures.** Authors declare no conflict of interest.

**Data availability.** Data underlying the results presented in this paper are not publicly available at this time but may be obtained from the authors upon reasonable request.

**Supplemental document.** See Supplement 1 for supporting content.
References

1. L. Alvarez W, "Simple or compound lenses having one or more elements with analytic function to create variable power," U.S. patent US3305294A (February 21, 1967).

2. K. P. Thompson and J. P. Rolland, "Freeform optical surfaces: a revolution in imaging optical design," *Opt. Photonics News* 23(6), 30 (2012).

3. A. Schleunitz and H. Schift, "Fabrication of 3D patterns with vertical and sloped sidewalls by grayscale electron-beam lithography and thermal annealing," *Microelectron. Eng.* 88(8), 2736-2739 (2011).

4. Q. Deng, Y. Yang, H. Gao, et al., "Fabrication of micro-optics elements with arbitrary surface profiles based on one-step maskless grayscale lithography," *Micromachines* 8(10), 314 (2017).

5. C. Williams, G. S. D. Gordon, T. D. Wilkinson, et al., "Grayscale-to-color: scalable fabrication of custom multispectral filter arrays," *ACS Photonics* 6(12), 3132-3141 (2019).

6. T. Guo, J. Evans, N. Wang, et al., "Monolithic chip-scale structural color filters fabricated with simple UV lithography," *Opt. Express* 27(15), 21646 (2019).

7. S. Abe, H. Hara, S. Masuda, et al., "Fabrication of vertical-taper structures for silicon photonic devices by using local-thickness-thinning process," *Jpn. J. Appl. Phys.* 61(SK), SK1005 (2022).

8. A. Grushina, "Direct-write grayscale lithography," *Adv. Opt. Technol.* 8(3-4), 163-169 (2019).

9. W. Henke, W. Hoppe, H.-J. Quenzer, et al., "Simulation and process design of gray-tone lithography for the fabrication of arbitrarily shaped surfaces," *Jpn. J. Appl. Phys.* 33(12S), 6809 (1994).

10. X. Li, Z. J. Tan, and N. X. Fang, "Grayscale stencil lithography for patterning multispectral color filters," *Optica* 7(9), 1154 (2020).

11. K. Sakai, T. Tsuzuki, J. Motoyoshiya, et al., "Relationship between molecular skeleton and stimulated-emission threshold in dilute thin films of linear-chain-structured fluorescent dyes," *Chem. Lett.* 32(10), 968-969 (2003).

12. M. M. Deshmukh, D. C. Ralph, M. Thomas, et al., "Nanofabrication using a stencil mask," *Appl. Phys. Lett.* 75(11), 1631-1633 (1999).

13. X. H. Li, Z. J. Tan, and N. Fang, "Grayscale stencil lithography for patterning multispectral color filters: supplemental document," *Optica*, 2020, https://doi.org/10.6084/m9.fishshare.12691826.v2.

14. W. G. Breiland and K. P. Killeen, "A virtual interface method for extracting growth rates and high temperature optical constants from thin semiconductor films using *in situ* normal incidence reflectance," *J. Appl. Phys.* 78(11), 6726-6736 (1995).

15. C. C. Katsidis and D. I. Siapkas, "General transfer-matrix method for optical multilayer systems with coherent, partially coherent, and incoherent interference," *Appl. Opt.* 41(19), 3978 (2002).

16. H. Van De Stadt and J. M. Muller, "Multimirror Fabry-Perot interferometers," *J. Opt. Soc. Am. A* 2(8), 1363 (1985).

17. S. H. Wemple and M. DiDomenico, "Optical dispersion and the structure of solids," *Phys. Rev. Lett.* 24(4), 193 (1970).

18. C. W. Tang and S. A. VanSlyke, "Organic electroluminescent diodes," *Appl. Phys. Lett.* 51(12), 913-915 (1987).

19. W. Holzer, A. Penzkofer, and T. Tsuboi, "Absorption and emission spectroscopic characterization of Ir(ppy)3," *Chem. Phys.* 308(1-2), 93-102 (2005).

20. S. Reineke, K. Walzer, and K. Leo, "Triplet-exciton quenching in organic phosphorescent light-emitting diodes with Ir-based emitters," *Phys. Rev. B* 75(12), 125328 (2007).

21. H. Bahsoun, T. Chervy, A. Thomas, et al., "Electronic light-matter strong coupling in nanofluidic Fabry-Pérot Cavities," *ACS Photonics* 5(1), 225-232 (2018).

22. L. Zhang, K. Ridier, O. Ye, et al., "Reversible switching of strong light-matter coupling using spin-crossover molecular materials," *J. Phys. Chem. Lett.* 14(30), 6840-6849 (2023).

23. S. Calvez, J. Bonneau, D. Bourrier, et al., "Shadow-mask evaporation for the fabrication of optical filters with spatially tailored thickness," in *Advanced Fabrication Technologies for Micro/Nano Optics and Photonics XVII*, G. Von Freymann, E. Blasco, and D. Chanda, eds. (SPIE, 2024), p. 49.

24. P.-J. Lapray, X. Wang, J.-B. Thomas, et al., "Multispectral Filter Arrays: Recent Advances and Practical Implementation," *Sensors* 14(11), 21626-21659 (2014).

25. Z. Yang, Y. Zhou, Y. Chen, et al., "Reflective Color Filters and Monolithic Color Printing Based on Asymmetric Fabry-Perot Cavities Using Nickel as a Broadband Absorber," *Adv. Opt. Mater.* 4(8), 1196-1202 (2016).

26. A. C. Kosger, A. Ghobadi, A. R. Rashed, et al., "Generation of additive colors with near unity amplitude using a multilayer tandem Fabry-Perot cavity," *Opt. Lett.* 46(14), 3464 (2021).

27. J. Bao, A. Yang, Y. Huang, et al., "One-year myopia control efficacy of spectacle lenses with aspherical lenslets," *Br. J. Ophthalmol.* *bjophthalmol* 106(8), 1 (2021).