Fano Resonant Optical coatings platform for Full Gamut and High Purity Structural Colors

doi:10.21203/rs.3.rs-2078826/v1

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Fano Resonant Optical coatings platform for Full Gamut and High Purity Structural Colors

https://doi.org/10.21203/rs.3.rs-2078826/v1

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Structural coloring is a photostable and environmentally friendly coloring approach that harnesses optical interference and Nanophotonic resonances to obtain colors with a range of applications including display technologies, colorful solar panels, steganography, décor, data storage, and anticounterfeiting measures. We show that optical coatings exhibiting the photonic Fano Resonance present an ideal platform for structural coloring- they provide full color access, high color purity, high brightness, controlled iridescence, and scalable manufacturing. We show that an additional oxide film deposited on Fano resonant optical coatings (FROCs) increases the color purity (up to 97%) and color gamut coverage range (> 99% coverage of the sRGB and Adobe color spaces). For coloring applications that do not require high spatial resolution, FROCs have a significant advantage over existing structural coloring schemes.

In nature, colors are mostly produced either through pigments or structures. While the former comes from molecular absorption, the latter, structural coloring (SC), is the result of optical interference from a structured surface. Structural colors offer several advantages over pigments- they are photostable, immune to chemical degradation, and environmentally friendly. In addition, a wide range of colors can be produced¹ and dynamically reconfigured ²using the same material. Furthermore, structured surfaces can have multiple functionalities, e.g., creating hydrophobic or antibacterial colored metals^3,4.

Several structural coloring schemes have been previously introduced including multilayer films⁵, thin film nanocavities⁶, plasmonic nanostructures ¹, dielectric nanostructures⁷, and photonic crystals⁸ with applications in decoration⁹, colorimetric sensing¹⁰, data storage¹¹, anticounterfeiting¹², display technologies¹³, colorful photovoltaic cells¹⁴, among others¹⁵. An ideal structural coloring platform should span a wide color gamut, producing colors with high and controllable purity- how monochromatic or pure the color is- and brightness - the relative intensity of the reflected color- and allowing control over the colors’ angle dependence. For many applications, it should also be scalable and inexpensive to fabricate. However, no existing scheme can satisfy all the above qualities simultaneously^1,7,9,14.

Recently, we proposed a new type of optical coatings that exhibits the photonic Fano Resonance effect ¹⁶. Fano Resonant Optical Coatings (FROCs) enjoy unique optical properties that cannot be reproduced with existing optical coatings such as metallic films, anti-reflective coatings, transmission filters, light absorbers, and dielectric mirrors. Figure 1a describes the composition of FROCs. FROCs are produced by coupling a broadband nanocavity (representing the continuum) with a narrowband Fabry-Perot nanocavity (representing a discrete state). The resonant interference between the nanocavities produces the well-known asymmetric Fano resonance line-shape. In this work, we develop a class of reflective FROCs by using a reflective and opaque material as substrates, which lead to a highly reflective resonant peak that corresponds to the narrowband nanocavity’s resonance. We investigate the color properties of FROCs numerically and experimentally. We show that the reflective FROCs are an ideal platform for structural coloring, producing colors spanning a wide color gamut with high brightness and high purity. The dependence of the color on the incident angle can be controlled through the cavity material, making FROCs suitable for a variety of applications that demand angle independence, e.g., decoration, or angle dependence, e.g., anti-counterfeit measures. Structural coloring with FROCs can find new applications where strong and broadband optical absorption and high purity colors are required, for example, colorful solar thermal generation panels, colorful photovoltaic panels⁸, colorful thermophotovoltaic panels, and colorful solar thermoelectric generators. These renewable energy sources often have the same color, black or dark blue, which makes them aesthetically unappealing.

Throughout this work, we compare the coloring performance of FROCs to Fabry-Perot (FP) thin film nanocavities since they are the closest platform in terms of structure and physics to FROCs ¹⁶. While Fabry-Perot nanocavities produce colors through selective absorption, mainly reflecting all colors except for the specific cavity resonance wavelength. On the other hand, FROCs produce colors through selective reflection (Fig. 1a). Figure 1b shows the measured p-polarized reflection spectrum of Fabry-Perot nanocavities consisting of Ag (20 nm)-TiO₂-Ag (100 nm) by varying the thickness of the TiO₂ film from 35 nm to 70 nm. Figure 1c shows the measured reflection spectrum of the same Fabry-Perot nanocavities after adding a 15 nm Ge layer to convert them into FROCs. The reflection lines produced by the FROCs can span the visible spectrum and target a relatively narrow range of wavelengths for each individual FROC by changing the cavity’s thickness.

To examine the structural coloring properties of FROCs vs. Fabry-Perot (FP) cavities, we calculate the colors produced from FP cavities vs. FROCs by varying the top metal film thickness and the cavity thickness. Figure 2a shows a swatch array for FP cavities consisting of a metal-dielectric-metal stack [top to bottom: Ag (Y nm)- TiO2 (X nm)- Ag (100 nm)] as a function of the top Ag film thickness and the TiO2 dielectric cavity thickness. A swatch is the perceived color for a person viewing the sample. The produced colors are Cyan-Magenta-Yellow (CMY) colors since FP cavities are selective absorbers and CMY colors are subtractive colors¹⁷. Note the restricted color palette from FP nanocavities. Figure 2b shows the corresponding purity of the FP cavities (see Methods for more details). The purity of FP cavities is limited since all the colors are reflected except within the absorbed wavelength range, making near-monochromatic reflection impossible. Figure 2c shows a swatch array of FROCs that are identical to the FP cavities with an added absorbing thin film [Ge (15 nm)] to generate the desired Fano resonance. The selective reflection from FROCs enables access to a wide range of hues from blue to red, including green. The colors enjoy significantly high purity as shown in Fig. 2d ¹⁸. The white point corresponds to the spectrum of the illuminant, i.e., white light. FROCs access the huge color gamut since they provide selective reflection at different wavelengths by simply changing the dielectric thickness.

Images of the reflected color for FROC and MDM structures with a TiO₂ thickness ranging from 35 nm to 150 nm are shown in Fig. 3a and Fig. 3b, respectively. FROCs are capable of reflecting blue, green, and red colors by simply increasing the dielectric thickness. Figure 3c shows a photograph of two FP cavities with CWRU and U of R letters “printed” on them by depositing a 15nm Ge layer and converting these regions to a FROC. By controlling the spatial distribution of the deposited layer, this printing method could be adapted for optical archival data storage and encrypting messages. Experimental reflection lines from different FP cavities (blue dots) and FROCs (red dots) are presented in the CIE 1931 color space (see Methods) that links distributions of electromagnetic wavelengths to visually perceived colors (Fig. 3d). The experimental results agree with the numerical calculations of colors produced by FROCs vs. FP cavities (see Supplementary Information Fig. S1, Fig. S2, and Fig. S3). High purity green colors were difficult to obtain using conventional FROCs and were obtained using silica capped FROCs (red stars) as we will discuss below.

High purity green and red colors are difficult to obtain. This agrees with the purity results and the corresponding colors of FROCs shown Fig. 2c and Fig. 2d. This is because obtaining red and green colors requires having high reflectance at longer wavelengths. However, the measured reflection from the Ge-Ag broadband absorber is > 0.15 at short wavelengths < 500 nm (Fig. 4a). Consequently, the color purity drops since other colors are reflected. To suppress the stronger reflection at shorter wavelengths, a 50nm silicon dioxide capping layer was added as shown in Fig. 4a. Figure 4b shows the measured reflection for a FROC that produces a green color with and without the SiO₂ capping layer. The SiO₂ capped FROC exhibits significantly reduced reflection at short wavelengths and relatively small change near the resonance peak. This combination leads to an overall increase in the colorimetric purity. Figure 4c and Fig. 4d show the swatch array and corresponding color purity of the SiO₂ capped FROC design, respectively. By comparing Fig. 4d and Fig. 2d, a wider range of colors and greater purities can be obtained as compared to the original FROC structure. A purity level of > 97% can be reached in the yellow region of the spectra (Supplementary information, Fig. S4 and Fig. S5) which is significantly higher than other ultrahigh purity structural color platforms⁷.

In addition, FROC’s iridescence can be controlled by simply changing the refractive index of the dielectric cavity. Figure 5a shows a swatch array (angle of incidence vs. cavity refractive index) for a range of FROCs with the same optical length $l=n t=700 nm$, where $n$ is the dielectric index and $t$ is its thickness, as a function of incidence angle. For high refractive indices, FROCs color over nearly the entire range of incident angles. Note that colors’ angle independence may not be a desired property in some applications, e.g., anti-counterfeit measures.

Another important metric is brightness, i.e., the reflected intensity compared to the incident light at the peak resonance wavelength. The calculated reflectance from FROCs is > 0.9 (Supplementary information, Fig. S2) and the measured reflectance ranges from 0.63–0.85 as shown in Fig. 5b. The lower reflectance in the measured films can be improved by depositing higher quality silver films and precisely controlling thickness ¹⁹. Finally, we assess the color range covered by silica capped FROCs. We obtain the CIE 1931 chromaticity coordinates of a stack of SiO₂ (50 nm) -Ge (15 nm)- Ag (48 nm)- TiO₂ (x nm)- Ag (100 nm) and vary the thickness of the TiO₂ layer. A total coverage of 42.9% is achieved at 0^o viewing angle (Fig. 5c). Adding a silica capping layer improves the covered color gamut of the bare FROC which is only 28% at 0 viewing angle. At 55^o viewing angle, the covered percentage of the CIE color space is 60% coverage (Supplementary information, Fig. S6). We also calculate the coverage of the sRGB color space (the standard color space for the web), and the Adobe RGB color space (the color space that encompasses most of the colors achievable on CMYK color printers). The theoretical silica capped FROCs exhibit high coverage of both color spaces, with 89.3% of the sRGB color space and 85.5% of the Adobe color space, at normal incidence. The coverage percentage increases to > 99% coverage of both color spaces at 55^o viewing angle (Supplementary information, Fig. S7). Access to a wide color gamut with a similar reflection profile has been realized recently with multipolar metasurfaces, which - in contrast with FROCs- require intense nanolithography⁷.

Figure 5d compares the structural coloring performance metrics of FROCs, dielectric nanostructures, plasmonic nanostructures, and photonic crystals. Indeed, FROCs outperform existing structural coloring methods by simultaneously offering high purity, access to a wide range of colors, high brightness, angular control, and cheap and scalable fabrication¹. Because thin film structural coloring in general has lower spatial resolution compared to nanostructures based structural coloring, the latter remains advantageous for high density coloring. Durable FROCs can be made with ceramic materials ²⁰. We believe that FROCs are particularly suitable for colored solar thermal panels as they are efficient in absorbing the solar spectrum ¹⁶ while reflecting an on demand, narrowband color. In addition, by using amorphous Si instead of Ge and utilizing the metal films in the FROC to act as electrodes, it is possible to realize colorful photovoltaic cells¹⁴ using the well-established thin film deposition technologies.

Sample fabrication:

The FROC films were deposited on a glass substrate (Micro slides, Corning) using electron-beam evaporation for Ge (3 Å s^− 1) and TiO₂ (1 Å s^− 1) pellets and thermal evaporation Ag (20 Å s^− 1), with the deposition rates specified for each material. The silica capped FROC films were deposited on a glass substrate (2948, Corning) using electron-beam evaporation for Ge (0.5 Å s^− 1), TiO₂ (1 Å s^− 1), and SiO₂ (0.8 Å s^− 1), and DC magnetron sputtering for Ag (2 Å s^− 1). All deposition materials were purchased from Kurt J. Lesker. Deposited layer thicknesses were measured with spectroscopic ellipsometry (J. A. Woollam).

Numerical Calculation Of The Reflection And Absorption Spectra:

Numerical reflectance and absorbance spectra were generated using a transfer matrix-based simulation model written in Mathematica and Python. Spectral optical constants for the multiple materials were obtained variously from the Brendel-Bormann model (Ag), fits to the experimental materials (SiO₂, TiO₂), and an amorphous experimental model for Ge. Transmittance was zero for all structures. Absorbance was calculated as the complementary to reflectance, or $A = 1-R$.

Reflection measurements: Experimental angular reflectance measurements were performed using a variable-angle high-resolution spectroscopic ellipsometer (V-VASE, J. A. Woollam). Sample transmittance was zero for all angles and wavelengths.

Color Analysis:

The reflectance spectra to CIE 1931-xyz colorspace conversation was performed in Python utilizing interpolations of the standard observer distributions.^21,22 The XYZ tristimulus values are given as:

$X = \frac{{\int }_{\lambda }^{ } S\left(\lambda \right) \alpha \left(\lambda \right) R\left(\lambda \right) d\lambda }{{\int }_{\lambda }^{ } S\left(\lambda \right) \beta \left(\lambda \right) d\lambda }$ , $Y = \frac{{\int }_{\lambda }^{ } S\left(\lambda \right) \beta \left(\lambda \right) R\left(\lambda \right) d\lambda }{{\int }_{\lambda }^{ } S\left(\lambda \right) \beta \left(\lambda \right) d\lambda }$, and $Z = \frac{{\int }_{\lambda }^{ } S\left(\lambda \right) \gamma \left(\lambda \right) R\left(\lambda \right) d\lambda }{{\int }_{\lambda }^{ } S\left(\lambda \right) \beta \left(\lambda \right) d\lambda }$,

with S as the illuminant spectrum, R as the spectral reflectance, k as a constant factor, and $\alpha , \beta , \gamma$ as the standard observer functions. The integration is over the visible spectrum. The CIE 1931-xyz values are then given as:

$x = Y/N$ , $y = Y/N$, and $z = 1-x-y$,

for $N = X+Y+Z$. Note that at constant luminance, the chromaticity is defined by x and y.

Color swatch arrays were generated by transforming the calculated chromaticity values into their sRGB equivalents using a matrix transform calculated from reference primaries with the D65 reference white and sRGB companding (IEC 61966-2-1 standard). Colors were generated with matplotlib in Python.

Excitation purity is calculated as:

$$p = | s - w| / |d - w|$$

where s, w, and d are the CIE 1931 (x, y) coordinates for the measured spectra point, white point, and dominant wavelength point, respectively.

Total CIE x-y space coverage is calculated as the area of the smallest convex hull encompassing all of the desired CIE (x, y) points. Area is presented relative to the area of the full visible light color gamut. Sufficient resolution was obtained in the numerical simulations to approach a smooth hull.

Authors Contribution:

M.E. developed the approach and designed the project. C.G., G. S., and M. H. supervised the project. N. H. and A. L. performed color analysis. A. L. and S. A. J. fabricated the samples. A. L., M. E., T. L. performed reflection measurements. M. E. wrote the manuscript with inputs from N. H. and A. L. All authors discussed the results.

Acknowledgments:

C. G acknowledges the support of the Army Research Office, The Bill and Melinda Gates foundation, and the National Science Foundation. G. S. and M. H. acknowledge the support of the National Science Foundation. This work made use of the High-Performance Computing Resource in the Core Facility for Advanced Research Computing at Case Western Reserve University.

Ethics declarations:

Competing interests

A patent application has been filed on the Fano resonance optical coating scheme in this work.

Data availability:

Data are available upon request from the corresponding authors.

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Yes there is potential Competing Interest. A patent application has been filed on the Fano resonance optical coating scheme in this work.

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Supplementary Information

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Fano Resonant Optical coatings platform for Full Gamut and High Purity Structural Colors

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