Supporting Information for “Encoding Mie, Plasmonic, and Diffractive Structural Colors in the Same Pixel”


 We present a 1D reflective multi-level structural color design that incorporates Mie, plasmonic, and diffractive mechanisms in the same pixel. Comprised of a metallodielectric grating made of TiOx nanowires sandwiched between Ag thin film and Ag substrate, the design can exhibit either a Mie resonance or a localized plasmonic resonance depending on the polarization of incident light, resulting in dramatically different color states. Due to the periodicity, the grating also diffracts light, providing an additional color state. Since diffraction can be turned on or off by the degree of coherence of the incoming light, both Mie and plasmonic colors can be modulated using objective lenses with different numerical apertures. Exploiting the different color generating modes, we encode four layers of information in a pixel array, where each layer is unveiled using a different combination of excitation and imaging settings. These results introduce new possibilities for data encryption, anticounterfeiting, and data storage.

.   In our previous work, we demonstrated a similar resonator in transmission mode, where scattering was reduced at the electric dipole resonance, enabling light at this wavelength to transmit through the resonator. 1 It was found that the thin Ag film on top of the TiOx nanowire serves to greatly enhance the resonance by confining the mode and suppress the scattering at the respective wavelength via formation of an opposite dipole moment in the Ag shell as shown in previous studies. 2,3 Although the Mie nanoresonator of our previous work and the one from our current study share geometrical similarities, a key distinction is that the current nanoresonator uses a reflective substrate rather than a transparent one. This difference, while subtle, leads to distinctive scattering and absorption behavior. Understanding the absorption peak is particularly important for reflective pixels since it defines the valley in an otherwise featureless reflection spectrum, forming the basis of subtractive color. In the case of the transmissive nanoresonator, the scattering minimum occurs close to the absorption peak position. However, for the reflective nanoresonator, the scattering minimum and absorption peak are further shifted apart from each other. This is attributed to the electric dipole resonance interfering with the reflected light, causing a Fano-like profile 4 in the scattering spectrum as depicted in Figure S3. Characteristic of the Fano line shape, the scattering cross section near the resonance falls sharply with increasing wavelengths, intersecting the absorption cross section before reaching its minimum value. This point at which the absorption and scattering cross sections are equal is known as critical coupling 5 and marks the point of peak absorption.
As a result, absorption is peaked at a lower wavelength than the point of minimum scattering. To further verify that the resonance wavelength is independent of the period, we simulated the total reflection spectra for nanoresonators by fixing the width and varying the periods ( Figure   S4a). For each width, the electric dipole resonance does not shift in wavelength over all periods, indicating that the pixel colors are not affected by the period for the given imaging conditions.

Supplementary Note 3. Details of subtle discrepancies in the intensity between measurement and simulations
We note that although the simulated spectra show consistent intensities for all three periods It is worth noting one fine spectral detail in the simulated results that is unresolved in our measurements due to surface roughness and irregularities. Close inspection at wavelengths between 400 and 500 nm and periods above 600 nm reveals the spectral signature of the 1 st SPP branch, where electric field distributions illustrate the characteristic SPP fields occurring at the Ag-air interface adjacent to the nanoresonator ( Figure S6a), which is a general phenomenon for plasmonic gratings as illustrated with a simple Ag lamellar grating in Figure   S6b. Figure S7. (a, b) Simulated (a) total and (b) 0 th order reflection spectra under p-polarized light (setting 2) as a function of period and wavelength for widths of 147, 233, and 319 nm. The simulation was performed with a planewave light source.

Supplementary Note 5. Simulated p-pol reflection spectra for fixed widths
One common effect observed with other plasmonic structural colors is the near-field coupling between neighboring units that gives rise to spectral modulations with separation distance.
While the near-field coupling can be used as a color tuning feature, it is often nonintuitive to predict. To see if this effect is also present in our nanoresonators, we calculated the reflection spectra over varying periods using a planewave source. Figure S7 shows that the resonance wavelength of the localized surface plasmon from a nanoresonator with fixed width remains unchanged despite changes in the period, confirming that it is largely free from the influence of neighboring modes. This is attributed to the trapezoidal shape of the nanoresonator cross section where the plasmonic sites on the top of the Ag shell are too far removed from equivalent neighboring sites even when the bases of the respective two resonators are slightly touching. This feature helps prevent spectral shifts of the localized plasmon resonance with period and facilitates prediction of accessible colors.

Supplementary Note 6. Diffraction equation
The relation between the incidence angle, grating period, and wavelength of the diffracted light collected by the objective lens can be found from the diffraction equation: where m, λ, P, and θ are the diffraction order, diffracted wavelength, period, and incident or diffracted angle with respect to the surface normal, respectively.

Supplementary Note 7. Design strategy for image encryption
Our encoding method takes advantage of the fact that the colors from s-pol light under a 0.9 NA lens are highly dependent on the nanoresonator width while the colors from sideilluminated 1 st negative order diffraction are predominantly determined by the period. This dependence of separate imaging modes on distinct spatial parameters allows two independent sets of images to be stored in the same pixel array by adjusting the nanoresonator widths for one image and periods for the other. imaging setting turns on both numbers (cyan numbers vs blue background) as shown in image Ⅲ .