Supporting Information. Two-Dimensional Active Tuning of an Aluminum. Plasmonic Array for Full-Spectrum Response

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1 Supporting Information Two-Dimensional Active Tuning of an Aluminum Plasmonic Array for Full-Spectrum Response Ming Lun Tseng 1,4, Jian Yang 2,4, Michael Semmlinger 1,4, Chao Zhang 1,4, Peter Nordlander 1,2,4, and Naomi J. Halas 1,2,3,4* 1 Department of Electrical and Computer Engineering, 2 Department of Physics and Astronomy, 3 Department of Chemistry, and 4 Laboratory for Nanophotonics, Rice University, Houston, Texas 77005, United States These authors provided equally important contribution to this work. * Correspondence and requests for materials should be addressed to N.J.H. ( halas@rice.edu).

2 Section I: Design of the plasmonic device for vivid color generation The building blocks of the plasmonic device are rectangular shaped aluminum nanoparticles. To achieve full color tuning, they should have a large scattering cross section across the whole visible spectrum. The simulated scattering cross section of an isolated structure (Lumerical FDTD Solutions) is shown in Supplementary Figure 1. The broad resonance of the nanoparticle allows it to scatter light from red to blue with comparable intensities. Vivid color scattering can be generated from far field interference. When a lattice is formed, light of different wavelengths constructively interferes in the far field at different observation angles. This effect can be described by the diffraction formula: λ = Dn(sin θ incidence + sin θ observe ) Where, λ is the wavelength which is constructively enhanced at observation angle θ observe. D is the lattice constant, n is the refractive index of the medium, and θ incidence is the incident angle.

3 Supplementary Figure 1: Simulated scattering spectrum of an isolated rectangular aluminum nanoparticle. The nanoparticle is 130nm long, 100nm wide and 35nm thick. It lies on a PDMS substrate and is covered with HSQ and PMMA pillars on the top. The simulated scattering spectrum shows a broad peak centered at around 550nm. Furthermore, using a beam stop, the collection cone of the objective used in this experiment can be controlled. Only a certain wavelength window supports constructive interference within the collection cone, while light for the rest of the spectrum is destructively suppressed: λ max = D x n(sin θ incident + sin θ colmax ) λ min = D x n(sin θ incident + sin θ colmin ) δλ = D x n(sinθ colmax sinθ colmin )

4 The wavelength detection window δλ is proportional to the lattice spacing in x direction. Supplementary Figure 2 shows the simulated far field signal collected by the objective with a collection angle of 1.7 degrees. The peak position and shape are in great agreement with the theoretical prediction. Therefore, by restricting the collection cone, it is possible to reduce the linewidth of the scattering peak and produce vivid colors, as shown in Supplementary Figure 3. Note that the choice of the incident angle of 68 degrees is arbitrary. Other incident angles could show similar color tuning effects if we re-designed the sample and modified the lattice constants. Supplementary Figure 2: Simulated scattering spectrum and theoretical prediction. The diffraction formula clearly predicts the upper and lower threshold of the wavelength window.

5 Supplementary Figure 3: Simulated scattering spectra with different collection angles. By limiting the collection cone, the scattering peak linewidth can be reduced. The grating theory only gives us the thresholds for the collected wavelength range. It does not take into account the detailed size, shape, material and the corresponding spectrum of the single nanoparticle. However, it served as an intuitive way for us to expect where the scattering peak would be and helped us with the design of the lattice. In fact, it agrees very well with the simulated scattering (see Supplementary Figure 2). However, for the spectra calculations, we used accurate FDTD simulations which consider all relevant details of the nanoparticles. The far field scattering spectra of the device is then simulated by combining the effects of single particle scattering and diffraction coupling. Since the length of the nanoparticle is larger than its width,

6 under transverse polarization, the scattering resonance would be blueshifted (see Supplementary Figure 4). However, because the diffraction effect is mostly dependent on the lattice constants, the collected dark field spectra would be identical in peak position and width. Transverse polarization would yield a lower scattering intensity because normal scattering was collected under a large incident angle. Supplementary Figure 4: Simulation results of the polarization dependence of the device. (a) Changing the polarization from longitudinal to transverse blueshifts the resonance, under normal incidence. (b) Transverse polarization yields a spectrum identical in lineshape, but lower in scattering intensity when compared to longitudinal polarization, under the 68-degrees oblique incidence. Due to the nature our experimental setup, the absolute value of the collected power does not give us much information about the efficiency. Therefore, we performed FDTD calculations to estimate the device efficiency. The calculation was done by integrating the far-field scattering power within the experimental collection angle, and then dividing it by the far-field scattering power into the whole semi-sphere at the air side of the sample. In this way, we obtained the portion of energy going into the detector with respect to the total reflected power. Then we multiplied this number with the percentage of reflection to get the scattering efficiency. There are two ways of defining a scattering efficiency for our device: one is an evaluation of the ratio

7 between the scattered signal intensity and the source intensity at the peak wavelength (peak efficiency), while the other is the ratio obtained by integrating over the whole visible spectrum (whole-spectrum efficiency). Both ratios are explained in Supplementary Figure 5 below. We have provided the calculated efficiencies for these two definitions for a range of collection angles, together with the corresponding color tuning trajectory in the chromaticity diagram (see the following Supplementary Figure 6). It can be observed from these calculations that the peak efficiency remains constant at around 6% for all collection angles. The whole-spectrum efficiency is lower than the peak efficiency, but it gradually increases as the collection angle is increased. In fact, we can reach around 2% whole-spectrum efficiency without sacrificing too much monochromaticity. In this context, we also want to point out that even without the beam stop (Supplementary Figure 6d), we can achieve much bluer colors than reported in previous works 1. Supplementary Figure 5: Two definitions of scattering efficiency. On the left panel, the ratio between the scattered signal and the source signal, η 1, at the peak wavelength is calculated. On the right panel, the ratio between the scattered signal and the source signal, η 2, integrated over the whole spectra is calculated.

8 Supplementary Figure 6: Calculated efficiency and chromaticity diagrams for different collection angles. The colors become less vivid when we gradually increase the collection cone from (a) 1.7 degrees to (b) 8 degrees, (c) 11 degrees and (d) 14.1 degrees. The collection cone in (a) corresponds to our experimental setup, while the 14.1 degrees in (d) correspond to completely removing the beam stop. For (b), (c) and (d), we added a horizontal stretch of 50% in the simulation. Section II: Influence of sample imperfections Since the sharp spectral profile of the plasmonic device arises from the lattice effect of the aluminum nanoparticle arrays, fabrication imperfections within unit cells do not have a large effect. For example, the building blocks do not necessarily have to be ideal rectangles. In fact, circular disks show almost identical scattering properties in the far field (Supplementary Fig. 7). However, inhomogeneity across the lattice (i.e. defects) deteriorates the periodicity and broadens

9 the peaks. A simulation of a lattice with alternating circular and rectangular nanoparticles shows a tail in the scattering spectra, confirming this effect (Supplementary Fig. 7). Supplementary Figure 7: Influence of defects on the scattering spectrum. The shape of the fundamental building block of the array does not influence the far field interference significantly, while the inhomogeneity within the lattice does. For practical applications, an elastic capping layer like PDMS can be coated on the devices to protect them from the environment. The addition of this dielectric background will redshift the plasmon resonance, but it could be compensated by shrinking the size of the aluminum nanoparticles. As stated above, the sharp profile of the peaks is due to far-field diffraction and depends almost solely on the lattice constant. Therefore, even if we do not compensate this

10 redshift, the resonance peaks will still lie at the same positions, but the scattering intensity across the visible regime might be more inhomogeneous. Section III: Size dependence Ideally, an infinite array of unit cells is required to generate perfect diffractive interference. However, that is neither possible nor necessary. As shown in Supplementary Figure 8, we studied how the scattering spectrum depends on the lattice size by analyzing plasmonic devices with different array areas. One can see that a 20μm 20μm lattice is sufficient to produce a very sharp scattering peak. When the area is as low as 10μm 10μm, the scattering peak starts to show significant broadening. We also fabricated arrays with an area of 5μm 5μm, but were unable to resolve them clearly in our dark field spectrometer. Therefore, we conclude that the device can be as small as 20μm 20μm without compromising any significant vivacity of the scattering colors.

11 Supplementary Figure 8: Size dependence of the plasmonic device. (a) SEM image of asfabricated arrays before they were transferred to the PDMS substrate. (b) Corresponding scattering spectra. Section IV: Dynamic color tuning According to the diffraction formula, the scattering window can be tuned by changing the lattice constant of the array. This can be realized by using a flexible substrate. PDMS was chosen because of its good elasticity. Under strain in the x-direction, PDMS will stretch in x but shrink in y according to the following formula: P x = P x0 (1 + ε) P y0 P y = 1 + ε Here ε is the amount of stretch. To reduce the tension in the PDMS and enhance stability, we make use of a 2D-stretching method. In the relaxed state, the device exhibits green color.

12 Stretching in the x/y direction red/blue shifts the peak, and hence full color tuning is achieved. The calculated absolute scattering spectra of a stretching series are shown in Supplementary Figure 9. We can clearly see that the height of the peaks follows the trend of the scattering spectrum of an isolated unit. It can also be observed that the scattering intensities at different colors are of similar magnitude. As we mentioned in part I, the linewidth of the peak is proportional to the lattice constant in the x direction, D x. Therefore, when we stretch in the y direction, D x decreases and the scattering peak blueshifts and gets sharper; when we stretch in the x direction, D x increases and the scattering peak redshifts and gets broader.

13 Supplementary Figure 9: Simulated absolute scattering peaks of a stretching series. The peak heights follow the trend of the scattering spectrum of an isolated unit. All peak intensities are of the same order of magnitude. Section V: Dynamic image switching The dynamic, multilevel image switching that is demonstrated in the main text (Fig. 6) relies on the sharp profiles of the scattering peaks. The three letters (O, W, L) are constructed of arrays with periods (x, y) = (429nm, 348nm), (400nm, 400nm), and (348nm, 429nm), and therefore produce scattering peaks at different wavelengths. The peak positions are at 430nm, 495nm, and 530nm for pattern L, pattern W, and pattern O, respectively. The images in Figure 5c were obtained by stacking the collected CCD images of the letters for cyan light excitation with a wavelength window of nm. Standard imaging techniques were used to enhance the image. Pattern W can be observed most strongly in the relaxed state, since its peak position falls within this excitation window. However, because the tail of the spectra of patterns O and L reach into the excitation window, they are slightly visible in the dark-field image as well. For an excitation wavelength of 490nm, the ratio of the maximum brightness of pattern W to the other two letters is 9.1. Similarly, L/O can be made the most dominant letter by horizontally/vertically stretching the PDMS substrate. The ratio of maximum brightness of L/O to the weaker letters was 29.7/9.6 for horizontal/vertical stretching. These values were calculated by dividing the strength of the brightest area of the dominant letter by the brightest area within the other two letters. The difference in these ratios can be explained by the asymmetry in the peak positions. Since the peak of L is farthest from the other two letters, there is the least overlap in this case, which leads to the best brightness ratio. In general, a larger peak separation will lead to a better switching effect, but also requires more stretching. The nonuniformity within the letters can be

14 attributed to defects in the array. These are most likely due to imperfections in the sample. Lastly, the image switching can also be accomplished with light of any other wavelength, by either stretching the substrate by the corresponding percentage, or designing the sample to have the corresponding periods initially. Our pattern switching device shows good color selectivity. To show this, we took the pattern O as an example. For the unstretched sample, the pattern O manifests itself as green, while the other two characters do not. When we stretch the sample horizontally, the scattering peak of O goes to 605nm, and we can see the character clearly under illumination at 605nm. Similarly, we see O clearly if the illumination is at 480nm for the vertical stretching case (see Supplementary Figure 10). In addition, our square lattice arrays also show good color selectivity (see Supplementary Figure 11).

15 Supplementary Figure 10: Pattern O under three different illumination wavelengths at different stretching conditions. In the unstretched case, the pattern O appears in the image under illumination at 530nm. In the horizontal/vertical stretching case, it appears when illuminating at 605nm/480nm, respectively.

16 Supplementary Figure 11: Stretchable aluminum device under three different illumination wavelengths at different stretching conditions. The device appears red, green and blue at 23% horizontal, 4% horizontal and 14% vertical stretching, respectively. Section VI: Elastic Limit of PDMS The stress-stain relationship in PDMS strongly depends both on the mixing ratio of base to curing agent 1, 2 and the curing temperature 3. One can define an elastic limit beyond which the material starts to show hysteresis. For common mixing ratios, this limit falls between % of strain. A higher ratio of base to curing agent will lead to a lower Young s Modulus and therefore a higher elastic limit. Kim 1 et al. have reported an elastic limit of around 100% for a mixing ratio of 15:1. The ratio used in this work was around 35:1, and it can therefore be safely assumed that the elastic limit is at least this high. Since the maximum strain was no larger than 35%, we are confident that all the strains used here fall far below the elastic limit. In this context, two-dimensional stretching is very useful for our PDMS-based device. With this method, full color tuning can be achieved without heavily stretching (i.e., beyond the linear regime of the

17 deformation of PDMS) the substrate. It not only prevents the sample from being damaged during the stretching process, but also improves the repeatability of the devices. Supplementary Figure 12 shows the self-constructed holder that was used for stretching the sample. The smallest increment in which we could repeatedly tune the movable arm is about 0.5mm. This corresponds to about 4% of strain. However, to more accurately determine the amount of stretch for any given measurement, we compared the sample dimensions in the optical images in the relaxed and the stretched state. We did not observe any material distortion within the stretching cycles reported in Supplementary Figure 13. Supplementary Figure 12: Photograph of self-constructed sample holder.

18 Normalized Scattering 1.0 initial 10x 20x 40x 60x 80x 100x Wavelength (nm) Supplementary Figure 13: Repeatability Test. The peak positions only shift slightly with repeated measurements. Section VII: Comparison with related work Our device can be tuned across the whole visible spectrum, while most previous devices can only tune across part of the visible spectrum, or lie in the IR spectrum. For example, John Rogers et al. have reported the results 7 on tuning the resonance of gold nanoparticles from the edge of visible regime up to around ~1.3μm, but gold is not suitable for this purpose because of its high loss related to the inter-band transition at high photon energy regime (< ~550 nm). Color pixels made of TiO2 and PDMS can show higher efficiencies because TiO2 is a loss-less material in the visible regime. However, for a stretchable device that is based on far-field interference of dipoles, the resonance of the unit cell needs to be broad across the visible regime, or the device will not

19 be able to tune across the full regime. Therefore, loss-free dielectric materials such as TiO 2 are not ideal for this type of full color tuning device 8. The working principle of our device is different from previously reported devices that are usually based on tuning the dipole coupling between nanoparticles The resonance quality of these devices will gradually become worse when the PDMS substrate is continuously stretched along a certain direction. On the contrary, in a far-diffraction based approach, only the lattice spacing is manipulated, and therefore the high quality of the produced colors can be sustained across the whole visible regime. Furthermore, even though the efficiency of the device is low due to the dark-field geometry, both the technologies of bright light sources and sensitive photodetectors are very mature nowadays. Therefore, for several applications such as ultracompact monochromators, monochromaticity and tunabiltiy are more important than efficiency. Furthermore, the elastic limit of PDMS under 1D stretching strategy prevents repeatable fullcolor tuning 9. Therefore, even though many works on tunable plasmonic devices based on stretchable substrate have been reported in recent years, full color generation/tuning on a single device has not been reported yet. Many of the devices from the mentioned references could be more monochromatic if dark field spectroscopy had been used instead of bright field. There are still several requirements for fullcolor actively tunable devices that have not been met in prior works. First, the lattice component should be an efficient light scatterer. That is why we are using plasmonic nanoparticles whose resonance is in the visible region. Second, the light scattering capabilities should be similar in magnitude across the whole visible spectrum. As we can see from some of the literatures

20 mentioned above, gold nanoparticles are good at scattering red light, but the scattering cross section at the blue part of the spectrum is too small. If the scattering power differs too much across the visible spectrum, the device will be extremely inhomogeneous in wavelengths, making it useless in practical applications. Here, we designed an aluminum nanoparticle with a broad resonance across the whole visible spectrum, and the resulting light scattering efficiency is similar in amplitude. (See Supplementary Figure 8) Third, the lattice constant of the array needs to be carefully designed, so that we can detect our desired visible colors under the particular incident and collection angle. Finally, we developed a 2D stretching approach and successfully avoided the problem of the elastic limit of PDMS, which we believe is important to the realworld applications of the PDMS device. Section VIII: Design of a MEMS-based stretchable device Introducing MEMS technology into our plasmonic device could be a promising way to make it electronically tunable for real word applications. PDMS is compatible with MEMS technology, and has been used in MEMS-based devices for photonics. 4-6, 15 A possible design is shown in Supplementary Figure 14. Here, two edges of the freestanding plasmonic device are anchored on the substrate while the other two can be moved by micromachined actuators. By applying a bias to one of the moveable actuators, the plasmonic device can be stretched along a specific direction, resulting in a change in the display color. Since the strains used in our device are relatively low (i.e., no large device deformation), the capability of achieving high tuning speeds mainly depends on the inherent elastic properties of the polymer substrate. It has been shown that when PDMS is stretched below the elastic limit, it

21 can support a stretching speed of up to a few khz. 4-6 Therefore, we believe that our device could be tuned with a similar ultimate speed if it was integrated into a high-speed MEMS system. Supplementary Figure 14: Schematic of a possible way to integrate our plasmonic device with MEMS technology. Two edges of the plasmonic device are fixed by anchors, while the other two edges are attached to movable frames. When a bias is applied to one of the frames, the PDMS substrate can be stretched accordingly.

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