Humidity- and Temperature-Tunable Metal–Hydrogel–Metal Reflective Filters

A tunable reflectance filter based on a metal–hydrogel–metal structure responsive to humidity and temperature is reported. The filter employs a poly(N-isopropylacrylamide)–acrylamidobenzophenone (PNIPAm–BP) hydrogel as an insulator layer in the metal–insulator–metal (MIM) assembly. The optical resonance of the structure is tunable by water immersion across the visible and near-infrared range. Swelling/deswelling and the volume phase transition of the hydrogel allow continuous reversible humidity- and/or temperature-induced tuning of the optical resonance. This work paves the way toward low-cost large-area fabrication of actively tunable reversible photonic devices.


Composition of the copolymer
Composition of the copolymer was determined by measuring an 1 H NMR spectrum (in DMSO-d 6 ) ( Figure S2). The polymer composition was estimated by comparing the integrals of the methyl peaks of the NIPAm monomer (6 protons per unit) and the aromatic signals of the acrylamidobenzophenone (9 protons per unit). The integral ratio (aromatic:methyl) is 1:32, and when the numbers of protons in each unit are taken into account, the composition is estimated to be 48:1 (NIPAm:BP). This is well in line with the ratio of monomers before polymerization (50:1). Figure S2: NMR of synthesized PNIPAm-BP copolymer.

Crosslinking characterization
The kinetics of crosslinking upon UV exposure was measured with UV-Vis spectroscopy.
Small amount of PNIPAm-BP ethanol solution (0.2 mg/ml) was sandwiched between two quartz plates and spectra were measured before and after 33 mW/cm 2 of 365 nm UV illumination ( Figure S3). During the UV illumination, the benzophenone is excited to a singlet state which rapidly undergoes intersystem crossing to a diradical triplet state. The formed radical reacts with a C-H in another polymer chain, forming a C-C bond and crosslinking the polymer (scheme in Figure S3). This will occur within the timespan of ∼10 min (inset of Figure S3). Figure S3: Absorbance spectra of PNIPAm-BP ethanol solution before and after UV irradiation.

Metal-hydrogel-metal samples
Several samples were fabricated with varied hydrogel layer thickness. The hydrogel thickness was controlled by a) solution concentration (2 concentrations were used, 20 and 40 mg/ml), and b) spin-coating speed (after first 10 s of predispensing at slow speed, rpm were increased to 1000-6000). Two samples with good uniformity providing the most consistent results (across the sample and between the measurement) were chosen for the main manuscript as VIS and NIR ones. Figure S4a shows overview of the VIS sample with the patterned top gold layer, it is possible to see the quality of both gold and hydrogel coatings, and high magnification Figure S4b illustrates the local porosity of thin top gold layer, enabling water penetration. Spectral measurements of all "VIS" samples in wet and dry state are presented in Figure S5. Figure S6 shows and example of the measurements at different points of one of "NIR" samples. Figure S1 shows the resonant wavelengths and corresponding Q-factors for the spectra presented in Figure 3 in the main text.  Figure S5: Reflectance spectra different samples with "dry" hydrogel layer thickness of around 90 nm before (dashed) and after (solid) immersion in water. Short dash -modeled spectra for the structures with different thickness of hydrogel layer. Figure S6: Reflectance spectra at different points of a sample with "dry" hydrogel layer thickness of around 250 nm before and after immersion in water. 3 Spectral measurement in the temperature-and humiditycontrolled environment The full spectroscopic maps for the data presented in Figure 5 in the main manuscript is shown in Figure S7. Figure S8 shows the example of the recorded humidity and temperature values (for Figure S7b) to illustrate the stability and precision of the control provided by the experimental setup. Figure S9 shows the FLIR camera image of the sample and Si wafer on the stage heated to to 50°C. Using the measurement we assumed that linear change of the stage temperature from ambient to 50°C corresponds to linear change of the sample temperature from ambient to 45°C.

Refractive index of PNIPAm hydrogel
The literature data 1 for the refractive index of PNIPAm hydrogel was confirmed with spectroscopic ellipsometry (with Woollam VASE ellipsometer). The real part of the refractive index of used PNIPAm-BP (48:1) hydrogel averaged across 4 samples with thicknesses of 84-99 nm is shown in Figure S10. In the modeling, we used the constant value of 1.503 for the refractive index, which corresponds to the experimental value at 650 nm wavelength.
There was no noticeable difference between the spectra modeled with this constant value and with full dataset from the ellipsometry measurement. The imaginary part was measured as 0-0.02 across the studied region and was disregarded in our modeling.