# Hyperspectral Molecular Orientation Mapping in Metamaterials
## 2 Experimental: Setups and Samples
### Gold-Polyimide-Gold MIM Metasurface and Properties  
The MIM structure was made on Si wafer coated by 200 nm of gold by evaporation (inset in Figure 1a). Then, vacuum polymerization of polyimide was carried out onto the gold film. Polyimide was formed using vapour deposition polymerization of pyromellitic dianhydride (PMDA) and \(4A^{\prime}\)-diaminodiphenyl ether (ODA) at temperatures of 150 \({}^{\circ}\)C (PMDA) and 110 \({}^{\circ}\)C (ODA) with substrate at 30 \({}^{\circ}\)C [23, 24]. After 30 min of deposition, thermal imidization treatment was carried out at 150\({}^{\circ}\)C in air for 1 hour. Polyimide layer formation was confirmed using infrared reflection-absorption spectroscopy and its thickness was measured by ellipsometry (Figure 1). The top layer of gold nanodisks arranged into a triangular lattice was fabricated using electron beam lithography (EBL), electron beam evaporation of 50 nm of gold and a lift-off process. Reflectivity and absorbance of MIM patterns was expected to be dependent on the portion of gold nanodisks occupied on the surface, which can be estimated as the area ratio of gold per unit cell \(S_{Au}=\frac{1}{2}\pi(\frac{d}{2})^{2}\) and the area of the unit cell \(S_{un}=\frac{\sqrt{3}}{4}\Lambda^{2}\): \(S_{Au}/S_{un}=\frac{8\pi}{9\sqrt{3}}\) for \(\Lambda=1.5d\) and is independent on \(d\). The Fourier transform IR (FTIR) spectral imaging was carried out with a PerkinElmer Spotlight 3000 spectrometer (Spectrum One, PerkinElmer Inc., Waltham MA, United States) with an IR microscope, the automatic xyz-stage with a sample holder, and 16 \(\times\) 1 linear detector array of Mercury Cadmium Telluride (MCT) sensors with a square pixel size of 6.25 \(\upmu\)m. Spectral analysis was carried out with 4 cm\({}^{-1}\) spectral resolution. Polyimide has several bands whose orientation can be used for determination of molecular alignment: 1780, 1745, 1520 and 1400 cm\({}^{-1}\) bands are for the C=O symmetric (chain axis), C=O asymmetric (ring tilt), C\({}_{6}\)H\({}_{5}\) aromatic rings (phenyl) tangential vibrations, and C-N-C axial stretch, respectively [25; 26; 27]. The parallel (to film) bands are at 1491 cm\({}^{-1}\) [n C-C; skeletal vibration of 1,2,4-tristubstituted benzene ring in diamine moieties] and 1501 cm\({}^{-1}\) [\(\
u\) C-C; skeletal vibration of 1,4-disubstituted benzene in diamine moieties], while the perpendicular band is at 1724 cm\({}^{-1}\) [\(\
u\) C=O; out-of phase asymmetric stretching of carbonyl bonds in amide rings) [25; 26; 27]. These bands are typically used for evaluating the molecular anisotropy of PMDA and ODA. The tilt of PMDA ring units and the chain axis shows a preferential orientation for films thinner than 15 nm [27]. This structural anisotropy decreases rapidly as a function of increasing film thickness but the chain axis direction maintains a constant value even for thick films, indicating that a good degree of anisotropy exists [27]. Upon crystallisation, the C=O stretching mode decreases by 8 cm\({}^{-1}\) from 1725 to 1717 cm\({}^{-1}\) due to an increase in chain conjugation in the ordered state [27]. Films thinner than 15 nm have lower packing efficiency, evidenced by the decrease in the transverse C-N-C stretch (1117 cm\({}^{-1}\)) for the thinner films. A better packing induces a higher degree of ring coplanarity and increased conjugation. Anisotropy and the main chain orientation at a range of temperatures (10-570 K) for polyimide films (0.1-300-\(\upmu\)m-thick) of different compositions were directly measured by temperature wave analysis (TWA) [28]. Temperature diffusivity has a strong increase at temperatures below 20 K due to an increase of the phonon mean free path (from 0.8 to 3.1 nm) in an amorphous phase. Temperature diffusivity differed \(\sim\)10 times due to the main chain orientation in the transverse direction of the film with anisotropy larger for thinner films [28]. A large anisotropy of temperature  
Figure 1: Sample and method.