# Hyperspectral Molecular Orientation Mapping in Metamaterials
## 3 Results and Discussion
### 3.1 4-pol. Chemical Mapping: Image Shifts and Baseline Correction
The most transparent spectral region was set as the same minimum for all four polarisations (without normalisation nor amplitude change). The negative \(A\) values were obtained at a 2380 cm\({}^{-1}\) CO\({}_{2}\) band and it is a normalisation artefact. Figure 3: Compensation for xy-plane shift in four-polarisation maps. (**a**) Principle of x and y-shifts for centring the maps; the images for all four polarisations was overlayed. (**b**) Absorbance maps of only two MIM regions (larger magnification as compared with Figure 2). One pixel is \(6.25\times 6.25\) μm\({}^{2}\) on the sample. (**c**) The cross-sectional absorbance for four polarisations (\(0\), \(\frac{\pi}{4}\), \(\frac{\pi}{2}\), \(\frac{3\pi}{4}\)) before and after compensation shifts. _Amp_ and the orientation angle \(\theta_{O}\) were extracted. Inset in Figure 5 shows the result in two MIM regions (patter #1 and #2). The difference in gold nanodisk diameter \(\Delta d=50\) nm and period \(\Delta\Lambda=1.5d=75\) nm contributed to a spectral shift in plasmonic band. This shift is visualised by plotting all the four polarisation absorbance spectra from both MIM regions (enclosed in the red boxes in the inset). Separate families of four polarisation spectra are most clearly distinguishable at the 3860 cm\({}^{-1}\) O-H stretch and 2985 cm\({}^{-1}\) vs. 2735 cm\({}^{-1}\) alkyl C-H bands. The plasmonic band defines location of the C-H band due to the E-field enhancement defined by an spectral overlap of the spectrally wide C-H and plasmonic bands. There is a weak absorbance at these bands which can be observed for the \(d>0.5\) um which shifts the plasmonic band into smaller wavenumbers (longer wavelengths). The absorbance values at the maximum of the bands \(\sim\)3000 cm\({}^{-1}\) are proportional to the local field enhancement and their spectral form is defined by the localised plasmon; the band centred at 2750 cm\({}^{-1}\) (\(\lambda=3.64\) um) and bandwidth of \(\pm 250\) cm\({}^{-1}\) (at the baseline) corresponds to the plasmonic spectral bandwidth of 0.67 um or 6 meV. By placing nanodiscs of different diameters over nanothin polymer film, the weak absorbance can be quantified. This also implies, that by creating anisotropic patterns of top-layer nanostructures in MIM metasurfaces, a strongly anisotropic response is expected as observed by using nanoparticles [30]. Enhancement of E-field component perpendicular to the surface (aligned with direction of propagation) is a feature which can probe not only lateral in-plane field enhancements [31]. More crowded absorption bands and lower wavenumbers are better resolved on the zoomed-in (right) spectrum in Figure 5. Clear spectral shifts are discernible due to plasmonic band interaction with polyimide absorbance bands around 1400 cm\({}^{-1}\) where shifts by \(\sim\)15 cm\({}^{-1}\) occurred; eye guide lines in the Figure 5b. It is however recognisable that there was no apparent difference between spectra measured at different polarisations. This implies that orientation of polyimide was not pronounced. This is understandable since the way the polyimide was formed is known to produce amorphous films. The orientation of polyimide is visualised next by plotting orientation angle \(\theta_{O}\) at several absorption bands (Figure 6). A random orientation was obtained from the best fit by four  
Figure 5: Absorbance map (top inset) and region-averaged absorbance spectra at different spectral windows: wide and narrow (at four polarisations after compensation shift and baseline correction). The spectra from the pattern #2 and #1 (\(d=550\) nm and 500 nm, respectively) are overlaid. See Figure 6 for detailed orientation map. One pixel is \(6.25\times 6.25\) μm\({}^{2}\) on the sample. polarisation orientation measurement at the 3850 cm\({}^{-1}\) O-H stretch band depicted by orientation of a bar while the length of the bar is proportional to the absorbance amplitude of the cos-fit at for orientation angles. More ordered absorbers were mapped at the 2735, 1400 and 1320 cm\({}^{-1}\) bands.