## 3 Materials and Methods  
In the experiment, a polymer film and an inorganic substrate material with an appropriate combination of refractive indices were selected. The polymer film (\(n_{1}\)), sandwiched between the substrate (\(n_{0}\)) and the air (\(n_{2}\)), forms the FP resonator. Collimated white light was incident from the side of the substrate at an angle slightly different from normal incidence, and the light transmitted from the opposing side was detected. The light reflected from the polymer film surface when incident from the substrate and the light emitted from the film after being incident on and multiply reflected in the film cancel each other out by destructive interference, which is observed as a sharp reflection dip (CPA).  
An MgF\({}_{2}\) plate (refractive index 1.38, 20 \(\times\) 20 \(\times\) 5 mm\({}^{3}\)) purchased from Oyo Koken Kogyo Co., Ltd. (Tokyo, Japan) was employed as the substrate, and polyvinylpyrrolidone K25 (PVP, refractive index 1.53 at 510 nm, extinction coefficient \(\approx\) 0.003) was purchased from FUJIFILM Wako Pure Chemicals Co. (Osaka, Japan). We chose PVP because it has a relatively high refractive index among polymers and can be deposited without water on MgF\({}_{2}\), which is slightly soluble in water. A PVP film was dip coated on the MgF\({}_{2}\) substrate. For dip coating, a linear guide (stroke length 100 mm, Heechoo, Shenzhen, China) was driven by a stepping motor (NEMA 17, 17 HS4401, RATTMMOTOR, Changzhou, China) and stepping motor controller (LD09, Longruner, Shenzhen, China). Its speed was controlled by an Arduino Uno (Arduino). The MgF\({}_{2}\) substrate was lifted from the chloroform solution containing 0.12 g/mL of PVP. The film thickness was adjusted to be uniform by varying the lifting speed from 4.08 to 4.04 mm/s.  
Figure 2a shows the experimental setup for observing CPA, and Figure 2b shows how the light beam was incident on the sample for CPA. The white light from a Xe lamp, a laser-driven light source (LDLS EQ-99 X, Energetiq Technology, Wilmington, MA, USA), whose luminous spot size is specified as \(100\times 180\)\(\upmu\)m\({}^{2}\), was collimated with a Cassegrain mirror to form the collimated beam of about 1.3-mm diameter just before sample incidence. The light was s- or p-polarized with a polarizer and then, incident from the side of the substrate at an incidence angle of nearly 90\({}^{\circ}\)(87.5\({}^{\circ}\) to 89.5\({}^{\circ}\)), as defined in Figure 2b. The multiply reflected light in the sample was emitted from the opposite side of the sample. The light was sent through a fiber bundle to a polychromator (Acton SpectraPro-300i, Acton Research Co., Acton, MA, USA) with a diffraction grating of 150 lines/mm, 500 nm blaze, and the spectrum in the wavelength range of 370 to 800 nm was detected with a CCD.  
All the calculations were performed by Python using Equation (1), where the wavelength dependence of the refractive indices of MgF\({}_{2}\) and PVP and that of the extinction coefficient of PVP were considered using the data in Refs. [20,21] as shown in Figure 3a,b. It is known that MgF\({}_{2}\) is a birefringent crystal, although its anisotropy is small (for example, \(n_{\rm o}=1.38040\) and \(n_{\rm e}=1.39233\) at 480 nm) [20], but we used the averaged refractive index for the calculation because the crystal axis is not specified in the substrate MgF\({}_{2}\).