## 5 Conclusions and Prospect  
We have theoretically explained and experimentally demonstrated that CPA is realized for an almost transparent sample in the visible light region, with a single dielectric layer sandwiched by semi-infinite dielectric layers. In the experiment, the transparent PVP film was deposited on the transparent MgF\({}_{2}\) substrate, where the PVP film sandwiched between MgF\({}_{2}\) and the air makes the FP resonator with TIR at the interface with air. Collimated white light from the lamp source was incident from the side of the substrate at an angle slightly deviating from normal incidence, and the minimum transmittance of over 90% (92.6%) dip was observed in the spectrum of the transmitted light emitted from the opposite side. The requirements (necessary conditions) for CPA in this configuration (a single layer slab thin film) are: (I) the transparent thin film must have a small amount of absorption, (II) \(n_{1}>n_{0}>n_{2}\), where \(n_{0},~{}n_{1},~{}\)and \(n_{2}\) are the refractive indices of the substrate, the thin  
Figure 5: (**a**) Reflection spectra (**p**- and **s**-polarized) of the transparent PVP film on the transparent MgF\({}_{2}\) substrate at \(\theta_{\text{air}}=88.5^{\circ}\) incidence in air, obtained from the side-to-side transmitted light spectra at 88.5\({}^{\circ}\) incidence, divided by the baseline transmitted light spectra at 90\({}^{\circ}\) incidence (= normal incidence from the side, thus transmitted through the substrate without being reflected by the PVP film), and normalized to 1 at the maximum value. The depth of the deepest reflectance dip is 92.6% at 594.7 nm for s-polarization and 89.4% at 559.4 nm for p-polarization. The raw reflection spectra, before normalization, and the baseline spectra are shown in Appendix B. (**b**) Calculated reflection spectra at the incidence angle \(\theta_{0}=88.92^{\circ}\) from MgF\({}_{2}\) to PVP, which corresponds to the incidence angle \(\theta_{\text{air}}=88.5^{\circ}\) in air, with the thickness of the PVP layer assumed to be 1480 nm. The deepest reflectance dip has a 99.28% depth at 783 nm for p-polarization. film, and the surrounding medium, and (III) grazing incidence. The condition (II) is so constructed that light can enter the thin film when incident from the side of the substrate, and large reflectivity at the thin film-substrate interface as well as TIR at the thin film-surrounding medium interface are realized. The need for grazing incidence (III) comes from the approximate CPA condition of Equation (4): For transparent materials, \(q\) is nearly zero, requiring \(|r_{01}|\) nearly equals unity. Although the reflectance increases with the refractive index difference, as evident from Equations (6) and (7), grazing incidence allows \(|r_{01}|\) to be as close to unity as possible, even when the refractive index difference is small. Therefore, there are countless other substrate-thin film combinations of transparent dielectric materials that can make CPA feasible, not only with the combination of solid-solid, but also with that of solid-liquid or liquid-liquid. For example, it has already been reported in Ref. [17] that it can be realized on a glass substrate (\(n_{0}=1.52\)), with an ITO thin film (\(n_{1}\approx 2\)) and water (\(n_{2}=1.33\)) as the surrounding medium. The proposed configuration is one of the simplest and most inexpensive methods to realize CPA, as far as we know, and it is expected to have a wide range of applications due to its simplicity and extensibility. It can be immediately applied to optical notch filters. It can also be applied to interfacial sensors, including WM sensors [13; 14; 15; 16], that detect refractive index changes at interfaces with high sensitivity, and optical modulators [17] that use the interfacial Pockels effect, both of which have already been realized. As a variation of WM sensors, a mechanical strain sensor, or bending sensor, is possible using polymer-polymer combination.