## 1 Introduction  
The concept and experiment of coherent perfect absorption (CPA), in which 100% absorption is achieved by enhancing the absorption, due to the interference of (generally multiple) coherent light beams in a Fabry-Perot (FP) resonator, was reported in 2010 [1, 2]. Since then, its realization in various systems has been reported and has made an impact [3]. The original proposal was to make two opposing coherent beams incident on a sample and to obtain complete absorption at the wavelengths that satisfy the CPA conditions. This also allows for effective nonlinear optical effects due to weak light, where a slight change in the intensity of one of the light beams changes the effective absorption coefficient. The method of making a single beam incident from one side of a sample and obtaining the counterpropagating beam from its reflection is now also regarded as single-channel CPA (SCCPA). Multichannel CPA provides better control of the CPA but requires a coherent laser light source and delicate alignment of the multiple laser beams, while SCCPA does not necessarily require a coherent light source and delicate alignment, but it needs a highly reflective structure on the sample. Historically, SCCPA has been studied as a Salisbury Screen, asymmetric FP resonator, or critically coupled resonator [4; 5; 6]. The principle is as follows. The light reflected from the sample surface and the light emitted from the sample after being incident to and reflected multiply in the sample have the same amplitude and opposite phases, resulting in destructive interference. It has attracted attention as a mechanism for obtaining absorption close to 100% at subwavelength thickness [7] due to its potential application for highly efficient photodetectors and solar cells. For SCCPA, the other side of the sample should be highly reflective in order to obtain the counter propagating light coherent with the incident light. For this purpose, metallic mirrors, or distributed Bragg reflectors (or dielectric multiple layers), have been mostly utilized [4; 5; 6] while there have not been many studies of SCCPA with total internal reflection (TIR), which readily realizes 100% reflection just by preparing an appropriate dielectric interface. For example, SCCPA with TIR was observed for a nanometer-order thick, lossy metallic film on a dielectric prism [8]. The cases of subwavelength lossy layers (\(\kappa\approx|n|\) where \(n\) and \(\kappa\) are the real and imaginary parts of the refractive index) sandwiched by dielectric layers have been extensively studied [7; 9]. Surprisingly, however, there has been no report on experimental realization of SCCPA using ordinary transparent dielectric layers with refractive indices of more than unity (\(n>1\gg\kappa\)) and using the layer interface as a reflective surface by TIR. On the other hand, although the relation to CPA has not been widely recognized so far, the detection of surface plasmon polariton (SPP) resonance, with a thin metal film deposited on the hypotenuse surface of a right-angle prism, has long been used as an interfacial sensor utilizing TIR. The principle that nearly 100% reflection dip is observed when the film thickness is adjusted to a certain level below 100 nm is nothing but critical coupling at which internal loss is equivalent to radiation coupling [10; 11; 12], i.e., SCCPA. In addition to the SPP sensor, a waveguide-mode (WM) sensor, which includes transparent dielectric layers on a prism, is also widely used [13; 14; 15; 16]. In the SPP (WM) sensor, the angle of incidence [10; 13; 14] or wavelength [11; 16] of light, which is resonant with SPP (WM), sensitively changes depending on the refractive index of the medium in contact with the metal film. In this paper, we report the observation of reflection dips close to 90% for a thin film of transparent polymer on a transparent substrate in the visible light region.