FIGURE 2.

Schematic illustration of the interaction mechanism between the THz evanescent field and sample and M-chip. (A) THz evanescent field (in orange color) occurs at the prism-substance interface, penetrates the cavity, and enters the upper wall of the M-chip. The angle of ATR prism slant side is 38.4°. The substance in the cavity of the M-chip can be either air (i.e., reference) or aqueous sample. A prism-substance interface and a substance-PDMS upper-wall interface are formed. As the side wall of the M-chip does not influence the analysis in our case, it is not shown for simplicity. (B) ${\stackrel{\sim }{r}}_{12}$ and ${\stackrel{\sim }{r}}_{23}$ are the Fresnel’s reflection coefficients of prism-substance interface and the substance-PDMS upper-wall interface, respectively. ${\stackrel{\sim }{\epsilon }}_{\mathrm{w}}$ , $\stackrel{\sim }{\epsilon }$ , ${\epsilon }_{\mathrm{S}\mathrm{i}}$ denote the complex dielectric constants of the PDMS, substance in the cavity, and silicon prism, respectively. ${\stackrel{\sim }{E}}_{\mathrm{i}\mathrm{n}}$ and ${\stackrel{\sim }{E}}_{\mathrm{o}\mathrm{u}\mathrm{t}}$ are the incident and emergent beams, respectively, and $\stackrel{\sim }{r}$ is the ultimate Fresnel’s reflection coefficient of the two-interface system. $\theta$ is the incident angle of the THz beam in the prism.