The effect of absorption of the sample medium on the surface-plasmon resonance (SPR) characteristics is analyzed by approximate analytical and exact numerical models. We show that absorption leads to specific changes in the value of reflectivity near the SPR angle and that these can be used for absorbance detection. The strongest absorption-induced change in reflectivity occurs at two values of metal film thickness (28 and 55 nm for a gold film and λ = 632.8 nm). Using a sample solution of Rhodamine 700 in ethanol, we measured the characteristic changes in the SPR angle and in reflectivity over the wavelength interval encompassing the strong absorption band at 610–680 nm. The possibility of the simultaneous determination of the refractive index and absorption from SPR measurements is demonstrated and has the potential for substance-specific detection.
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