Measuring the absorption coefficient of biological materials using integrating cavity ring-down spectroscopy

Abstract

A number of imaging modalities rely on the exact knowledge of both the absorption and scattering properties of cells and organelles. We report a simple method for accurate and precise measurement of the optical absorption coefficient of biological samples, even in the presence of strong scattering. The technique is based on cavity ring-down spectroscopy, but the traditional mirrored cavity is replaced with a high-reflectivity integrating cavity. The Lambertian behavior of the cavity walls creates an isotropic field inside the cavity, thereby eliminating the effects of scattering in the sample. Thus, integrating cavity ring-down spectroscopy (ICRDS) provides a true, direct measurement of the absorption coefficient, as opposed to the net attenuation. We demonstrate the effectiveness of this technique by measuring the absorption coefficient of retinal pigmented epithelium cells. Furthermore, we demonstrate that ICRDS is insensitive to scattering effects using suspensions of copolymer microspheres and an absorbing dye solution. These results are compared with measurements made using a more traditional transmission-style setup. This technique will have an impact on the field of nanoscience, where optical characterization of nanoparticles is still done using a conventional spectrometer that is only capable of providing measurements of the extinction coefficient.