Refractive-index anisotropy and optical dispersion in films of deoxyribonucleic acid

We have determined the refractive indices in the directions parallel and perpendicular to the surface plane of films of deoxyribonucleic acid (DNA) and their wavelength dispersion. These parameters are fundamental for understanding the properties of waveguiding structures containing DNA-based photonic materials. The orientation of DNA molecules in films and their optical properties are sensitive to the film fabrication and environmental conditions influencing the structure. Prism coupling measurements show ambient-humidity- related changes in the refractive index, birefringence, and anisotropy of the alignment of the DNA molecules in the films studied. These films were 0.5-5 urn thick, were prepared by both spin coating and casting from aqueous solutions containing 0.1-3 wt % DNA, and were measured in ambient air with relative humidities of 37-58%. The optical properties of the films and the orientation of the DNA molecules are discussed with respect to the mechanism for the formation of the polymer liquid-crystalline phases during film deposition. The dispersion of the refractive indices in films of native DNA has been derived from interference fringes in absorption and reflection spectra in the wavelength range of 350-2700 nm through the fitting of the positions of the fringes with the Sellmeier dispersion formula in combination with the prism coupling data.