Radiation transfer in layers of densely packed aggregates of hydroxyapatite nanoparticles was studied for a spectral range from 300 to 1100 nm using diffuse reflectance measurements and the modeling of the light transfer properties of the layers. The studied samples of dispersed biogenic hydroxyapatite were obtained from animal bone material (bovine bones) using fast pyrolysis followed by grinding and pressing into tablets. A distinctive feature is the high reflectivity (high albedo) of the obtained samples, which is practically independent of the wavelength in the studied spectral range and comparable to the reflectivity of the diffuse reflectance standard based on Spectralon. The modeling of the light transfer properties of the studied samples within the framework of the effective medium theory (using coherent potential approximation) made it possible to establish the weak dependence of the mean scattering-free path and the mean transport-free path of light propagation in the medium on the wavelength, which is consistent with the features observed in the experiment. Possible prospects for the use of nanostructured hydroxyapatite as photonic material are discussed.

The optical properties of a composite material consisting of close-packed (with a volume fraction of about 0.4) polydisperse bio-hydroxyapatite particles in a sodium silicate matrix are examined in the near UV, visible and near IR ranges (between 0.3 μm and 1.0 μm) based on measured diffuse reflectance spectra and theoretical data. The theoretical analysis of wavelength-dependent light transfer in the composite was performed using Monte Carlo simulations and the coated coherent potential approximation as a modification of the frequency-dependent effective medium theory. The recovery of the effective values of the scattering anisotropy parameter for the studied system at various wavelengths based on a comparison of the measured and calculated diffuse reflectance of composite layers of different thicknesses made it possible to assume a decrease in this parameter from 0.9 to 0.4 under the transition from the near UV to the near IR interval. The negative dispersion of the wavelength dependences of the diffuse reflectance observed in the experiments is considered in relation to the negative dispersions of the scattering mean free path and scattering anisotropy expected for the dependent scattering regime. Possible applications of hydroxyapatite-based composites as photonic media with a disordered close-packed structure are discusse