Photophoresis of Irradiated Spheres: Evaluation of the Complex Index of Refraction

The photophoretic force on micron-sized particles within the continuum regime has been evaluated for a range of size parameters and complex indices of refraction. A dimensionless quantity, the asymmetry factor divided by the absorption efficiency, J/Q_abs, which can be obtained both experimentally and theoretically, is shown to provide a useful measure for determining the complex refractive index. For large size parameters, J/Q_abs provides a measure of the extinction coefficient, k; it is insensitive to the real part of the refractive index, n, and depends weakly on the size parameter α. For small size parameters, J/Q_abs depends strongly on α, n, and k. n and k can be obtained by measuring J/Q_abs as a function of a.