Influence of surface-mode-enhanced local fields on photophoresis

The radiometric force (photothermal) on a sphere is investigated theoretically in the surface-mode region ($-1>\epsilon >-2\mathrm{}$). Closed-form expressions obtained for this force in the transition regime ($K_n\sim 1$) reveal a quasi-double-resonant character, with contributions from both dipole and quadrupole surface modes. Full Mie calculations verify these asymptotic expressions and extend the range of the calculations to an optical size (the ratio of the circumference to the wavelength) near 1. Applications of the theory to surface plasmons in Ag and surface phonons in SiC show that excitation of the quadrupole mode causes a reversal in the radiometric force with a factor-of-${10}^4$ increase in magnitude. The overall force (radiometric plus radiation pressure) is evaluated, and photophoretic spectra are presented.