Optical trapping near resonance absorption

Expressions for radiation-induced forces are presented for the case of a Rayleigh particle near the focus of a Gaussian laser beam at near-resonant conditions. Classical electromagnetic theory was used to obtain the dependence of the scattering and gradient forces on the incident laser frequency, the beam convergence angle, and the spatial position of the particle with respect to the focus. Approximative numerical analysis performed for particles with a single resonant absorption peak demonstrates the occurrence of up to 50-fold enhanced trapping forces at near-resonant frequencies. The use of this technique of gradient force enhancement may provide optical tweezers with enhanced trapping strengths and a degree of specificity.