Short wavelengths for enhanced stiffness optical trapping of subwavelength particles

For a small particle in a Gaussian laser beam, the trapping force arises as a competition between gradient forces pulling the particle to the beam focus and scattering forces pushing the particle along the optical axis. Prior studies have shown that the balance of these competing forces causes particles which are approximately the same size as the beam waist of the trapping light to be optimally trapped. However, there is a clear scaling between trap stiffness and wavelength in 3D Optical tweezers that has not yet been fully studied. In this work, we compare optical trapping of silica nano-particles using both violet (405 nm) and infrared (1064 nm) light. We obtain a substantial improvement in the stiffness per unit of power for a subwavelength particle. We show trapping of 300nm diameter silica spheres with a trap stiffness of 11 pN/um/mW using 405nm light, which is a factor of 20 enhancement compared to what is achieved with a trapping wavelength of 1064nm.