Enhancement of axial optical trapping force using a pair of axicons

Usually, the potential along the axial direction of a widely implemented single-beam gradient trap is weaker than in the lateral direction, therefore great effort should be taken to enhance the axial trapping force. This is especially the case when one wants to trap and manipulate a particle within a great depth, where spherical aberration becomes large due to mismatch of refraction index and thus degrade the beam focusing. Various theoretical studies and experiments have demonstrated that the trapping forces exerted on microparticles depend on the properties of both particle and laser beam, which including particle size, shape, relative index with respective to the surrounding, optical wavelength, polarization, beam convergence, spot size, beam profile.