Optical trapping of colloidal particles and cells by focused evanescent fields using conical lenses.

We demonstrate advantages in terms of trapping force distribution and laser efficiency that come from using a telescopic pair of conical lenses ('axicon') to generate a ring-like beam, that in conjunction with a high NA objective is used for direct optical trapping with a focused evanescent field near a surface. Various field geometries are considered and compared. First, a Gaussian beam and a laser beam focused on the back focal plane of the objective are compared with each other, and they are scanned across the inlet aperture of the objective. This allows to detect the point of total internal refraction, and to study the trapping power near the surface. We confirm that the hollow beam generated by the conical lenses can generate an evanescent field after a high NA objective lens, and that micron-sized particles can be trapped stably. Finally, we apply the focused evanescent field to erythrocytes under flow, showing that cells are trapped against the flow and are held horizontally against the surface. This is a different equilibrium condition compared to conventional single beam traps, and it is particularly favorable for monitoring the cell membrane. We foresee the integration of this type of trapping with the imaging techniques based on total internal refraction fluorescence (TIRF).