Double optical tweezers for 3D photonic force measurements of Mie scatterers

The ability to observe quantitatively mechanical events in real time of biological phenomena is an important contribution of the Optical Tweezers technique for life sciences. The measurements of any mechanical property involves force measurements, usually performed using a microsphere as the force transducer. This makes the understanding of the photonic force theory critical. Only very sensitive and precise experimental 3D photonic force measurements for any particle size will be able to discriminate between different theoretical models. In particular it is important to obtain the whole photonic force curve as a function of the beam position instead of isolate particular points. We used a dual trap in an upright standard optical microscope, one to keep the particle at the equilibrium position and the other to disturb it. With this system we have been able to obtain these force curves as a function of x, y and z position, incident beam polarization and wavelength. We investigated the optical forces for wavelengths in and out of Mie resonances of dielectric microspherical cavities for both TM and TE modes and compared the experimental results with the calculations performed with different models for the optical force.