Experimental and theoretical study of the transient rotation of isotropic transparent microparticles in astigmatic optical tweezers

Micrometer-sized rod-like non-birefringent transparent particles trapped by an astigmatic laser beam having el- liptical intensity profile spontaneously rotate until they become aligned with the major axis of the beam cross-section. The observed rotation confirms a simple picture based on orbital angular momentum transfer from the light beam to the trapped particle. A model which regards the particle as a thin cylindrical lens with effective focal lengths f1 and f2 has allowed to calculate the applied optical torque and, hence, the time law governing the particle alignment. The time evolution of the particle alignment is studied for different rod-like particles by analysing the sequence of frames re- corded by a CCD camera. The time evolution of the angle a formed between the main axes of the body and the laser shape results in good agreement with the theory for bodies fully immersed in the spot size of the laser. Discrepancies observed for rod-like particles longer than the minor elliptical axis of the beam cross-section, were removed by de- veloping a more general model taking into account the partial overlapping between the body and the laser profile.