Ultrasensitive force spectroscopy measurement of single particle light scattering by the use of optical tweezers

Abstract

Optical tweezers (OT) rely on the radiation pressure to trap and manipulate microscopic particles and living microorganisms. Because the optical forces vary from hundreds of femto to tens of pico Newtons, OT can be used as an ultra sensitive force measurement tool to study interactions involving very small forces. We use a double tweezers to perform ultra sensitive measurement of the force due to the scattering of light as a function of its wavelength, in other words, to perform a Force Spectroscopy. Our results show not only the Mie resonances but also a selective coupling to either the TE, TM or both microsphere modes using the light polarization and the beam positioning. Mie resonances have usually been observed by scattering measurements. Very few reports of levitation experiments observed these resonances directly through the force. The double tweezers system has the advantage and flexibility of a stable restorative force measurement system. The experimental results show excellent agreement with Gaussian shaped beam partial wave decomposition theory. The understanding of the optical scattering forces in dielectric microspheres under different incident beam conditions is important as they have been used as the natural force transducer for mechanical measurements. Our results show how careful one has to be when using optical force models for this purpose. The Mie resonances can change the force values by 30-50 %. Also the results clearly show how the usually assumed azimuthal symmetry in the horizontal plane no longer holds because the beam polarization breaks this symmetry.Optical tweezers rely on the radiation pressure to trap and manipulate microscopic particles 1-2 . The recent review by David Grier 3 shows its important use in many fields of research from physics to life sciences. The optical forces involved are very small, in the range of hundreds of femto to tens of pico Newtons, thus optical tweezers have also been used as a mechanical properties measuring tool for microscopic phenomena where the forces involved are in the same scale. Typically, it has been used to measure forces, stiffness or elasticities of membranes or single DNA macromolecule 4-8 . Force measurements are usually performed with restorative systems like springs. The optical tweezers, being one of the smallest force stable trap, can be itself used to characterize optical forces involved in scattering of particles. Its ultrasensitiveness can be useful for experimental characterization of hard to measure very low force fundamental phenomena. The understanding of the optical scattering forces in dielectric microspheres under different incident beam conditions is very important as they have been used as the natural transducer for force and other mechanical measurements. Our results show how careful one has to be when using optical force models for mechanical properties measurements.The optical force measurements were usually calibrated by the hydrodynamics force in microspheres at very low Reynolds number flow or through the Brownian ther...