Measurement of the optical stiffness of photophoretic force tweezers in air

We report on a method of measuring the stiffness in photophoretic force tweezers in air by using an experimental configuration with two counter-propagating hollow beams. By setting the optical traps of both beams in the same focal plane of the camera, we are able to record the flight procedure of a trapped particle into the other trap after the initial trapping beam is switched off. Then, the stiffness of optical trap can be obtained by fitting the relationship of particle position vs time during the flight procedure. It is found that the measurement is tolerant to the distance between the traps and laser power variation. In addition, the measured stiffness is demonstrated applicable in determining the size of aerosol particles. The reliable method provided in this work is not only useful in studying the physical behavior of signal particle under atmosphere condition but also meaningful in the applications of aerosol studies, such as in situ aerosol characterization and the local surrounding environment sensing.