Measurement of mass by optical forced oscillation of absorbing particles trapped in air

Jinda Lin, Jianliao Deng, Rong Wei, Yong-qing Li, and Yuzhu Wang

We demonstrate the application of optical forced oscillation to measure the mass of an absorbing microparticle trapped in air. When the light intensity is modulated sinusoidally, the particle in the trap undergoes forced oscillation, and the amplitude of the oscillation depends directly on the modulation frequency. We obtain the stiffness of the optical trap and the mass of the trapped particle by fitting the experimental data of the amplitudes versus the modulation frequencies with a simple spring model. The fitting results show that, for a certain particle, the stiffness varies linearly with the trapping light intensity while the mass is constant. The density of the microparticle is also estimated, which has potential to classify different kinds of absorbing particles, such as C and CuO.

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