Abstract: A focused laser beam can be used to trap and manipulatea particle in liquid, which has found applications instudying living cells. The characteristics of theinteraction between the particle and the laser beam cangive information of the particle. We put forward amethod for determination the spring constant of anoptical trap through external excitation and lock-indetection. The external sinusoidal excitations of thetrapped particle can be introduced by electrostaticforce through applying a sinusoidal voltage upon thetransparent conductive glass substrates composing thesample cell. A sensor using the feedback effect of alaser diode with two external cavities is also proposedfor monitoring the particle oscillation. Thisself-mixing interferometry has the capability ofabsolute measurement of the particle displacement,which is important to determine the interactionstrength of a laser-particle trapping system. !9
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