In this paper, we begin with a brief overview of optical trapping of micro- and nano- particles and of various techniques for the measurement of optical force constants in the linear spring model. We then move on to introduce two complimentary approaches to implement optical forced oscillation of the trapped particle, one by an oscillatory optical tweezers, and the other by chopping (i.e., switching on-and-off) one of the beams in a twin set of optical tweezers. In each implementation, we have measured the steady state amplitude and phase of the oscillating particle as a function of frequency (from ~ 10Hz to 600Hz) with the aid of a quadrant photo-diode in conjunction with a lock-in amplifier. For the case of optical forced oscillation of a "free" particle involving only the optical force and the viscous drag, the experimental data fit fairly well the theoretical curve obtained from the simple linear spring model; both the optical force constant and the viscosity of the surrounding fluid can be deduced with fairly high precision as the fitting parameters from the best fit of the experimental data to the theoretical curves.
When one or more external forces, in addition to the optical force and the drag force, were applied to the oscillating particle via mechanisms such as protein-protein interaction or DNA stretching, the oscillating amplitude and phase varied in response to the external forces. Preliminary data showing the change in oscillating amplitude and phase as a function of time in response to external forces will be presented, and potential biomedical applications of this approach will be discussed.