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Figure 1: Forces at work in optical tweezers

Qualitative picture of the origin of the trapping force. [a] Lateral gradient force of a Gaussian laser beam profile. Since rays p1 and p2 have different intensity, the momentum changes of these rays (Δp1 and Δp2, respectively) differ in magnitude, causing a net reaction force on the refracting medium in the direction of highest intensity. The x–projection of this force Δpx tends to counteract a displacement from the laser beam axis, pulling the particle to the center of beam. [b] Axial gradient force towards the focus of the trapping light. The white arrows indicate the net restoring force in the respective directions. Note that the scattering component due to reflection by the particle is not indicated.