Intracavity feedback optical trapping

Several research groups have integrated feedback control with optical trapping to improve performance, e.g., for force or position control. Among the different proposed approaches, using the feedback when trapping inside a laser cavity stands out for several reasons, namely, trapping can occur at lower optical intensities, reducing photodamage, and with low numerical aperture lenses, simplifying setup design. This is possible because the trapped particle position alters the cavity losses, triggering an intrinsic feedback on the trapped particle. Here, we analyze the behaviour of intracavity optical trapping with a single beam and with counter-propagating beams. The single-beam configuration features a well-known nonlinear feedback effect, because the beam power changes as the square of the particle displacement from trapping position. Instead, the counter-propagating-beam configuration feedback effect acts on both beams and can not be described by the same model.