Fig. 2.

Schematic of the optical path of dual beam-steering optical trapping microscope. Both the primary trapping beam and the secondary detection beam have two-dimensional acousto-optical deflectors (AOD) in the optical path. Two telescope lens pairs collimate, expand and image the back focal plane of the objective onto both AODs. First, a probe is trapped and the secondary beam is steered across it while the detection QPD signal is recorded to calibrate β. Then the trap beam is alternately oscillated and held stationary in a sequential measurement for both the active-passive calibration of trap stiffness and for broadband active microrheology measurements.