Abstract
We demonstrate a fast and direct calibration method for systems using a single laser for optical tweezers and particle position detection. The method takes direct advantage of back-focal-plane interferometry measuring not an absolute but a differential position, i.e. the position of the trapped particle relative to the center of the optical tweezers. Therefore, a fast step-wise motion of the optical tweezers yields the impulse response of the trapped particle. Calibration parameters such as the detector's spatial and temporal response and the spring constant of the optical tweezers then follow readily from fitting the measured impulse response.
Highlights
Optical tweezers are ideally suited for manipulation of micron and submicron-sized particles
We demonstrate a fast and direct calibration method for systems using a single laser for optical tweezers and particle position detection
A fast step-wise motion of the optical tweezers yields the impulse response of the trapped particle
Summary
Optical tweezers are ideally suited for manipulation of micron and submicron-sized particles. There may be considerable variability in the axial position of the particle with respect to the tweezers center which affects the lateral calibration coefficients and which can only be minimized by sufficient averaging This situation was remedied by Vermeulen et al [8] who acquired the detector response by very rapidly scanning the trapping laser beam over the particle (so fast as not to allow any appreciable particle motion). This method enables online calibration of each trapped particle used in an experiment, but does not simultaneously yield the tweezers spring constant. A power spectrum density (PSD) analysis of the particle position in the optical tweezers circumvents such practical limitations and is capable of providing the optical tweezers spring constant as well as the linear detector response [7]
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