In photonic force microscopes, the position detection with high temporal and spatial resolution is usually implemented by a quadrant position detector placed in the back focal plane of a condenser. An objective with high numerical aperture (NA) for the optical trap has also been used to focus a detection beam. In that case the displacement of the probe at a fixed position of the detector produces a unique and linear response only in a restricted region of the probe displacement, usually several hundred nanometers. There are specific experiments where the absolute position of the probe is a relevant measure together with the probe position relative the optical trap focus. In our scheme we introduce the detection beam into the condenser with low NA through a pinhole with tunable size. This combination permits us to create a wide detection spot and to achieve the linear range of several micrometers by the probe position detection without reducing the trapping force.
© 2012 Optical Society of America
Original Manuscript: March 26, 2012
Revised Manuscript: June 14, 2012
Manuscript Accepted: July 17, 2012
Published: August 22, 2012
Vol. 7, Iss. 11 Virtual Journal for Biomedical Optics
Ignacio A. Martínez and Dmitri Petrov, "Back-focal-plane position detection with extended linear range for photonic force microscopy," Appl. Opt. 51, 5973-5977 (2012)