title: Back-focal-plane position detection with extended linear range for photonic force microscopy.
abstract: 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.
authors.affiliation: ICFO—The Institute of Photonic Sciences Av. Carl Friedrich Gauss num 3, 08860 Castelldefels (Barcelona), Spain.
authors.forename: Ignacio A
authors.initials: IA
authors.lastname: Martínez
authors.forename: Dmitri
authors.initials: D
authors.lastname: Petrov
mesh_terms: D004867:Equipment Design; D018625:Microscopy, Atomic Force / Q000295:instrumentation* / Q000379:methods; D008863:Microspheres; D052898:Optical Tweezers; D010316:Particle Size