Backscattering position detection for photonic force microscopy

Abstract

An optically trapped particle is an extremely sensitive probe for the measurement of pico-and femto-Newton forces between the particle and its environment in microscopic systems ͑photonic force microscopy͒. A typical setup comprises an optical trap, which holds the probe, and a position sensing system, which uses the scattering of a beam illuminating the probe. Usually the position is accurately determined by measuring the deflection of the forward-scattered light transmitted through the probe. However, geometrical constraints may prevent access to this side of the trap, forcing one to make use of the backscattered light instead. A theory is presented together with numerical results that describes the use of the backscattered light for position detection. With a Mie-Debye approach, we compute the total ͑incident plus scattered͒ field and follow its evolution as it is collected by the condenser lenses and projected onto the position detectors and the responses of position sensitive detectors and quadrant photodetectors to the displacement of the probe in the optical trap, both in forward and backward configurations. We find out that in the case of backward detection, for both types of detectors the displacement sensitivity can change sign as a function of the probe size and is null for some critical sizes. In addition, we study the influence of the numerical aperture of the detection system, polarization, and the cross talk between position measurements in orthogonal directions. We finally discuss how these features should be taken into account in experimental designs.

Macroscopic quantum resonators (MAQRO)

“…Its mean position can then be used to determine the acceleration of the spacecraft. While DECIDE is based on optically trapped nanospheres, it is also possible to optically trap significantly larger spheres [7] and to optically read out changes in their position (see, e.g., [84]). While the goal of CASE is to achieve acceleration sensitivities similar to state-of-the-art capacitive sensors (∼10 −12 ms −2 as in GOCE [56] or MICROSCOPE [81]), CASE in its currently suggested form exhibits several limitations due to the heating of the center-of-mass motion by the trapping laser and the sensitivity of the read-out mechanism.…”

Optical manipulation of nanoparticles: a review

“…The signal can be collected from either back scattered light or forward scattered light with the aid of probe laser beam, in general the trapping beam itself [95,96]. The detector measures the geometrical centre of the trapped particle, in three dimensions, over a given time scale.…”

Back-focal-plane position detection with extended linear range for photonic force microscopy

“…2(a)]. Following [4,6,23], it gives the linear range of the position detection of about several hundred nanometers. The incident beam clipping cannot be useful in this case because it reduces the axial gradient trapping force and the optical trapping becomes impossible.…”

“…According to the models of one-dimensional displacement of a spherical scatterer in the objective focal plane developed in [4][5][6]22,23] both for Rayleigh scatterers and Mie scatterers, an expansion of the detection spot extends the linear range of detection but reduces the position sensitivity.…”