Construction of multiple-beam optical traps with nanometer-resolution position sensing

Abstract

We describe the design and construction of two different types of multiple-beam optical tweezers, each equipped with nanometer-resolution position detectors. Multiple optical traps can be created either by splitting a laser beam in two parts, based on its polarization, or time-sharing a single beam among several different locations. The advantages and disadvantages of optical tweezers based on either scheme are discussed, along with details of specific implementations. Various ways to detect microscopic movements of an optically trapped object are presented and compared, including designs that are relatively insensitive to absolute location of a trapped particle within the field of view. Two of many possible applications for such instruments are illustrated: the detection of molecular steps by kinesin motor molecules, and determinations of the stiffness of single microtubules.

Mechanical Unfolding of the Beet Western Yellow Virus −1 Frameshift Signal

“…Lasers used for the optical trapping are required to be in the wavelength region of 800 nm to 1100 nm to minimize photo-damage and water-heating (64,67). For precise positioning of the specimen, a computer-controlled, high resolution piezo-driven XYZ stage was mounted on a manually operated microscope stage.…”

“…To determine the displacement of the particle from the trap center, the change in the direction of a second laser beam focused onto the particle and scattered or deflected due to bead motion is measured by collecting this forward scattered laser light and projecting it onto a quadrant photodiode in the back focal plane (68,69). In principle, the optical tweezers can be used as a laser …”

“…Particle-imaging detectors such as a CCD camera should be much less affected by the bead-proximity issues, but have their own problems (68). One should realize that at the same time as the detection signal is modified, the optical tweezers' properties such as spring constant and harmonic nature of the potential may also be affected.…”

Design strategies for optimizing holographic optical tweezers set-ups

“…These special displays can be updated at video rates, so that with every new diffractive element a completely different optical potential is formed at the sample plane. Furthermore, holographic optical tweezers have an advantage over other methods of dynamic light array generation, such as time sharing [7], in that the modulator spatially modifies the phase of the incoming wavefronts. Wave-front control easily permits three-dimensional positioning of the traps as well as the creation of beams with special characteristics, such as Bessel or Laguerre-Gaussian beams [8] that carry angular momentum.…”

The µPIVOT: an integrated particle image velocimeter and optical tweezers instrument for microenvironment investigations

“…In general, an optical trap is modeled as a linear mechanical spring with the trap force F trap = k * Δx, where Δx is the particle displacement from the trap center (actual focal point) and k is the trap stiffness (Visscher et al 1996). This linear approximation is only valid for small particle displacements.…”