The application of optical traps has come to the fore in the last three decades. They provide a powerful, sterile and noninvasive tool for the manipulation of cells, single biological macromolecules, colloidal microparticles and nanoparticles. An optically trapped microsphere may act as a force transducer that is used to measure forces in the piconewton regime. By setting up a well-calibrated single-beam optical trap within a fluorescence microscope system, one can measure forces and collect fluorescence signals upon biological systems simultaneously. In this protocol, we aim to provide a clear exposition of the methodology of assembling and operating a single-beam gradient force trap (optical tweezers) on an inverted fluorescence microscope. A step-by-step guide is given for alignment and operation, with discussion of common pitfalls.

Using a dual-beam fiber optic trap, we have experimentally observed bistability and hysteresis in the equilibrium separations of a pair of optically bound dielectric spheres in one dimension. These observations are in agreement with our coupled system model in which the dielectric spheres modify the field propagation, and the field self-consistently determines the optical forces on the spheres. Our results reveal hitherto unsuspected complexity in the coupled light-sphere system.

The accurate determination and control of the wavelength of light is fundamental to many fields of science. Speckle patterns resulting from the interference of multiple reflections in disordered media are well-known to scramble the information content of light by complex but linear processes. However, these patterns are, in fact, exceptionally rich in information about the illuminating source. We use a fibre-coupled integrating sphere to generate wavelength-dependent speckle patterns, in combination with algorithms based on the transmission matrix method and principal component analysis, to realize a broadband and sensitive wavemeter. We demonstrate sub-femtometre wavelength resolution at a centre wavelength of 780 nm, and a broad calibrated measurement range from 488 to 1,064 nm. This compares favourably to the performance of conventional wavemeters. Using this speckle wavemeter as part of a feedback loop, we stabilize a 780 nm diode laser to achieve a linewidth better than 1 MHz.

At the microscopic level, light-matter interactions can organize colloidal matter via a process known as optical binding. Optical binding refers to the creation of arrays of microparticles formed in the presence of laser fields, the inter-particle spacing being determined by the refocusing and/or scattering of the laser fields by the microparticles. In this paper we investigate one-dimensional optically bound arrays of microparticles using a femtosecond dual-beam optical fiber trap, and develop a means to visualize the field intensity distributions responsible for the optical binding using two-photon fluoresence imaging from fluorescein added to the host medium. The experimental intensity distributions are shown to be in good agreement with numerical simulations, thereby validating our new approach to visualizing the fields responsible for optical binding, and the physical model of optical binding as due to refocusing of the fields by the microparticles.

Optical binding along the axis of two counterpropagating laser fields may be used to organize microparticles into longitudinal, spatially separated, arrays. Here we investigate correlations between the displacements of two optically bound microparticles from their equilibrium positions due to noise. Measurement of the decay time of the correlation functions of the center of mass and relative normal modes is shown to provide an in situ method to determine the optical restoring forces acting on the bound particles, thereby providing a test of our physical understanding of longitudinal optical binding.