Applications of closed-loop feedback control with holographic optical tweezers

SINCE their invention, optical tweezers have found a host of applications, primarily as micro- manipulation tools and sensitive force transducers. The development of techniques such as holographic optical tweezers has enabled the manipulation of multiple particles in three dimensions. Recent advances in computation power and display technologies have afforded the opportunity to explore applications of real-time manipulation and particle tracking, utilising closed-loop feedback control, the principle aim of this work. We begin by investigating the use of holographic optical tweezers as an autonomous micro- fabrication device, to construct three dimensional colloidal crystals. We develop a bespoke microfluidic construction environment, and demonstrate tracking and control algorithms to iden- tify, capture and manipulate microspheres into required arrangements. Our algorithm calculates the paths of particles from any desired initial to final configurations, and avoids inter-particle collisions, and the undiffracted beam. The development of multiple trap systems has also increased interest in the trapping and tracking properties of lower symmetry structures. In this light, we investigate the behaviour of multiply trapped silica micro-rods, and demonstrate real-time tracking in two dimensions. The precision of our method is estimated, and the translational and rotational stiffness coefficients are evaluated using thermal motion analysis and Stokes' drag. We measure the variation of these stiffness coefficients relative to the displacement of the traps from the ends of the micro-rods, and find good agreement with theory. We go on to use our micro-rod probes to image the surface of an oil droplet in a manner analogous to scanning probe microscopy, with closed-loop feedback control of the micro-rod's position. As. the resolution of our surface images is limited by the tracking accuracy of the micro-rods, we investigate the imaging capability of another form of high aspect-ratio probe - a cigar shaped, silica shelled form of Diatom algae. This probe is held using two optical traps centered at least lOμm from its tip, enabling highly curved and strongly scattering samples to be imaged. We demonstrate the advantages of this technique by imaging the surface of the soft alga Pseudopediastrum, while it is alive and unadhered to the surface. The resolution is currently equivalent to confocal microscopy, but as it is not diffraction limited, there is scope for significant improvement by reducing the tip diameter and limiting the thermal motion of the probe. Motivated by improving our imaging technique, we further investigate the degree of control that can be exercised over our Diatom probe. By position clamping translational and rotational modes in different ways, we are able to dramatically improve the position resolution, with no reduction in force sensitivity. We also demonstrate control over rotational-translational coupling, and exhibit a mechanism whereby the average centre of rotation of our probe can be displaced away from its centre, a feature that could potentially be used to damp existing couplings within the motion of more complex optically trapped structures. Finally, we explore three dimensional tracking of our Diatom probes using stereo-microscopy. The full five degree of freedom stiffness matrix is used to calculate forces and torques exerted at the probe tip. Together, we hope the techniques demonstrated in this thesis. represent a step towards a novel and flexible form of all optical scanning probe microscopy.