Gavin Sinclair

Interactive approach to optical tweezers control

We have developed software with an interactive user interface that can be used to generate phase holograms for use with spatial light modulators. The program utilizes different hologram design techniques, allowing the user to select an appropriate algorithm. The program can be used to generate multiple beams and can be used for beam steering. We see a major application of the program to be in optical tweezers to control the position, number, and type of optical traps.

3D manipulation of particles into crystal structures using holographic optical tweezers

We have developed holographic optical tweezers that can manipulate many particles simultaneously in three dimensions in order to create micro-crystal structures that extend over many tens of microns. The technique uses specific hologram-design algorithms to create structures that can be dynamically scaled or rotated about arbitrary axes. We believe the generation and control of pre-determined crystal-like structures have significant potential in fields as diverse as photonic-crystal construction, seeding of biological tissue growth and creation of metrological standards within nanotechnology.

Assembly of 3-dimensional structures using programmable holographic optical tweezers

The micromanipulation of objects into 3-dimensional geometries within holographic optical tweezers is carried out using modified Gerchberg-Saxton (GS) and direct binary search (DBS) algorithms to produce the hologram designs. The algorithms calculate sequences of phase holograms, which are implemented using a spatial light modulator, to reconfigure the geometries of optical traps in many planes simultaneously. The GS algorithm is able to calculate holograms quickly from the initial, intermediate and final trap positions. In contrast, the DBS algorithm is slower and therefore used to pre-calculate the holograms, which are then displayed in sequence. Assembly of objects in a variety of 3-D configurations is semi-automated, once the traps in their initial positions are loaded.

Interactive application in holographic optical tweezers of a multi-plane Gerchberg-Saxton algorithm for three-dimensional light shaping.

Phase-hologram patterns that can shape the intensity distribution of a light beam in several planes simultaneously can be calculated with an iterative Gerchberg-Saxton algorithm [T. Haist et al., Opt. Commun. 140, 299 (1997)]. We apply this algorithm in holographic optical tweezers. This allows us to simultaneously trap several objects in individually controllable arbitrary 3-dimensional positions. We demonstrate the interactive use of our approach by trapping microscopic spheres and moving them into an arbitrary 3-dimensional configuration.

Defining the trapping limits of holographical optical tweezers

Abstract We report on the limitations of using a spatial light modulator (SLM) within optical tweezers to produce both lateral and axial displacements. We find that lateral displacements of optical traps are limited by the optical efficiency of the SLM, whereas the axial displacements are limited by the abberations of the objective lens. In addition, we show the SLM can be used for correcting abberations arising from trapping deep within the sample. The maximum possible lateral and axial displacements were 50 μm and 40 μm, respectively.

Surface-enhanced resonance Raman scattering in optical tweezers using co-axial second harmonic generation

Silica particles were partially coated with silver, and a suitable chromophore, such that they could be simultaneously trapped within an optical tweezers system, and emit a surface-enhanced resonance Raman scattering (SERRS) response. A standard 1064 nm TEM00 mode laser was used to trap the bead whilst a frequency doubling crystal inserted into the beam gave several microwatts of 532 nm co-linear light to excite the SERRS emission. The con fi guration has clear applications in providing apparatus that can simultaneously manipulate a particle whilst obtaining surface sensitive sensory information.

Three-dimensional optical trapping of partially silvered silica microparticles

We demonstrate three-dimensional trapping of micrometer-diameter silica particles, partially coated with silver, within conventional optical tweezers. Although metallic particles are usually repelled from the beam focus by the scattering force, we show that transparent spheres partially coated with silver can be trapped with efficiencies comparable with dielectric particles. The trapping characteristics of these particles are examined as a function of metallic coverage, and the application of these particles to surface-enhanced resonance Raman scattering is investigated.

Semi-automated 3D assembly of multiple objects using holographic optical tweezers

The micromanipulation of objects into 2-dimensional and 3-dimensional geometries within holographic optical tweezers is carried out using a modified Gerchberg-Saxton algorithm. The modified algorithm calculates phase hologram sequences, used to reconfigure the geometries of optical traps in several planes simultaneously. The hologram sequences are calculated automatically from the initial, intermediate and final trap positions. Manipulation of multiple objects in this way is semi-automated, once the traps in their initial positions are loaded.

Three-dimensional structures in optical tweezers

We use holographic optical tweezers to trap multiple micron-sized objects and manipulate them in 3-dimensions. Trapping multiple objects allow us to create 3-dimensional structures, examples of which include; simple cubes which can be rotated or scaled, complex crystal structures like the diamond lattice or interactive 3-dimensional control of trapped particles anywhere in the sample volume.

Optical trap quality in extended 3-D structures built using holographic optical tweezers

We have recently demonstrated how holographic optical tweezers can be used to build and dynamically manipulate extended 3-D structures. Although successful trapping can be maintained even when a large number of traps are simultaneously manipulated, in general a gradual degradation of trap quality is observed as the number of traps increased. This degradation is partly attributed to the increased 3-D size of the structures. To build and control such large structures the high numerical aperture focusing objective lens has to operate away from its design conjugate for most of the traps, and therefore aberrations will be significant even for high quality objective lenses. A second effect is the decreasing efficiency of the liquid crystal spatial light modulators as they are required to display holograms that contain high spatial frequencies. However these factors do not appear to account fully for the observed weakening of the traps, and it is likely that a reduction of contrast in the trapping optical field also plays an important role. We examine the effects individual optical traps have on each other when they are in close proximity. Techniques that may be used to mitigate the reduced contrast will also be discussed.