This paper demonstrates the use of micron sized beads, modified with fluorescent dyes, as non-invasive sensors to probe the local changes in pH, within a microfluidic channel. To achieve this, amine modified polystyrene spheres (either 3 microm or 6 microm in diameter) were functionalised with the pH sensitive fluorochrome SNARF-1 to produce point sensors. The modified beads were trapped at defined positions close to a pair of integrated planar gold microelectrodes within the channel, using optical tweezers. Both transient and steady-state electrochemical potentials were applied to the microelectrode pair in order to generate changes in the local pH, associated with electrolysis. The functionalised beads indicated the pH changes in the channel, measured as a change in the fluorescence signal, generated by the immobilised pH sensitive dye. Responses were measured with temporal resolutions of between 1 and 200 ms, whilst the spatial resolution of the pH gradients was limited by the size of the beads to 3 microm.

SNARF-1 fluorochrome was used to functionalize 3μm diameter latex spheres making them sensitive to the pH of their environment, manifested as a change in their fluorescence. The fluorescence emission at 580nm was excited using a filtered xenon arc lamp at 515nm. A solution of functionalized latex spheres was placed between gold microelectrodes in a microfluidic channel. Optical tweezers were used to trap and manipulate the spheres in the vicinity of the microelectrodes, to map out the pH profile in the electrolyte solution, induced by passing 20 microsecond transient current pulses through the microelectrodes.

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

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.

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.

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.

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.

Surface-enhanced resonance Raman scattering (SERRS) is considerably more efficient than Raman spectroscopy, thereby enabling the detection of single molecules. Strong and short-lived SERRS emissions (<100 ms) from single, partially silver-coated, dye-adsorbed microparticles have been detected within optical tweezers (see picture). Full spectra, recorded from single particles, clearly show that the emission is due to SERRS.

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.

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.

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.

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.

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.