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We use patterned 3D multi-spot illumination to perform neuronal multi-site stimulation in rat brain tissue. Using a spatial light modulator, we holograpically project 3D light fields for multi-site two-photon photolysis of caged neurotransmitters to generate synaptic inputs to a neuron. Controlled photostimulation of multiple synapses from various locations in the dendritic tree provides a way to analyze how neurons integrate multiple inputs. Our holographic projection setup is incorporated into a two-photon 3D imaging microscope for visualization and for accurate positioning of specific uncaging sites along the neuron's dendritic tree. We show two-photon images and the neuron's response to holographic photostimulation of synapses along dendrites.

We demonstrate a multi-functional system capable of multiple-site two-photon excitation of photo-sensitive compounds as well as transfer of optical mechanical properties on an array of mesoscopic particles. We use holographic projection of a single Ti:Sapphire laser operating in femtosecond pulse mode to show that the projected three-dimensional light patterns have sufficient spatiotemporal photon density for multi-site two-photon excitation of biological fluorescent markers and caged neurotransmitters. Using the same laser operating in continuous-wave mode, we can use the same light patterns for non-invasive transfer of both linear and orbital angular momentum on a variety of mesoscopic particles. The system also incorporates high-speed scanning using acousto-optic modulators to rapidly render 3D images of neuron samples via two-photon microscopy.

We use spatially sculptured light for user-interactive micromanipulation of mixed yeast cell populations to analyze growth behavioural patterns. There is negligible absorption in the near-infrared region of the light spectrum making it suitable for direct manipulation of individual cells in a growing population. Rather than using a single-beam optical trap, multiple cells are manipulated using a system based on the Generalized Phase Contrast (GPC) method, which allows arbitrary trapping configurations i.e. control over the number of traps, and the size/shape of each trap. This enables the cells to be selectively trapped in all three-dimensions (3D) and manipulated in real-time while under direct observation. Here, we impose controlled experiments using these multiple 3D optical traps to show the alteration of growth patterns in mixed cultures of Saccharomyces cerevisiae and Hanseniaspora uvarum experiencing spatially constrained conditions.

We are engaged in applying the Generalized Phase Contrast (GPC) method that enables lossless light conversion of spatial phase patterns to highly photon efficient light distributions. The GPC-method can be used in a number of applications requiring parallel light-beam encoding such as in advanced user-controlled optical micro-manipulation, wavefront sensing and generation for common-path interferometry and adaptive optics, optical phase-only encryption and integrated micro-optical implementations. In this work, we will outline the concept for a complete GPC-platform for advanced and user-interactive manipulation of fluid-borne colloidal structures with state-of-the-art controllability and versatility in 3D space and time, hence true 4D. Real-time reconfigurable light patterns with sub-micron accuracy are obtained from a direct map of phase patterns addressed on programmable phase-only spatial light modulator devices. A graphical user interface enables real-time, interactive and arbitrary control over the dynamics and geometry of synthesized light patterns. Experimental demonstrations have shown that GPC-driven micro-manipulation can be used for guided assembly of particles in a plane, control of particle stacking along the beam axis, manipulation of multiple hollow beads and real-time sorting of inhomogeneous mixtures of micro-particles. These experiments illustrate that GPC-driven micro-manipulation can be utilized not only for the improved synthesis of functional microstructures but also for their non-contact and parallel actuation crucial for sophisticated micro-fluidic based lab-on-a-chip demonstrations in the future.

Optical elevators are constructed from counter-propagating coaxial beams with orthogonal linear polarizations, the strengths of which are controlled by a spatial polarization modulator. Each of these optical potential wells draws a dielectric particle towards the optical axis where it finds a stable transverse position. Adjustment of the relative strengths of the counteracting scattering forces induced by the opposing beams enables control over the axial trap position of the particle within the confines of the optical elevator. We have devised a power-preserving scheme using a spatial polarization modulator to vary the intensity ratio of the two polarization states. The proposed method has the potential of extending the dynamic range in axial positioning of optically trapped particles surpassing the range offered by other techniques.

We investigate the potentials of using a far-field technique for tunable coupling to high-order guided modes of photonic crystal fibers. The field distribution matched for coupling to the fiber is generated by the optical Fourier transform of grating-based phase patterns dynamically encoded on a spatial light modulator. Tuning the parameters of phase-only binary diffractive patterns can modulate both amplitude and phase of the coupling field. Experiments demonstrate tunable and spatially controllable coupling to the second-order mode of a commercially available index-guided micro-structured fiber with a triangular lattice air-hole structure.

We report a new method for generating multiple optical tweezers that enables beam-steering functionality and polarization state control for each trapping beam. The method employs a spatial light modulator (SLM) that imparts a two-dimensional (2-D) phase distribution onto a linearly polarized input beam from a CW-laser (λ = 830nm). A microlens array is used to divide the beam resulting in a matrix of point sources that are subsequently imaged onto the sample plane of a microscope as trapping beams. For beam steering, the plane of polarization of the input field is aligned parallel to the slow axis of the SLM, on which an array of phase grating patterns is encoded. The blazed grating parameters dictate the positional deflections of the optical traps. When the input polarization plane makes a 45°-angle with the SLM slow axis, an elliptic polarization state is produced for each optical trap and the degree of ellipticity depends on the phase difference introduced between the extraordinary and ordinary output field components at each SLM sub-aperture. It is known that elliptically polarized light changes its angular momentum when focused to a birefringent particle thereby inducing an axial rotation of the trapped object. We experimentally demonstrate the two modes of operation: (1) controlled lateral deflection of simultaneous trapped polystyrene microspheres and (2) polarization-induced rotation of simultaneously trapped calcite crystals.

We demonstrate the use of independently controlled doughnut beams for trapping and manipulation of an array of low-index microstructures. These types of particles are generally difficult to handle using conventional optical traps due to the inverse effect of the gradient force, which repels the particles from the region of stronger light intensity. The configurable multiple doughnut beams are generated using a nearly loss-less phase-to-intensity conversion of a phase-encoded collimated coherent light source. The two-dimensional phase distribution corresponding to the optical trap pattern is encoded using a computer-programmable spatial light modulator enabling each trap to be moved arbitrarily along the transverse direction. Experiments show trapping and manipulation of hollow “air-filled” glass micro-spheres suspended in aqueous medium.

We analyze the performance of implementing the generalized phase contrast method in a planar-integrated micro-optics system. A 4-f lens system with a phase-only filter at the Fourier plane is fabricated in a 50 mm-diameter fused silica substrate using multi-level multimask lithography. A phase distribution at the input plane is converted into a high-contrast intensity distribution at the output plane. The capability of the system for visualization of phase is evaluated for its application in a compact optical decryption system.

We demonstrate the flexibility of a multiple-beam trapping system that enables interactive manipulation of fluid-borne colloidal structures with advanced controllability and versatility that can lead to light-powered microdevices performing multiple functions in a "lab-on-a-chip". A straightforward phase-imaging operation forms the basis for the efficient generation of arbitrarily shaped trapping beam configurations. The multiple-beam trapping pattern is a direct map of the phase variation encoded on a programmable phase-only spatial light modulator (SLM). A graphical user interface that encodes desired phase patterns onto the SLM enables interactive and independent control over the dynamics and geometry of each trapping beam. Experimental results show that the system can be used for guided assembly of particles in a plane, control of particle stacking along a beam axis, and real-time sorting of inhomogeneous mixtures of microspheres. These experiments illustrate that multiple beams generated by the system can be utilized not only for the improved synthesis of functional microstructures but also for their non-contact and parallel actuation crucial for sophisticated microfluidic-based lab-on-a-chip demonstrations in the future.

We propose the use of a multiple-beam optical tweezer (MOT) system for the formation and active manipulation of an assembly of micro- and nano-scale particles. This method enables the study of arrays of particles for practical applications in all-optical control of an array of pumps, switches and valves for micro-fluidic processes, bio-chemical sensing or lab-on-a-chip devices and guided assembly of colloidal structures to form photonic crystals. A fully dynamic MOT system is constructed using the generalized phase contrast (GPC) method and a phase-only spatial light modulator (SLM). The GPC method enables loss-less light projection and direct transformation of an input phase pattern to an array of high-intensity beams. This straightforward process enables an adjustable number of traps and instantaneous control of the position, size, shape and intensity of each trapping-beam by encoding the appropriate phase pattern on the SLM. We present experimental results showing trapping and dynamic manipulation of multiple micro-spheres in a liquid solution.