Peter John Rodrigo

Four-dimensional optical manipulation of colloidal particles

This work has been funded by the European Science Foundation through the Eurocores-SONS program and partially by an internal grant awarded by Riso National Laboratory.

Real-time three-dimensional optical micromanipulation of multiple particles and living cells

Counterpropagating light fields provide a stationary optical potential well for a Brownian particle. Introducing variability in the relative strengths of the counterpropagating beams allows us to create a more general configuration-the optical elevator. An optical elevator dynamically controls the axial location of the potential minimum where the particle finds a stable equilibrium position. We describe the implementation of multiple real-time reconfigurable optical elevators with the generalized phase contrast method for dynamic manipulation of polystyrene spheres and yeast cells S. cerevisiae in three dimensions.

Optical microassembly platform for constructing reconfigurable microenvironments for biomedical studies

Cellular development is highly influenced by the surrounding microenvironment. We propose user-reconfigurable microenvironments and bio-compatible scaffolds as an approach for understanding cellular development processes. We demonstrate a model platform for constructing versatile microenvironments by fabricating morphologically complex microstructures by two-photon polymerization (2PP) and then assembling these archetypal building blocks into various configurations using multiple, real-time configurable counterpropagating-beam (CB) traps. The demonstrated capacity for handling feature-rich microcomponents may be further developed into a generalized microassembly platform.

Actuation of microfabricated tools using multiple GPC-based counterpropagating-beam traps.

We explore the functionalities of a generalized phase contrast (GPC) -based multiple-beam trapping system for the actuation of various microfabricated SiO2 structures in liquid suspension. The arrays of optical traps are formed using two counterpropagating light fields, each of which is spatially reconfigurable in both cross-sectional geometry and intensity distribution, either in a user-interactive manner or under computer supervision. Design of microtools includes multiple appendages with rounded endings by which optical traps hold and three-dimensionally actuate individual tools. Proof-of-principle demonstrations show the collective and user-coordinated utility of multiple beams for driving microstructured objects. The potential to integrate these optically powered microtools may lead to more complex miniaturized machineries - a closely achievable goal with the real-time reconfigurable optical traps employed in this work.

Interactive light-driven and parallel manipulation of inhomogeneous particles

A light-driven micromanipulation system with real-time userfeedback control is used to simultaneously trap colloidal suspensions enabling a unique interactive sorting capability and arbitrary patterning of microscopic particles. The technique is based on a straightforward phase-tointensity conversion generating multiple beam patterns for manipulation of particles in the observation plane of a microscope. Encoding of phase patterns in a spatial light modulator, which is directly controlled by a computer, allows for dynamic reconfiguration of the trapping patterns, where independent control of the position, size, shape and intensity of each beam is possible. Efficient sorting of microsphere mixtures of distinct sizes and colors using multiple optical traps is demonstrated.

Real-time interactive 3D manipulation of particles viewed in two orthogonal observation planes.

The generalized phase contrast (GPC) method has been applied to transform a single TEM00 beam into a manifold of counterpropagating-beam traps capable of real-time interactive manipulation of multiple microparticles in three dimensions (3D). This paper reports on the use of low numerical aperture (NA), non-immersion, objective lenses in an implementation of the GPC-based 3D trapping system. Contrary to high-NA based optical tweezers, the GPC trapping system demonstrated here operates with long working distance (>10 mm), and offers a wider manipulation region and a larger field of view for imaging through each of the two opposing objective lenses. As a consequence of the large working distance, simultaneous monitoring of the trapped particles in a second orthogonal observation plane is demonstrated.

Dynamic array of dark optical traps

A dynamic array of dark optical traps is generated for simultaneous trapping and arbitrary manipulation of multiple low-index microstructures. The dynamic intensity patterns forming the dark optical trap arrays are generated using a nearly loss-less phase-to-intensity conversion of a phase-encoded coherent light source. Two-dimensional input phase distributions corresponding to the trapping patterns are encoded using a computer-programmable spatial light modulator, enabling each trap to be shaped and moved arbitrarily within the plane of observation. We demonstrate the generation of multiple dark optical traps for simultaneous manipulation of hollow “air-filled” glass microspheres suspended in an aqueous medium.

Helico-conical optical beams: a product of helical and conical phase fronts

Helico-conical optical beams, different from higher-order Bessel beams, are generated with a parallel-aligned nematic liquid crystal spatial light modulator (SLM) by multiplying helical and conical phase functions leading to a nonseparable radial and azimuthal phase dependence. The intensity distributions of the focused beams are explored in two- and threedimensions. In contrast to the ring shape formed by a focused optical vortex, a helico-conical beam produces a spiral intensity distribution at the focal plane. Simple scaling relationships are found between observed spiral geometry and initial phase distributions. Observations near the focal plane further reveal a cork-screw intensity distribution around the propagation axis. These light distributions, and variations upon them, may find use for optical trapping and manipulation of mesoscopic particles.

2D optical manipulation and assembly of shape-complementary planar microstructures

Optical trapping and manipulation offer great flexibility as a non-contact microassembly tool. Its application to the assembly of microscale building blocks may open new doors for micromachine technology. In this work, we demonstrate all-optical assembly of microscopic puzzle pieces in a fluidic environment using programmable arrays of trapping beams. Identical shape-complimentary pieces are optically fabricated with submicron resolution using two-photon polymerization (2PP) technique. These are efficiently assembled into space-filling tessellations by a multiple-beam optical micromanipulation system. The flexibility of the system allows us to demonstrate both user-interactive and computer-automated modes of serial and parallel assembly of microscale objects with high spatial and angular positioning precision.

Real-time interactive optical micromanipulation of a mixture of high- and low-index particles

We demonstrate real-time interactive optical micromanipulation of a colloidal mixture consisting of particles with both lower (n(L) < n(0)) and higher (n(H)> n(0)) refractive indices than that of the suspending medium (n(0)). Spherical high- and low-index particles are trapped in the transverse plane by an array of confining optical potentials created by trapping beams with top-hat and annular cross-sectional intensity profiles, respectively. The applied method offers extensive reconfigurability in the spatial distribution and individual geometry of the optical traps. We experimentallydemonstrate this unique feature by simultaneously trapping and independently manipulating various sizes of spherical soda lime micro -shells (n(L) >> 1.2) and polystyrene micro-beads (n(H) = 1.57) suspended in water (n(0) = 1.33).