The phenomenon of light's momentum was first observed in the laboratory at the beginning of the twentieth century, and its potential for manipulating microscopic particles was demonstrated by Ashkin some 70 years later. Since that initial demonstration, and the seminal 1986 paper where a single-beam gradient-force trap was realized, optical trapping has been exploited as both a rich example of physical phenomena and a powerful tool for sensitive measurement. This review outlines the underlying theory of optical traps, and explores many of the physical observations that have been made in such systems. These phenomena include 'optical binding', where trapped objects interact with one another through the trapping light field. We also discuss a number of the applications of 'optical tweezers' across the physical and life sciences, as well as covering some of the issues involved in constructing and using such a tool.

https://iopscience.iop.org/article/10.1088/0034-4885/76/2/026401/pdf

Optical trapping and binding

This work primarily aims to fabricate and use two photon polymerization (2PP) microstructures capable of being optically manipulated into any arbitrary orientation. We have integrated optical waveguides into the structures and therefore have freestanding waveguides, which can be positioned anywhere in the sample at any orientation using optical traps. One of the key aspects to the work is the change in direction of the incident plane wave, and the marked increase in the numerical aperture demonstrated. Hence, the optically steered waveguide can tap from a relatively broader beam and then generate a more tightly confined light at its tip. The paper contains both simulation, related to the propagation of light through the waveguide, and experimental demonstrations using our BioPhotonics Workstation. In a broader context, this work shows that optically trapped microfabricated structures can potentially help bridge the diffraction barrier. This structure-mediated paradigm may be carried forward to open new possibilities for exploiting beams from far-field optics down to the subwavelength domain.

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Wave-guided optical waveguides.

Leveraging developments in microfabrication open new possibilities for optical manipulation. With the structural design freedom from three-dimensional printing capabilities of two-photon polymerization, we are starting to see the emergence of cleverly shaped ‘light robots’ or optically actuated micro-tools that closely resemble their macroscopic counterparts in function and sometimes even in form. In this work, we have fabricated a new type of light robot that is capable of loading and unloading cargo using photothermally induced convection currents within the body of the tool. We have demonstrated this using silica and polystyrene beads as cargo. The flow speeds of the cargo during loading and unloading are significantly larger than when using optical forces alone. This new type of light robot presents a mode of material transport that may have a significant impact on targeted drug delivery and nanofluidics injection. Light-driven microtools that pick up and drop off cargo through light-induced convection currents have been made by a team in Denmark. The tools, which resemble racing cars in shape, are fabricated, positioned and used to manipulate cargo by light. Jesper Gluckstad at the Technical University of Denmark and colleagues used two-photon polymerization to produce the ‘chassis’ and ‘wheels’ of the tools and then deposited metal layers in their interiors using electron-beam physical vapour deposition. The car is maneuvered by illuminating its wheels with four trapping beams. When the metal layers are illuminated by light, they heat up and generate convection currents that can be used to suck up and spew out cargo—silica and polystyrene beads in this case. The researchers anticipate that such tools could be used for drug delivery.

/pdf/light-driven-micro-tool-equipped-with-a-syringe-function-2eidkx6srd.pdf

Light-driven micro-tool equipped with a syringe function

Optics is usually integrated into robotics as part of intelligent vision systems. At the microscale, however, optical forces can cause significant acceleration and so optical trapping and optical manipulation can enable the noncontact actuation of microcomponents. Microbeads are ubiquitous optically actuated structures, from Ashkin's pioneering experiments with polystyrene beads to contemporary functionalized beads for biophotonics. However, micro- and nanofabrication technologies are yielding a host of novel synthetic structures that promise alternative functionalities and new exciting applications. Recent works on the actuation of synthetic microstructures using optical trapping and optical manipulation are examined in this review. Extending the optical actuation down to the nanoscale is also presented, which can involve either direct manipulation of nanostructures or structure-mediated approaches where the nanostructures form part of larger structures that are suitable for interfacing with diffraction-limited optical fields.

/pdf/gearing-up-for-optical-microrobotics-micromanipulation-and-1be7h6q6kv.pdf

Gearing up for optical microrobotics: micromanipulation and actuation of synthetic microstructures by optical forces

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.

Optical microassembly platform for constructing reconfigurable microenvironments for biomedical studies

Optical trapping has enabled a multitude of applications focusing, in particular, on non-invasive studies of cellular material. The full potential of optical trapping has, however, not yet been exploited due to restricted access to the trapped samples, caused by high numerical aperture objectives needed to focus the trapping laser beams. Here, we use our recently developed biophotonics workstation to overcome this limitation by introducing probing and spectroscopic characterization of optically trapped particles in a side-view geometry perpendicular to the trapping beams rather than in the traditional top-view geometry parallel to the trapping beams. Our method is illustrated by CARS and fluorescence spectroscopy of trapped polystyrene beads. The side-view geometry opens intriguing possibilities for accessing trapped particles with optical as well as other types of probe methods independent from the trapping process.

/pdf/independent-trapping-manipulation-and-characterization-by-an-18d1jzmkku.pdf

Hans-Ulrik Ulriksen

Papers

Independent trapping, manipulation and characterization by an all-optical biophotonics workstation