Optical Tweezers: Laser Manipulation of Microparticles

Abstract

Although the forces of radiation pressure have been well known to physicists, 1 for a long time it has been difficult to study them in the laboratory due to the presence of temperature gradients causing thermal forces which could obscure the radiation pressure. It has only been since 1970, after the advent of suitable laser sources, that practical applications could be investigated, beginning with the experimental demonstration by A. Ashkin of acceleration and trapping of micron-sized particles using the force of radiation pressure from a cw visible laser.2 Additional steps were made in the following years, 3–6 but a major practical advancement was achieved in 1986 when Ashkin and coworkers demonstrated optical trapping of dielectric particles (in the size range from about 25 nm up to 10 μm) in water solution by a single-beam gradient force radiation-pressure trap. 7 An year later, Ashkin and Dziedzic coined the term «optical tweezers» when reporting the demonstration of the optical trapping and manipulation of individual viruses and bacteria in aqueous solution by laser radiation pressure. They «have used the trap as an optical tweezers for moving live single and multiple bacteria while being viewed under a high-resolution optical microscope».8 During the last decade there has been increased attention to the use of optical tweezers: this occurred mainly in microbiology, but optical tweezers are expected to play an important role in the future development of nanometric-scale technology for manipulation, control and analysis of ultra-small structures.