Unsteady Force Fields in Optics

Wave phenomena in laser beams are used to position particles for examination under microscopes. The theory is well-developed for this application, and we have drawn on this field to tie our acoustic field results to what can be achieved in the various parts of the electromagnetic spectrum. The theoretical development is based on the work of Zemanek et al (see reference below). We will post the theory here as the web site evolves - thanks for your patience.

"Space Tweezer": Laser Beam Optical Tweezer for Space Applications: NASA experiment. Credits: Northrop Grumman


Gomez-Medina, R., San Jose, P., Garcia-Martin, A., Lester, M., Nieto-Vesperinas, M., Saenz, J.J., "Resonant Radiation Pressure on Neutral Particles in a Waveguide". Physics Review Letters, 84, 4275 (2001).

Harada, Y., Asakura, T., "Radiation forces on a dielectric sphere in the Rayleigh scattering regime". Optics Communications 124 (1996) p. 529-54. Elsevier Science.

McCormack, E., "Investigation of the Feasibility of Laser Trapped Mirrors in Space". NIAC Phase 1 project, poster presentation, Houston, June 2002. http://www.niac.usra.edu/studies/

Zemanek, P., Jonas, A., Sramek, L., Liska, M., "Optical trapping of Rayleigh particles using a Gaussian standing wave". Optics Communications, 151 (1998) p. 273-285. Elsevier Science, 1998.

Optical tweezer set for launch 30 November 2001 A fully automated optical tweezer has been developed and will be installed in the International Space Station in 2005. http://optics.org/articles/news/7/11/30/1

Light beams lift metal particles 7 September 1999 Optics.org http://optics.org/articles/news/5/9/6/1

Excerpts: "Doughnut shaped light beams can support and manipulate metallic particles that are large enough to see. Miles Padgett and colleagues at the University of Glasgow in the UK have developed optical tools that can manipulate micrometre scale opaque objects, such as metal particles... first to trap metal particles this size in three dimensions. Conventional optical tweezers work due to the refraction of light passing through the transparent particles. Padgett says that opaque particles present additional challenges because refraction is no longer involved. "Instead it is the reflection that is important. We use a Laguerre-Gaussian beam pointing upwards to support a metal particle against the pull of gravity. The Laguerre-Gaussian beam is a bright ring with a hole in the middle. The metal particle sits in this hole, supported by light 'bouncing' off the bottom." This beam is important as it has angular momentum so allows objects to be rotated."

Lasers set micro-objects in a spin 3 May 2001 Optics.org http://optics.org/articles/news/7/5/6/1

Excerpts: "A new laser-trapping technique moves and rotates microscopic objects. UK scientists have developed a new laser-trapping technique to rotate micron-sized particles. By trapping objects within the interference pattern of a helix-shaped "Laguerre-Gaussian" (LG) laser and a plane wave, Kishan Dholakia and colleagues from St Andrews University, Scotland, were able to rotate objects ranging from microscopic silica balls and glass rods to a hamster chromosome. "Previous methods have proven useful in specific applications, but have serious shortcomings for general applications in rotating optical microcomponents and realizing optical micromachines", report the research group in Science 292 912-914. The group's latest development builds on the well established "optical tweezers" technique. Here, scientists harness the optical-gradient force by directing a focused laser at a particle. Differences in the refractive index of the particle and its surroundings force the particle into the region of highest light intensity within the beam. However, Dholakia and colleagues combined two specialized lasers to form the helix-shaped LG laser and found that by manipulating the orientation of one of the light beams, they could "spiral" the helix laser and the trapped particle with it. "

Rod of light traps particles 18 October 2001 Optics.org http://optics.org/articles/news/7/10/18/1

Excerpts: "..Kishan Dholakia and co-workers believe that this is the first demonstration of particle manipulation with a Bessel light beam (Opt. Comm. 197 239). The center of a Bessel beam does not spread out and acts like a rod of light in which particles can be captured. "A variety of particles were used, including silica spheres, elongated glass fragments, chromosomes and E. coli bacteria cells," said Dholakia. The researchers initially manipulated and tweezed a variety of particles in two dimensions. These included hamster chromosomes, E. coli bacteria cells and 5 µm-diameter silica spheres. The group then managed to show further manipulation by stacking, aligning and guiding these particles. ...Dholakia explains: "One particle is tweezed in the central portion of the beam. You then translate the beam and hovering particle over another particle (at a lower level) and it too gets pushed up into the beam just below the first particle. We have been able to stack a total of nine particles in this manner." Higher laser powers, typically greater than 60mW, were required compared to the 35mW used in the initial tweezing experiments. .. "We believe this method of optical manipulation will find several interdisciplinary uses including optical micromachines, colloid research and biological studies," said Dholakia. Optical tweezers make use of the reflection and refraction of light at a dielectric surface. Light possesses momentum. An interaction between light and matter can result in an exchange of momentum. In the case of the linear momentum of light, this can be used to trap microscopic particles. If a dielectric sphere is placed in a focused light beam, it is drawn into the region of highest light intensity due to the change in momentum it experiences. The Bessel beam was generated using a conical glass element known as an axicon. A standard Gaussian beam was incident on the axicon and the emergent beam consisted of light and dark rings, with a light central maximum. The non-diffracting centre had a propagation distance of a few millimeters, approximately 40 times the Rayleigh range of a Gaussian beam. This feature offered enhanced optical guiding, which was previously limited to the Rayleigh range of the Gaussian beam. "