ScienceWeek

APPLIED PHYSICS: ON OPTICAL MANIPULATION

The following points are made by David G. Grier (Nature 2003 424:810):

1) A new generation of techniques that use the forces exerted by carefully sculpted wavefronts of light offers precisely the level of access and control needed for rapid progress at the frontiers of several branches of science and engineering. In particular, optical forces are ideally suited to manipulating mesoscopic systems, which are characterized by length scales ranging from tens of nanometers to hundreds of micrometers, forces ranging from femtonewtons to nanonewtons, and time scales ranging upward from a microsecond. In biology, this range covers many of the inter- and intracellular processes responsible for respiration, reproduction and signalling. In physics and chemistry, it corresponds to the still-puzzling interface between classical and quantum mechanical behavior, which is made all the more perplexing by the general inapplicability of statistical many-body theory in this realm. Fulfillment of the promise of mesoscopic engineering has been held back by the need for tiny motors to drive micromachines and for robust human-scale interfaces with atomic-scale nanotechnology. Until quite recently, the options for manipulating, analyzing and organizing mesoscopically textured matter have been limited. The advent of flexible multifunctional optical traps meets this need.

2) Many of the most powerful optical manipulation techniques are derived from single-beam optical traps known as "optical tweezers", which were introduced by Arthur Ashkin, Steven Chu and their coworkers at AT&T Bell Laboratories(1,2). An optical tweezer uses forces exerted by a strongly focused beam of light to trap small objects. Although the theory behind optical tweezers is still being developed, the basic principles are straightforward for objects either much smaller than the wavelength of light or much larger. Small objects develop an electric dipole moment in response to the light's electric field, which, generally speaking, is drawn up intensity gradients in the electric field toward the focus. Larger objects act as lenses, refracting the rays of light and redirecting the momentum of their photons. The resulting recoil draws them toward the higher flux of photons near the focus(3). This recoil is all but imperceptible for a macroscopic lens but can have a substantial influence on mesoscopic objects.

3) Optical gradient forces compete with radiation pressure resulting from the momentum absorbed or otherwise transferred from the photons in the beam, which acts like a fire hose to blow particles down the optical axis. Stable trapping requires the axial gradient force to dominate, and is achieved when the beam diverges rapidly enough away from the focal point. For this reason, optical tweezers are usually constructed around microscope objective lenses, whose high numerical apertures and well corrected aberrations focus light as tightly as possible. Optical tweezers can trap objects as small as 5 nm (4,5).

4) In summary: Optical tweezers use the forces exerted by a strongly focused beam of light to trap and move objects ranging in size from tens of nanometers to tens of micrometers. Since their introduction in 1986, the optical tweezer has become an important tool for research in the fields of biology, physical chemistry and soft condensed matter physics. Recent advances promise to take optical tweezers out of the laboratory and into the mainstream of manufacturing and diagnostics; they may even become consumer products. The next generation of single-beam optical traps offers revolutionary new opportunities for fundamental and applied research.

References (abridged):

1. Ashkin, A., Dziedzic, J. M., Bjorkholm, J. E. & Chu, S. Observation of a single-beam gradient force optical trap for dielectric particles. Opt. Lett. 11, 288-290 (1986)

2. Ashkin, A. History of optical trapping and manipulation of small-neutral particle, atoms, and molecules. IEEE J. Sel. Top. Quantum Elec. 6, 841-856 (2000)

3. Ashkin, A. Forces of a single-beam gradient laser trap on a dielectric sphere in the ray optics regime. Methods Cell Biol. 55, 1-27 (1998)

4. Svoboda, K. & Block, S. M. Optical trapping of metallic Rayleigh particles. Opt. Lett. 19, 930-932 (1994)

5. Ke, P. C. & Gu, M. Characterization of trapping force on metallic Mie particles. Appl. Opt. 38, 160-167 (1999)

Nature http://www.nature.com/nature

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TRANSPORT OF BOSE-EINSTEIN CONDENSATES WITH OPTICAL TWEEZERS

The following points are made by T.L. Gustavson et al (Phys. Rev. Lett. 2002 88:020401):

1) Since the achievement of Bose-Einstein condensation in dilute gases of alkali atoms in 1995, intensive experimental and theoretical efforts have yielded a great deal of progress in understanding many aspects of Bose-Einstein condensation [1,2]. Bose-Einstein condensates are well-controlled ensembles of atom useful for studying novel aspects of quantum optics, many-body physics, and superfluidity. Condensates are now used in scientific studies of increasing complexity requiring multiple optical and magnetic fields as well as proximity to surfaces.

2) Conventional condensate production techniques severely limit optical and mechanical access to experiments due to the many laser beams and magnetic coils needed to create the condensates. This conflict between cooling infrastructure and accessibility to manipulate and study condensates has been a major restriction to previous experiments. So far, most experiments are carried out within a few millimeters of where the condensate was created. What is highly desirable is a condensate "beam line" that delivers condensates to a variety of experimental platforms. Transport of charged particles and energetic neutral particles between vacuum chambers is standard, whereas it is a challenge to avoid excessive heating for ultracold atoms. Thus far, transport of large clouds of atoms has only been accomplished with laser-cooled atoms at microkelvin temperatures [3,4]. Condensates are typically a few orders of magnitude colder and hence much more sensitive to heating during the transfer.

3) The authors report a demonstration of an application of optical tweezers that can transfer Bose condensates over distances of at least 44 centimeters (limited by the vacuum chamber) with a precision of a few micrometers. This separates the region of condensate production from that used for scientific studies. The "science chamber" has excellent optical and mechanical access, and the vacuum requirements in this region may well be less stringent than those necessary for production of condensates. The authors suggest this technique is ideally suited to deliver condensates close to surfaces, e.g., to microscopic waveguides and into electromagnetic cavities. The authors report they have used this technique to transfer condensates into a macroscopic wiretrap located 36 centimeters away from the point where the condensates were produced.

References (abridged):

1. W. Ketterle, D. Durfee, and D. Stamper-Kum, in Proceedings of the International School of Physics Enrico Fermi, edited by M. Inguscio, S. Stringari, and C. Wieman (IOS Press, Tokyo, 1999), p. 67.

2. F. Dalfovo, S. Giorgini, L. P. Pitaevskii, and S. Stringari, Rev. Mod. Phys. 71, 463 (1999).

3. M. Greiner, I. Bloch, T. W. Hansch, and T. Esslinger, Phys. Rev. A 63, 031401 (2001).

4. T. Kishimoto, P. Schwindt, Y.-J. Wang, W. Jhe, D. Anderson, and E. Comell, DAMOP/DAMP Poster (London, Ontario, Canada, 2001).

Phys. Rev. Lett. http://prl.aps.org

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