Optical tweezers

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Glass nanoparticles in an optical trap

An optical tweezer , also known as an optical trap or dipole trap , is a photonic device for manipulation, i.e. H. for holding and moving, the smallest objects. The function is based on the fact that light exerts a force on microscopic objects (e.g. microspheres , individual biological cells , cell organelles or even atoms ) , thereby drawing the objects to the focus of a strongly focused light beam.

A typical design reflects a laser beam into an optical microscope , which is thereby focused in the object plane . The parts to be manipulated must be transparent at the wavelength used . Once the laser has been set so that the object is in focus, any positional deviation causes it to be drawn back into focus by the transmission of impulses during refraction .

In addition to focusing optics, holographic bundling of the laser light is also used.

By using a second laser with a wavelength that is absorbed by the object (usually ultraviolet ), you also have a cutting instrument ( micro laser scalpel ) available.

History and discovery

The first scientific study of forces on particles on the order of micrometers , caused by scattering of light and gradient forces , was published in 1970 by Arthur Ashkin , then a physicist at Bell Laboratories  (USA). A few years later, Ashkin and colleagues reported the first observation of the possibility of capturing microscopic particles in three dimensions with the help of a strongly focused light beam. This discovery was the basis for the development of the optical trap.

One of the co-authors of this paper was Steven Chu , who developed the technique of laser cooling and the storage of atoms. For the development of methods for cooling and trapping atoms with the help of laser light , he received the Nobel Prize in Physics in 1997 together with the theoretical physicist Claude Cohen-Tannoudji and William D. Phillips .

In an interview, Steven Chu described how Ashkin was the first to describe optical tweezers as a method of holding atoms. Ashkin was able to catch large particles (10–10,000 nm in diameter). Chu improved this technique to smaller particles down to 0.1 nm in diameter.

The first work in which living biological objects (cells) were successfully manipulated with optical tweezers comes from Ashkin and Dziedzic. Arthur Ashkin received the Nobel Prize in Physics 2018 for his work on optical tweezers and their application to biological systems.

functionality

The optical forces that optical tweezers exert on a silicone or latex bead in micrometer or nanometer size are between a piconewton and more than a nanonewton. These forces are sufficient to keep freely diffusing particles in water calm or to influence biological molecules in a physiologically relevant manner. Optical tweezers are mostly used to manipulate particles in solution (e.g. in water or in air).

A small dielectric sphere which is significantly smaller than the incident wavelength interacts with the electromagnetic field of an incident light beam by an electric dipole induced is. The resulting interaction between the induced dipole and the inducing field leads to a force along the electrical field gradient ( gradient force / dipole force ) in the direction of the location of maximum light intensity .

A second effect is superimposed on this force, which can be interpreted as the intuitive classic radiation pressure . Reflection and refraction of the light beam on the surface of the bead lead to a momentum transfer according to the rules of momentum conservation . This effectively creates a force and thus movement of the bead in the direction of propagation of the light beam.

If the beam is focused enough, the gradient force outweighs the force due to the radiation pressure. It is possible to manipulate the location of a bead in a plane that is perpendicular to the laser beam; the bead “follows” the beam. In detail, from electrodynamics, the light force can be separated semiclassically into dipole force and spontaneous force, the latter generating the above-mentioned “radiation pressure”.

With special beam shaping optics, the “self-healing” properties of Bessel rays can also be used for optical tweezers.

The wavelength is chosen so that the light is hardly absorbed by the cell's chromophores . Due to the large surface-volume ratio of the particles, the absorbed energy is also quickly released into the surrounding water.

Web links

Individual evidence

  1. ↑ Optical dipole traps
  2. ^ A. Ashkin: Acceleration and Trapping of Particles by Radiation Pressure . In: Physical Review Letters . tape 24 , no. 4 , January 26, 1970, p. 156-159 , doi : 10.1103 / PhysRevLett.24.156 .
  3. ↑ A. Ashkin, JM Dziedzic, JE Bjorkholm, Steven Chu: Observation of a single-beam gradient force optical trap for dielectric particles . In: Optics Letters . tape 11 , no. 5 , May 1, 1986, pp. 288-290 , doi : 10.1364 / OL.11.000288 .
  4. ↑ A. Ashkin, JM Dziedzic: Optical trapping and manipulation of viruses and bacteria . In: Science . tape 235 , no. 4795 , March 20, 1987, pp. 1517–1520 , doi : 10.1126 / science.3547653 .
  5. ↑ Stockholm: The Nobel Prize in Physics goes to three laser researchers . In: Spiegel Online . October 2, 2018 ( spiegel.de [accessed October 2, 2018]).
  6. ^ Christian Schmitz, Joachim Spatz , Jennifer Curtis: High-precision steering of multiple holographic optical traps . In: Optics Express . tape 13 , no. 21 , October 17, 2005, p. 8678-8685 , doi : 10.1364 / OPEX.13.008678 .
  7. ↑ J. Arlt, V. Garces-Chavez, W. Sibbett, K. Dholakia: Optical micromanipulation using a Bessel light beam . In: Optics Communications . tape 197 , Issue 4–6, October 2001, ISSN  0030-4018 , p. 239–245 , doi : 10.1016 / S0030-4018 (01) 01479-1 (English, delmarphotonics.com [PDF; accessed on August 7, 2016]).
  8. ↑ a b Miles J. Padgett: Optical Tweezers. CRC Press, 2010, ISBN 978-1-420-07414-7 , p. 36.