Peeking and poking at particles with light

Philipp Treutlein, Department of Physics, Uni Basel
Barbara Treutlein, Department for Biosystems Science and Engineering, ETH Zürich, Basel

Nature is made of discrete building blocks - atoms and molecules in the physical world, large biomolecules and cells in living systems. To precisely manipulate these building blocks, to study their function one at a time and to assemble them into larger systems has been a long-standing dream of science. With the invention of the optical tweezer and related laser trapping and manipulation tools, this dream has come true. One half of this year's Nobel prize in physics goes to Arthur Ashkin, who pioneered these techniques since the early 1970s.

The fact that light can exert forces on matter had been known for a long time, postulated by Kepler to explain comet tails and finally observed in the lab in delicate precision experiments in the early 1900s. However, the optical forces were much too weak to be of practical use. This changed with the invention of the laser in 1960, which provided an intense and coherent source of light that can be tightly focused and precisely aligned. Soon after the first lasers became available, researchers began to study optical forces as a tool to manipulate small particles.

Arthur Ashkin at Bell Laboratories in New Jersey reported a first success in 1970, demonstrating radiation pressure forces on small particles in water and air [1], followed soon by the demonstration of optical levitation with a gravito-optical trap [2]. In 1986, Ashkin and his co-workers reported the first all-optical single-beam trap [3], which soon became known as "optical tweezers".

Two different types of forces can be distinguished in optical trapping: the scattering force, which is proportional to the light intensity and directed along the beam's propagation direction, and the gradient force, which is proportional to the intensity gradient and points along this gradient. It is the latter force which is employed in optical tweezers to confine polarizable particles to the intensity maximum of a tightly focused laser beam.

The optical trapping techniques developed by Arthur Ashkin kick-started two entire fields of research in different domains of science. In atomic physics, laser traps and related laser cooling schemes were soon used to trap clouds of atoms and to cool them to microkelvin temperatures. Experiments with such ultracold atoms in laser traps made it possible to study quantum physics with unprecedented control and precision. This led, among other things, to the observation of novel states of matter, such as Bose-Einstein condensation, and to the development of the most accurate atomic clocks and other atomic precision measurement devices. The optical trapping techniques for ultracold atoms have been continuously refined, and it is now possible to use computer-controlled arrays of optical tweezers to arrange individual atoms in nearly defect-free three-dimensional patterns of arbitrary shape. These techniques currently play an important role in the development of quantum technologies.

Optical tweezers have also enabled an entirely new set of precision experiments in the life sciences, which has revolutionized the field of single-molecule biophysics. Already in 1987, Arthur Ashkin used optical tweezers to trap living bacteria without harming them [4]. For the first time it became possible to perform controlled mechanical manipulations of individual living cells, benefitting from the fact that the optical forces can be applied to these objects in their natural environment under ambient conditions. Ashkin noticed that even structures within the cell could be moved and manipulated by the optical force. A major breakthrough came in the 1990s, when scientists realized that optical tweezers can reveal the mechanical properties of individual motor proteins, which are macromolecules that move and transport cargos along the skeleton of a cell. When an individual motor protein is attached to a dielectric bead held in an optical trap, the motor's movement can be measured through the forces it exerts on the bead. In this way, the 8 nm steps of the motor protein kinesin along its track could be measured for the first time. Since these first studies, the resolution of optical tweezers has dramatically improved such that it is even possible to observe the 3.4 Å steps that the RNA polymerase takes as it reads the genetic code.

About 50 years after it started, optical trapping of particles continues to be a dynamic and exciting field of research. Most recently, techniques for optical levitation have been used in the new field of optomechanics to trap dielectric nanospheres in vacuum and cool them close to the quantum mechanical ground state. Such experiments could lead to novel tests of quantum physics with massive, mesoscopic objects and find applications in precision force sensors operating at the nanometer scale. In the life sciences, optical tweezers have become a widely used tool for interrogating and manipulating individual biomolecules and subcellular structures. Recent advances include the application of optical holography to simultaneously use thousands of optical traps in high-throughput experiments and the integration of optical tweezers with super-resolution microscopy to simultaneously and correlatively visualize and manipulate molecular interactions with sub-piconewton and sub-nanometer resolution. This shows that the work of Arthur Ashkin, which has now been honored with the Nobel Prize in physics, continues to have significant impact even today.

[1] A. Ashkin, Acceleration and trapping of particles by radiation pressure, Phys. Rev. Lett. 24, 156 (1970).
[2] A. Ashkin and J.M. Dziedzic, Optical levitation by radiation pressure, Appl. Phys. Lett. 19, 283 (1971).
[3] A. Ashkin, J.M. Dziedzic, J.E. Bjorkholm and S. Chu, Observation of a single-beam gradient force optical trap for dielectric particles, Opt. Lett. 11, 288 (1986).
[4] A. Ashkin and J.M. Dziedzic, Optical trapping and manipulation of viruses and bacteria, Science 235, 1517 (1987).

[Published: October 2018]