Optical tweezers

Optical tweezers make use of a laser beam to provide an attractive force (in the range of piconewtons to femtonewtons) to physically move microscopic objects (on the order of nanometers to tens of micrometers in diameter) with high precision. In this way, optical tweezers are a microscopic version of the tractor beam concept popular in many science fiction stories. Optical tweezers have had particular success in studying a variety of biological systems in recent years.

History and development

File:Ashkin c.jpg

Arthur Ashkin, Father of the optical tweezers

In 1970, the detection of optical scattering forces and gradient forces on micrometre sized particles was first reported in the scientific literature by Arthur Ashkin (Ashkin, 1970)^[2], a scientist working at Bell Labs. Years later, Ashkin and colleagues reported the first observation of what is commonly referred to as an optical trap today (Ashkin et al., 1986)^[3], i.e. a tightly focused beam of light capable of holding microscopic particles stable in three dimensions.

One of the authors of this seminal 1986 paper, Steven Chu, would go on to make use of optical tweezing techniques in his work on cooling and trapping of atoms. This reserach would earn Chu the 1997 Nobel Prize in Physics^[4]. In an interview [5], Steven Chu described how Askhin had first envisioned optical tweezing as a method for trapping atoms. Ashkin was able to trap larger particles (10 to 10,000 nanometers in diameter) but it fell to Chu to extend these techniques to the trapping of atoms (0.1 nanometers in diameter).

Professor Steven Chu giving a seminar at The Chinese University of Hong Kong

Excerpts from the interview

"There was another really wonderful scientist there named Arthur Ashkin, an older department head. I started talking to him casually, in the hallways. He had this dream: "Wouldn't it be nice if you can hold on to an atom with light?" He had tried to pursue this dream in the early seventies, in the mid seventies, but it wasn't really working. They did some very key experiments demonstrating the fundamental forces, but it wasn't looking like they were getting closer to really holding on to atoms with light. Finally the management told Ashkin, "It doesn't look like it's going to work; you've got to move on to other things." But then I came on board, and I was this new, young person who he could corrupt"[6]...Steven Chu

In the 1980s Steven Block, Howard Berg and Herchel Smith first applied the technology to the biological sciences, using it to grasp a bacterium in order to study bacterial flagella.^[7] Throughout the 1990s and afterwards, researchers like Carlos Bustamante, James Spudich, and Steven Block pioneered the use of optical trap force spectroscopy to characterize molecular-scale biological motors. These molecular motors are ubiquitous in biology, and are responsible for locomotion and mechanical action within the cell. Optical traps allowed these biophysicists to observe the forces and dynamics of nanoscale motors one molecule at a time; optical trap force-spectroscopy has since led to greater understanding of the stochastic nature of these force-generating molecules.

Optical tweezers have proved useful in other areas of biology as well. For intance, in 2003 the techniques of optical tweezers were applied in the field of cell sorting; By creating a large optical intensity over the sample area filled with micro-biological sample, the cell can be sorted by its intrinsic optical characteristics, M.P.Macdonald et al [8], Brian A. Koss et al[9]. Optical tweezers have also been used to probe the cytoskeleton, measure the visco-elastic properties of bio-polymers, and study cell motility.

Optical tweezers in brief

The optical forces exerted by optical tweezers on a micrometre-sized or nanometer-sized silica/latex sphere range from less than one piconewton to greater than a femtonewton, a range of loads which can easily immobilize a freely diffusing particle in water or apply physiologically relevant perturbations to single bio-molecules. In most instances, optical tweezers are performed in a fluid environment (e.g. water or air). A simplified description of the mechanism of optical tweezers is presented in the image below at [10]

A small dielectric sphere interacts with the electric field created by a beam of light, creating an induced dipole about the sphere. The sphere is therefore drawn along the electric field gradient through this dipole interaction to the point of the highest intensity of the light. In addition to this gradient force, the bead is subject to a scattering force caused by the reflection of light off the sphere surface. This causes the bead to be pushed forward by the light. However, if the beam is strongly focused the intensity gradient overcomes the push of the beam.

Physics of optical tweezers ^[11]

The theoretical analysis of optical micromanipulation is based on analysis of how linear momentum of light is transferred to microparticles.

Ashkin proposed two different optical micromanipulation regimes based on the microparticle size (diameter) with respect to the wavelength of the laser used for the optical trap.

In atmospheric science, it is well-known that particle in air scatters light respective to their size. When light scatters in the Rayleigh regime, the scattering particles size is much smaller than the wavelength of the light, thus results in an angular separation of colors that is responsible for the reddish color of sunset and the blue of the sky (selective scattering). When light scatters in the Mie Regime, the particles causing that scattering are larger in size than the wavelengths of light, such as pollen, dust, smoke, water droplets. Mie scattering is responsible for the white appearance of the clouds.

Following that same fashion, Ashkin proposed that optical micromanipulation can be analysis by two separate methods namely, ray optics approach for Mie particles (diameter of particle d > λ wavelength of light) and electric dipole approximation for Rayleigh particles (d < λ).

Within the ray optics analysis, rays tracing of the refraction and reflection process through the microsphere is sufficient to analyze the optical trapping (seen in the image above).

The simple ray optics equation is developed based on geometrical optics. The ray tracing indicates a directional change of linear momentum with respect to time. Hence, this rate of change of momentum of light rays, by Newton’s 2nd Law of motion (photon as particles), will result in a physical force. Similarly, there will be a reaction force from the sphere acting on the light rays. A crude interpretation would be that a region of higher intensity of light will possess more N photons and thus higher momentum and force due to the change of momentum.

By using a simple ray and force vector diagram, the microsphere will experience two different optical forces due to the Gaussian intensity profile (different of input and output momentum with respect to the sphere). The net forces from the imbalance action and reaction forces pulls the sphere towards the region of highest intensity region of the Gaussian beam. The resultant force is termed as the gradient force. Also there is a strong on-axis scattering forces from the scattering light rays where the sphere is encounters axial force along the beam’s propagation direction.

For a true three-dimensional optical tweezers, the Gaussian beam will be to be focused using a high numerical aperture(NA>1.0) microsope objective to achieve the optimum optical gradient force just around the focal point of the microscopic objective.

Using the conservation of momentum, the total force acting onto the sphere will be the summation of all the force acting onto it i.e. the scattering force, gradient force.

On the other hand, in the Rayleigh regime, the particles are not as restricted in particle shapes. In general, the smaller the particle the less the trapping power is required. In most cases, the dipole model and the conservation of momentum model are being used for explaining the working mechanisms of the optical tweezers with respect to the shape of the particles. Hence, the dipole model is used. The electromagnetic radiation of the light will induce dipole to form with the particles. The scattering force comes from the reflected and absorbed light. This vector force has a magnitude proportional to the intensity of light and a direction point toward the propagation of laser light. Furthermore, when light interacts with the particles, polarization occurs. This polarization will cause the particle to experiences a gradient force. This vector force has a magnitude proportional to the gradient intensity and a direction pointing towards the direction of intensity gradient.

Detail process of construction of an optical trapping setup can be found have been listed down by Steven M. Block,[12]

Optical tweezers based on alternate laser beam modes

Since the first introduction of Optical Tweezers based on a single Gaussian beam (fundamental laser mode TEM00) by A.Ashkin [13] in the 1986.

The concept of a single beam optical tweezers have seen to incorporate a series of high order laser beams i.e Hermite Gaussian beam (TEMxy), [Laguerre-Gaussian beams](LG) (TEMpl) and Bessel beams (Jn).[14] have been explored as alternative optical tweezers.

Optical tweezers based on Laguerre Gaussian beam have the unique capability of trapping particles that are optically reflective and absorptive. Laguerre-Gaussian beam also possess a well-defined orbital angular momentum that can rotate the particles [15],[16]. This is accomplished without external mechanical or electrical steering of the beam. Note that by imparting polarization of the light in circular manner using waveplate can allow a Gaussian beam to possible intrinsic spin angular momentum[17].

Besides the use of Laguerre Gaussian beam for optical tweezing, both zeroth and higher Bessel Beams also possess a unique tweezing ability namely, ability to trap and rotate multiple particle millimeter apart and even around obstacles [18][19].

Micromachines can be driven by these unique optical beam due to their intrinsic rotating mechanism due to spin and orbital angular momentum of light. [20]

Optical tweezers in manifold

A typical setup has only one or two laser beams. More complex optical tweezing operations that require multiple laser beams would need extensive modification to the optical tweezing set-up by incorporating laser steering devices such as acoustic optical modulator and spatial light modulator

One of the main use of the multiple optical traps to configure microparticles into predetermined position and achieve a suitable crystalline structure The three main optical tweezers group that made use of spatial light modulators for optical trapping are David Grier's group at New York University, Miles Padgett's group at the University of Glasgow, and Jesper Gluckstad's group at the Riso Institute.

Optical tweezers based on optical fibers

The fiber optical trap relies on the same principle as the optical trapping just that the laser is being deliver through Optical fiber. If one end of the optical fiber tip is being moulded into lens-like facet, that lens tip will act as a focusing (converging) point for the high optical gradient trap to be formed. [21].

On the other hand, if the ends of the fiber are not being moulded, the laser exiting the fiber will be diverging and thus a stable optical trap can only be realised through the balanced of the gradient and the scattering force from two opposing ends of the fiber, seen in the figure below. The gradient force will trap the particles the transverse direction, while the axial optical force from the scattering force the two counter propagating beams emerging from the two fibers. The equilibrium z-position of such a trapped bead is where the two scattering forces equal each other out. This work was pioneered by A. Constable et al, Opt. Lett. 18,1867,(1993), and followed by J.Guck et al Phys. Rev. Lett. 84, 5451(2000), who made use of this technique to stretch microparticles. By manipulating with the input power into the two end of the fiber, there will be an increase of a "optical stretching" that can be used to measure viscoelastic properties of cells, with sensitivity sufficient to distinguish between different individual cytoskeletal phenotypes. i.e. human erythrocytes and mouse fibroblasts. A recent test has seen great success in differentiating of cancerous cells from non-cancerous ones.[22]from the two opposed, non-focused laser beams.

Optical tweezers in a 'landscape' (cell sorting)

One of the more common cell sorting systems make use of flow cytometry through fluorescent imaging. In this method, a suspension of biologic cells is sorted into two or more containers, based upon specific fluorescent characteristics of each cell during an assisted flow. By using an electrical charge that the cell is "trapped" in, the cell are then sorted based on the fluorescence intensity measurements. The sorting process is undertaking by an electrostatic deflection system that diverts cell into containers based upon their charge.

In the optically actuated sorting process, the cell are flowed through into an optical landscape i.e 2D or 3D optical lattices. Without any induce electrical charge, the cell would sorting based on their intrinsic refractive index properties and can be re-configurability for dynamic sorting. Mike MacDonald, Gabe Spalding and Kishan Dholakia, Nature 426, 421-424 (2003)[23] made use of diffractive optics and optical elements to create the optical lattice. An automated cell sorter was described at the University of Toronto in 2001, but made use of mechanical parameters as opposed to spatial light modulation

On the other hand, K. Ladavac, K. Kasza and D. G. Grier, Physical Review E 70, 010901(R) (2004)[24] made use of the spatial light modulator to project an intensity pattern to enable the optical sorting process.

The main mechanism for sorting is the arrangement of the optical lattice points. As the cell flow through the optical lattice, there are forces due to the particles drag force that is competing directly with the optical gradient force(See Physics of an Optical Tweezers) from the optical lattice point. By shifting the arrangement of the optical lattice point, there is a preferred optical path where the optical forces are dominate and biased. With the aid of the flow of the cells, there is a resultant forces that is directed along that preferred optical path. Hence, there is a relationship of the flow rate with the optical gradient force. By adjusted the two forces, one will be able to obtain a good optical sorting efficiency.

Competition of the forces in the sorting environment need fine tuning to succeed in high efficient optical sorting. The need is mainly with regards to the balanced of the forces; drag force due to fluid flow and optical gradient force due to arrangement of intensity spot.

Scientists at the University of St. Andrews have received considerable funding from the UK Engineering and Physical Sciences Research Council (EPSRC) for an optical sorting machine. This new technology could rival the conventional fluorescence-activated cell sorting.[25][26]

Optical tweezers based on evanescent fields

An evanescent field [27] [28] is a residue optical field that "leaks" during total internal reflection. This "leaking" of light fades off at an exponential rate. The evanescent field has found a number of applications in nanometer resolution imaging (microscopy); optical micromanipulation (optical tweezers) are becoming ever more relevant in research.

In optical tweezers, a continuous evanescent field can be created when light is propagating through an optical waveguide (multiple total internal reflection). The resulting evanescent field has a directional sense and will propel microparticles along its propagating path. This work was first pioneered by S. Kawata and T. Sugiura, in 1992 (Opt. Lett. 17 (11), 772 (1992)). Kawata showed that the field can be coupled to the particles in very close proximity on the order of 100 nanometers.

This direct coupling of the field is treated as a type of photon tunnelling across the gap from prism to microparticles. The result is a directional optical propelling force.

A recent updated version of the evanescent field optical tweezers make use of extended optical landscape patterns to simultaneously guide a large number of particles into a preferred direction without using a waveguide. It is termed as Lensless Optical Trapping (“LOT”) [29]. The orderly movement of the particles is aided by the introduction of Ronchi Ruling that creates well-defined optical potential wells (replacing the waveguide). This means that particles are propelled by the evanescent field while being trapped by the linear bright fringes. At the moment, there are scientist working on focused evanescent fields as well [30]

Optical tweezers: an indirect approach

Ming Wu, a University of California Berkeley Professor of electrical engineering and computer sciences invented the new optoelectronic tweezers.

Wu transformed the optical energy from low powered light emitting diodes (LED) into electrical energy via a photoconductive surface. The ideas is to allow the LED to switch on and off the photoconductive material via its fine projection. As the optical pattern can be easily transformable through optical projection, this method allow a high flexibility of switching different optical landscapes.

The manipulation/tweezing process is done by the variations between the electric field actuated by the light pattern. As the particles will be either attracted or repelled from the actuated point due to the its electrical charges. Particles being suspended in a liquid will be susceptible to electrical charge due to the ions in the liquid, this is known as dielectrophoresis.

One clear advantage is that the electrical conductivity between a different cells. Living cells have a lower conductive medium while the dead ones have minimum or no conductive medium. The system may be able to manipulate roughly 10,000 cells or particles at the same time.

See comments by Professor Kishan Dholakia on this new technique, K. Dholakia, Nature Materials 4, 579-580 (01 Aug 2005) News and Views.

Optical Binding: Making optical Matter

COMING SOON!!!

Measuring the optical forces

At present the restoring force is measured on single or dual optical tweezers ([Photonic force microscope|Photonic Force Microscope]) [31][32]. Recent works have started the measurement of the optical forces in Holographic optical tweezers to enable high quality parallel optical tweezing process [33],[34], [35],[36].

The basic principle behind optical tweezers is the optical momentum transfer associated with refraction of light through the particles thereby bending optical light source. A change in the refraction in both the transverse and axial manner provide a force acting on the object. If the optical beam possess a Gaussian beam profile, the particle should have a strong restoring force that retain the particles at the centre of the beam. This restoring force pulls the particles into the center.

When the particles are displaced from the center of optical trap, the restoring force acts as an optical spring. This means that the optical force can be modelled onto a spring constant model as such that optical force is proportional to the displacement of the particles out of the trap. In actual practice, due to the thermal energy of the surrounding, there is always an inherent Brownian motion (noise). The displacement can be accurately measured by projecting the image of the particles onto a quadrant photodiode. The quandrant photodiode can be used to measure nm-scale displacements[37].

Optical tweezers community

Research groups

• ATOM3D is a joint research project comprising 7 European optics group with optical tweezing expertise, supported by the EC Sixth Framework Programme (FP6). [38]

• Block Laboratory at Stanford University, USA [39]

• Bustamante Laboratory at the University of California, Berkeley USA [40]

• Grier Lab at New York University, USA [41]

• Gross Laboratory at University of California, Irvine, USA [42]

• Lang Laboratory at MIT, USA [43]

• Meiners Laboratory at The University of Michigan, USA [44]

• Oddershede Lab at the Niels Bohr Institute, University of Copenhagen, Denmark [45]

• Perkins Lab at University of Colorado at Boulder, USA [46]

• Wang Lab at Cornell University, USA [47]

• Wuite Single Molecule Group at The Vrije Universiteit Amsterdam, The Netherlands [48]

• Optical trapping Group in University of St Andrews, Scotland, UK [49]

• Optical Tweezers Group in ICFO [50]

• Optical Tweezers at Umeå University, Sweden [51]

• Optical trapping Group in University of Queensland, Australia [52]

• Optics tweezing in University of Glasgow, Scotland [53]

Other resources

Extensive link pages to other academic groups in optical tweezing

• Extensive links to the worldwide Optical trapping community, hosted at University of St. Andrews, Scotland. [54]

• Jaffar's Group, Anna University, INDIA[55]

Professional Paper reviewing Optical Tweezer

• A. Ashkin, "Optical trapping and manipulation of neutral particles using lasers"[56]

• Neuman, K.C., and Block S.M Review on Optical Trapping method[57]

• M. Lang and S. Block, A Resource Letter on Optical Tweezers[58]

• K.Dholakia on Recent review of state of the art tweezers[59]

• D. McGloin on Review of Bessel beam optical tweezers[60]

• David Grier on A revolution in optical manipulation[61]

• Special Edition of Journal of Modern Optics A selection of optical tweezers papers from some of the leading groups[62]

• A more detailed list of references can be obtained from the online manuscript written by Justin E Molloy [63] and Miles J Padgett[64] titled Lights, Action: Optical Tweezers[65]posted online

Web resources

• Tweezers for information on the mechanical device from which optical tweezers take their name
• What are optical tweezers [66]
• Recent Optical Tweezers Journal Papers [67]

Multimedia links

• BBC Frontier covering the technique of Optical Tweezers 2003[68]

• Movies showing positioning and rotation controlled by optical tweezers [69]

• Videos of optical tweezers being used on bacteria.

Commercial optical tweezer systems

Commercial suppliers of optical tweezer systems include (in alphabetical order)

• Arryx
• Cell Robotics
• Elliot Scientific
• MMI Molecular Machines & Industries
• PALM

References

1. ^ Ashkin, A. Phys. Rev. Lett. 24, 156-159, (1970)
2. ^ A Ashkin, J M Dziedzic, J E Bjorkholm and S Chu, Opt. Lett. 11, 288-290, 1986.

1. ^ Hill, Murray (November 1987). "wrote the book on atom trapping". Retrieved June 25, 2005.
   Interview conducted for internal newsletter at Bell Labs. Contains confirmation of Ashkin as the inventor of optical trapping and provides information on the 1997 Nobel Prize in Physics.