optical tweezers

optical tweezers
Name	Optical Tweezers
Caption	A typical schematic of an optical tweezers instrument.
Classification	Scientific instrument
Inventor	Arthur Ashkin
Manufacturer	Various, including Thorlabs, Bruker, JPK Instruments
Related	Atomic force microscopy, Magnetic tweezers, Confocal microscopy

optical tweezers are a scientific instrument that uses a highly focused laser beam to provide an attractive or repulsive force to physically hold and manipulate microscopic, dielectric objects. The technique, which is a type of optical trap, allows for the non-contact handling of particles ranging in size from tens of nanometers to several micrometers. It has become a foundational tool in biophysics, soft matter physics, and analytical chemistry for measuring tiny forces and displacements at the molecular scale.

Principles of operation

The fundamental operation relies on the transfer of photon momentum from a tightly focused laser to a microscopic particle. When a laser beam is focused through a high-numerical aperture microscope objective, it creates a strong intensity gradient. A dielectric particle within this gradient experiences forces due to light scattering and the gradient force; the latter pulls the particle toward the region of highest intensity, typically the beam's focal point. The scattering component, or radiation pressure, tends to push the particle along the direction of beam propagation. Precise balance of these forces creates a stable three-dimensional trap. The theoretical framework is often described using electromagnetic theory and the Rayleigh scattering or Mie scattering regimes, depending on particle size relative to the wavelength of light.

Instrumentation and setup

A typical apparatus is built around an inverted optical microscope, such as those from Nikon or Zeiss, to allow observation. The core laser source is often a high-power, continuous-wave Nd:YAG laser operating at a infrared wavelength to minimize damage to biological samples. The beam is steered and expanded before entering the microscope and being focused by a high-numerical aperture oil immersion objective. Precise sample positioning is achieved with a piezoelectric stage or nanopositioning system. Detection of particle position and force is accomplished by projecting the trapped particle's image onto a quadrant photodiode or using back focal plane interferometry, with data acquisition managed by software from companies like National Instruments.

Applications in biology

This technology has revolutionized the study of molecular motors and biomolecule mechanics. Pioneering work used it to characterize the stepwise motion and force generation of the motor protein kinesin along microtubules. It is extensively used to study the mechanical properties of DNA by tethering a single molecule between a trapped bead and a surface, allowing measurement of supercoiling, replication, and transcription forces. Investigations into the immune system have measured the binding forces between antigens and antibodies. Research at institutions like Marine Biological Laboratory and Max Planck Institute has employed it to study the mechanics of bacterial flagella and cell membrane elasticity.

Applications in physics and chemistry

In soft matter physics, it is used to probe colloidal interactions, phase transitions in complex fluids, and the rheology of polymer solutions. It can measure the van der Waals force and Casimir effect at microscopic scales. In analytical chemistry, it serves as a sensitive probe for surface chemistry and catalysis on single particles. The technique has been instrumental in studying Brownian motion and testing fluctuation theorems like the Jarzynski equality, bridging statistical mechanics and thermodynamics. Experiments have also manipulated carbon nanotubes and characterized the optical binding forces between multiple trapped particles.

History and development

The foundational discovery was made by Arthur Ashkin and colleagues at Bell Labs in 1986, following earlier work on radiation pressure on atoms. Ashkin demonstrated the first stable three-dimensional trap using a single focused laser beam, for which he was awarded the Nobel Prize in Physics in 2018. Key subsequent developments included the first biological application in 1987, trapping live *E. coli* bacteria without optical damage. The 1990s saw the technique combined with fluorescence microscopy and its application to single-molecule biophysics, notably by Steven Block at Stanford University and Carlos Bustamante at the University of California, Berkeley. Commercial systems later became available from companies like Cell Robotics and Lumicks.

Limitations and challenges

Primary constraints include the maximum trapping force, typically on the order of piconewtons, which limits the study of very strong molecular interactions. High laser intensities can cause photodamage in biological samples through reactive oxygen species generation or localized heating. The technique is generally restricted to dielectric particles or those that can be attached to dielectric microspheres; highly reflective or absorbing materials are difficult to trap stably. Calibration of the trap stiffness and precise force measurement requires careful consideration of Brownian noise and detector calibration. Furthermore, the complexity of the instrumentation and the need for vibration isolation, such as on an optical table, present significant practical hurdles for widespread adoption.

Category:Scientific techniques Category:Biophysics Category:Optics