Optical tweezers

The Nanophysics and Soft Matter group has considerable research expertise in optical trapping and micromanipulation, which is a major research area. As part of our ongoing work in this field, we maintain several optical tweezers workstations, optimised for a variety of tasks.

Technique overview

Optical tweezers are a micromanipulation technology in which the gradient forces associated with tightly focussed laser light are exploited to exert forces on dielectric particles. By using a high numerical aperture objective lens, a laser beam is brought to a focus in a liquid sample chamber, forming a trap for micron-sized particles (see image below). These traps make an ideal environment for the investigation of systems such as dielectric colloidal particles and single cells.

Multiple optical traps can be generated and controlled by imposing a diffractive optical element (DOE) in a plane optically conjugate with the back-aperture of the objective lens. We use liquid crystal spatial light modulators (LCSLMs) for this purpose, with a "gratings and lenses" algorithm running on a graphics card generating the necessary patterns to produce arbitrary arrays of laser focii in the microscope's image plane.

In the "ray optics" description of optical confinement, we consider the effect of a photon deflected by its passage through a dielectric sphere of greater refractive index than the surrounding medium. As the photon's momentum is altered by refraction, the sphere receives a small "kick" from each photon that passes through it. The net effect of these "kicks" is illustrated by the large black arrows, and acts to draw the sphere towards the focus of the beam.

Our optical tweezing programme is run in close consultation with our collaborators at Glasgow University. For more information, there is a detailed Wikipedia article on the topic of optical tweezers.

The optical tweezers undergoing alignment. Note the familiar inverted microscope (top right of the image). The green scatter is produced a laser which is pumping the Ti:sapphire laser used to produce confinement at 800nm.

Example applications

A cardiomyocyte being interrogated using a 2um silica microsphere. By monitoring the position of the microsphere relative to the optical trap, we can make extremely sensitive force measurements of the cell membrane.

Paragraph about the range of forces accessible, and a highlight of the aqueous environments being ideal for cells. Also mention that biological material is typically not very absorbing in the near infra-red where our equipment is centred, meaning you can in many cases directly manipulate living cells.

Optical tweezers have found high profile application in a number of areas:

  • Investigation of the interactions between different cell lines in suspension;
  • Measurement of the stepping motion of molecular motors, for example Kinesin and Mysosin-V;
  • Extremely low-force measurement of surface topographies and free volumes within, for example, agar networks;
  • The guided assembly of repeating structures from simple dielectric building blocks, e.g. spheres;
  • Investigation of photo-sensitive cells, including the guiding of ganglia development in neurons.

Cells in suspension

Optical tweezers can be exploited to study the behaviour of single cells in suspension, as well as the behaviour of small populations of cells as they are brought into close proximity with one another. The "light touch" of optical tweezers are ideal for investigations in physiologically relavent in vitro conditions.

Who to contact

For information relating to optical tweezers, please contact Merv Miles or Dave Phillips.