What are optical tweezers...
...and how do they trap things using a laser?
Optical tweezers use a laser to trap microscopic dielectric objects, and a dielectric is defined as an electrical insulator that can be polarised (charged) by an application of an electric field.
In our scenario, a glass or polystyrene bead approximately 1 micron (µm) in diameter is a microscopic dielectric object.
Light can be described as an electromagnetic wave which, in classical physics, is a synchronised oscillation of an electric and magnetic field.
Modern physics describes light as having wave-particle duality in that it can behave both as wave or a particle, a photon... and photons can exert a physical pressure as they transfer the momentum of their movement to an object in front of them.
The greater the number of photons, the higher the force exerted. Yet even with the intense light from a near infra-red laser beam, the pressure on an object is still only a few trillionths of a newton - which is the international unit of force. However, this piconewton (pN) force is sufficient to hold and manipulate our microscopic bead.
The bead is attracted to the electric field, and the laser beam's focus or 'waist' is the area with the strongest pull. Once in the waist, it is trapped and can be moved around with ease.
This ability to move things on the sub-nanometre scale has enabled studies in the life sciences previously thought impossible. Research into the properties of DNA has advanced significantly due to the laser trapping ability of optical tweezers.
Scientists can sort or track viruses, living cells and bacteria with optical tweezers, even fold or unfold proteins by 'glueing' beads to these biological structures that can then be manipulated with the laser beam.
Measurements can be made as to how much force is required to do this, and these types of studies are typically used to determine the properties of molecular motors and DNA.
To avoid damage to the experiment, near infra-red laser beams operating with a wavelength of around 1 µm are often used. Biological samples, being mostly water, have a low absorption of this wavelength of invisible light, so tend not to quickly cook. Green lasers are also used in some cases.
Elliot Scientific can supply fully integrated optical tweezer systems comprising microscope, lasers, imaging system, optional force measurement, specialist software and the complex opto-mechanical design off the shelf, making it all work straight out of the box.
We can sometimes also add an optical tweezer capability to your existing microscope, so do contact us for more details about photonic force microscopes.
For more information please read: A Practical Guide to Optical Trapping
Tweezer Systems Tweezer Videos Application Notes
The first optical trapping systems were single beam open architecture lab experiments derived from the Ashkin paper published in 1986. Physics researchers then developed more complex double and multiple trap systems.
In 2003 Elliot Scientific and Professor Kishan Dholakia of St. Andrews University started a collaboration to develop an 'off the shelf' fully integrated optical trapping system. This meant that, for the first time, trapping experiments did not need a PhD researcher to build and operate optical tweezers... Elliot Scientific could deliver a unit that worked straight out of the box rather than taking six months to build and set up.
We started with single trap units, then dual beam traps, finally moving on to computer controlled multiple trap systems with force measurement and particle trapping included.
These integrated microscope systems comprise lasers, imaging system, optional force measurement, specialist software and superior opto-mechanical design. They are delivered to allow Bio/Microscopy researchers to start their relevant optical trapping experiments on day one. The technique becomes a useable tool and is no longer a research project.
Without the early work of Ashkin we would never have been able to develop the systems that we have today. - Mike Elliot