Trapping
fluorescent particles with Arago spots. Credit: European Physical Journal E.
By
exploiting a particular property of light diffraction at the interface between
a glass and a liquid, researchers have demonstrated the first optical tweezers
capable of trapping nanoscale particles.
Optical
tweezers are a rapidly growing technology, and have opened up a wide variety of
research applications in recent years. The devices operate by trapping
particles at the focal points of tightly focused laser beams, allowing
researchers to manipulate the objects without any physical contact. So far,
optical tweezers have been used to confine objects just micrometers across—yet
there is now a growing desire amongst researchers to extend the technology to
nanometre-scale particles. In new research published in EPJ E, Janine Emile and
Olivier Emile at the University of Rennes, France, demonstrate a novel tweezer
design, which enabled them to trap fluorescent particles just 200 nanometres
across for the first time.
If made
available for widespread use, nanoscale optical traps could be used for
experimental procedures requiring extreme degrees of precision—including direct
measurements of nanoscale forces, alterations of cell membranes, and
manipulations of viruses and DNA strands. Emile and Emile's design was based
around "Arago spots': bright points of light which form in the centers of
circular shadows, as light diffracts around the objects creating them. In
addition, they relied on the principle of 'total internal reflection' – where
light rays hitting a glass-liquid interface at just the right angle are
perfectly reflected.
In the
experiment, the duo fired a perfectly aligned laser beam onto the interface
between a glass plate, and a liquid containing suspended fluorescent
nanoparticles; with an opaque circular disk partially blocking its path. The
resulting Arago spot was then totally reflected at the interface, creating an
exponentially fading wave which ran out from the spot in all directions.
Finally, suspended nanoparticles could be positioned inside this donut shaped
wave, and excited by a separate laser to emit light themselves. The resulting
forces imparted by these light waves caused the particles to become tightly
confined at the Arago spot. With further improvements to this setup, nanoscale
optical tweezers could soon open new opportunities for research, in areas
ranging from medicine to quantum computing.
Source: SciencePOD.