Microparticles feel the pinch

10 Nov 2006

Optical tweezers help to measure the refractive index of micrometre-sized particles.

A new method for measuring the refractive index of micrometre-sized particles has been developed at Australia’s University of Queensland. The technique involves laser trapping in optical tweezers and could lead to significant improvements in the study of a wide range of microparticles including single biological cells, paint pigments and smog (Phys. Rev. Lett. 97 157402).

The refractive index (RI) is a fundamental property of how light interacts with matter and knowing its value for microparticles is vital to a wide range of scientific research. Although several measurement techniques are currently available, they all have serious drawbacks including incompatibility with other common laboratory tools - such as conventional microscopes – that are used to study microparticles.

The RI of a single spherical microparticle (1-5 micrometre diameter) was measured in a laser trap, which used one beam of laser light in an optical "tweezers" system that confines the microparticle to a small focal point. The RI was determined to within 1% accuracy from the "stiffness" of the trap, which is a measure of the forces required to keep the microparticle under confinement. The stiffness was determined by measuring the particle’s thermal motion using light from a second laser and a photodetector.

Queensland’s Greg Knöner told PhysicsWeb: "This is the first time that an individual microparticle’s refractive index has been measured in a laser trap and, as an added bonus; the measurement technique is relatively simple". Indeed, unlike other methods that rely on the capture of scattered light using rotating stages and/or an array of detectors, this new technique can be implemented on a standard microscope by the addition of two mirrors.

An advantage of the new technique is that the particles do not have to be suspended in special liquids, which could damage biological specimens such as single cells or alter crystalline growth. Another unique feature of the new technique is that it can be used to study polydisperse solutions, which contain particles of different sizes. Together, these features could be exploited in automated systems that rapidly test microparticles for desirable pharmacological or other properties.

The group is now extending the technique so it can be used to study non-spherical objects. "This involves the development of new measuring techniques that account for the orientation of a non-spherical particle in the trap as well as the development of new models to simulate the interaction between the laser beam and the object," explained Knöner.