Kishan Dholakia, Peter Reece and Min Gu from the University of St Andrews, UK, examine how light can move and sort biological objects, and be used for studying physics and chemistry at the microscopic scale.

Holding and moving objects using light might seem like science fiction, but it is actually science fact - at the microscopic scale.  The characteristics of light and light-matter interactions on a small scale have allowed some astounding scientific advances in the last forty years. The invention of the laser opened up many new research fields including optical micromanipulation - where light exerts a force, to hold (trap) and move objects.  This force comes from transferring momentum possessed by light to objects. For example if an object refracts the light, the light's momentum will change as it bends. Naturally the momentum of a single quantum of light, the photon, is very small, meaning these forces are barely a million millionth of a Newton.

Such forces cannot hope to move macroscopic objects, but are perfect for holding and moving objects the size of single cells or smaller. A tightly focused beam of light can move objects this size without damage, as carefully choosing the light wavelength avoids absorption and ensures the force transfer is non-invasive. 
In the biological world, this technique can measure exceptionally precise and miniscule forces where macromolecules are tethered to microscopic beads.  Here we are in the realm of molecular motors that convert chemical energy to mechanical work. Examples include the actin-myosin system which operates using energy from chemical reactions, typically the hydrolysis of adenosine triphosphate (ATP) to adenosine diphosphate (ADP) and phosphate. Trapping allows this process to be observed in real time. The exact positions of trapped particles are observed to much higher accuracies than the wavelength of light, as detectors can take in to account an objects centre of gravity, meaning the movement of biological molecules can be observed with amazing sensitivity.  Transcriptional elongation of Escherichia coli RNA polymerase motion can even be monitored on a DNA template with its motion recorded at the ngstrom level.

Light can do much more than just make accurate force measurements.  It can move single or multiple droplets for subsequent mixing and study chemical reactions - opening up new chemistry including combinatorial chemistry using only picolitres of reagents, studying coagulation dynamics and micro-reactions. Optical forces combined with fluidic forces are also used in the area of microfluidics for lab-on-a-chip research.

Novel photonics in the form of arrays of traps or the implementation of new light patterns (rather than the standard circular mode pattern from most lasers) is core to many recent advances. If extended light patterns are created instead of one single beam, an array of light spots (akin to a set of egg-boxes) are generated. These arrays of traps are known as a potential energy landscape.  The motion of a particle across this optical array is like a small ball moving along a slanted corrugated roof, where gravity causes the object to move across the roof, but the exact trajectory followed is dependent upon the slant of the corrugation in the roof. Extending this analogy, arrays of traps can be created so that when particles flow over them they deflect objects to a degree based on their affinity to the light. This method allows the separation (sorting) of cells and colloidal particles without adding fluorescent markers. This is just one example of what these landscapes may achieve: their impact should reach a wide range of colloidal and soft matter research, and may even help us better understand superconductivity.

Optical micromanipulation has been around for over 35 years, with its impact significantly expanding recently.  These new applications and insights make the field more dynamic and exciting than ever - light has 'caught' more than the imaginations of scientists. 

Read Kishan Dholakia et al's tutorial review on 'Optical micromanipulation' in issue 1, 2008 of  Chem. Soc. Rev.