Sub-micron particles, such as molecules, are too small to be seen. Scientists use different methods to indirectly observe them and study their properties. One of these methods is to study light rays that are scattered by these particles.
Light can interact with an object in different ways — it is reflected, refracted, transmitted or absorbed in different measures, depending on the object it is interacting with. In general, light, when it interacts with an object, is randomly scattered in all directions.
When the object in question is very small, of the scale of a few nanometres (a billionth of a metre) or less, most of the light incident on it goes along undisturbed, taking no note of the particle. This is because these particles are smaller than the wavelength of light and, therefore, do not interact strongly with light waves. Very occasionally though, not more than a few times in a billion, light waves do interact with the particle. Detecting these scattered light waves can provide some very important information about the particle light has interacted with.
One of the things that scientists study is whether the scattered light has the same energy it had before hitting the particle, or whether there was a change in energy levels. In other words, whether the interaction was “elastic” or “inelastic”.
One particular type of inelastic scattering, in which a change in the energy of the light is effected due to the vibrations of the molecule or material under observation, leading to a consequent change in wavelength, is Raman Scattering (or Raman Effect) — named after the physicist Sir C V Raman who discovered it in the 1920s, and for which he won the Nobel Prize in 1930.
While Raman Scattering is a very effective way of gaining some information about the object under observation, it is also an extremely weak phenomenon. For several years now, Dr G V Pavan Kumar and his team at the Indian Institute of Science Education and Research (IISER), Pune, have been trying to look for ways to enhance the effects of both Raman and elastic scattering, so that the phenomena can be studied more easily. They have been looking at increasing the number of light waves undergoing Raman Scattering, and also aligning the scattered waves in a particular direction so that all of them can be picked up by a sensor or detector.
In a recent paper in Nano Letters, Dr Pavan Kumar and his team reported how they achieved this through innovative use of special properties of metals at nano scales. The metal they used extensively was silver. A nano silver wire coupled with the layer of molecules under observation showed very interesting results. Apart from enhancing the strength of Raman Scattering, the silver wire acted like a “wave guide antenna”, directing the scattered waves at a particular angle. The effect was seen to be strengthening further when the set-up was placed on a gold nano film.
To ensure that they were studying the scattered light only from the desired molecule and not from the silver wire or the gold foil, the experimenters took readings of scattered light from each of the individual materials before combining them. The team designed and built a special microscope, called Fourier Plane Raman Scattering microscope, to measure the enhancement of Raman Scattering, as well as to detect the exact direction from which the scattered light waves emerged.
The signals received by the microscope can give very good information about the vibrational motion of molecules in nano-cavity, their orientations with respect to each other, and the angular distribution of the scattered light with high accuracy and precision. Dr Pavan Kumar and his team are continuing with their studies to see how these experiments can be tweaked to get even better results down to single-molecule sensitivity.
Also, they are extrapolating the Fourier microscopy methods to elastic and nonlinear light scattering to study the structure and dynamics of soft matter such as colloids, liquid crystals and active matter, which has conceptual connections to biological cells, membranes, and tissues.