Dr G V Pavan Kumar’s group at IISER Pune, in collaboration with researchers at Center for Nanoscale Materials, Argonne National Laboratory, USA, have experimentally shown for the first time, that Raman and fluorescence components of scattered light behave differently when confined to a plasmonic nano-cavity. This is a compact and cost-effective system that will enable researchers to discern Raman signals from other types of molecular emission such as fluorescence, which so far has been possible only through sophisticated and expensive techniques.
The study has shed new light on fundamental questions related to how quantum electrodynamic properties of molecular cavities affect scattering of light. Moreover, the system devised by the researchers could also be employed to develop ‘optical antennae’ coupled to single molecules.
Cavities can enhance or supress some properties of molecules that are trapped within them. For example, arresting molecules in extremely small cavities can drive them to emit photons in specific directions. Just like antennae in modern cell phones transmit radio waves in a directed manner, molecules arrested in a cavity can be used to channel molecular emission to a direction of choice.
However, when such a molecular antenna emits two different signals (for instance, Raman + fluorescence) this can cause complications in ‘reading’ of the signals. It would be very useful to direct the two signals at different angles, such that they do not interfere with each other.
About Raman Scattering (named after Nobel Prize winning Physicist Sir CV Raman)
When light strikes a material or a molecule, scattering is a dominant process. A small proportion of the scattered light has a frequency different from that of the incident beam. This is called ‘inelastic scattering’; and the phenomenon is known as Raman scattering or Raman effect if it arises due to vibrational energy states of the molecules or materials. Raman scattering is an inherent property of every material, with different materials having distinct Raman signals. This signal is thus like a molecular fingerprint and studying it can give unique insights into the properties of that material.
Dr. Pavan Kumar says, “We designed a cavity using a combination of two nanostructures – silver nanowire placed on gold thin film. This imparts different directionalities to different signals emanating from the molecules arrested in the cavity between the wire and film. One can thus separate the emissions from these molecules in terms of their emission angles.”
So far, it has been difficult and expensive to separate Raman emission from the other molecular emissions as they spectrally overlap. This advancement by Dr Pavan Kumar and team thus has significant implications for studying Raman scattering of individual molecules.
“Our nano-cavity is special as it can separate Raman scattering from background molecular emissions by changing their emission angles. This system is quite economical and finds extensive utility in understanding the molecular emission processes at the level of individual molecules – an idea which is at the heart of quantum physics.”, comments Dr. Pavan Kumar.
This paper titled “Differential Wavevector Distribution of Surface-Enhanced Raman Scattering and Fluorescence in a Film-Coupled Plasmonic Nanowire Cavity” has been accepted for publication in Nano Letters and is authored by Adarsh B. Vasista, Harshvardhan Jog, Tal Heilpern, Matthew E. Sykes, Sunny Tiwari, Deepak K. Sharma, Shailendra K. Chaubey, Gary P. Wiederrecht, Stephen K. Gray, and G. V. Pavan Kumar. (Nano Lett., 2018, 18 (1), pp 650–655; DOI: 10.1021/acs.nanolett.7b05080).
This work received funding from DST-Nano Mission Grant, Govt. of India; Center for Energy Science; and an IUSSTF grant.
– Reported by Apurva Barve, with inputs from G V Pavan Kumar