Raman scanning made stronger
doi:10.1038/nindia.2011.87 Published online 22 June 2011
The intensity of the Raman scattering signal can be enhanced by two orders of magnitude by using photonic nanojets, according to new research. The finding would improve the efficiency of Raman based sensors used in fields ranging from healthcare to security.
Raman spectroscopy is widely used to analyse chemical composition and phase transition of materials. C. V. Raman won the Nobel Prize in 1930 for this optical effect named after him. Using condensed sunlight, Raman discovered a new kind of radiation, in which the scattered light had a different frequency from that of the illumination. The scattering agents could be gases, vapours, liquids, crystals or amorphous solids.
The change in spectrum provides a unique identity tag to molecules — a 'finger-print'. However, the Raman scattering signal is weak. Several mechanisms such as surface-enhanced Raman scattering, tip enhanced Raman scattering, shell-isolated nanoparticle-enhanced Raman spectroscopy and resonance Raman scattering have been developed later for improving the Raman scattering signal.
Researchers from the Indian Institute of Technology Madras, Chennai have now found that by suitably selecting certain experimental parameters, they can enhance the Raman signal by approximately two orders of magnitude.
They drew from recent reports in which scientists had enhanced the Raman scattering signal by photonic nanojets emerging from single microparticles. Earlier research had shown enhancement in Raman scattering by using silica microparticles and polystyrene microspheres.
The IIT researchers tweaked these experiments further to change the size of the microspheres. They noticed that the enhancement depended upon the pump wavelength, the refractive index of the microsphere and the collection objects. They reported the variation in the enhancement factors of 'Raman peaks' of silicon wafer and cadmium ditelluride (CdTe2) thin films due to these parameters.
'Raman technology' finds application in small, inexpensive and powerful lasers, fast digital image processors for space and communication industries and surface-enhanced spectroscopy.
"With the Raman scanner, a hand-held device weighing less than 250g, it is now possible to scan a surface non-intrusively and in real time to detect super microscopic traces of a wide variety of molecules from pathogens and drugs to explosive chemicals," says lead researcher Prem Bisht, a professor of physics at IIT Madras. The new finding will help increase the efficiency of such scanners, he says.
Danthan, V. R. et al. Enhancement of Raman scattering by two orders of magnitude using photonic nanojet of a microsphere. J. Appl. Phys. 109, 103103 (2011) | Article |