How I wonder…. How you are …
Scientists are those who always look towards Nature with the exclaimed eyes of a child, always amazed to
discover Her hidden treasures, without attempting to conquer Her, but to realize Her in totality with great
care and tenderness. They bear the spirits of the ancient philosophers in their hearts, the eternal fire of urge
to probe to the intricate details of the Universe, existence and participation of human mind in every part of
this endless Cosmic dynamism. Sir Chandrashekhar Venkata Raman, born at Trichinopoly on November
7th, 1888, also lit the sacred fire of eternal questions in his heart.
Sir C.V. Raman, in 1907, came to Calcutta as an officer in the Finance Department of the Government of
India. In his spare times, he started research in the Indian Association for Cultivation of Science (IACS),
present Kolkata. In 1917 under the invitation of Sir Asutosh Mukherjee, Raman joined the newly founded
Science College of the Calcutta University as its first Palit Professor of Physics and continued his research .
Attracted by Raman’s fame a legendary group of brilliant and dedicated students from all parts of India
gathered at Calcutta and started their scientific career, which included S.K. Mitra, K.S. Krishnan, K.R.
Ramanathan, L.A. Ramdas, S. Bhagavantam, K. Banerjee, to name only a few. It was in this laboratory, in
association with K.S. Krishnan, Raman effect was discovered on February 28, 1928 for which Raman
received the Nobel Prize in 1930. The 28th day of February is celebrated as National Science Day.
The journey of a thousand miles started with a single step …….the child of Nature, always amazed to
discover Her hidden treasures, asked to himself, How does sea water appear blue ? The search for the
answer opened the wormhole of a new Universe of immense scientific and technological possibilities.
Blue is the Ocean………..
Every system in this Universe, whether in macroscopic or microscopic scale, exhibits non-linearly to an
extent depending on their properties , over a certain range of the input variable(s), as the system-properties
themselves are ‘changed’ because of the input variable(s).
When electromagnetic radiation passes through a medium (like sun-light traveling through sea-water), it
interacts with the molecules of the same. The fundamental question thus arises here “Does light change the
properties of the medium through which it propagates ?”
Without taking resort to the elegant theory of Quantum Electro Dynamics (QED), which offers a very
accurate picture of interaction of radiation with matter, it can be simply visualized that the incident photons
(quantized electromagnetic radiation) interact with the electron cloud of the molecules during their passage
though the medium. The electric field (E) associated with the radiation changes the electric dipole moment
(P) of the electron-cloud according to the relation
P =αE
The proportionality constant α is termed as polarizibility, which is a measure of the ‘deformability’ of the
electron cloud due to the electric field. Higher the value of α , more deformed is the electron cloud for a
given electric field. However, this linear relation no longer holds when the polarizibility itself becomes a
function of the electric field and the medium exhibits optical non-linearity.
What happens inside the medium to make it optically non-linear ? The degree of freedom of a molecule in a
medium is a function of a number of factors like the number of atoms in the molecule, the molecular
geometry, type of the bonds, the Coulomb interaction between atoms and molecules, etc.. Accordingly, the
molecule can vibrate at certain specific modes. The corresponding frequencies of the vibration modes (the
states of vibrating molecules) define the vibrational energy levels of the molecules. When photons pass
though such molecules of the medium, they can interact in three possible ways as discussed below.
First, some of the incident photons can interact with the electron clouds of the molecules so that the
molecules acquire the necessary energy from them (photons) to support a vibration mode. In this case the
energies of those scattered photons decrease by the amount of the vibrational energy of the molecules. This
is an example of inelastic scattering of photons with a red shift (frequency of the scattered photons
decrease). The spectroscopic signatures of these energy-transitions are known as Stoke’s lines.
Secondly, some of the photons can acquire the vibrational energy from the vibrating molecules in the
medium. Thus the energy of those photons increase by the amount of the vibrational energy of the
molecules. This, again, is an example of inelastic scattering of photons with a blue shift (the frequency of
the scattered photons increase). The spectroscopic signatures corresponding to these energy transitions are
known as Anti-Stoke’s lines.
The third possibility is that the photons interact with the molecules without any transfer of energy. The
energy of the scattered photons remain same as that of the incident photons. This, however, is an example
of elastic scattering (also known as Rayleigh scattering) of photons.
It is notable that unlike the third case, in the first two cases of inelastic scattering , the building blocks of
the medium (the molecules) either gained or lost energy, leading to changes in their optical properties.
These two cases, therefore, represent the non-linear optical phenomena, collectively known as the Raman
Effect , after the discoverer, Sir C.V.Raman.
What is the probability that an incident photon would undergo a Rayleigh or Raman scattering ?
Semiclassical treatments assume the molecules of a medium obeying the Maxwell-Boltzmann distribution
and using electromagnetic wave picture of radiation one can derive the probability of each scattering process. However, the path integral method using Feynman propagators offer a more accurate description.
Only 1 part in 107 photons undergo Raman effect, the majority undergo Rayleigh scattering. The best way to determine this probability experimentally is to compare the optical intensities of the spectroscopic lines corresponding to Rayleigh scattering (unchanged frequency), Anti-Stoke’s scattering (blue shifted) and Stoke’s scattering (red shifted). Among Raman-scattered photons, very less numbers undergo Anti-Stokes scattering, because such scattering requires interaction between photons and already vibrating molecules, and at room temperature the Maxwell-Boltzmann distribution predicts very less number of molecules
already in the vibrational state.
Technological Applications of Raman Effect
There are several applications of Raman effect in technological regime, some of them are mentioned below.
The list, however, is not exhaustive.
Remote Thermometry
The ratio of the intensities of the Anti-Stoke’s and Stoke’s lines is a measure of temperature. It is therefore
possible to ‘sound’ a remote target by electromagnetic wave and analyse its Raman spectra to obtain its
temperature very precisely. This method, as compared to the conventional black-body-radiation technique,
offers more signal-to-noise ratio.
Non-Destructive Testing (NDT)
By simply illuminating the sample by a laser beam and analyzing the Raman spectra, it is possible to
extract the information regarding a sample, without destroying it.
Real-Time Monitoring of Chemical Reactions
Chemical reactions can be monitored by elegant Raman spectra technique which is non-invasive and real
time. The scattering time is typically 10-14 s and hence continuous, real time, non invasive monitoring of
chemical reactions is possible.
Stimulated Raman Scattering (SRS) in Optical Fibres
In this technique, energy from a high frequency radiation is transferred to a low frequency radiation to
amplify it. This technique is used towards Erbium Doped Fibre Amplifiers (EDFA) technique to amplify
the transmitted optical signal though the optical fibre by ‘pumping’ optical power from a higher frequency
radiation.
Other applications of Raman effect are in bio-medical instrumentation for gas monitoring in anesthetic and
respiratory systems ; study of crystallography, etc.. In other words, this non-linear optical phenomena
pervades almost every field of technological interest, thereby proving it to be a very important
instrumentation principle in different regimes.
Resolution on the National Science Day
On the National Science day, we take the oath to realize our participation in this Cosmic dynamism as a
part of Nature, to have a crystal-clear mind , where the image of the Universe finds itself undistorted,
unperturbed….which answers the eternal queries of mankind with the simplicity of a child. We will keep
our signatures in this Universe on the screen of time and ink of energy, for our descendents to re-discover
our joyous participation in this endless dynamism.