Preamble to the discovery of Raman Effect

Today is India’s National Science Day. It celebrates the discovery of Raman effect on 28th February, 1928.

For more details on the discovery of the effect, and various human aspects related to it : you can see my past blogs here, here, here and here.

In this blog, I will briefly discuss about some of the work that directly influenced Raman’s thinking that further led to a remarkable discovery that we know by his name.

All creative pursuits are motivated by ideas from the past. No one gets their ideas in vacuum. All of us are influenced by the information which we perceive and receive. This means consciously or subconsciously the world that we are creating, both in our minds and in reality, is fundamentally influenced by the information in the world.

The discovery behind the Raman effect is no exception to this particular principle. In his formative years, C V Raman was heavily influenced by the research of Rayleigh and Helmholtz, and some classical thinkers including Euclid. Raman was also closely following the development of quantum mechanics in the early 1920s, and he was keenly studying the theoretical and experimental developments in this field.

Two aspects which played a crucial role in motivating Raman’s (Nobel prize winning) work was Compton scattering and Kramers-Heisenberg formula.

Compton scattering was as outstanding experimental achievement that revealed two aspects of light-matter interaction. First, it demonstrated inelastic scattering of electromagnetic radiation interacting with a quantum object (in this case free electrons) in the laboratory frame. Second is that it laid a foundation to revisit the wave-particle duality of light from an experimental viewpoint. Raman and Krishnan’s main paper on light scattering starts by explicitly referring to Compton effect, and motivates observation for optical analogue of Compton scattering.

To quote from Raman’s Nobel lecture :

“In interpreting the observed phenomena, the analogy with the Compton effect was adopted as the guiding principle. The work of Compton had gained general acceptance for the idea that the scattering of radiation is a unitary process in which the conservation principles hold good.”

Next is the Kramers-Heisenberg formula. This mathematical description gives the scattering cross section of a photon interacting with a quantum object (in this case electron). This formula uses second-order perturbation theory, and evokes the famous ‘sum of all the intermediate states’ for non-resonant optical interaction. PAM Dirac played a vital role in deriving this formula from a quantum mechanical framework of radiation. An important and logical consequence of this formula is the emergence of stimulated emission of radiation, and this has had deep implications in understanding LASERs. Raman was keenly studying the formula and made a brilliant conceptual connection between laboratory observation and this formula that revealed the scattering cross-section.

Again to quote from Raman’s Nobel lecture:

“The work of Kramers and Heisenberg, and the newer developments in quantum mechanics which have their root in Bohr’s correspondence principle seem to offer a promising way of approach towards an understanding of the experimental results.”

The above two concepts were important ideas that motivated Raman scattering experiments. Importantly it highlights the jugalbandi between theoretical intuition with concrete experimental observations, which forms the bedrock of modern physics.

Newton famously mentioned about the discoveries he made by ‘standing on the shoulders of the giants’. Various people involved in creative pursuits including scientists acknowledge the fact that new ideas emerge from convergence/mutation of old ideas. The harder part of creativity in science, or for that matter any art form, is to choose the right ideas to combine so that the ’emergent’ new idea has greater value compared to the individual parts. In that sense, science is a great form of creative activity that not only combines old ideas to create new valuable ideas, but also transforms the perspective of the individual seed ideas. Thus ideas combine and evolve.

So let us combine good ideas and evolve. Happy Science Day !

Roadmap for optical tweezers is on arxiv

I am delighted to share the arxiv link to Roadmap For Optical Tweezers :

This document brings together researchers from 52 different affiliations across the globe to look into the accomplishments and future directions of optical tweezers.

My contribution towards roadmap appears on page 136, topic number 31 on — Raman scattering in (thermo)plasmonic tweezers.

In there, I discuss how plasmonic platforms can be used to generate attractive optothermal forces to trap and interrogate molecules and nanoparticles, down to single copy limit. I also discuss the challenges and opportunities of such a process.

Optical tweezers is one of the most powerful optical tools that finds utility not only in fundamental physics, but also in diverse applications including biology and medicine.

Ever since the Nobel prize-winning work of Arthur Ashkin, optical tweezers have evolved and continues to evolve into various forms. This roadmap article aims to capture this evolution, and to discuss the emerging capabilities and challenges of optical tweezers.

A big thank you to

Giovanni Volpe
Onofrio M. Maragò
and Halina Rubinzstein-Dunlop

for their leadership in this field and for the herculean task of coordinating and editing this roadmap….

The roadmap has been submitted to Journal of Physics : Photonics (IOP)

Talks on C.V. Raman – YouTube links

Below are the YouTube links to the 2 talks I gave on C.V. Raman on the occasion of India’s Science Day

The first talk is about : C.V. Raman: A brief History

Organized by IISER Pune Science Activity Centre
Age group 6 to 100: Students, Teachers, Science Enthusiasts and all Members of the Public

The second talk is about : C. V. Raman : History of Ideas

Organized by Science Club IISER Pune

Target audience: Science students and researchers

Raman and Science Day

Raman was an extraordinary scientist, great communicator of science, and a very interesting human being with strong opinions and independent thought…. his scientific journey is an ‘audacity of hope’. His human endeavour.. from Bowbazar to what makes Raman what he is…and what he will be remembered for..

Raman’s work was deeply influenced by many great classical thinkers, and Euclid was one of them. To quote Raman

“Not until many years later did I appreciate the central position of geometry to all natural knowledge. I can give a thousand examples. Every mineral found in Nature, every crystal made by man, every leaf, flower or fruit that we see growing, every living thing from the smallest to the largest that walks on earth, flies in the air or swims in the waters or lives deep down on the ocean floor, speaks aloud of the fundamental role of geometry in Nature. The pages of Euclid are like the opening bars in the Grand Opera of Nature’s great drama. They lift the veil and show to our vision a glimpse of the vast world of natural knowledge awaiting study.”

To know more, you may want to attend the announced talks..

Happy Science Day !

Announcement: my webinars on Science Day

This year, on India’s Science Day – 28th Feb, I will be giving 2 webinars

1)9:45 AM to 10:15 AM:
C.V. Raman: A brief History

This is a pre-recorded talk

Organized by IISER Pune outreach.
IISER Pune Science Activity Centre
Age group 6 to 100: Students, Teachers, Science Enthusiasts and all Members of the Public

2) 6pm – live talk
C. V. Raman : History of Ideas

Organized by Science Club IISER Pune

Target audience: Science students and researchers

Zoom Link:

Probably, these talks will be put on YouTube too. I shall post the links when available

57. Single nanoparticle driven thermoplasmonic tweezer : single-molecule SERS

We have a new paper published in Journal of Physical Chemistry Letters on “Single Molecule Surface Enhanced Raman Scattering in a Single Gold Nanoparticle-Driven Thermoplasmonic Tweezer”

Thanks to the fantastic effort by Sunny Tiwari, and excellent support by Utkarsh Khandelwal (former IISER-P undergrad) and Vandana Sharma from my group, we have been able to combine single molecule Raman scattering with a specialized nanoscale optical tweezer.

The uniqueness of this tweezer platform  is that the optical trapping process is driven by the thermo-plasmonic potential created by a SINGLE, 150nm GOLD NANOPARTICLE. Concomitantly, the same field can be used to perform single-molecule Raman spectroscopy. Kind of  “ek teer mae do shikar” strategy Smile

Using this system, not only we push the limits of optothermal trapping of a single nanoparticle (see video) at low laser powers, but also create a platform for deterministic transport of reversible colloidal assembly in a fluid.

We envisage that our nanometric plasmonic tweezer can be harnessed to trap and tweeze biological entities such as single virus and bacteria. Another possible application of our study is to create reconfigurable plasmonic metafluids in physiological and catalytic environments, and to be potentially adapted as an in vivo optothermal tweezer.

All the videos related to this study can be found on our lab’s Youtube channel :

DoI of the published paper :

preprint version on arxiv :

44. Beaming light with a bent-nanowire

We have a new publication in Journal of Physical Chemistry Letters on the “Beaming Elastic and SERS Emission from Bent-Plasmonic Nanowire on a Mirror Cavity”

In short, we show, how by bending a nanowire we can narrowly beam the light scattered from molecules (see adjoining picture).

Optical emission from quantum objects such as atoms and molecules are very sensitive to their local surroundings. One of the current challenges in controlling optical emission from molecules at subwavelength scale is to narrow their scattering directivity. In the context of molecules, controlling light scattering at sub-wavelength scale has utility in optical trapping of molecules, molecular QED, cavity molecular mechanics, molecular quantum optics and many other areas of research. 

Thanks to the great effort by Sunny Tiwari in my lab, who in the middle of the pandemic, tirelessly executed the idea of beaming elastic and Raman scattering emission from molecules in the vicinity of a bent plasmonic silver nanowire resting on a metallic mirror.  He was ably supported by Adarsh (now at ETH), Dipta and Shailendra. Together, they experimentally confirmed the beaming characteristics from this geometry and corroborated with elaborate numerical simulations.

This work further motivates questions related to directivity control for single photon emitters and can be potentially harnessed for momentum-space engineering of nano-optical forces……

we say bend the light like a nanowire…Smile

DOI of JPCL article :

arxiv version :

38. K.S. Krishnan et al., – students related to the discovery of Raman effect

Above picture : A group of 5 students of Raman. Front row- Left to Right 1) S. Vekateswaran, whose observations on the polarized ‘weak fluorescence’ of glycerine in early 1928 started the last lap of investigations which led to the discovery of the Raman effect. 2) K. S. Krishnan, he was 31 when this photograph was taken. 3) A. S. Ganesan – spectroscopist, later editor Current Science, who worked with Raman. He compiled the first bibliography of the Raman effect which Rutherford submitted to the Nobel Committee when he proposed Raman for the Nobel Prize. Back row. 1) C. Mahadevan, who later became renowned geologist who did his post-graduate work with Raman on X-ray studies of minerals. He was present at the Indian Association for the Cultivation of Science in Calcutta during the momentous discovery of the Raman effect and he has left graphic accounts of what happened then. Right S. Bhagavantam, another renowned student of Raman, who worked with him after the discovery of the Raman effect and is well-known for his application of group theory to the Raman effect. Reproduced from Current Science, Vol 75, NO. 11, 10 DECEMBER 1998

Today is National Science Day in India. We celebrate this day in commemoration of the discovery of the Raman effect. I have previously written about the significance of this day.

One of the important aspects of the discovery of the Raman effect is the role played by the then student K.S. Krishnan, who went on to be become a distinguished scientist and the founding Director of National Physics Laboratory, Delhi. There were also a few others who played a part in this discovery too (see picture)

Raman Research Institute has an excellent repository of the collected works of Raman. It also has a lot of content about Raman.

Of the many documents, the one which caught my attention was an interview of K.S. Krishnan by S. Ramaseshan, which was published in Current Science. Below I reproduce a few excerpts from the article:

“I (Ramaseshan) said there was a view that he (Krishnan) years discovered the Raman effect for Raman and this  view had again surfaced. His reply was ‘It is a blatant misrepresentation. The best I can say is that I participated actively in the discovery”

Krishnan goes on to say how it all started with Raman taking the initiative. In fact, Krishan vividly describes the scene :

‘The story starts in the early Febrauary 1928 when Professor (Raman) came to
my room and said “I want to pull out of the theoretical studies in which
you have immersed yourself for the 2 or 3 years. I feel it is not quite healthy
for a scientific man to be out of touch with actual experimentation and experimental facts for any length of time’

Interestingly, Raman and Krishan fell out of each other, and this interview has some snippets of this controversy. The article has some comments by S. Chandrasekhar on the credit of discovery behind Raman effect, in which he attributes Raman and Krishan’s collaborative approach towards the discovery, and mentions about the importance of exchanging ideas between two researchers working on a problem.

Overall, I must mention that the interview and the historical anecdotes in the document are riveting to say the least, and also showcases the complexity and sociology of a scientific discovery.

Science, per se, is objective. But pursuit of science has a human element, which makes it complex and interesting…

So always remember that as we commemorate the effect named after a person, but there are a few more people who have contributed to it. After all, science is a collective human endeavor.

Happy Science Day !

My Metaphoric Oxygen

There is no Frigate like a Book
To take us Lands away
Nor any Coursers like a Page
Of prancing Poetry –
This Traverse may the poorest take
Without oppress of Toll –
How frugal is the Chariot
That bears the Human Soul –

                             BY EMILY DICKINSON

Generally speaking, scientists are natural philosophers: they observe nature, ask questions, hypothesize an answer, test them through experiments and extend this exploration by escaping into the universe of ideas in books and journals. New ideas emerge from this exploration and join the chorus, and the intellectual journey continues. In my own research on light scattering, I have been deeply influenced by ideas of various fellow-explorers. For me, journal papers and books encompass the “metaphorical oxygen” for creativity and knowledge. Below I introduce you to some classic books which keep my research alive.

  1. Absorption and Scattering of Light by Small Particles
    • Author(s): Craig F. Bohren and Donald R. Huffman
      • Comments: There are two kinds of authors who write textbooks. One is the ‘boring kind’ and the other is the ‘Bohren kind’. If you want to fall in love with light scattering (and science in general), read books and articles by Craig Bohren. It will not only deeply influence your thinking, but also will show how a textbook can, and should, evolve a subject systematically. This particular classic has some of the most important ideas related to how light behaves when it interacts with matter comparable to the wavelength of light, and forms the bedrock on which a lot of contemporary research, including nanophotonics and plasmonics, is pursued. This book has wit, humour and a touch of poetry jumbled up together as flowing river of knowledge. To give you a spirit of their writings, let me reproduce the first paragraph of their introduction


  1. Light Scatteing by Small Particles
    • Author(s): H.C. van de Hulst
      • Comments: The first edition of this book was published in 1957, by the author was a legendary astronomer. This book has a beautiful description of single and multiple-scattering phenomenon, and describes specific situations where they apply. Written with an astrophysical viewpoint, it elegantly combines depth and breadth in a lucid way. This book has perhaps served as inspiration to most of the books written on light scattering.
  1. The scattering of light and other electromagnetic radiation
    • Author(s):  Milton Kerker
    • Comments: Some researchers have remarkable ability to choose problems that have far reaching consequences beyond the next research paper. Milton Kerker was one such legend. His research papers and this book has not only influenced the way physics of light scattering is studied, but has had deep impact on utilization of light scattering in various branches of science and technology. This 600 odd page book is indeed a masterpiece, and in a unique way caters to almost all kinds of researchers who are interested in light scattering.
  2. Dynamic Light Scattering with applications to chemistry, biology and physics
    • Author(s): Bruce J. Berne and Robert Pecora
      • Comments: A majority of the matter in biology and chemistry are suspended in a fluid. When an object in a medium undergoes Brownian motion, it influences the way a light beam scatters and traverses through that medium. This book explain the how and why of this fascinating topic. Written by experts in chemical physics, this classic serves as the foundation for light scattering in soft-condensed matter physics.
  1. Molecular Light Scattering and Optical Activity
    • Author(s): Laurence Barron
      • Comments: Historically, light scattering by molecules has been studied by legends such as Rayleigh, Raman and many more. Interestingly, all these legends emphasized the connection between polarization of scattered light and structure of matter. In this book, Barron puts together these ideas in a very elegant way, and motivates and develops the phenomenon of optical activity from a molecular physics viewpoint. Given that a majority of biomolecules are chiral in nature, the insight that one obtains by reading this book has direct implication in understanding the structure and dynamics of biomolecules such as amino acids, proteins and DNA.
  1. Scattering, Absorption, and Emission of Light by Small Particles
    • Author(s): MI Mishchenko, LD Travis, AA Lacis
      • Comments: Mischchenko is a scientist at NASA, and his books on light scattering have had great influence in aerosol science, radar technology and many more. The T-matrix codes based on this book forms a very important tool across the research community that works on weather prediction and pollution monitoring.
  1. Wave Propagation and Scattering in Random Media (Vol 1 and 2)
    • Author(s): Akira Ishimaru
      • Comments: This classic from late 1970s was one of the elaborate attempts to put together wave propagation and scattering in a random media on a rigorous mathematical foundation. This 2 volume book has solutions to various mathematical problems that one encounters in light scattering physics, and makes an important connection to transport theory of light in a medium.
  1. Optical Scattering Measurement and Analysis
    • Author(s): John C. Stover
      • Comments: If you are interested in experimental aspect of light scattering, this is one of the best books. It is essentially a field guide, which tells you how to quantitatively make a light scattering measurement, and what aspects to look-out for. This is a very good book for students who want to get a hands-on experience in light scattering.
  1. LASER LIGHT SCATTERING, Basic Principles and Practice
    • Author(s): Benjamin Chu
      • Comments: Chu’s book develops the topic of laser light scattering in terms of both experimental aspect and theoretical foundations. Importantly, it connects the topics of light scattering to optical spectroscopy, and shows how one can obtain meaningful information about light-matter interaction.
  1. Mesoscopic Physics of Electrons and Photons
    • Author(s): E. Akkermans and G. Montambaux
      • Comments: Quantum mechanical entities such as electrons and photons can be confined in space and time. Depending on the geometry of confinement, very interesting physics such as weak and strong localization can emerge. This book looks at the physics of confined electron and photon from a unified viewpoint. It highlights similarities and difference between the electrons (fermions) and photons (bosons).
  1. The Raman Effect: A Unified Treatment of the Theory of Raman Scattering by Molecules
    • Author(s): Derek A. Long
      • Comments: Written by a pioneer in the field, this book till date remains the most rigorous treatment on Raman scattering of light from a theoretical viewpoint. Based on quantum mechanical arguments, this book relies on perturbation theory, and clearly shows the connection between structure of molecules and how they influence the scattered light.
  1. Principles of Surface Enhanced Raman Spectroscopy and Other Plasmonic Effect
    • Author(s): Eric C Le Ru and Pablo G. Etchegoin
      • Comments: The most definitive book written on surface enhanced Raman scattering by two physicists whom I greatly admire. This book gives unified treatment of plasmonics and surface enhanced inelastic light scattering, and is written in a style catering to physics audience. The book has a lot of details and explanations, and also serves as excellent introduction to plasmonics and vibrational spectroscopy. Given that the authors themselves are pioneers in single-molecule Raman scattering, their insight into single molecule optics in plasmonic field is fascinating. Unfortunately, Etchegoin succumbed to cancer, and I could never meet him. However his great ideas and thoughts stay on…
  1. Introduction to Wave Scattering, Localization and Mesoscopic Phenomena
    • Author(s): Ping Sheng
      • Comments: Random lasing is an emerging topic of research in nanophotonics. The fact that one can have random structures assembled in space and time, and yet achieve spatial and temporal coherence is quite remarkable. This book brings together insights from wave scattering and mesoscopic physics to show how light behaves when confined to small volumes compared to wavelength of light. The insights obtained from this book are heavily used in the literature on random lasers.
  1. Fundamentals of Atmospheric Radiation
    • Author(s): Craig F. Bohren and Eugene E. Clothiaux
      • Comments: Bohren weaves his magic…..again. Although the title of this book indicates atmospheric radiation, the way the authors treat the topic of absorption, emission and scattering of light is fascinating. This book gives a broad viewpoint of interaction of light with matter, and shows one can and should treat the subject coherently. The references and problems are very relevant and interesting, and I have found some gems while reading through this text.