Author: G.V. Pavan Kumar

Namaste, Hola & Welcome from G.V. Pavan Kumar. I am a Professor of Physics at the Indian Institute of Science Education and Research, Pune, India. My research interests are : (1) Optics & Soft Matter: Optically Induced Forces – Assembly, Dynamics & Function; (2) History and Philosophy of Science – Ideas in Physical Sciences. I am interested in the historical and philosophical evolution of ideas and tools in the physical sciences and technology. I research the intellectual history of past scientists, innovators, and people driven by curiosity, and I write about them from an Indian and Asian perspective. My motivation is to humanize science. In the same spirit, I write and host my podcast Pratidhvani – Humanizing Science.

Sagan’s quote

Keynote address at CSICOP conference in 1987. “Do Science and the Bible Conflict?” Book by Judson Poling, p. 30, 2003

Even before the age of social media, Carl Sagan played a prominent role in communicating science to the public. His book ‘Cosmos’ is a classic, and his public lectures (a few are on YouTube) are certainly worth watching.

For me what is very impressive about Sagan’s communication is that he is not condescending by any means. This attitude is probably one the most important aspect in communicating science to society. The moment the public feels that scientific thinking is an elitist’s endeavour, they tend to repel.

Not only was Sagan an excellent scientist, but also a person who articulated the role of science in society, especially by contrasting it to religion and politics. A very famous quote of his is reproduced here, that highlights the value of scientific thinking, and how it elevates the human mind.

Quote reference: Keynote address at CSICOP conference in 1987. “Do Science and the Bible Conflict?” Book by Judson Poling, p. 30, 2003

Black hole image and optical vortex – an analogy

The recent image of the black hole at the center of the milky way has been spectacular. When I teach a course, I generally emphasize analogies across the sub-disciplines of physics. In the below video I draw some analogies between black hole image and an optical vortex.

About the black hole images : https://iopscience.iop.org/journal/2041-8205/page/Focus_on_First_Sgr_A_Results

Some work from our group on optical vortex : ACS Photonics 6, 1, 148–153 (2019) https://doi.org/10.1021/acsphotonics.8b01220

Book on singular optics : https://www.google.co.in/books/edition/Singular_Optics/H-WVDQAAQBAJ?hl=en&gbpv=1&printsec=frontcover

Science paper on optical analog of event horizon https://www.science.org/doi/epdf/10.1126/science.1153625

Scientific Philosophy and Mental Health

Many a times, we are oblivious to the impact that Science has on an individual mind. It gets drowned in the collective impact. Curiosity is probably the most natural feeling of humans. If done right, Science cultivates and elevates this feeling. It may positively impact mental well-being too.

Increasingly, in Indian academia, mental well-being of all the stakeholders (students, faculty, admin etc.,) has emerged as an urgent and important issue that needs attention. Especially in a country such as India, where a loose order emerges out of chaos in almost all aspects of life, it is important to stay connected with oneself. Scientific curiosity may cater to this vital need.

The connection with oneself via science, or broadly speaking with any form of curiosity (arts/sports), is something we must harness. A major part of human development is to connect with oneself AND with the society. In this age of social media, sometimes, we may forget the former and focus only on the later. We will have to remind ourselves that being curious about anything is not being ‘childish’ but being human. Scientific curiosity and questioning is fundamental to our living, and this affects everything we do in our life.

To conclude let me quote from Bertrand Russell’s The Value of Philosophy :

“Philosophy is to be studied, not for the sake of any definite answers to its questions, since no definite answers can, as a rule, be known to be true, but rather for the sake of the questions themselves; because these questions enlarge our conception of what is possible, enrich our intellectual imagination, and diminish the dogmatic assurance which closes the mind against speculation; but above all because, through the greatness of the universe which philosophy contemplates, the mind also is rendered great, and becomes capable of that union with the universe which constitutes its highest good.”

Science as in philosophy, does cater to the highest good of humanity. What we under-appreciate is that it goes beyond the call, and impacts an individual’s mind. Academia should be a place to foster such an impact at various levels: individual, local and global. Perhaps that is the meaning of an “University”.

More surprises in Optical Momentum…

Electromagnetic momentum is a topic with rich history dating back to Maxwell, Poynting, Minkowski, Abraham, Einstein, and many more1.
It has also led to new questions, and an intriguing controversy in electromagnetism2.

An interesting and contemporary question to ask is: what is the behavior of optical momentum in artificial materials ?

One class of artificial materials is the near zero-refractive index (NZI) materials.

What are NZI materials ? The general definition of refractive index from a material view point is that it is proportional to square root of a product: dielectric permittivity (ε) and magnetic permeability (μ) of the given material.

n = (εμ)½ 

 If either of these material values go to zero at a given wavelength of light, then the refractive index goes to zero or close to zero. Such a situation creates new opportunity for enhanced or supressed light-matter interaction. See this popular review on NZI materials3

A recent theoretical paper4 addresses the consequence of evolution of optical momentum in NZI media.
This analysis has thrown a few fundamental surprises that are fascinating such as : absence of interference in Young’s double slit experiments, and some new opportunities in optical cloaking thanks to quantum nature of light. To quote the authors4 :

being inside an NZI materials would lead to an infinite uncertainty on position and zero uncertainty on momentum. Conceptually, this implies that since the resolution is poor and no correct image can be formed, an object of any shape and material can be “hidden” in a NZI material.

There are a few more interesting prospects, and of course, all of them are yet to be verified with experiments.

If you are interested in this topic, I strongly recommend this recent, popular level article5

1.           M. Buchanan, “Minkowski, Abraham and the photon momentum,” 2, Nature Phys 3(2), 73–73, Nature Publishing Group (2007) [doi:10.1038/nphys519].

2.           S. M. Barnett and R. Loudon, “The enigma of optical momentum in a medium,” Philosophical Transactions of the Royal Society A: Mathematical, Physical and Engineering Sciences 368(1914), 927–939, Royal Society (2010) [doi:10.1098/rsta.2009.0207].

3.           “Optics & Photonics News – Zero-Index Platforms: Where Light Defies Geometry,” <https://www.optica-opn.org/home/articles/volume_27/july_august_2016/features/zero-index_platforms_where_light_defies_geometry/> (accessed 5 May 2022).

4.           M. Lobet et al., “Momentum considerations inside near-zero index materials,” 1, Light Sci Appl 11(1), 110, Nature Publishing Group (2022) [doi:10.1038/s41377-022-00790-z].

5.           “Exotic Materials Through Momentum’s Looking-Glass,” <https://www.optica-opn.org/home/newsroom/2022/may/exotic_materials_through_momentum_s_looking-glass/> (accessed 5 May 2022).

OAM + SAM -New paper from my lab

We have a new paper from our lab to appear in the journal : Laser & Photonics Reviews

on “Simultaneous detection of spin and orbital angular momentum of light through scattering from a single silver nanowire”

preprint version on arxiv : https://arxiv.org/abs/2111.14919

Light can carry orbital angular momentum (OAM) and spin angular momentum (SAM). This momentum can be transferred to an object that is interacting with the light. What we show is the experimental proof of concomitant detection of OAM and SAM in the coherent light scattering signatures from a single, silver nanowire. Essentially, the nanowire acts like a slit, and scatters the light. During this scattering process, the distribution of light in momentum space gets altered according to the spin (polarization) and orbital (topological charge) state illuminating the nanowire.

A notable point is that unlike other (metamaterials) methods, this unambiguous detection scheme does not require sophisticated nanofabrication methods and is mainly founded on fundamental principles of vectorial light scattering in the momentum space.

This experimental work (with a good dose of theoretical optics) was mainly due to the sustained efforts of an outstanding PhD student in my lab : Diptabrata Paul (about to finish PhD !)

He had excellent support and inputs from our PhD alumni Deepak K Sharma (now a postdoc/research scientist at ASTAR, Singapore).

Going further, this study motivates some interesting questions, of which we are interested in exploring the direct transfer of OAM and SAM at sub-wavelength scale to nanoscale objects including (macro)molecules. This will have some interesting manifestation on optical forces and torques at sub-wavelength scale, and we intend to study them in detail. This can be studied in a unique set-up that we have built in our lab that combines nano-optical tweezers with momentum-space imaging microscope. Look out for some studies in this direction from our lab.

We will spend a lot time…in momentum space :)

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

Nanowire kink as an antenna for 2D material

https://link.springer.com/article/10.1140/epjs/s11734-022-00511-y

A nanowire kink on a mirror can influence light scattering wavevectors and direct photoluminescence from a monolayer of a 2D material at sharp angles.

Shailendra, Sunny Tiwari , Asutosh from my group in collaboration with my colleague Atikur and his student Gokul show this unconventional nanowire antenna concept, experimentally.

The link to publication in European Physics Journal: Special Topics is above. This paper is part of a special issue on Photonic Materials

Arxiv link : https://arxiv.org/abs/2203.00391

Connection between science and empathy

Apart from ideas, and the utilitarian, materialistic benefits,what can science offer to the society? This is a question I repeatedly ask myself in understanding a related question: ‘why I do what I do?’. This question, in my opinion, is also at the heart of social relevance of the pursuit of science.

A vital aspect which scientific research can indirectly teach and train its practitioners and its beneficiaries is the ability to empathize.

Empathy towards a fellow living creature, and not just human beings, requires oneself to suspend ones ego and understand something from a different perspective. This act needs patience, and the result is almost always enriching.

A quote (mis?)attributed to Plato puts it succinctly:

“The highest form of knowledge is empathy, for it requires us to suspend our egos and live in another’s world. It requires profound purpose larger than the self.”

One of the interesting aspects of scientific research is to study an idea or an object of interest from various different viewpoints. This ability to look at a particular thing from various conceptual angles enriches the understanding, and concomitantly clarifies the purpose.

Many a times one would be able to obtain an unexpected insight by looking at something from a different viewpoint.

The pursuit and the spirit of scientific enquiry essentially requires the same attributes as empathy, and hence the connection.

It is astonishing fact that we are witnessing a war among human beings in this day and age. Human beings are the most dominant creatures of our planet. This domination has already caused a severe problem in the form of climate change, and has drastically affected our own well being. War is the last thing you want at any circumstance.

If we have to overcome these problems,  we cannot ignore science or empathy. In an essense, ignoring them is like reversing the benefits of human intellectual evolution.

We humans can do far better than this…

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 Bengaluru..is 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 !