Toys, Geim and Gupta

Recently I came across an editorial in Nature Physics, titled as Physics is our playground, which emphasized how playfulness has had an important role in some of the major inventions and discoveries in physics.

A particular example of this is the discovery of graphene, and how it has evolved into one of the most important topics in condensed matter science. Nowadays graphene is used as ‘Lego’ blocks to build higher order structures and the so-called ‘Van der Walls’ heterostructures are one of the most exciting applications of 2D materials. What started as a playful project in the lab has now turned out to be an important part of emerging technologies.

Two important inferences can be drawn from the playful attitude towards doing science :

First is that making modular elements and stacking them creatively can lead to emergence of new structures and function. Anyone who has used lego blocks can immediately relate to it.

Second is that toys are powerful research and teaching aids. Please note, that I emphasized research and teaching here. This is because toy-models are ubiquitous in research, and they help us create modular state of a problem in which unnecessary details are discarded and only the essential parts are retained. This way of thinking has been extremely powerful in science and technology (for example : see ball and stick models in chemistry and mega-construction models in civil engineering )

When it comes to toys and education, there is no better example than the remarkable Arvind Gupta (see his TED talk). His philosophy of using toys as thinking aids is very inspiring. Being in Pune, I have had a few opportunities to attend his talks and interact with him (as part of an event at science activity center at IISER-Pune), and I found his approach both refreshing and implementable. Importantly, it also showed me how creativity can emerge from constraints. To re-emphasize this, let me quote APS news article on Andre Geim :

“Geim has said that his predominant research strategy is to use whatever research facilities are available to him and try to do something new with the equipment at hand. He calls this his “Lego doctrine”: “You have all these different pieces and you have to build something based strictly on the pieces you’ve got.””

Now this is an effective research strategy for experiments in India !

Review article – Plasmonic colloids + Optical Vortex

Appended is a link to arxiv preprint of an invited review article that I wrote as part of a special issue on nanophotonics in the Indian Journal of Physics and Applied Physics. The issue is edited by Dr. Achanta Venugopal (TIFR/NPL).

In this review, I discuss about assembly and dynamics of plasmonic colloids under the influence of optical vortex fields.

The abstract reads :
Structured light has emerged as an important tool to interrogate and manipulate matter at micron and sub-micron scale. One form of structured light is an optical vortex beam. The helical wavefront of these vortices carry orbital angular momentum which can be transferred to a Brownian colloid. When the colloid is made of metallic nanostructures, such as silver and gold, resonant optical effects play a vital role, and the interaction leads to complex dynamics and assembly. This brief review aims to discuss some recent work on trapping plasmonic colloids with optical vortices and their lattices. The role of optical scattering and absorption has important implications on the underlying forces and torques, which is specifically enunciated. The effect of spin and orbital angular momentum in an optical vortex can lead to spin-orbit coupling dynamics, and these effects are highlighted with examples from the literature. In addition to assembly and dynamics, enhanced Brownian motion of plasmonic colloids under the influence of a vortex-lattice is discussed. The pedagogical aspects to understand the interaction between optical vortex and plasmonic colloids is emphasized.

I welcome comments on the review.

arxiv :

New paper : Microsphere can narrow emission from a 2D material on a mirror

We have new paper appearing in Applied Physics Letters on how a dielectric microsphere placed on a 2D material deposited on a mirror can act as an optical antenna (see left panel for the schematic of the geometry and an optical image of the realized antenna).

The experimental and simulation efforts were mainly driven by our dynamic PhD student Shailendra Kumar Chaubey, who is very passionate about nanophotonics of 2D materials. He along with Sunny Tiwari and Diptabrata Paul explicitly show how experimental parameters such as sphere size and location of focusing can influence the photoluminescence emission from a WS2 monolayers. The experiments were mainly possible thanks to our collaboration with my colleague Atikur Rahman and his student Gokul, who continue to produce fantastic 2D materials for our nanophotonics study.

Interestingly, the emission from the WS2 monlayers can be as narrow as 4.6 degrees (see right side panel of the figure) which is one of the narrowest angular spread at room temperature. We also capture the energy-momentum photoluminescence spectra from WS2 monolayers, which is convoluted with the beautiful whispering gallery modes of the microsphere (see parts (a) and (d) of the figure).

We envisage such ’emission engineering’ using a simple microsphere can be further harnessed to control emission from quantum and nonlinear photonic 2D materials. Also, it raises new questions on how local photonic density of states can be tailored by altering the local environment around quantum emitters in solid state materials.

Arxiv version of the paper :

12 Years as a faculty member in India – 12 lessons

Today I complete 12 years as a faculty member at IISER-Pune. I have attempted to put together a list of some lessons that I have learnt so far. A disclaimer to note is that this list is by no means a comprehensive one, but a text of self-reflection from my viewpoint on Indian academia. Of course, I write this in my personal capacity. So here is it..

  1. People first, infrastructure next – As an experimental physicist, people and infrastructure at working place are of paramount importance. When I am forced to prioritize between them, I have chosen people over infrastructure. I am extremely fortunate to have worked with and continue to work with excellent students, faculty colleagues, and admin staff members. I think a good work place is mainly defined by the people who occupy it. I am no way neglecting the role of infrastructure in academia, especially in a country like India, but people have a greater impact in academic life than infrastructure.
  2. Create internal standards – In academia, whether you like it or not, there will be evaluations and judgments on your research and teaching. Generally, every academic ecosystem has its own standards for evaluating people. These standards are generalized principles and are not customized to an individual. Therefore, it was important for me to realize what good work meant and how to judge myself. As long as internal standards are high, and are properly met with consistency, the external evaluation becomes secondary. This attitude totally frees up the mind, and helps me to get better compared to my previous self. This also means I can appreciate others work without having to compare myself to them.
  3. Constancy and Moderation – When it comes to any work, it is important to be consistent over a long period of time. An equally important aspect is to moderate the amount of work done for a period. Most of the important work in research happens in units of months if not years. Therefore, constant effort with moderation keeps the motivation high, and makes the work enjoyable. Binge-working is attractive, but for intellectual work it is ineffective.
  4. Writing is a great tool to think – One of the most underestimated tools of thinking is writing. Not just formal writing, but the process of external articulation of thought on paper does wonders. Countless times, I have obtained, clarified and developed my thoughts only after I started writing about it. Writing is integral part of research and not just a communication tool.
  5. Importance of philosophy – Ever since I was an undergrad, I have been interested in philosophy of science. I had never taken a formal course on philosophy, but I have gradually started appreciating the role of philosophy of science. Specifically, it has catered to my question on ‘why I do what I do?’
  6. Teaching as social responsibility – In the Indian academia, scientific social responsibility is a buzz word. For me, the greatest social responsibility as an academic comes in the form of teaching. In larger scheme of things, some how we tend to neglect the impact of conventional teaching on students. Also, this impact is not easily measured. But the joy one obtains seeing a student do well is priceless. This local impact is what I value more because the feedback is there to see, right in front of you.
  7. Science, sports and arts : a trinity – I love outdoor sports including running, swimming, cricket etc. Equally, I enjoy listening to music, reading poetry and appreciating any form of art from any culture. I have found that it has not only made me a better person, but also has positively impacted my work. Taken as a trinity, these human endeavours continue to enrich our lives.
  8. Emphasis on mental and physical health – The most important policy that I have set in my research group is this: To do effective and enjoyable work, your mental and physical health should be good. There is no point in doing good work at the expense of your health. Therefore the priority will always be good health followed by good work.
  9. Social media : effective if used properly – Social media has a unique reach. If you are in India, and you want to connect to the scientific world, social media is an excellent platform. Given the size and diversity of India, social media can also reach people from various backgrounds and languages. If used responsibly, social media can have great impact on how science is communicated within India and how India connects to the scientific world.
  10. Renewed interest in applied mathematics and coding – During my BSc (Maths, Physics, Electronics) and MSc (Physics), I had excellent training and exposure in mathematics. During the lock-down period, I rejuvenated my interest in applied mathematics (especially nonlinear dynamics) and have started coding too. This has added a new impetus to my research and should reflect in the coming years.
  11. Professor as a post-doc – A research strategy I found useful in my work is to treat myself as a post-doc in the lab. Given that, in India retaining a long-term post-doc is difficult, many of the skills and thought processes cannot be effectively transferred in lab. A long term research problem needs sustained effort in the lab. As an experimental faculty it is easy to get caught up in activities outside the lab and lose touch with the (optical) bench work. Thanks to the lock-down period, I was the only one in the lab for almost 6 months,and I restarted my experimental work. I value that time and I see great benefit in this approach.
  12. Self-mentoring – A lot of academic advice is written by people working in the west. A few of them are general and applicable to Indian academia, but many of them are not. In such a situation, as an academic I try to derive inspiration by reading, especially about people who have done great work in India. I have been deeply influenced by people such as M. Visvesvaraya and Ashoke Sen. 

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 :

Some work from our group on optical vortex : ACS Photonics 6, 1, 148–153 (2019)

Book on singular optics :

Science paper on optical analog of event horizon

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 :

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 🙂

47. Nagendra, Weinberg and some memories…

IIA days…

It was late summer/early monsoon season of 2003, in Bangalore. The BTS bus travel from Rajajinagar to Koramangala via Majestic used to take 90 min or more. This commute, which I did for about 2 to 3 months, as summer student at Indian Institute Astrophysics (IIA) is still etched in my memory. I had just finished my first year MSc (Physics), and was seriously hooked on to physics in general, and astrophysics in particular. My summer project was on second solar spectrum guided by Prof. K. N. Nagendra (KNN) at IIA. It was he who introduced me to the fabulous world of polarization optics in the context of solar physics. This opened my eyes to the spectacular world of photon transport through an inhomogeneous medium, and hence multiple scattering of light. It was KNN who also introduced me to the classic : Radiative Transfer by Subrahmanyan Chandrasekhar. My first task as a summer student was to read the first chapter of this book and understand the representation of polarized light using Stokes parameters. The summer of 2003, was also the first time I encountered the power of computational methods to solve scientific problems, and ever since then I have deeply appreciated the role of computers in solving scientific problems. This introduction to computational physics and polarization optics (in the form of Jones, Stokes and Muller matrices) has turned out to be an important concept which I still use in my research. I thank KNN for this.

Recently, I was shocked to know that Prof. KNN passed away. His death was untimely, and a very sad news to me and many of the people who knew him. My condolences to his family, friends and students.

Weinberg inspires…

Recently, I also came to know about the sad demise of Steven Weinberg. Thanks to a special paper on Introduction to Quantum Electrodynamics in the final semester of my MSc, I learnt a bit about Weinberg as we were introduced to some aspects of unification of weak and electromagnetic forces. Also, with great enthusiasm, I learnt a lot from his fascinating book : The First Three Minutes: A Modern View of the Origin of the Universe. Undoubtedly, the scientific world has lost a great thinker.

The greatest impact of Weinberg on me was in a different context. In summer of 2004, I was selected for a PhD position at JNCASR. Prof. Chandrabhas had agreed to take me in as a PhD student, and I was elated and excited to join his group. I still remember the first time I visited his lab (after the selection) sometime in late May or early June 2004. As I entered the lab and opened that famous sliding door, there was a print-out of an article which was pasted right beside the door. This article was the Four Golden Lessons by Steven Weinberg, which was then recently published in 2003. This was literally, the first article I read as a PhD student in the lab, and has deeply impacted my work.

I still revisit the four golden lessons, time and again, and has been extremely useful throughout my career. As a tribute to him, below I reproduce the third lesson, which I think is worth contemplating :

My third piece of advice is probably the hardest to take. It is to forgive yourself for wasting time. Students are only asked to solve problems that their professors (unless unusually cruel) know to be solvable. In addition, it doesn’t matter if the problems are scientifically important — they have to be solved to pass the course. But in the real world, it’s very hard to know which problems are important, and you never know whether at a given moment in history a problem is solvable. At the beginning of the twentieth century, several leading physicists, including Lorentz and Abraham, were trying to work out a theory of the electron. This was partly in order to understand why all attempts to detect effects of Earth’s motion through the ether had failed. We now know that they were working on the wrong problem. At that time, no one could have developed a successful theory of the electron, because quantum mechanics had not yet been discovered. It took the genius of Albert Einstein in 1905 to realize that the right problem on which to work was the effect of motion on measurements of space and time. This led him to the special theory of relativity. As you will never be sure which are the right problems to work on, most of the time that you spend in the laboratory or at your desk will be wasted. If you want to be creative, then you will have to get used to spending most of your time not being creative, to being becalmed on the ocean of scientific knowledge. (emphasis is mine)

Thank you, KNN and Weinberg…for some golden lessons…

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 :

Letter to My Undergrad Students

This semester I was teaching an advanced physics lab course (4th year BS-MS). Below is an email I sent to them. You may find it interesting :


Image of a plasma discharge experiment in the UG physics lab at IISER-Pune

Dear Students of PHY430,

I hope all of you are doing fine and staying safe where ever you are. Given that we are part of an advanced lab course, compensating for the lost time via internet is not feasible. To fill in the gap, I am writing to you about something you may find interesting and useful. So here it is:

  • Ventilators : By now you may be very familiar with this terminology. Essentially, it is a medical device that helps you to mechanically breathe, and has turned out be a vital component in fighting the extreme medical cases of COVID epidemic. In this regard, I want to inform about the efforts of my colleagues Sunil Nair and Umakant Rapol, who are actively involved in design and development of low-cost ventilators. As you may recognize, both of them are experimental physicists, and their knowledge and intuition has been put to excellent use during the pandemic. In an essence, their involvement in this venture shows how a strong foundation in physics can not only solve deep queries in fundamental aspects of science, but also can cater to an emergency situation. This is one of the important lesson of this course : the skills and knowledge that you gain as part of experimentation in a lab can be transferred and implemented to solve problems outside a lab.
  • A Book recommendation: Talking about experimental skills, I thought of recommending an excellent book by Matthew Crawford titled “Shop Class as Soulcraft: An Inquiry Into the Value of Work”. This is a kind of an autobiographical exposition by the author, who majored in Physics, obtained a Ph.D. in political philosophy, and worked in policy circles of Washington D.C. for a brief period, and quit this job to become a motorcycle mechanic and an academic author. This book dives deep into the philosophy of why working with hands (and brains) is a deeply satisfying venture as a career and life-style. If you are not able to read the book, here is an excellent excerpt by the author himself.
  • Lab reports: Do send me the report of the experiments that are due to be evaluated. I know some of you may or may not have good access to internet, so timelines can be flexible (2 weeks from today). Also, you may not have access to research material. In that case, do co-ordinate with your lab partners, and let me know if I can be of some help in this regard.

Finally, keep your experimental spirits high. After all, everything at home is a kind of lab equipment to explore


Stay safe,