VISMAYA – History & Philosophy of Science – Page 43 – HUMANIZING SCIENCE : PEOPLE, IDEAS, TOOLS & WONDER

Soft Matter Physics + Photonics – my renewed interest

Malleshwaram is one of the oldest parts of Bangalore. I studied BSc (Physics, Maths, Electronics) in MES College which is at the 15^th cross of Malleshwaram. Apart from the college day memories of eating Dosae at CTR, other memorable aspects of my student life were playing cricket at Malleshwaram ground, and regularly visiting IISc and Raman Research Institute (RRI), which were not far from Malleshwaram. Particularly, the library at RRI was the place I spent most of my time during BSc and MSc. Two Professors at RRI with whom I interacted a lot were Prof. G.S. Ranganath and Prof. G. Srinivasan (both are retired now). I owe a lot of my interest in science to these two gentlemen. I was always interested in optical physics, and thanks to the interaction with Prof. G Srinivasan, I really got interested in optical phenomena in astronomy and astrophysics (I even did a rotation curve experiment using the radio telescope at RRI).

Thanks to this excitement, during my MSc Physics at Bangalore University, I did my summer research project at Indian Institute of Astrophysics, Bangalore with Prof. K.N. Nagendra, who introduced me to solar astrophysics. In fact, my project was on second solar spectrum and polarization of light in stars such as sun. Gradually, as I learnt more about optics in stellar environment, I increasingly became interested in optics of everyday life, and started exploring optics of rainbow, soap bubbles and other common objects.

Concomitantly, during BSc and MSc days, I and some of my classmates used to visit RRI and interact with Prof. G.S. Ranganath. He was the one who introduced us to soft-matter physics. Importantly, he impressed upon me the fascinating world at the interface of soft-matter physics and optics. I strongly recommend one of his books, which discusses some of these topics.

This introduction to soft-matter physics and interactions with Prof. Ranganath has had a profound impact on my research career. So much so, that I joined Prof. Chandrabhas’s lab at JNCASR for my Ph.D. to work on a (then) newly emerging topic of surface enhanced Raman scattering, which had a unique blend of colloids (a prototypical soft-matter) and light scattering, and it perfectly suited my research interest. During my Ph.D., I had a fantastic and thrilling experience of working on topics related to interaction of metal colloids with biological macromolecules using Raman scattering microscope as a tool. Thanks to the deep knowledge of Prof. Chandrabhas on optics and optical spectroscopy, and a variety of research at JNCASR, I got introduced to the fascinating field of optical microscopy, Raman scattering and soft-matter physics. Then during my post doc, first at ICFO-Barcelona, I got introduced to near-field optics and single-molecule imaging, and then at Purdue University, I learnt a bit of cell biology and used plasmonic light scattering to study some questions in bio-imaging.

Ever since I started my own research group in 2010 at IISER-Pune, my research interest evolved in topics such as nanowire plasmonics, spin and orbital angular momentum of light, whispering gallery modes in microspheres, single-molecule Raman scattering, and Fourier-plane optical microscopy and spectroscopy. As of Feb 2020, 6 Ph.D. students and around 9 MS students have graduated working on the abovementioned topics. The main focus, for about 10 years, has been on nanophotonics, and on some topics related to soft-matter physics, especially on colloids.

Starting Jan 2020, our emphasis and research orientation will be mainly towards ‘soft-photonics’. The motivation of this research is to explore some emerging questions at the interface of soft matter physics and micro- and nano-photonics. There are two important objectives to this research:

To study structure and dynamics of mesoscale soft-matter including colloids, liquid crystals, and complex fluids using a variety of techniques that we have developed for the past 10 years in the area of nanophotonics and single-nanoparticle optics.
To explore new opportunities in meso- and nano-photonics using soft-matter systems such as colloids, liquid crystals, droplets and bubbles, as a platform.

In a way, for the past 5 years or so, we have been implicitly working on these objectives. But from 2020 onwards, we will be mainly focusing on these objectives, and will be orienting all our efforts towards this direction.

This explicit reorientation is for the following reasons:

The interface of soft-matter physics and photonics provides some new opportunities to study some interesting questions in fundamental physics (such as topology, pattern formation, emergence and single-macromolecule dynamics) and applications (optofluidics, optical antennas, aerosol optics and gastronomy)
Light scattering and “quantitative” optical microscopy have emerged as powerful tools to study structure and dynamics of soft-matter. Given that our lab has laid a strong foundation in these tools for 10 years or so, it is an obvious extrapolation of our capabilities.
Thanks to the interaction with my soft-matter colleagues at IISER-Pune and many friends/researchers across India and outside, I have been “re-hooked” to soft matter physics. Given that the Indian research community on soft-matter is growing in number and has a good mix of experiments and theory, further motivates me to pursue this direction.
Perhaps the most important reason is that it renews my interest in science and reminds me of the fundamental reason of why I became a researcher: to enjoy what I do!

As a consequence of this renewed interest, I intend to write blogs oriented towards soft matter physics + photonics and wish to use this platform to educate myself and communicate my excitement with all of you.

Let me conclude by quoting “a poem from an experiment of soft matter” by Boudin, which is also the concluding part of the Nobel lecture of Pierre de Gennes:

“Have fun on sea and land

Unhappy it is to become famous

Riches, honors, false glitters of this world

All is but soap bubbles”

Importance of a failed experiment

India’s recent mission to reach the moon, Chandrayaan 2, has spurred a lot of interest, and I am glad that it is getting the attention it deserves. As we know, the space vehicle was supposed to land on the south pole of the moon but lost communication with earth just before the touchdown. The initial goal of landing the spacecraft was not achieved as per the expectation. The efforts of the people involved in this mission are indeed laudable. Given the drive, commitment and financial support that ISRO has, I am sure they will achieve greater things in the future.

This event is also a good occasion to talk about the importance of failed experiments in science, and below is my take:

This semester I have been teaching an advanced physics lab course to the 4^th year BS-MS students. There are about 23 students, and we have been performing some experiments on concepts such as Thermionic emission, statistics in radioactive decay, electron spin resonance, Zeeman Effect, etc. As you may guess, all these experiments have deep connection to quantum mechanics, and its manifestation is evidenced in the lab. These experiments are designed such that we can test some hypothesis by formulating them as a question, and the experiments aim to reveal an answer to the posed question. As part of the process, the students explore the basic theory behind the experiment, understand the rationale behind the instrumentation utilized, and perform measurement and analyse the error in them. They are expected to record their observations, and finally submit a report in the form of a small research paper.

Many a times, the experiment that the students perform do not work according to the plan. So they need to troubleshoot the problem, and understand why things are failing. This stage of troubleshooting is where one LEARNS about how to do an experiment. After a careful analysis, they figure out where the problem was, and rectify it to proceed further. This whole process requires attention to details, better understanding of the instruments under use, and importantly a lot of patience. In a way, a lab course, if done in the right spirit, is one of the most fulfilling aspects of science education because it interfaces the abstract knowledge with the real world. So our understanding of the physical world is not only enriched but also we gain some degree of control over it, which is kind of empowering, so to speak.

Now what about experiments in a research lab? Well, the story is even more interesting in this situation. A majority of the times, the experiments that we design in a research lab DOES NOT work. In fact, we will not even know whether the direction we are taking is indeed the most accurate and appropriate one. Therefore, a careful design of experiments guided by hypothesis, and an educated “guesstimation” plays a vital role. Even with all precautions, we may fail to perform the experiments according to the plan. So the question is: how does one react during such a situation?

This is where the training we get in the laboratory courses is very vital. We need to fine tune our thinking to know what it is to do an experiment. Given the high probability of failure, we need to consider every experiment as a path to learn something new. This means that the negative result what we get should be considered as a feedback to our thought process.

With this new information from the failed experiment, there are at least two important prospects: First is that it will improve our understanding about the current situation, and throw some light on corrections that we need incorporate in our experiments. The second and more interesting aspect is that it can lead to a completely new direction of research which we may have otherwise ignored. This emergence of new direction is what makes experimentation very interesting. The new, uncharted path that a failed experiment can take us may result in some major discovery or inventions. History of science has a few examples of experiment with negative results that have led to major breakthroughs (for example Michelson-Morley experiment). A caveat to add is that not all negative experiment may result in a breakthrough. Generally speaking, paying attention to the failure is imperative to learn something new, and the same goes with experimentation. In an essence, true progress in experiments (and science in general) can be achieved only by revising it further. Let me conclude by quoting Peter Medawar (Advice to a Young Scientist (1979), 94):

“All experimentation is criticism. If an experiment does not hold out the possibility of causing one to revise one’s views, it is hard to see why it should be done at all.”

Conversations in Research Life

Knowledge-sharing — Image credit : https://www.queensu.ca/connect/grad/page/3/

This semester I am teaching a course on interdisciplinary optics to about 200+ sophomore undergraduate students. The class encompasses diverse audience with varied interest, and I have been exploring some topics at the interface of optics and other disciplines. After we finish a class, I invariably have a conversation with a few students who have specific questions not only on the topics I have been teaching, but also on general optical phenomena. Since these questions arise totally out of interest of the students, I strongly encourage them and spend as much time as possible to address them. What I have found during these conversations is that the quality of questions is very good. I have found that I can answer only a few of them, but invariably it makes me think about it in greater detail, and further motivates me to consult relevant literature so that I can address the question in depth. This process of informal conversation is one of the enriching experiences of teaching. This has made me think about the role of conversation in science, especially in an informal way, and how it has influenced my thinking as a researcher.

During my undergraduate and postgraduate years, I frequently visited Raman Research Institute (RRI) in Bangalore. I did spend a lot of time in the RRI library, which I think is one of best in the country, especially for science literature. Thanks to great conversations and encouragement of Prof. G. Srinivasan and Prof. G.S. Ranganath, who were scientists at RRI (now retired), I was able to learn very interesting aspects of astronomy, astrophysics, optics, thermodynamics and soft-matter physics. Over informal conversations with them, I learnt interesting questions as diverse as: how stars form? What is the role of surface tension in formation of a soap bubble? Can an electron move faster than light in a specific medium? How diamonds shine light? How to measure colour? And many more…

Questions like these were per se not part of any curricula, but what I realized that the process of answering them took me on a mini intellectual-journey so to speak, and this process has had a tremendous influence on me and my work. What was fundamental to this process was the informal conversation that I had not only with the above-mentioned gentlemen, but also with my friends.

When I started my Ph.D. at JNCASR, I had a great set of batchmates with diverse interest in atomic, molecular and optical physics, high energy physics, condensed matter physics, all sub-disciplines of chemistry, molecular biology, ecology and fluid mechanics. Invariably, our informal conversations during coffee-break, lunch and dinner used to revolve around explaining some everyday phenomena from various viewpoints. These conversations were never meant to be serious. In fact most of the time it was a joke that we were trying to explain, but invariably, the humour was built on the relevant research an individual was doing, and this added great flavour to discussion, and ended up as a joyful learning experience. Surprisingly, the memories of certain moments that we spent during these informal conversations have still remained intact in my mind, and I cherish them.

After finishing my Ph.D., I moved to Barcelona, and then to the US. The informal conversations still played a critical role in my everyday research. Being in labs with great scientific, cultural, linguistic and artistic diversity, what I learnt was that the same science that I had learnt was viewed with different spectacles. It means analogies to explain a specific scientific concept depended on the person’s personal history. This added tremendous variety and richness to informal conversations on science. It also helped me appreciate diversity in viewpoints, and a bit of understanding on different cultures of doing science.

In my own research group, informal conversations on science and research play a very important role in our everyday research. Majority of the time we spend asking questions that help clarify our research, and further add new insights to the work we are doing. This process is generally through a conversation. Research students also learn from each other by talking on science in an informal way, and this percolation of knowledge is as important as reading research papers, and attending talks. Note that other form of scientific communication such as journals, research talks, posters are still the bedrocks of research, but the informal conversations on work plays a significant role in how we perform our research.

This human interaction through conversation is the reason why attending conferences and meetings is still a very important part of scientific life. Even when we have read research papers of an author, we obtain new insight on the same work when we converse with the author, in person. This valuable interaction adds a new dimension to our thoughts, and gives us an opportunity to express ideas which sometimes may get lost in formal communication channels.

Another intriguing but equally interesting aspect of doing science is to have an informal conversation with oneself on concepts and questions we are trying to address. Invariably, I end up understanding something when I try to explain it, first to myself and then to others. This process of “self-talk” is a very useful way to clarify ideas and identify a loophole in my own arguments. What is also interesting is that what we call as “our own thoughts”, are essentially words and images that we have borrowed from an external source. A quote attributed to Alan Watts nicely summarizes this point:

We seldom realize, for example, that our most private thoughts and emotions are not actually our own. For we think in terms of languages and images which we did not invent, but which were given to us by our society.

Verbal conversations are important part of human interaction. We learn, unlearn and relearn many things by talking to each other in an informal setting. Not only we exchange ideas during conversations, but also create new ones. Let me end by quoting the Oxford scholar Theodore Zeldin:

Conversation is a meeting of minds with different memories and habits. When minds meet, they don’t just exchange facts: they transform them, reshape them, draw different implications from them, and engage in new trains of thought. Conversation doesn’t just reshuffle the cards: it creates new cards.

Expression as Exploration

“How often I found where I should be going only by setting out for somewhere else.”
R. Buckminster Fuller

About a month ago, I had an opportunity to interact with school students who were on the verge of transitioning from 10^th and 11^th grade. This event was part of a tech-fest organized by College of Engineering, Pune. The topic of discussion was “what scientist does in everyday life?” The students were very communicative (surprise!) and asked many questions (another surprise!), which was heartening. During the interaction, one of the issues we discussed was the importance of note-taking, as part of any serious observation in science, art or any other creative pursuit.

One of the curious questions asked by a student was the following: “If there are so many technological tools that are available to us today, why should we at all write by hand? Why don’t we directly learn typing on a computer instead of handwriting?”

This was an important question, and I did mention that writing by hand has not only the benefit of processing thoughts more effectively, but also provides a sense of creation that may be lost while typing a text. Furthermore, symbolic representation, manipulation and thought processing – as done in mathematical thinking or calligraphy – is more conducive and convenient in the hand written form.

I also pointed out that there is some scientific evidence which indicates that handwritten notes have greater impact on processing the information in our brain, than when the same notes are typed on a device. I told that there is a form of elegance and individuality that a handwritten displays, which may not be represented in a text that is typed. I mentioned that writing in general and handwriting in particular, was not only a form expression but also as form of exploration. I indicated that just like music, writing has a psychological benefit of its own. It helps you to explore your thoughts and creates a sense of connection with oneself. Interestingly, it will also take you on a journey which you may not anticipate. The quote at the beginning of this blog sums it up nicely. Writing is a form of exploration, and by merely writing, we are taken to new worlds which we had not envisaged or planned to go.

In this blog I give 2 examples of a scientist and a writer, who have effectively used handwritten text in their work and have deeply impacted their respective fields. The choice is purely personal, as they are inspirational to me. Here we go….

Marie Curie

Curie photo — Image Credit : India Today

Cutting-edge science in early 1900s, especially in experimental physics and chemistry has had a great impact on modern society. Among the many who thought deeply about the nature of matter, Marie Curie’s contribution stood out. As a dedicated researcher, she not only developed elaborate experimental methods by herself to unveil the secrets of radioactivity, but also silently built a school of thought where dodgy, experimental exploration motivated new questions and directions in natural science. Below text is a snapshot from Marie Curie’s notes which describes the sample preparation in her lab. Interestingly, the mentioned texts of Marie Curie are still radioactive (and kept under isolation), and will remain radioactive for another 1500 year!

Marie Curie's notebook — Image credit : Wellcome Images

Virginia Woolf

A literary giant who is surely one of the pioneers of modernist thought process, kept a diary for herself all throughout her life. In my opinion she was a great humanist who redefined the art of narrative from a modern perspective. What’s more, her texts are so quotable that anybody who reads them will get a new viewpoint of the world which we had never seen. Below I reproduce a copy of her handwritten page of her famous book “A room of one’s own”. In this text, the story is still in the making, but you can see how a cluttered text at that time has evolved into a masterpiece now.

A Room of One's Own, by Virginia Woolf — Image Credit : Cambridge University

Well….preaching without practice is always hollow. When I was interacting with the students regarding handwritten text, they asked me whether I do write by hand. And my answer was yes, and below is a small handwritten note from my own notebook:

Pavan — Snapshot of text from my notebook

Handwritten text has its own aesthetic value, and I believe it should be retained as long as human expression exists.

Virginia Woolf once famously wrote,

“Thoughts without words… Can that be?”

last part modified

Physics Nobel….via Geetanjali

Every year the Science club at IISER-Pune organizes “Nobel evening – an event that has been a part of IISER since its inception and has thus become tradition”. This is essentially a gathering of IISER community where public lectures are delivered on the latest Nobel prizes.

On 22nd Oct 2018, we had this year’s Nobel evening, and in there I gave a talk on 2018 Nobel Prize in Physics. It was indeed an honour to talk about the work which has created such a deep impact on both science and technology.

While I was preparing for the talk over the weekend (talk was on a monday), I took a break and casually looked into my collection of books. I happened to skim through Tagore’s Geetanjali. As I went through the beautiful verses of this epic poem, what immediately struck me was its 44th verse. I could see an interesting analogy between the 44th verse, and the 2018 Nobel prize in physics. So I decide to use this analogy in closing of my talk. The whole talk had 25 slide. Below I show the 3 slides from my talk : the first one being the opening slide and last two are my closing slides that contain the analogy I mentioned about….

first slide

Geeta last slide

A Random Walk in Edinburgh

Diffusion is a simple yet fascinating physical phenomenon. By merely observing how an object moves around in a medium as a function of time, there is a lot of stuff one can learn about the environment, about the diffuser and about the interaction between diffuser and its environment. Over the last few days, I have been studying some papers related to trajectory of individual nanostructures in liquid environment, and have learnt some interesting aspects such as sub-diffusion and super-diffusion.

Concomitantly, I came across one of the better poems I have read in recent times on travelling: Childe Harold’s Pilgrimage by Lord Byron George. This is a long poem, but a couple of stanzas are worth a read:

There is a pleasure in the pathless woods,
There is a rapture on the lonely shore,
There is society where none intrudes,
By the deep Sea, and music in its roar:
I love not Man the less, but Nature more,
From these our interviews, in which I steal
From all I may be, or have been before,
To mingle with the Universe, and feel
What I can ne’er express, yet cannot all conceal.

Roll on, thou deep and dark blue Ocean—roll!
Ten thousand fleets sweep over thee in vain;
Man marks the earth with ruin—his control
Stops with the shore;—upon the watery plain
The wrecks are all thy deed, nor doth remain
A shadow of man’s ravage, save his own,
When for a moment, like a drop of rain,
He sinks into thy depths with bubbling groan,
Without a grave, unknelled, uncoffined, and unknown

These two readings (of the paper and the poem) were on the same day, and I felt a connection between diffusion and travelling. It kept lingering on my mind for the next few days, and I felt going deeper and exploring it further. So, I went back to my archived files on my laptop and started exploring some photographs I have taken over the years of travel. During this exploration, I found some of my travelogue related to Scotland when I visited that beautiful country in June 2013. During that trip, I and some of my colleagues were mainly visiting Glasgow University. During the last leg of the trip, we visited the University of Edinburgh and Edinburgh city for a day.

It was a bright, sunny day on 21 June 2013 (generally 21 June is the longest day of the year). We took an early morning train from Glasgow to Edinburgh. Around 9am in the morning, we were at University of Edinburgh, and we visited the departments of physics and chemistry. This was followed by talks by us and a few researchers at the university. After our interaction and lunch, we had about 5 to 6 hours to spend in the city of Edinburgh before we could take our train back to Glasgow. So, we went to the city centre, and I decided to visit the travel information desk. I wanted to know if I could see around the city within 5 hours or so, and what places could I visit on feet. One of my favourite activities, especially when I am travelling alone, is to take a random walk around the city (of course with a map), and explore the places on feet. I have found these “on-feet” explorations can closely connect you to the place, and importantly slows one down so that one can pause, observe and grasp the local environment in its details. In an essence, this “confined Brownian motion” can lead to some interesting insight and thoughts.

Coming back to Edinburgh, I gathered all the information of possible sites I could visit on feet within 4 to 5 hours, and here are a few things I explored during the walk:

Statue of Sherlock Holmes:

Although a fiction-character, Sherlock Holmes has a real statue in Edinburgh! Anybody who has read Sherlock Holmes also knows its author Aurthur Conan Doyle cannot miss this place. Close by to the Holmes’ statue is a pub named The Conan Doyle (see below)

Conan doyle

Bronze statue of Adam Smith:

Adam Smith

A 10 feet long monument of Adam Smith cannot be ignored. The celebrated economist, philosopher and author of “The Wealth of Nation”, is one of jewels in the crown of Scotland.

Statue of James Clerk Maxwell:

Interestingly, this was the hardest thing to find in the city. It was at a remote corner of the town, and very few people knew that there is indeed a statue of Maxwell in Edinburgh city. It took me almost 45 to 60 minutes to explore the statue. I was almost about to give up, but somehow I did not want to….so I went ahead, and found this statue of the celebrated physicist. It was a happy moment!

The famous Scotch Wishky trail:

Whisky trail

How could one miss this! This was one of the easiest things to find on my path, and what I found inside this trail was nothing short of breath-taking variety of Scotch.

The cliff edge:

Sky and green

This was strictly-speaking not during the walk in the city, but just before that, and is perhaps the picture that has stuck in my mind all the while when I think about Scotland. Unconsciously, when I think of diffusion in space and time, this is the same picture that comes back to my mind. There is something unique about a person at cliff edge, all by himself exploring his universe…I find it kind of philosophical and fascinating…..and also goes well with abovementioned poem.

Brain is a strange thing. It forces us to connect the unconnected, and the above content is just an example. Towards the end of the trip, I sat on the train back to Glasgow. I started listening to the music on my headphone, and the song on my playlist was Hotel California. As I relaxed back in the seat, I was struck by these lyrics :

Last thing I remember, I was
Running for the door
I had to find the passage back to the place I was before
‘Relax’ said the night man,
‘We are programmed to receive.
You can check out any time you like,
But you can never leave!’

I had checked out of Edinburgh, but my mind has never left that random walk…..

Curiosity as a Career

Every morning, I have an interesting task at home. I prepare filtered-coffee and boil milk as soon as I wake up in the morning. Both these processes are supposed to be mundane task, but over the years I have found it to be one of the most intriguing things one can do in kitchen. To make the task engaging, I have been measuring the rate at which half a liter of milk boils and when does it reach the point where it is about to spill over from the container (of course, I do not spill it over, else I will be devoid of my morning coffee…no way). Over many years of this task, I have found that the parameters of milk boiling vary as a function of temperature, humidity, shape of the container, the pressure of the gas supplied in the stove, the content and age of the milk. I have also found some interesting methods to stop the milk spill over even while it is still under boil. In an essence, I start my day with a curious-experiment in the kitchen, and I look forward to it every day.

Radial patterns in a tomato slice on my kitchen slab

Curiosity as life – Tasks like boiling milk, preparing coffee, playing with tooth-paste, running in rain, watching clouds, creating soap bubbles, watching water flow, slicing vegetables (see image), dusting the house, cleaning a window pane, washing shoes and drying an umbrella are common to all of us. If you look at these tasks closely, one can connect them to a lot of interesting science. I have found great joy in doing so, and have turned out be an integral part of my life. An important off-shoot of this way of looking at things is that I hardly get bored. Every trivial thing that I observe has something intriguing, and this has had a profound influence on how I approach my life. Invariably, while exploring my curiosities, I find myself losing the feel for time, and one goes into the state of flow. That is a happy place to park your mind.

Scientists’ dilemma – ‘Impact on society’ is touted as the modern mantra for doing research. A scientist is strongly encouraged, especially by funding agencies, to work on research problems that have relevance to a large community. Even among scientific communities, novel solutions to research problems are often encouraged and are highly valued and rewarded. So, a scientist is always looking for problems that can have greater impact, either conceptually or technically. Influenced by this external push, the priority of what one has to do is always under question. Critically, this puts a scientist in a dilemma: should I work on problems that are curiosity-driven or should I work on problems that have largest impact to the society? This conundrum is especially sharp if one is a scientist whose research requires large infrastructure and financial assistance. Related to this dilemma is the debate of basic vs applied research, and has inspired concepts such as Pasteur’s quadrant. I do research for my living and most of time is spent on it. I and my research group think on the “why and how” of our research, and it is important for us to resolve this dilemma.

Resolving the dilemma, personally – Given that we do laboratory-based experimental research, I have to ensure that we secure research funds to keep it up and running. Concomitantly, I have to cater to my curiosity, without which I will not be able to sustain my interest in the work I do. Over the years, balancing these concerns has influenced the work I do. An important aspect of resolving the above-mentioned dilemma has been to spend long hours on identifying and choosing a research problem that caters to my curiosity and has relevance to the research community. The process of choosing a research problem is not a simple one, but in my opinion, is perhaps the most important step in doing research. After all, the question one defines will eventually guide the answer we can find; hence every minute we spend on it is priceless.

Light and light scattering has been central to all the stuff I do in my research. I am also intrigued by science in everyday life. So, the best possible thing to do was to study light-matter interaction. This inspired me to look for problems that can cater to my interest and a large research community, and may potentially have applications that can impact the society -all of this without having to sacrifice my curiosity. Over the years, this intention has guided me to pursue research at the interface of optical physics and biochemistry; nano-plasmonics, advanced optical instrumentation, and in recent times on plasmon-soft matter interactions. All these areas that I have been working-on are strongly rooted in my curiosities. I have deliberately picked these fields such that I never have to sacrifice on what I like to do.

Parting thoughts – Generally, among research students, there is a concern about their future, and how they can retain their curiosity and pursue their career. Invariably, they are sandwiched between what they like and what the external-world tells them to like. If these two things do not overlap, there is always frustration. For such situations, I have a suggestion: follow your curiosity and be cognizant of the fact that curiosity-driven life not only feeds your brain, but also your stomach. Just by following curiosity, a lot of people including myself, have been able to build a career out of it. What is further encouraging is that there is enough room in the society for our curiosities to develop and flourish, provided we take the effort to connect our curiosity to a relevant research problem out there. This exploration will take time, and we must remain patient until it yields. The onus of connecting our curiosity to external relevance is ours, and we must take the initiative. As the saying goes:

IF IT IS TO BE, IT IS UP TO ME!

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.

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

Bhoren

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.

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.
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.

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.

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.

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.

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.

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.

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).

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.

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…

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.

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.

Trapping Questions and Evolving Answers

A voice said, Look me in the stars
And tell me truly, men of earth,
If all the soul-and-body scars
Were not too much to pay for birth.

—- “A Question” By Robert Frost

In research, as in life, humble questions can sometimes lead to profound answers. A curious question flying as a passing thought in the mind of a researcher can equally lead to some important discoveries and inventions. Furthermore, what starts as a simple question, evolves into a creature that the questioners themselves would have not envisaged. This evolution of thought in various directions is fascinating to say the least, and history of science is dotted with such examples.

Take for example Arthur Ashkin of Bell Labs, who in late 1960s, asked the following question:

“is it possible to observe significant motion of small particles using the forces of radiation pressure from laser light?”

Note- at that point of time, lasers were still a relatively new invention, and people were looking for an application. In that context, it was indeed an interesting question to ask about the effect of laser beam on a small particle which may be immersed in fluid or in vacuum. After all, radiation pressure should have some effect on the motion of particles, as evidenced in the case of comet tails.

With this question, Ashkin embarked on a journey that conceptually and literally pushed and revolutionized a large part of our science and technology based on lasers. Ashkin’s question led to the realization of laser-based optical trap of microscopic objects, which further evolved into a major experimental tool not only in physics but also in biology and chemistry.

Below figure shows the conceptual schematic of Ashkin’s experiment, in which he introduced two counter-propagating laser beam which created an optical potential to stably trap an object in space and time. The physics of optical trapping itself in intriguing, in which, the compelling battle between forces due to in-line pushing and orthogonal pulling will be eventually won by the pulling component. A stable energy minimum is achieved at the center of the focused laser beam, in which the object of interest happily resides. Of course, parameters such as refractive index of the object and the medium play a critical role, so does the alignment of laser beam and its wavelength.

trap Ash — Optical schematic of the first optical trap created by Arthur Ashkin. Adapted from the original paper [1]*.

There are two important aspects to Ashkin’s work. One is that he pursued on a simple question that lead to an important observation, which has had far researching consequences not only in physics but also in biology and allied research areas, and the second point is that a few people, in his own lab felt that the discovery was not important. In the first chapter of his book, Ashkin describes a very interesting situation after he had performed this seminal work:

It may be interesting and instructive to recall the initial reactions of other scientists to paper [1]*, which described the earliest trapping work. At Bell Labs., before a manuscript could be sent out to a journal it had to undergo an internal review to make sure it would not tarnish the laboratory’s excellent reputation in research. Since paper [1]*was intended for Physical Review Letters, it was sent to the theoretical physics department for comment. The Bell Labs, internal reviewer made only four points: (i) there was no new physics here, (ii) the reviewer could not actually find anything wrong with the work (this is a reminiscent of the famous Pauli insult, when he commented on some work he thought worthless that “it is not even wrong!”), (iii) the work could probably be published somewhere, and (iv) but not in Phys. Rev. Lett.This four-point internal referee report from the theoretical group greatly distressed me, and so I went to my boss, Rudi Kompfner, inventor of the traveling wave tube, whom I greatly admired. Rudi, a man usually slow to anger, simply said, “Hell, just send it in!” As it turned out, I had no problem whatever with the Physical Review Letters reviewers. In 1999, paper [1]* had the honor of being selected as one of the 23 seminal papers on atomic physics reprinted in the compilation, “The Physical Review — The First Hundred Years”, edited by Henry Stroke, American Institute of Physics Press and Springer Verlag (1999) on the occasion of the centennial of the American Physical Society.

There are at least two important lessons in this story: a) not always one can instantaneously judge the importance of a research work and b) the notion of “new physics” depends on how you look at a topic and judge its implication. To see how a new result can connect to something else requires a kind of broad view of science well beyond the boundaries of the “known unknowns”.

Going further, Ashkin did not stop his train of questions. He writes that he was intrigued by the observations which further motivated him to explore on the following topics:

Could traps be observed for macroscopic particles in other media such as air or even in a vacuum? Could optical manipulation be used as a practical tool for studying light scattering, for example, and other properties of small macroscopic particles?

Evolution of Ideas

After some resistance, slowly the physics community started taking notice of Ashkin’s experiments, and paid more attention towards the simple yet powerful methods he was developing. What followed was indeed a revolution. The methods he developed immediately caught the attention of two very diverse research communities – one was of atomic physicists and other one was of biologists. Whereas the former were interested in trapping and cooling atoms, the later were in desperate search for non-invasive optical tools that could trap and manipulate cellular and sub-cellular objects. Optical trapping indeed catered enormously towards these research efforts. It not only led to “new and interesting physics”, but also some wonderful experiments in soft-matter and biological sciences. In order to give you a gist of the way Ashkin’s work evolved, below I give a table of interesting research results. As you will see, the papers themselves discuss topics and problems that were not envisaged by Ashkin, but the influence of his ideas percolated deep and wide.

Year	Link to the relevant papers and my comments
1982	Electromagnetic mirrors for neutral atomsThis paper theoretically proposed use of evanescent optical fields at dielectric-vaccum interface to reflect neutral atoms. The concept of radiation pressure at an interface was emphasized.
1986	· Three-dimensional viscous confinement and cooling of atoms by resonance radiation pressure· Experimental observation of optically trapped atoms These were the foundational experiments on laser cooling and trapping of atoms, which went on to win the 1997 Nobel Prize in physics. Note that Ashkin missed out on the prize!
1989	Optical Binding This introduced a fascinating concept of binding microscopic objects with long range optical forces facilitated by electromagnetic fields. This topic is still of great interest, and still inspires a variety of experiments.
1992	Movement of micrometer-sized particles in the evanescent field of a laser beam This paper was a pioneering contribution towards movement of particles in fluids using an evanescent wave of laser beam.
1993	Direct observation of kinesin stepping by optical trapping interferometry The abstract of this paper is worth a read and tells a compelling story :“Do biological motors move with regular steps? To address this question, we constructed instrumentation with the spatial and temporal sensitivity to resolve movement on a molecular scale. We deposited silica beads carrying single molecules of the motor protein kinesin on microtubules using optical tweezers and analysed their motion under controlled loads by interferometry. We find that kinesin moves with 8-nm steps.”
1996	Optical vortex trapping of particles This was one of the first experiments to use vortex beams to trap objects. In conclusion of the paper, the authors envisage trapping application based on holograms, which were created soon after the proposal.
1997	Theory of nanometric tweezerA first significant jump towards extrapolating optical trapping to sub-wavelength scales. The idea of utilizing a metal nano-tip to trap dielectric objects was proposed. This paper laid an excellent foundation for optical manipulation at nanometer scale.
1998	Optical tweezer arrays and optical substrates created with diffractive optics This literally added new dimensions to optical trapping, where a diffractive optical element, a static hologram in this case, was introduced in the optical scheme. This laid the foundation towards parallel trapping on conventional set-up, and has turned out to be extremely useful for applications in soft-matter physics and biological applications.
2001	Force of surface plasmon-coupled evanescent fields on Mie particles This theoretical paper compares how evanescently-excited surface plasmon polaritons at metal-dielectric interface can exert more force on Mie particle compared to a dielectric-dielectric interface, thus creating a platform for film-based plasmonic manipulation of micro-objects.
2006	Surface Plasmon Radiation Forces This was the first report that experimentally showed how surface plasmon from a metal interface exerted about 40 times more force on a micron sized particles compared to a dielectric interface. Importantly, this paper measure the trapping potential depths created by surface plasmon on a metal-film.
2007	Parallel and selective trapping in a patterned plasmonic landscape This was perhaps THE BREAKTHROUGH experiment in plasmonic trapping, that showed how gold nano-disc could create parallel traps of micron scale object at significantly lower power compared to optical trapping. A majority of plasmon trapping experiments nowadays derive their inspiration from this paper
2009	Self-induced back-action optical trapping of dielectric nanoparticles This experimental paper is one of the first reports which harness the feedback from the trapped 50 nm object to improve the performance of the trap. This significantly reduces the power of laser one needs to use for trapping experiments and represents truly a nanometric optical trap.
2010	Laser Printing Single Gold NanoparticlesOptical trapping forces are harnessed to printing individual gold nanoparticles on glass substrates. This has opened up new opportunities to directly fabricate nanostructure from colloidal phase onto a surface of interest.
2012	Subkelvin Parametric Feedback Cooling of a Laser-Trapped Nanoparticle To quote the authors “Using a single laser beam for both trapping and cooling we demonstrate a temperature compression ratio of four orders of magnitude”. This opens a new avenue to perform optical tests of quantum mechanics using isolated nanoparticles.
2014	Plasmofluidic Single-Molecule Surface Enhanced Raman Scattering from Dynamic Assembly of Plasmonic NanoparticlesThis is an experiment report from my group where we showed that one could not only create a large scale plasmonic trap of multiple nanoparticles, but also one can utilize it to perform single-molecule spectroscopy.
2016	Direct Measurement of Photon Recoil from a Levitated Nanoparticle Another experimental breakthrough where the photon recoil from a single nanoparticle is measured.
2018	Opto-thermoelectric nanotweezers This experimental paper shows how optical, thermo-plasmonic and electric fields can be combined to trap and manipulate nano-object in fluids.

To conclude I will again quote Ashkin, who makes an important observation in an editorial he wrote on the occasion of commemorating 50 years after the discovery of laser:

As we look to the future, what can we anticipate? Certainly much more of the present hot fields such as: single atom studies; properties and behavior of single biological molecules such as mechanoenzymes and nucleic acids; mechanical properties of single molecules and tissue; studies of particle arrays; and particle separation schemes. Of course, we cannot anticipate serendipitous discoveries. We can only hope to recognize them when they occur.

After all, one question leads to another….and the rest is evolution…you see!

Science Behind the Science Day

Every year on 28th February India celebrates National Science Day. Although, there is quite a bit of enthusiasm in this celebration, especially in academic environment including schools, universities and research institutes, the general public does not pay too much attention to it. Partly because symbolic days and symbolism are just that: symbols – superficial representation of something bigger. They do not encompass the complete picture, and they are too many in number.

Raman + Krishnan Effect : So, why does India celebrate National Science day on 28th February ? Well, on this day, way back in 1928, there was an experimental observation performed which turned out to be an important discovery in science. The main players in this discovery were two scientists from India: C.V. Raman and K.S. Krishnan. These men, after a long, sustained effort and with limited experimental resources, discovered a new type of secondary radiation, and the effect what is now known as Raman effect.

Stokes and anti-Stokes: The experiment that they were performing was to look at monochromatic (single colour) light scattering from molecules in a liquid. What Raman and Krishnan found was that scattered light had three components in terms of energy: the first and the dominant part of the scattered light had the same energy as the incident light, the second most dominant part of the scattered light had its energy lower than the incident light, and the third component, which was the weakest, had its energy greater than the incident light. These three components are called Rayleigh, Stokes and anti-Stokes light, respectively. The Stokes and the anti-Stokes components of the scattered light, together encompass the inelastic scattering in the process, and formally represent Raman scattering of light from molecules. What is remarkable on the part of Raman and Krishnan is that they experimentally observed these feeble, inelastic scattering components with ingenious experimental design. The story of this discovery is beautifully captured in the book The Raman and his effect by G. Venkatraman (who has also written a biography of C.V. Raman). In this book (page 46), we get a glimpse of the lab notes of K.S. Krishnan during this historical phase of experimental observation, which is reproduced below:

February 17, Friday
Prof, confirmed the polarisation of fluorescence in pentan vapour.
I am having some trouble with the left eye. Prof, has promised to
make all the observations himself for some time to come.
February 27, Monday
Religious ceremony in the house. Did not go to the Association.
February 28, Tuesday
Went to the Association only in the afternoon. Prof, was there and we
proceeded to examine the influence of the incident wavelength on the
phenomenon. Used the usual blue-violet filter coupled with uranium
glass, the range of wavelengths transmitted by the combination being
much narrower than that transmitted by the blue-violet filter alone.
On examining the track with a direct vision spectroscope, we found
to our great surprise that the modified scattering was separated from
the scattering corresponding to the incident light by a dark region

Note that it is this February 28, that we celebrate as National Science Day.

Mud + gold – I see an important lesson from this reading – that lab note books are an excellent window to the world of observations that a researcher experiences. Most of the time what is written in a lab notebook is routine stuff, but once in a while there is something important that pops out from it. The analogy is similar to digging for gold in the soil. Most of the time it is mud what you get, but once in a while you extract the precious metal. But without the effort of digging, one will never be able to extract the gold, and a lab notebook is the place where you record your efforts. So, what comes out as “success” is a small part of this greater effort. It is a tiny bit of a greater whole, and worth every bit.

new type of — Title and a part of the abstract of the paper reporting Raman effect (reference: *Nature* **volume121**, pages501–502 (31 March 1928))

Coming back to Raman and Krishnan, they published (see above) their experimental observation which caught the attention of scientific community around the world (‘western’ world to be more precise). And in 1930, Raman went on to win the Nobel prize in physics, and the rest is history. I need to emphasize that this discovery was made purely to address a quantum mechanical effect in optical regime. Specifically, the researchers were addressing an optical analog of the Compton effect, and they had no immediate applications in their mind. However, in the current age, Raman scattering spectroscopy, has bloomed into one of the most important scientific concepts, and a vital tool in science and technology, including applications in clinical bio-medicine and homeland security.

Prof. Wolfgang Kiefer showing his Raman scattering instrument which he has set-up in the basement of his house

The celebration: It has been 90 years since this important discovery, and to celebrate the discovery, there was a conference at IISc, Bangalore. Some of my students and I were part of it. In there, various researchers across the world including many from India, discussed about Raman scattering and its implication over the past 90 years. One of the highlights of the conference was a lecture by Prof. Wolfgang Kiefer (given on 28th February, National Science Day), who is a legend in the community of Raman scattering. Prof. Kiefer has been working on Raman scattering for more than 50 years. Now, he is retired, but amazingly, maintains a lab in the basement of his house, in which he has set-up Raman scattering experiments (see picture above), and pursues his curiosity with child-like enthusiasm. He gave an overview of his work done with his illustrious students from the past, and beautifully blended science, humor and humanity in a single talk. To listen to him was a pleasure and inspiration, and I will remember this for ever. On a personal note, I was actually celebrating the science day without realizing it !