28th Feb of every year is celebrated as National Science Day in India. I have previously written about the science behind the National Science Day. This day is associated with the discovery of Raman effect. Raman had a great legacy and influence on Indian science. In addition to being a great scientist, CV Raman encouraged the pursuit of science (with exceptions).
One of his legacies was the impression and influence he had on some close members of his family. Below is a small list of his illustrious nephews who made significant contributions in science.
Raman’s younger sister, Sitalakshmi, had 5 sons.
3 brothers : Pancharatnam, Ramseshan, Chandrasekhar. Image courtesy : Indian Academy of Sciences
S. Chandrasekhar was an astrophysicist who went to win the Nobel Prize in Physics
The real impact of science and technology, is not only in the materialistic gains of a society but also in the way it elevates the thought process of a society. Science as a pursuit of human knowledge influences thinking of human beings, and hence plays a vital role in shaping the character and culture of any individual, family, community, country and the world.
We should also remind ourselves that “impact of a scientist” cannot be judged merely by counting the number of papers/patents they publish nor by the high-office they hold in corridors of (scientific and political) power. If anything, such a judgment of impact should be left to the posterity.
On a related note, Kameshwar Wali, physicist and biographer of Subrahmanyan Chandrasekhar writes :
Chandra often quoted from a letter of his friend Edward A Milne during his Cambridge years:
“Posterity, in time will give us our true measure and assign to each of us our due measure and humble place; and in the end it is the judgement of posterity that really matters. He really succeeds who preserves accordingly to his lights, unaffected by fortune, good or bad. And it is well to remember there is no correlation between posterity and the judgement of contemporaries.”
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 4th 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.”
“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 10th and 11th 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….
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!
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.
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:
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.
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….
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)
Bronze statue of 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:
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:
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…..
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 –
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.
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
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.
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.
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.
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.
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.
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.
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.
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.
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).
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.
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…
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.
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.
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.
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.
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!
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.
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.
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!
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.
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 !
BORN AGAIN: Today I opened the google webpage and to my surprise found the doodle (picture above) celebrating birthday of Max Born. He was not only a great physicist who contributed immensely to quantum mechanics and other branches of physics (including optics), but also a mentor to many great physicists including Fermi, Heisenberg, Pauli, Wigner, Teller, Emil Wolf and many more.
Every student who has studied physics, is aware of quantum mechanical wavefunction (ψ). Given a quantum system and its environment (electron in an atom, for example), wavefunction is a fundamental quantity that one can compute, and forms the basis to understand the system in greater detail. When quantum mechanics was evolving in early 1900s, the question of how to physically interpret the meaning of wavefunction was at the forefront. It was Max Born who gave the statistical interpretation for the wavefunction, which later fetched him a Nobel prize in 1954.
Born identified the importance of interpretation of the wavefunction, and its connect to the realistic, observable parameter. To quote Born from his Nobel lecture :
“The problem was this: an harmonic oscillation not only has a frequency,
but also an intensity. For each transition in the array there must be
a corresponding intensity. The question is how to find this through the
considerations of correspondence? “
This quest set forth an intense programme in physics and motivated people like Heisenberg, Schrodinger, Bohr, and Einstein to find an answer. Interestingly, Born’s work was heavily inspired by Einstein’s work. To quote Born from his Nobel lecture:
“But the decisive step was again taken by Einstein who, by a fresh
derivation of Planck’s radiation formula, made it transparently clear that the
classical concept of intensity of radiation must be replaced by the statistical
concept of transition probability.”
Further, he adds
“Again an idea of Einstein’s gave me the lead. He had tried to make the duality of particles light quanta or photons – and waves comprehensible by interpreting the square of the optical wave amplitudes as probability density for the occurrence of photons. This concept could at once be carried over to the ψ-function: |ψ|^2 ought to represent the probability density for electrons (or other particles).”
Reading Born’s Nobel lecture, two things struck me : first was that science is never done in isolation. Every single idea is inspired by another idea. Second, physical optics has a major influence on interpretation of quantum mechanics. Max Born was no stranger to optics. In fact, he was one of the pioneers of classical optics, and I am not surprised that he could make some vital connections between physical optics and quantum mechanics.
My personal copy…..standing tall and heavy :)
THE BOOK: This brings me to the most famous book written in optics(see picture above) by none other than Max Born and Emil Wolf (Emil Wolf was the last research assistant of Max Born, and a well know optical physicist) The book is titled “Principles of Optics”, but in optics community we call it “Born and Wolf”. The first edition of this book appeared in 1959, and has never gone out of print. Currently, it is in its 7th edition and is 951 pages thick !
As described in the preface (first edition of Born and Wolf), several people urged Born to translate his 1933 book: “Optik” from german to english. By 1950s, optics had evolved and had made inroads into atomic physics, molecular spectroscopy, solid-state physics and various other branches of science and technology. So, they had to write the book from scratch taking new ideas into consideration.
“Born and Wolf” explains optical phenomenon through the eyes of Maxwell’s theory, and has become the foundation on which various aspects of classical optics can be studied in a mathematically rigorous fashion. In fact, it also lays foundation to various quantum optical phenomenon including coherence and correlation functions, on which Emil Wolf’s contribution has been immense.
For me, chapter 13 on “Scattering from homogeneous media” is the highlight of this book. It starts with elements of scalar theory of scattering by expaining the first-order Born approximation followed by discussion on scattering from periodic potential. The best part is the discussion on multiple scattering, which in a sense lays the foundation to study various important optical phenomenon including diffraction tomography and optical cross-section theorem (or more famously known as Optical theorem). Also, the 13th chapter has a very interesting discussion on concept of far-field and its connection to scattering of electromagnetic waves.
Actually, the book is very well known for its treatment on diffraction theory and image formation. It gives a very strong footing to attack problems in imaging, aberration and inteferometry using Maxwell’s equation and related boundary condition. It also, highlights optics of metals, which has now transformed and evolved into a sub-field of optics and photonics – plasmonics.
Origins of the book: The writing of this book has a historical context. Emil Wolf was a research assistant (post-doc) of Max Born and joined him after his Ph.D. He recollects his experiences with Born and about writing this book in an interesting article. Below is an interesting quote:
“Through Gabor I learned in 1950 that Born was thinking of preparing a
new book on optics, somewhat along the lines of his earlier German book
Optik, published in 1933, but modernized to include accounts of the more
important developments that had taken place in the nearly 20 years that
had gone by since then. At that time Born was the Tait Professor of Natural
Philosophy at the University of Edinburgh, a post he had held since 1936,
and in 1950 he was 67 years old, close to his retirement. He wanted to find
some scientists who specialized in modern optics and who would be willing
to collaborate with him in this project. Born approached Gabor for advice,
and at first it was planned that the book would be written jointly by him,
Gabor, and H. H. Hopkins. The book was to include a few contributed
sections on some specialized topics, and Gabor invited me to write a section
on diffraction theory of aberrations, a topic I was particularly interested in
at that time. Later it turned out that Hopkins felt he could not devote
adequate time to the project, and in October of 1950, Gabor, with Born’s
agreement, wrote to Linfoot and me asking if either of us, or both, would
be willing to take Hopkins’ place. After some lengthy negotiations it was
agreed that Born, Gabor, and I would co-author the book.”
Wolf writes about Born and his working style:
“In spite of his advanced age Born was very active and, as throughout all
his adult life, a prolific writer. He had a definite work routine. After coming
to his office he would dictate to his secretary answers to the letters that
arrived in large numbers almost daily. Afterward he would go to the adjacent
room where all his collaborators were seated around a large U-shaped
table. He would start at one end of it, stop opposite each person in turn,
and ask the same question: “What have you done since yesterday?” After
listening to the answer he would discuss the particular research activity and
make suggestions. Not everyone, however, was happy with this procedure.
I remember a physicist in this group who became visibly nervous each day
as Born approached to ask his usual question, and one day he told me that
he found the strain too much and that he would leave as soon as he could
find another position. He indeed did 80 a few months later. At first I too
was not entirely comfortable with Born’s question, since obviously when one
is doing research and writing there are sometimes periods of low productivity.
One day when Born stood opposite me at the U-shaped table and asked,
“Wolf, what have you done since yesterday?” I said simply, “Nothing!” Born
seemed a bit startled, but he did not complain and just moved on to the next
person, asking the same kind of question again.”
Wolf also gives an account of why Gabor pulled-out, and how Wolf had to play an unexpected, but vital role in writing this book:
“…..Gabor soon found it difficult to devote the necessary time to the project, and it was mutually agreed that he would not be a co-author after all, but would just
contribute a section on electron optics. So in the end it became my task to
do most of the actual writing. Fortunately I was rather young then, and so
I had the energy needed for what turned out to be a very large project. I
was in fact 40 years younger than Born. This large age gap is undoubtedIy
responsible for a question I am sometimes asked, whether I am a son of the
Emil Wolf who co-authored Principles of Optics with Max Born!”
Wolf also praises Born’s open-mindness to various branch of physics:
“Optics in those days-remember we are talking about optics in pre-laser
days-was not a subject that most physicists would consider exciting; in fact,
relatively little advanced optics was taught at universities in those days. The fashion then was nuclear physics, particle physics, high energy physics, and
solid state physics. Born was quite different in this respect from most of his
colleagues. To him all physics was important, and rather than distinguish
between “fashionable” and “unfashionable” physics he would only distinguish
between good and bad physics research.”
Emil Wolf is now 95 years old, and is still a very active researcher. His recent paper was in 2016 on partially coherent sources and their scattering from a crystal. Wolf’s books are classics in optics, and continues to raise probing questions and important connections in sub-branches of optics.
In an essence, great science books are written with love and passion to communicate the excitement of science. Born and Wolf certainly does that, and continues to inspire us to learn optics from the masters themselves.
“The work for which the Nobel Prize has been awarded to me is of a kind which has no immediate effect on human life and activity, but rather on human thinking. But indirectly it had a considerable influence not only in physics but in other fields of human endeavour.
This transformation of thinking in which I have taken part is however a real child of science, not of philosophy: it was not the result of speculation, but forced upon us by the observed properties of Nature.”
Max Born and Emil Wolf, your work and your books have transformed our thinking, and the way we see light and matter. Thank You !
There are very few people in human history who have combined arts and science like Leonardo da Vinici. He was a polymath: a great painter, inventor, sculptor, scientist and as you will see – a keen observer of nature, and many more. One of the great aspects of Leonardo is that he recorded his observation as texts, which gives us a deep and direct insight into his thinking.
All of us have been captured by the beauty of sun lit sky. It has made us gaze and wonder about its colour. Of course, it has inspired a countless number of artist and scientist to ask the question : what is the origin of colours of a sun-lit sky ? Leonardo himself was fascinated by this question, and led him to view this question both as a painter and as a scientist. Thanks to the great work of J.P. Richter, who has translated the Literary works of Lenardo da Vinici into english (available free online), we obtain a direct peek into the mind of Leonardo which is an everlasting treasure trove: more you dig more you get.
The title page of the translated book
In the collected works, what has caught the attention of scientists is a chapter titled “Aerial Perspective”. In there, Leonardo is trying to converse with his fellow painters on how to create perspective in paintings. While doing so, he makes some vital observation and proposes hypotheses, and further discusses about some experiments to test them. Leonardo was a keen observer. His approach to art was heavily influenced by an analytical way of looking at the problem at hand. In his writings, he appeals to painters to pay close attention to angles and perspectives in the geometry. In order to attain precision he gives elaborate explanation based on his observation. Below is an example where he explains how to represent the atmosphere in paintings.
“Why the atmosphere must be represented as paler towards the lower portion? Because the atmosphere is dense near the earth, and the higher it is the rarer it becomes. When the sun is in the East if you look towards the West and a little way to the South and North, you will see that this dense atmosphere receives more light from the sun than the rarer; because the rays meet with greater resistance.”
It is remarkable how his efforts to create a painting inspired him to go deeper and hypothesize a physical phenomenon. Below sentences reveals a connection he makes between colour of the sky and the presence of “insensible atoms”.
“ I say that the blueness we see in the atmosphere is not intrinsic colour, but is caused by warm vapour evaporated in minute and insensible atoms on which the solar rays fall, rendering them luminous against the infinite darkness of the fiery sphere which lies beyond and includes it”
Although, now we know that light scattering from molecules (not atoms) as the reason for colourful sky, we need to really appreciate Leonardo’s quantum leap of thought. Remember, his texts are dated around late 1400s or early 1500 AD, where the presence of atoms and molecules were not yet verified. As a person with scientific aptitude, Leonardo not only hypothesized, but also tested them with experiments. Below he refers to a beautiful experiment with smoke and the perception of colour arising due to the background.
“Again as an illustration of the colour of the atmosphere I will mention the smoke of old and dry wood, which, as it comes out of chimney, appears to turn very blue, when seen between the eye and the dark distance. But as it rises, and comes between the eye and the bright atmosphere, it at once shows of an ashy grey colour; and this happens because it no longer has darkness beyond it, but this bright and luminous space.”
To me this is nothing but a first rate example of looking at nature through a scientific eye, and adapting this view as means to a certain end. It is a tribute to Leonardo who paid such meticulous attention to details, and attempted to explain an unexplained physical phenomenon – all in the name of getting a painting right ! This is also a wonderful example of how aesthetics and science combined in the mind of Leonardo, which further led to some breathtaking work. After all, science and arts are two aspects of human expression, and Leonardo combined them effectively.
There is another lesson we can learn from such endeavours: observations play a key role. Sometimes, when a student is working in a laboratory, she or he may wonder why one should keep records of ones observations. Well, to them I say – look at Leonardo, he took an important step to write down his observations and this served not only as a template for further exploration, but also clarified his thoughts about the phenomenon he was interested in. Writing this way serves two purposes: one is to record the observation at the moment of exploration and other is to seed new thoughts and questions that can be derived out of these recordings. You can surely get a lot out of this approach – give it a try.
Coming back to the sky – what is the exact origin of its colour? It took almost 400 years after Leonardo’s observations for someone to come up with an ‘accurate’ answer. And that person was Lord Rayleigh (actual name: John William Strutt). In his remarkable research paper published in 1899, Rayleigh explained the blue of the sky as due to molecular scattering. The opening paragraph of this paper is historic and reproduced below-
“This subject has been treated in papers published many years ago. I resume it in order to examine more closely than hitherto the attenuation undergone by the primary light on its passage through a medium containing small particles, as dependent upon the number and size of the particles. Closely connected with this is the interesting question whether the light from the sky can be explained by diffraction from the molecules of air themselves, or whether it is necessary to appeal to suspended particles composed of foreign matter, solid or liquid. It will appear, I think, that even in the absence of foreign particles we should still have a blue sky.”
The final statement is significant as it recognizes that light scattering can occur purely due to molecules in air, even after discounting the contribution of suspended particles. Rayleigh gave his famous formula in 1871, which drew an inverse relationship between the intensity of scattered light from a very small particle (compared to wavelength of light) and the fourth power of the wavelength of light. In other words, smaller the wavelength of light (violet-blue in case of visible light), more will it be scattered from the molecules in the sky, and hence the blue colour.
One may wonder why not a violet coloured sky. After all, if the inverse relationship between scattering intensity and wavelength holds good, then according to visible colour distribution (VIBGYOR), violet should be the dominant colour of the sky. The answer to this puzzle is a complex one. Mainly because what we perceive as blue is due to a combination of at least three concomitant effects: Rayleigh scattering, human perception and the background in which the scattered light from molecules are observed. Although the colour of the sky is beautiful to perceive from earth, the intricate understanding of the optical processes in atmosphere of planets, including earth, is still a work in progress.
The foundations laid by Leonardo opened a new line of thought, and Rayleigh put forth an important explanation that forms the basis for a majority of studies on light scattering since 1900s. We will revisit Rayleigh and his work many times in this blog; meanwhile enjoy watching the sky with your scientific eye – after all sky is no limit for science!
VISMAYA - History & Philosophy of Physics 