Soft Matter Optics – talk at ACS -India

About 2 years ago (22nd May 2020), when all the academic activities were online, I gave a talk on “Soft-Matter Optics: A Cabinet of Curiosities” organized by American Chemical Society as part of India Science Talks. Below is the embedded video of the online talk.

Link to ACS website can be found here.

In there, I give a broad overview of how interesting optical function can emerge from the complex world of soft matter. In addition to this, I have emphasized how optics can be harnessed to study structure and dynamics of soft-matter systems including colloids, liquid crystal and some biological matter. The target audience are new PhD students and anyone who is entering the field of light-soft matter interaction.

Updates – COVID times

First of all, my condolences to all people who have lost someone directly or indirectly due to pandemic. Second, my salutations to all the health and essential workers who are striving hard to keep the world breathing. Third, my sympathies to all the free-willing minds who have been locked down. This outbreak has indeed changed our lives and life-style, and has confined most of the humankind spatially, and has metaphorically frozen us in time. Also, it has given us some time for self-introspection on what it is to be an individual in a society, and how actions of individuals and local community can affect the globe. In an essence, what we may be witnessing is a classic case of butterfly effect.

So, what am I up to in the past month or so ?

  • Research work: Now that all my research-group members are away from the institute, it has had an effect on our research. Although online platforms have kept us connected, and we are making slow progress in writing some papers and performing some computer simulations, it can never substitute two important things: experimental work in a lab, and the in-person interaction during research. On personal research front, I have been studying some interesting concepts on liquid crystals, and their related meso-photonics effects. That will be a topic of another blog in future.
  • COVID-related research: For the past year of so, I have been informally interacting with some researchers at Bharat Electronics Limited, Pune on topics related to nanphotonics and optofluidics. Thanks to the recent developments, we have initiated collaboration on research related to COVID. We will be exploring some on-chip optical microscopy and plasmonic methods to detect and interrogate pathogens in our local environment (including virus and virus-like particles). I will update you as we make some progress.
  • An interesting book: Over the past fortnight or so, I have been reading an interesting book titled : Fizzics – The Science of Bubbles, Droplets and Foams. It is a semi-technical/popular science book written by F. Roland Young, who has done considerable research on bubble cavitation and sonoluminescence. This book has some fascinating discussion on questions such as:
  • What is the origin of the sound when we crack our knuckles ?
  • Why and how do bubbles rise in a bottle of champagne ?
  • How to compute a math puzzle using a soap film ?
  • and many more…
  • My posts going further – Henceforth, I wish to post short blogs more frequently. Once in a while, I will post longer essays.

cheers and stay safe !

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!

Biological Lasers – Alive and Emitting


Laser, laser burning bright,

In the lab, through the night

What immortal ideas

Could frame thy emission process?

modified from William Blake’s ‘The Tyger


Light Amplification by Stimulated Emission of Radiation or LASER is a light source which is ubiquitous in the world around us. They are extensively used in scientific laboratories to study phenomenon spanning various sizes: from astronomy to sub-atomic particles. A laser has three vital characteristics: they are a monochromatic, coherent and highly directional in nature, which make them unique emitters of light. Unlike conventional light sources, such as tube-lights, lasers carry a relatively large amount of energy in a confined volume, and hence can propagate over a longer distance and strongly interact with matter. These properties of lasers have not only made them a fascinating topic of fundamental research, but also play a critical role in various applications.

Vital Quest: Almost all the lasers that have been produced are made of abiotic matter, that is, matter that does not have life. An interesting question to ask is: can we create a laser out of a living system, such as biological cell?

The answer is YES. In 2011, two optical physicists, Malte C. Gather and Seok Hyun Yun, then at Harvard Medical School, came up with an interesting experiment. They transfected green florescent proteins inside a human embryonic kidney cell, and placed the cell between two high-quality mirrors, and optically pumped the cells with blue pulsed light (see figure 1(a)). Interestingly, this experiment resulted in lasing action at around 513nm wavelength from various parts of the biological cell (see figure 1(b)), and thus a biological laser was realized. The green laser light originated from the transfected green florescent protein inside the cell, which essentially acted as a gain media inside an optical cavity. The emitted light had rich spatial structure as evidenced in figure 1(b), and this structure depended on the local distribution of the green fluorescent proteins.  This is the first report, to my knowledge, where a living system, such as a biological cell, has emitted laser light, and has literally created tremendous ‘excitement’. So one may ask, what the uses of such a system are.

Figure 1:  (a) Experimental schematic to realize biological laser. The system consists of a human-embryonic-kidney-cell transfected with green florescent protein. This cell was placed between two high quality mirrors which are separated by a distance ‘d’ (representing an optical cavity). The cell was optically pumped by blue pulsed light (465 nm wavelength).  This resulted in laser emission from the cell. (b) Optical image of a lasing human embryonic kidney cell. The green light (wavelength around 513nm) emanating from the cell is the laser light. The scale bar is 5 μm and the colour bar represents increasing intensity from dark to light shade. Figures reproduced with permission from Nature Photonics, 5, 406-410 (2011).


Bright Future: The research on biological lasers is in its infancy. There are many interesting prospects of such lasers, and I outline a few of them:

  • They can act as a localized source of light and heat, which may further drive certain mechanical, thermal and chemical reactions in organelles and compartments of a cell.
  • If placed in an appropriate location inside living systems, biological laser can be harnessed as light sources in biomedical applications, where the reach of certain surgical instruments is constrained.
  • The emanating light is due to stimulated emission, which has narrow spectral width. In contrast to spontaneous emission, which is the process behind conventional light emitters, lasers have very narrow spectral widths, and hence can be utilized for spectral multiplexing and discrimination of species inside a cell.

 Voyage ahead: There are still many unexplored aspects of a biological laser. Below I mention three of them from different viewpoints:

  • From optical physics viewpoint, a biological laser can be treated as an optical cavity with a randomly-scattering gain media. It would be interesting to explore the localization, propagation and directional emission of light in such a randomized medium inside a living system.
  • From chemistry viewpoint, it is interesting to ask if one can design and synthesize bio-compatible macromolecules or nano-materials which can be placed inside a cell such that it leads to efficient self-lasing without external stimuli.
  • From biology viewpoint, it is vital to know what will be the fate of a cell if it keeps emitting laser light. Specifically, it would be interesting to know how cells can adapt to an in-built laser source. Furthermore, if a cell with biological laser splits into to two, will it carryover its ability of lasing to the next generation?

As with all interesting inventions and discoveries, biological lasers have opened many interesting questions to be explored. It has room for contribution from various branches of science and technology, and may open new avenues by bringing together biology and laser photonics. Let me conclude by quoting Feynman:

“…..A biological system can be exceedingly small. Many of the cells are very tiny, but they are very active; they manufacture various substances; they walk around; they wiggle; and they do all kinds of marvelous things—all on a very small scale. Also, they store information…..”

Well Mr. Feynman, now they can even emit laser light!