Geo-fractal…somewhere over south India

I am always amazed to see fractal-like patterns….this one at a geographical scale…captured somewhere over south India..

I took the photo on my trip back to Pune from IIT Madras…

Thanks to IIT-M physics department for their invitation for colloquium…

Special thanks to Basudev Roy and Nirmala for hosting…. greatly enjoyed the discussion with many faculties and students..

In my talk, I mainly spoke on topics at the interface of statistical optics, Brownian motion and pattern formation..
Was delighted to see (and meet) Profs. Balki, Suresh Govindarajan Sunil Kumar Arnab Pal and many more in the audience.

The photo, retrospectively, captures the essence of the science discussed…

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.

Review article – Plasmonic colloids + Optical Vortex

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

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

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

I welcome comments on the review.

arxiv :

New paper :”Optothermal evolution of active colloidal matter in defocused laser trap”

We have a new paper to be published in ACS Photonics on “Optothermal evolution of active colloidal matter in defocused laser trap”

In the context of non-equilibrium statistical mechanics, it is relevant to ask : how does a system of (active) Brownian particles respond to environmental cues ?

Structured light in the form of an optical trap can facilitate a platform to create unconventional, environmental cues in which Brownian particles can dynamically assemble and evolve as a function of space and time.

In this work, we utilize a simple defocused laser trap and study the evolutionary dynamics of thermally active colloids (polystyrene spheres infused with iron oxide). We observe a variety of (nonlinear) dynamical states including hovering of a pair, a kind of synchrony among the assembled colloids in the trap (see video).

Thanks to the great efforts of Dipta and Rahul from my group, we have been able to study and unveil the complex forces at play. The dual contribution of optical potential of the laser and thermophoretic interaction of colloids were revealed by systematic experiments and numerical simulations leading to this elaborate report.

This study further motivates interesting questions in the context of microscopic heat engines where the light and heat created in an optical trap can lead to some interesting nonlinear dynamics of soft matter systems. Another prospect is to characterize structured optothermal fields using Brownian motors as microscopic probes in an optical trap. More on this in the coming month….

all videos related to the work on our lab youtube channel

preprint version of the paper on arxiv :

I will post the published version of the article when available

58. Optothermal Pulling of colloids using Nanowire Plasmons – my talk at Compflu 2021

Linked is my recorded-talk presented at Compflu 2021 today (13th Dec) in the session : Active and Living Matter.

I discuss our recent work on optothermal pulling, trapping and assembly of micro-colloids under the influence of thermoplasmonic field of a single silver nanowire.

The talk was recorded on 2nd Dec 2021, so the reference on conclusion-slide is not updated.

57. Single nanoparticle driven thermoplasmonic tweezer : single-molecule SERS

We have a new paper published in Journal of Physical Chemistry Letters on “Single Molecule Surface Enhanced Raman Scattering in a Single Gold Nanoparticle-Driven Thermoplasmonic Tweezer”

Thanks to the fantastic effort by Sunny Tiwari, and excellent support by Utkarsh Khandelwal (former IISER-P undergrad) and Vandana Sharma from my group, we have been able to combine single molecule Raman scattering with a specialized nanoscale optical tweezer.

The uniqueness of this tweezer platform  is that the optical trapping process is driven by the thermo-plasmonic potential created by a SINGLE, 150nm GOLD NANOPARTICLE. Concomitantly, the same field can be used to perform single-molecule Raman spectroscopy. Kind of  “ek teer mae do shikar” strategy Smile

Using this system, not only we push the limits of optothermal trapping of a single nanoparticle (see video) at low laser powers, but also create a platform for deterministic transport of reversible colloidal assembly in a fluid.

We envisage that our nanometric plasmonic tweezer can be harnessed to trap and tweeze biological entities such as single virus and bacteria. Another possible application of our study is to create reconfigurable plasmonic metafluids in physiological and catalytic environments, and to be potentially adapted as an in vivo optothermal tweezer.

All the videos related to this study can be found on our lab’s Youtube channel :

DoI of the published paper :

preprint version on arxiv :

52. Optothermal pulling and trapping..with nanowire plasmons

We have a new paper published in the journal ‘Soft Matter’ titled : Optothermal pulling, trapping, and assembly of colloids using nanowire plasmons

When a silver nanowire is optically illuminated under certain conditions, they propagate surface plasmons. These surface electromagnetic waves not only propagate light at subwavelength scale, but also generate heat along the nanowire.

A question of interest to us: can we use the quasi one-dimensional optothermal potential of a nanowire-plasmon to trap and assemble soft, microscale matter ?

Motivated by this question – Vandana, Sunny and Dipta from my research group, performed optical trapping based experiments to show an interesting pulling and trapping effect on dielectric colloids (see video). Furthermore, by increasing the concentration of the colloids, an emerging two dimensional crystal was observed. Interestingly, the formation of this two dimensional assembly was found to be sensitive to the optical polarization at the excitation point on the nanowire.

Thanks are also due to other co-authors: my colleague Vijayakumar Chikkadi and his student Rathi for helping us to implement the particle tracking code on python.

Optical trapping and tweezing is a fascinating area of research. By adding plasmons to the mix of things, these optical effects become intriguing. Importantly, they facilitate a platform to explore questions in non equilibrium statistical mechanics including optically driven active matter…

Afterall, more is different…

DOI of article :

Link to arxiv preprint:

All videos here :

27. Question of Life

This link has an interesting article by Paul Davies on an emerging question in science : “Does new physics lurk inside living matter?”
Ever since Schrodinger asked and wrote about “What is Life ?”, biology has always been within the grasp and underneath the metaphoric lens of physicists. Although this question has always drawn attention of physicists, a serious effort to address it was lacking in mainstream physics. This situation has changed, especially in past decade or so, thanks to evolution of physical tools and biology going quantitative and welcoming physics into their life…literally.
In recent times, many of the questions in biology have been re-casted as questions in mainstream physics, which makes it very appealing for researchers who wish to quantitatively measure things in a complex systems, and understand the mechanistic aspects of life and life-like objects (think bio-robots). Importantly, biology has readily offered a spectacular platform by opening itself for quantitative scrutiny. With new experimental tools, and a broad theoretical base of statistical physics, physics of living systems has arrived as a major sub-domain of physics.
From a physical science viewpoint, it is important to know how we go about addressing the questions raised by Davies in his article. The answer may be found by addressing some auxiliary questions at interface of soft matter science, fluid dynamics, statistical physics and information science. This pool of answers may get us closer to the frontiers of biology, and who knows, it may shine light on new questions, which would have otherwise gone unnoticed by the biologists themselves.
In my opinion, the experimental tools to address these questions need to come from various branches of science including chemistry, molecular, organism and evolutionary biology. As you may see, it requires an inclusive effort from various disciplines of science and technology, and mainstream physics has a vital role to play.
Time has also come, especially for the Indian physics community, to take this question seriously, and integrate with the above-mentioned domains, and pursue this fascinating aspect of life-science. A mere glance at any new issue of Phys Rev Lett or Nature Physics clearly says that biology has arrived in physics…big time…
After all, humanity is curious to know : what is life ? Physics may have some interesting answer(s)…

25. Ants@IISER as Active Matter

Nowadays, collective motion in active matter is one of the happening topics in the science of condensed matter, with a motivation in understanding biology at scales spanning from molecules to flock of birds. There is also a lot of contemporary research in active and driven natural systems and soft-robots at various length scales. Of my own interest is to understand how light can drive collective motion in synthetic colloids and other soft-systems in a fluid, and how they can lead to emergence of new assemblies.


Today, when I was walking in the IISER Pune campus, I came across a group of ants carrying food (see video above). It is amazing to see how coordinated is the movement of ants when carrying an object which is much larger than their individual weight (see video). One of the observations you can make is that how ants change their collective direction with minimum communication. How they do it is a fascinating question to explore. Undoubtedly studying such collective motion can lead to deeper understanding of not only the behaviour of ants and non-equilibrium systems, but also in designing adaptable soft-robots for various environments.

IISER Pune campus is quite rich in flora and fauna, and there is a lot to learn just by looking around the natural resources on campus. I hope to explore this rich environment in the context of soft matter systems, and report to you in this blog.