I mainly spoke about emergent Brownian dynamics of laser-heated colloids under optical confinement. Below is the link to the talk.
I concluded my talk quoting P W Anderson’s essay “More is Different“
“The constructionist hypothesis breaks down when confronted with the twin difficulties of scale and complexity. The behavior of large and complex aggregates of elementary particles, it turns out, is not to be understood in terms of a simple extrapolation of the properties of a few particles.
Instead, at each level of complexity entirely new properties appear, and the understanding of the new behaviors requires research which I think is as fundamental in its nature as any other.”
P.W. Anderson ‘More is Different’ Science, 177, 4047 (1972)
On 1st Dec 2022, I gave a talk on “Structured-Light Scattering : Implications in Momentum Space” as part of a discussion meeting on STRUCTURED LIGHT AND SPIN-ORBIT PHOTONICS held at International Center of Theoretical Sciences, Bangalore.
I mainly spoke about topological light scattering in the frame work of angular momentum of light and absorptive effects in optothermal tweezers created by structured light.
Below is the embedded video link to my talk. The playlist also has many other interesting talks related to the topic.
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.
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….
This document brings together researchers from 52 different affiliations across the globe to look into the accomplishments and future directions of optical tweezers.
My contribution towards roadmap appears on page 136, topic number 31 on — Raman scattering in (thermo)plasmonic tweezers.
In there, I discuss how plasmonic platforms can be used to generate attractive optothermal forces to trap and interrogate molecules and nanoparticles, down to single copy limit. I also discuss the challenges and opportunities of such a process.
Optical tweezers is one of the most powerful optical tools that finds utility not only in fundamental physics, but also in diverse applications including biology and medicine.
Ever since the Nobel prize-winning work of Arthur Ashkin, optical tweezers have evolved and continues to evolve into various forms. This roadmap article aims to capture this evolution, and to discuss the emerging capabilities and challenges of optical tweezers.
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
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.
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…
We have a new paper to appear in Applied Physics Letters
The work is about “Experimental observation of transverse spin of plasmon polaritons in a single-crystalline silver nanowire”
Circularly polarized laser beams carry spin angular momentum. Such “spinning” photons have a plethora of applications including optical spanners, optical information processing, chiro-optics, micro-gyroscopes, nonlinear dynamics of matter at micro and nanoscales, and many more.
An interesting question to ask is : can we generate spin states with surface electromagnetic waves such as surface plasmon polaritons ? Unlike freely propagating optical laser beams, surface electromagnetic waves can be harnessed for sub-wavelength optical interaction on a chip, and have risen to importance.
In this paper, we experimentally show how transverse spin can be generated by nanowire surface plasmons. Thanks to the outstanding effort of my group members Chetna Taneja and Diptabrata Paul, we were able to image and measure the spin density of such (quasi) one-dimensional surface electromagnetic waves in a single-crystalline silver nanowire.
A prospect that we are interested in is to transfer this spin angular momentum to objects such as individual nanoparticles and molecules in a trap, which can, hopefully, create some interesting (nonlinear) dynamic states.