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.
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 new paper appearing in Applied Physics Letters on how a dielectric microsphere placed on a 2D material deposited on a mirror can act as an optical antenna (see left panel for the schematic of the geometry and an optical image of the realized antenna).
The experimental and simulation efforts were mainly driven by our dynamic PhD student Shailendra Kumar Chaubey, who is very passionate about nanophotonics of 2D materials. He along with Sunny Tiwari and Diptabrata Paul explicitly show how experimental parameters such as sphere size and location of focusing can influence the photoluminescence emission from a WS2 monolayers. The experiments were mainly possible thanks to our collaboration with my colleague Atikur Rahman and his student Gokul, who continue to produce fantastic 2D materials for our nanophotonics study.
Interestingly, the emission from the WS2 monlayers can be as narrow as 4.6 degrees (see right side panel of the figure) which is one of the narrowest angular spread at room temperature. We also capture the energy-momentum photoluminescence spectra from WS2 monolayers, which is convoluted with the beautiful whispering gallery modes of the microsphere (see parts (a) and (d) of the figure).
We envisage such ’emission engineering’ using a simple microsphere can be further harnessed to control emission from quantum and nonlinear photonic 2D materials. Also, it raises new questions on how local photonic density of states can be tailored by altering the local environment around quantum emitters in solid state materials.
Light can carry orbital angular momentum (OAM) and spin angular momentum (SAM). This momentum can be transferred to an object that is interacting with the light. What we show is the experimental proof of concomitant detection of OAM and SAM in the coherent light scattering signatures from a single, silver nanowire. Essentially, the nanowire acts like a slit, and scatters the light. During this scattering process, the distribution of light in momentum space gets altered according to the spin (polarization) and orbital (topological charge) state illuminating the nanowire.
A notable point is that unlike other (metamaterials) methods, this unambiguous detection scheme does not require sophisticated nanofabrication methods and is mainly founded on fundamental principles of vectorial light scattering in the momentum space.
This experimental work (with a good dose of theoretical optics) was mainly due to the sustained efforts of an outstanding PhD student in my lab : Diptabrata Paul (about to finish PhD !)
He had excellent support and inputs from our PhD alumni Deepak K Sharma (now a postdoc/research scientist at ASTAR, Singapore).
Going further, this study motivates some interesting questions, of which we are interested in exploring the direct transfer of OAM and SAM at sub-wavelength scale to nanoscale objects including (macro)molecules. This will have some interesting manifestation on optical forces and torques at sub-wavelength scale, and we intend to study them in detail. This can be studied in a unique set-up that we have built in our lab that combines nano-optical tweezers with momentum-space imaging microscope. Look out for some studies in this direction from our lab.
We have a new paper in Materials Research Bulletin to be published in a special issue on Recent Advances in Functional Materials
The paper is about “Sub-wavelength plasmon polaritons channeling of whispering gallery modes of fluorescent silica microresonator”
Individual spherical objects, such as a silica-microsphere, when excited with a laser under certain conditions, exhibit a set of optical resonances called as “whispering gallery modes” (WGMs). These modes are very sharp (high Q value) and can be harnessed as optical resonators. An interesting prospect is to channel the WGMs through a nanoscale plasmonic waveguide, such as a single silver nanowire, and study the optical emission.
Motivated by this prospect, Sunny Tiwari and Chetna Taneja from my group experimentally show how to channel WGMs through a plasmonic silver nanowire waveguide. They go a step ahead and show the spectral and angular characteristics of such a hybrid optical system. These experiments motivate further questions related to micro-resonances and angular spectrum distribution in dielectric-plasmonic hybrid systems, and can be harnessed to design compact micro-lasers and on-chip couplers. With some effort, they can also be optically trapped and manipulated.
A new collaborative paper in Optics Express on modal and wavelength conversions in plasmonic nanowires
Work done by Adrian, Deepak K Sharma et al, as part of Ifcpar Cefipra grant
We show that plasmonic nanowire-nanoparticle systems can perform nonlinear wavelength and modal conversions and potentially serve as building blocks for signal multiplexing and novel trafficking modalities. When a surface plasmon excited by a pulsed laser beam propagates in a nanowire, it generates a localized broadband nonlinear continuum at the nanowire surface as well as at active locations defined by sites where nanoparticles are absorbed (enhancement sites). The local response may couple to new sets of propagating modes enabling a complex routing of optical signals through modal and spectral conversions. Different aspects influencing the optical signal conversions are presented, including the parameters defining the local formation of the continuum and the subsequent modal routing in the nanowire.