New paper : Microsphere can narrow emission from a 2D material on a mirror

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.

Arxiv version of the paper :

Nanowire kink as an antenna for 2D material

A nanowire kink on a mirror can influence light scattering wavevectors and direct photoluminescence from a monolayer of a 2D material at sharp angles.

Shailendra, Sunny Tiwari , Asutosh from my group in collaboration with my colleague Atikur and his student Gokul show this unconventional nanowire antenna concept, experimentally.

The link to publication in European Physics Journal: Special Topics is above. This paper is part of a special issue on Photonic Materials

Arxiv link :

44. Beaming light with a bent-nanowire

We have a new publication in Journal of Physical Chemistry Letters on the “Beaming Elastic and SERS Emission from Bent-Plasmonic Nanowire on a Mirror Cavity”

In short, we show, how by bending a nanowire we can narrowly beam the light scattered from molecules (see adjoining picture).

Optical emission from quantum objects such as atoms and molecules are very sensitive to their local surroundings. One of the current challenges in controlling optical emission from molecules at subwavelength scale is to narrow their scattering directivity. In the context of molecules, controlling light scattering at sub-wavelength scale has utility in optical trapping of molecules, molecular QED, cavity molecular mechanics, molecular quantum optics and many other areas of research. 

Thanks to the great effort by Sunny Tiwari in my lab, who in the middle of the pandemic, tirelessly executed the idea of beaming elastic and Raman scattering emission from molecules in the vicinity of a bent plasmonic silver nanowire resting on a metallic mirror.  He was ably supported by Adarsh (now at ETH), Dipta and Shailendra. Together, they experimentally confirmed the beaming characteristics from this geometry and corroborated with elaborate numerical simulations.

This work further motivates questions related to directivity control for single photon emitters and can be potentially harnessed for momentum-space engineering of nano-optical forces……

we say bend the light like a nanowire…Smile

DOI of JPCL article :

arxiv version :

42. New paper on WGMs via plasmonic nanowire

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.

arxiv link to the paper :

DOI of the published paper:

My Metaphoric Oxygen

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 –

                             BY EMILY DICKINSON

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.

  1. Absorption and Scattering of Light by Small Particles
    • Author(s): Craig F. Bohren and Donald R. Huffman
      • 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


  1. Light Scatteing by Small Particles
    • Author(s): H.C. van de Hulst
      • 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.
  1. The scattering of light and other electromagnetic radiation
    • Author(s):  Milton Kerker
    • 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.
  2. Dynamic Light Scattering with applications to chemistry, biology and physics
    • Author(s): Bruce J. Berne and Robert Pecora
      • 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.
  1. Molecular Light Scattering and Optical Activity
    • Author(s): Laurence Barron
      • 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.
  1. Scattering, Absorption, and Emission of Light by Small Particles
    • Author(s): MI Mishchenko, LD Travis, AA Lacis
      • 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.
  1. Wave Propagation and Scattering in Random Media (Vol 1 and 2)
    • Author(s): Akira Ishimaru
      • 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.
  1. Optical Scattering Measurement and Analysis
    • Author(s): John C. Stover
      • 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.
  1. LASER LIGHT SCATTERING, Basic Principles and Practice
    • Author(s): Benjamin Chu
      • 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.
  1. Mesoscopic Physics of Electrons and Photons
    • Author(s): E. Akkermans and G. Montambaux
      • 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).
  1. The Raman Effect: A Unified Treatment of the Theory of Raman Scattering by Molecules
    • Author(s): Derek A. Long
      • 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.
  1. Principles of Surface Enhanced Raman Spectroscopy and Other Plasmonic Effect
    • Author(s): Eric C Le Ru and Pablo G. Etchegoin
      • 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…
  1. Introduction to Wave Scattering, Localization and Mesoscopic Phenomena
    • Author(s): Ping Sheng
      • 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.
  1. Fundamentals of Atmospheric Radiation
    • 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.

Polluted air…..why do we care ?

Delhi airport : A peek from the widow of the flight at around 9.30 in the morning

If you are an Indian researcher, you cannot escape a visit to Delhi. For the last few years, I have been visiting Delhi for various research related reasons: conference, grant meeting etc. A few days ago I had an opportunity to visit Delhi for a half-a-day meeting.

For me, Delhi embodies a rich feeling of delicious north Indian (street) food, extreme temperatures (by Indian standards), loud taxi music, an assorted flavor of Hindi dialects, and of course, national politics. Of late, Delhi also has gained a lot of attention in another matter: pollution. In winters, for several years now, smog (smoke + fog) has been a major problem, and has drastically perturbed the lives of Delhi citizens. This problem is not confined to Delhi. Various parts of India are not doing great either.

Anyway, as soon I landed in Delhi, it was foggy (see picture), and the visibility was poor. Clearly, there was something in the air, and it was not pleasant.  I wondered about all the kids who travel to school in such an air, and the possible effects on their health. There were several questions running through my mind:  Why is the polluted air the way it is ? How does one quantify pollution? What are the effective methods to detect pollution, and how can it be contained effectively? I knew some scientific aspects of air pollution, but I was curious about how at all air quality was measured and quantified. Below are some facts related to pollution and some interesting connection to light scattering.

There are several reasons for air pollution. In India, some of the major reasons include crop and biomass burning, emission from automobiles and industries, dust etc. There are mainly 8 kinds of pollutants: PM10, PM2.5, NO2, SO2, CO, O3, NH3, and Pb. A majority of the problems are caused by the so-called particulate matter. These tiny objects which can cause severe harm to human beings can be mainly classified as PM10 and PM2.5, where PM stands for particulate matter and the numbers 10 and 2.5 represents the size of such particles in microns. To give you a comparison, the width of our hair is approximately 100 microns in thickness. So imagine a particle which is thinner than your hair entering your respiratory system. This inhalation causes severe trouble to your lungs and the worst part is that it can cause irreversible damage to the inner walls of your respiratory tracts. Even more disturbing fact is that smaller the size of the particle deeper is the penetration in to our system, and greater harm it does to human well-being.

So, how to detect these small particulate matter? There are several ways to detect these tiny objects of which I found two methods to be interesting and effective.

First one is based on light scattering. Generally, the instrument used to monitor air quality using light scattering is called as nephlometer (In Greek nephos means cloud). This is a powerful and compact instrument that can continuously detect and monitor density of particulate matter. The measurement is based on Mie scattering (named after Gustav Mie, more about him in future), where the size of the scatterer is generally comparable to the wavelength of light. It assumes that the scattering particles are spherical in nature and isotropic in composition. It works on the basic principle that when you shine light through smog (at the ground level), the intensity of the scattered light carries characteristic signature of the size of the particle and its concentration. More specifically, the intensity of the scattered light depends on two important ratios. One is the ratio of particle size to wavelength of light and second is the ratio of refractive index of the particle to its surrounding medium. By calibrating the instrument for known particles and concentrations, the unknown size and concentration of the pollutant can be determined. (If you are interested to learn more, see this old research paper). As mentioned earlier, the measurement assumes the scatterer to be spherical and isotropic, which is not the usual case in the air. So corrections due to variation in shape and compositions have to be taken into consideration in this measurement. However, one of the major advantages of this measurement is that it is quick and portable, and hence a lot of air quality measurements are based on these instruments.

Alternatively, if one needs very accurate measurement of particle size, the instrument to use is Tapered Element oscillating microbalance (TEOM).  In this a tiny piece of tapered glass acts like a tuning fork. This tuning fork vibrates at a specific frequency which can be measured with reasonable accuracy. As one may guess, if something is moving, the speed of movement can be affected by adding weight on the moving object. In this case, the vibrating piece changes its frequency as soon as a small particle is in contact with it. The difference in the frequency is now related to the mass of the particle. Thus by using simple physics, one can obtain a powerful instrument to monitor air quality. Apart from the above-mentioned methods, there are various approaches to monitor air-pollution. Each of them have pros and cons, and are utilized depending on the situation.

Coming back to my Delhi trip, I finished my work, and headed towards the airport in a taxi. I casually asked the driver whether he was worried about the pollution in his city. He did mention that it was a concern, but after a brief pause he grinned and said – “odd-even phir sae shooru ho ra hain, business badega” (odd-even is starting gain, business will go up (note: odd-even was eventually stalled this time)). I grinned back at the driver, and remembered a quote of Charles Kettering : “The only difference between a problem and a solution is that people understand the solution”.