Hermit photons coaxed to interact via one-dimensional confinement!

Photons, the particles of light, do not contain any charge and hence typically do not interact with each other. In fact, they are so aloof that when you shine a beam of light from a torch onto one from another, they cross each other’s path and move on as if the other beam does not exist.

While this property makes them excellent candidates for long distance communication, it also serves as a hindrance for applications in optical information processing and realization of all-optical circuits, where photon-photon interaction at the level of individual photons (for example, controlling the motion of a single photon with another photon) is highly desired. The question now is this: How do we achieve photon-photon interactions at the level of individual photons?

One way would be to tap into what is known as nonlinear optical behavior, which is exhibited by certain materials that change their refractive index as a function of the intensity of incident light. As a consequence, the propagation of light in the interior of such materials can be controlled by another beam. This gives rise to effective interaction between otherwise noninteracting light beams. Intensity-dependent refractive index is routinely employed in many nonlinear optical devices in laboratory optics and in technology. Could this feature of nonlinear optics be used to make individual or at most a few photons interact?

Unfortunately, the electric fields (and hence the intensity) associated with individual photons is tiny; therefore a vast number of photons are required to realize optical nonlinearity in the interior of any known nonlinear optical material. Let us take a minute here to ponder this question: why do we need a large number of photons to generate nonlinear behavior in a material? Answering this question could point one in the right direction towards achieving nonlinearity with a few photons. Let us consider a cylindrical light beam of diameter ‘d’ incident upon a material made up of atoms. The chance that one photon will “see” one atom in the material is given by the ratio of the effective size of the atom as seen by a photon and the transverse area of the beam. The word, “see” is in inverted quotes because of the following: how well a photon sees an atom is dependent on how well the energy of the photon matches the energy levels of the atom; the closer they are more the chance of interaction. When the energy of the photons matches the energy levels of the atom, the effective size of the atom is approximately the square of the wavelength of the incident photon. The beam diameter is usually much greater than the photon wavelength, and hence the chance of one photon interacting with an atom in bulk materials is very very small. Right away, we see that to improve the chances of interaction and hence nonlinearity we need a lot of photons (which we do not want, since we are aiming for nonlinearity with a few photons) or—here is our aha moment—reduce the diameter of the light beam ‘d’ to such a small value, in fact far below the wavelength of photons, and couple it to an artificial atom whose effective size is far greater than any existing real atom.

Enhanced atom-photon coupling with just a few photons, leading to strong optical nonlinearity, is achieved by confining the photons into tiny spaces with dimensions much smaller than the wavelength of the photons. This has been experimentally realized by squeezing the photons along with individual atoms in confined geometries such as waveguides. One-dimensional waveguides such as superconducting transmission lines can confine microwave photons to deeply sub-wavelength sizes in their transverse dimensions. When we couple an atom (a superconducting qubit here) with a large transition dipole (equivalent to a large effective size) to such confined photons, strong optical nonlinearity may be manifested with a just few photons. In such a case, photons are transversely focused to an area comparable with the scattering cross-section of the atom, and their electric field at the atom becomes large enough to excite the atom (which means that atom absorbs energy from the incident photon and goes to a higher energy state) with very high probability.

In a recent colloquium, Dr. Dibyendu Roy from the Raman Research Institute along with international collaborators has discussed the topic of photons interacting strongly when confined to a one-dimensional geometry from experimental and theoretical perspectives.

To know more about this new exciting research field, please visit



Figure caption: A superconducting qubit (indicated in orange) embedded in a one dimensional transmission line waveguide. The waveguide confines light to deeply sub-wavelength dimensions in the transverse direction.


‘Organic’ material set to make solar energy truly ‘green’

IGs - Part 9 (1)_0Illustrations: Purabi Deshpande

Scientists believe that inorganic semiconductors used in making solar cells may have hit the efficiency ceiling besides being environment hazard.

In my opinion, silicon technology has reached a sort of limit regarding efficiency. It is around 20% efficient right now, but it hasn’t seen a huge leap in efficiency in a long time. Production of electronic grade silicon pollutes the environment significantly and manufacturing silicon wafers indigenously on a large scale is still a challenge,” says Prof. Sandeep Kumar, of the Raman Research Institute (RRI), Bangalore, who specializes in Soft Condensed Matter. 

Going ‘organic’ is the alternative, says Prof. Kumar, who along with his team of researchers, is studying the use of organic materials like polymers or carbon fullerenes in solar cells. “Organic photovoltaic (OPV) research has received a good momentum in the past decade as promising sources of alternative energy due to their low cost, ease of fabrication, roll-to-roll processing, flexibility, and lightweight, he says.

Traditionally, inorganic semiconductors like silicon and its alloys have been used in making solar cells due to their appropriate electrical properties and wide availability. In addition to being an environmental hazard, some scientists believe that silicon-based solar cells may have also hit the efficiency ceiling. The power conversion efficiency (PCE) of OPV’s has crossed the 10% mark and is increasing day by day”, he explains.

In a recently published study, Prof. Kumar of the Raman Research Institute in collaboration with researchers at the Center of Material Sciences, University of Allahabad, and National Physical Laboratory, New Delhi, have used indigenously developed Discotic Liquid Crystals (DLC) to build effective organic solar cells that could become an eco-friendly alternative to conventional solar cells. This study was funded by the Department of Electronics and Information Technology (DEITY) and published in the Liquid Crystals Journal.

“DLCs were discovered at the Raman Research Institute in 1977. They are of fundamental importance not only as models for the study of energy and charge migration in self-organized systems but also as functional materials for device applications such as one-dimensional conductors, photoconductors, light emitting diodes, photovoltaic solar cells, field-effect transistors and gas sensors”, remarks Prof. Kumar.

DLCs are made of stacks of disc-like molecules in a state of matter that lies between those of a liquid and a solid crystal, like powdered sugar, which is made of solid crystals, but collectively behaves like a liquid taking the shape of the vessel in which it is stored.  Earlier studies have shown that DLCs efficiently conduct electricity along their length with very little loss, effectively making them molecular wires.   In this study, the researchers have used DLCs in solar cells to increase their efficiency.

The world is witnessing a shift in its primary energy sources, with fossil fuels giving way to renewable sources like solar and wind. India is well along this line of transformation with 10GW of electricity currently produced from solar energy, compared to just 2.6 GW in 2014. This shift to solar energy across the world has revived the solar cell (also known as the photovoltaic cell) industries, and now increased investments pour into this sector.

A solar cell, in principle, converts the energy of light falling on it, into electricity. The light conversion in OPV cells is based on charge generation at the interface between two different organic semiconductors, followed by their separation and migration toward opposite electrodes. The researchers of the study sandwiched a layer of DLC between an active layer (PCDTBT and PCBM) and a molybdenum trioxide buffer layer.

“We have explored the use of DLC as an additive in classical organic photovoltaic cells and observed significant improvement in conversion efficiency from 1.24% to 5.14%. The enhancement is attributed to the better charge mobility in the ordered system due to the presence of columnar phase of the DLC”, exclaims Prof. Kumar.

Although solar is hailed as a ‘green’ energy source, the mining and the manufacturing process associated with silicon based solar cells releases large amounts of pollutants, overshadowing the eco-friendly status of solar power. Organic solar cells, on the other hand, overcome these shortcomings and with studies like this, they may soon surpass the efficiency of silicon-based cells, finally providing us a true ‘green’ source of power.

RRI at the India International Science Festival, IISF 2017

The third edition of the India International Science Festival (IISF, 2017), jointly organized by the Ministry of science and technology, Ministry of Earth sciences and Vijnana Bharati was held between 13-16 October at Anna University, IIT and CSIR-CLRI campus in Chennai. IISF is one of the largest outreach events of its kind and showcases achievements in Indian science to the general public. One of the many events at IISF 2017 was a “Mega Science and Technology Expo: theme based pavilions in Engineering, Water, Healthcare, Agriculture, Environment, Strategic Sectors, etc”.

Raman Research Institute participated in this expo with an eye catching pavilion that had the shape of the antenna element that forms the International MWA Radio Telescope array in Western Australia, in which RRI has been a partner for the last decade and for which RRI built the digital receivers. For the benefit of visitors we had an explanatory “Why does the RRI stall have this weird shape?” at the entrance to our pavilion.

Our pavilion included posters highlighting the research achievements at our Institute this past year as well as models of a radio telescope, antenna and detectors. We also had a screen that constantly replayed images of the Milky way galaxy and distant universe at a range of frequencies from Gamma rays down to radio frequencies. Incidentally the image of the radio sky that was part of the slide show resulted from the sky survey using the MWA telescope.

The posters discussed (i) Cold Atoms – A chill cocktail of atoms and ions at near absolute zero temperatures (ii) Theoretical Physics – “Understanding the Universe – One equation at a time” (iii) Two posters on Theoretical Astrophysics – “Understanding the Dynamic Universe” (iv) Two posters on Radio Astronomy – “Seeing the Unseen” (v) Quantum Key Distribution – “A new security paradigm” (vi) POLIX: X-ray vision – “India’s own eye in the sky” (vii) Liquid crystals – “The fourth state of matter”. Apart from the above we also had posters that gave an overview of our Institute along with posters on the Knowledge Communication we do – PhD, postdoc, visiting student and research assistants programmes.

Over the course of the four days more than five thousand people from all walks of life – high-school students, undergraduates, graduates, PhD students, academics from the scientific fraternity, Ministry officials and also the general public visited our pavilion. A few elderly visitors were very interested in the shape of the pavilion as well as the science and with childlike enthusiasm bombarded us with questions and gently asked us if we could give our answers in the local vernacular. The same was the request from many school students who had made the trip to Chennai from remote locations in this vast country of ours. For many it was their first experience to any event of this kind and we hope they returned with positive memories and a good appreciation for science in general and our Institute’s research in particular. It is worth noting that a few alumni of our institute also visited us and appreciated our efforts.

The pavilion was ably managed by our PhD students Rishab Chatterjee (LAMP), Sagar Suthradhar (LAMP) and our visiting student Urvashi Nakul (AA). The interactions were not limited to questions and answers but to in depth scientific discussions with PhD students from other institutes. Needless to say, there was never a dull moment and everyone involved came back richer from the experience. We hope to continue such public engagements in the years to come.


International Science School at University of Sydney (Australia) for senior school students

ISS2017photoFor the sixth time in a row Indian senior high school students had a chance to participate in the Professor Harry Messel International Science School (ISS for short) at the University of Sydney in Australia. Talented science students from eight countries (that included Australia, China, India, Japan, New Zealand, Thailand, United Kingdom and United States, each of whom won a University of Sydney Physics Foundation scholarship) attended the 2-week lectures and activities event at the ISS2017, as it is popularly referred to. The awarded scholarship completely covers the entire 2 weeks of the Science School for each of the students.

“Future Power” was the theme this time, covering a wide array of fields that included Fusion, Fission, solar power, wave power, power from biogas, electricity grids and storage devices apart from most current research efforts and out-of-the-box thinking in fields of materials science tackling energy-related problems being faced today. The one word that comes to mind when describing the ISS is “quality”. Whether it is do with the lectures, the hands-on activities, the engineering challenges, the design and planning of the entire 2-weeks of the science school or the local arrangements, there is satisfaction all round, with any mishaps or problems that arise each day (of which there are several!) handled in a sensitive and satisfying manner. Experts in the particular fields from around the world give the lectures. Pitched at 1st or 2nd year undergraduate level the students are drawn into the beauty and intricacies of the subject areas, which this time (given the ISS theme of Future Power) also included a multi-disciplinary fare of policy and social as well as environmental considerations.

To give a background to India’s participation in this, one of the oldest of international science schools (since 1962), the journey began in 2007 when Indian students participated in the Sydney International Science School for the first time. Raman Research Institute, upon request from the University of Sydney School of Physics, agreed to facilitate the selection.

This year, the sixth year of participation for Indian students, was a particularly memorable one with one of the students, Poorvi Hebbar, having been chosen for the top and coveted Len Basser award for Science Leadership at the science school (as did Nruthya Madappa, the very first time that Indian students participated in 2007).

The Australian Minister for Industry, Innovation and Science lauded the Indian participant saying:“Poorvi Hebbar, attending from India, was selected from the 132 high-performing senior science students at the school. The students, including five Indigenous students, come from schools from all states and territories who have been joined by 46 from China, India, Japan, New Zealand, Thailand, the UK and the USA. She has been described as being a quiet achiever, who is diligent and very innovative in her ideas, always showing great interest in the lectures and activities and easily and openly sharing her knowledge and enthusiasm, inspiring her peers and drawing their respect.”

As many as 30 talented Indian students have been to the Sydney International Science School over the years, facilitated by the Raman Research Institute. The themes of the past six ISS that were attended by Indian students have covered topics such as Ecoscience, Light and Matter, From Genes to Galaxies, Nanoscience, BIG: Big ideas, big experiments, big challenges — big science.

No doubt the next ISS to be held in 2019 will be eagerly awaited.  It is going to cover humanity’s adventures in space and promises to once again thrill and fire the imagination of the those lucky few who will be there!



Milk, Coffee’s best mate? Maybe not in the quantum regime!

What happens when you pour cold milk over hot coffee? Given enough time and provided you resist the temptation to swirl the mixture around with a spoon, the milk will completely spread into the coffee and you end up with a delicious mug of coffee albeit a little less warm. The heat from the hot coffee atoms is transferred to the cold atoms of milk until they reach a steady state. However, do start sipping your coffee soon or else the molecules in the coffee will transfer most of their heat to the molecules in the air.

Similarly, if you thrust the end of a metallic rod, which is a good conductor of heat, into a flame it would heat the rod and before long you start feeling it at the other end at your hand. Thus, by some mechanism the heat from a hot region is transferred to the cold region. Also, we know that if by accident a metallic rod we hold touches a live wire, electrocution results!

What is the mechanism by which heat and electricity is conducted along metals? Any student of science would tell you that in a good conductor it is due to the freedom that energy carriers (phonons) and charge carriers (electrons) enjoy, which is just not there in an insulator.

This explanation hardly satiates our curiosity and we often want to know the answer at a deeper level. For that we have to dive into the metal and go underneath the surface. What do we see? We would see a sea of electrons in a regular periodic arrangement of positive ions. In metals, electrons are the predominant carriers of energy and charge. The electrical conductivity is directly proportional to the average length an electron can travel in the metal before it gets scattered by an ion. This is the classical picture, which considers electrons like particles colliding with bigger sized ions, something like in a game of marbles.

However, the classical picture is incomplete at the subatomic scale. Since the ions are positively charged they have an electric field (or in other words a potential) associated with them and since they are arranged in a periodic fashion the electron sees a periodic potential. We know from quantum mechanics that electrons have a wave nature. In fact, all matter has an inherent waviness associated with it: even us humans, although in our case the wavelength would be unimaginably small and unfortunately meaningless. Google “De Broglie wave” sometime. The electronic wave function within some energy bands is spread (or delocalized) over the periodic potential of the positive ions.

One interesting question to ask is what would happen if we make the potential aperiodic, thus introducing disorder into the system? Let’s do just that by adding impurities; for example, substituting some Aluminum ions with Copper ions in a lattice of Aluminum ions. The electron sees an aperiodic potential because the substituted Copper ions will have a different potential. The electronic wave function that was earlier spread out tends to localize around the impurity. In the classical model, this can be thought of as a reduction in the average length that an electron can travel leading to a drop in conductivity. Lets keep adding more impurity just to see what happens: beyond a certain amount of impurity the electronic motion stops altogether resulting in the material becoming an insulator. This means the wave function becomes localized around the disorder. This is known as Anderson localization after its discoverer and Nobel Laureate P. W. Anderson.

Anderson localization is due to wave interference between multiple scattering paths (an easier way to think about this is to imagine that the electron zig zags around the impurity resulting in a net zero motion) and is well understood for particles/waves that do not interact with each other in a disordered (with impurities) medium. One may then ask: We saw that electrons are present in abundance in a metal, and there is every possibility they will interact and if they do what happens when we include these interactions? Further investigations of localization in disordered media in the presence of short-range interactions between quantum particles (electrons, for example) led to the concept of many-body localization (MBL).

Let us try to understand many body localization with a macroscopic analogy. Going back to our earlier example of cold milk and hot coffee, this time instead of pouring milk into the coffee let’s add one drop in the center of the cup. In normal systems our everyday experience of milk and coffee mixing completely leaving no trace of where the drop of milk hit the coffee holds true. However, in an ideal MBL system, even after a very long time the milk drop will still stay where it fell as well as maintain its coldness.

Dibyendu Roy, from the Raman Research Institute, along with his collaborators Rajeev Singh from Bar-Ilan University and Roderich Moessner from Max Plank Institute for the Physics of Complex Systems, have recently published a paper on MBL and have the following to say about their work:

MBL is a recently discovered state of solids. It is an insulator and results from the interplay of disorder and interaction between particles. The nature of MBL in more than one spatial dimension is not entirely understood. We have studied properties of MBL in one-dimensional long-range models and made an analogy to infer features of MBL in higher-dimensional short-range models.

To read this interesting paper please click https://journals.aps.org/prb/pdf/10.1103/PhysRevB.95.094205




Scientists may have discovered the most massive planet orbiting two stars

“Data will talk to you if you’re willing to listen to it”, said Jim Bergeson – a well known computer engineer. And that’s probably what scientists from Raman Research Institute (RRI), Bangalore, and Hans Raj College, University of Delhi, seem to have done. While combing through data on orbit of a binary system of star (MXB 1658-298), collected over a 40 years, 1976-2016) period, they have noticed discrepancies in the orbital period of these stars. They attribute this to an invisible ‘third’ object, probably the most massive planet revolving around a binary star, which we have ever known.

In a binary system of stars, two stars orbit each other and in some cases, one of the stars in the binary is a compact star, like a neutron star or a black hole that accretes matter from its companion. In the case of MXB 1658-298, an eclipsing X-ray binary star system, the massive star is believed to be a very dense neutron star, which is actively eating up material from its companion, making it interesting for scientists to study. This system of stars has now become even more appealing after scientists involved in this study discovered discrepancies in the time between the X-ray eclipses.

While comparing observational data obtained from several X-ray telescopes, during three active periods of the binary stars between 1976 to 1978, 1999 to 2001, and 2015 to 2016, the researchers found that the time between X-ray eclipses did not match theoretical predictions. Instead, there appeared to be an invisible force affecting the orbital period of the stars. The invisible force, they think, might be a massive planet (around 20 to 25 times the mass of Jupiter) orbiting the stars. “This work didn’t start as a search for an exoplanet. We were studying the evolution of this X-ray binary – MXB 1658-298. We were just keeping track of the orbital period of the binary”, says Prof. Biswajit Paul, Professor at RRI, explaining the context behind this study.

Detecting a planet around a far-away star can be quite a challenge. When looking through a telescope, the light reflected from the planet is drowned out by the brightness of its parent star or stars, requiring alternate methods to observe them. One way to detect an exoplanet is to measure the dip in the amount of light reaching us, when its orbit is between its parent star and our line of sight from Earth. But in the case of a binary system, knowing the orbital period of the two stars would also help in detecting an exoplanet, as it interacts with the stars through gravity, sometimes changing the orbital period of the stars. For this discovery, the researchers used the latter principle and developed a new technique of measuring the periodic delay in X-rays, to identify the existence of an exoplanet.

“If the arrival of the X-ray eclipses slows down, then it means the orbit is expanding and if the eclipses appear faster than expected,, then the orbit is decaying. Such an evolution of the orbit could be because of the mass transfer between the stars. But in this case, in addition to a decay of the orbit we found that the time of the eclipse was oscillating, which means the binary period was slowing down and then speeding up over a two-year period. This is what prompted us to propose that there could be a third body, which is causing the oscillation in the orbital period. Now, by measuring the change in orbital period, we can calculate the mass of the third body”, explains Prof. Paul.

Although not conclusive, the prediction now opens up new avenues of study on MXB 1658-298. “While there are more than 2000 exoplanets that are known, only 20 ‘circumbinary’ planets (planets that revolve around binary stars) have been discovered. This one is the most massive, it is at a great distance from us, and it is around a LMXB (Low Mass X-ray Binary), which are usually very old systems. So it would be very interesting to study this system further”, signs off Prof. Paul, talking about the enthusiasm this study has created.

About the authors:

Prof. Biswajit Paul is a Professor at the Astronomy and Astrophysics Group at RRI

Contact: bpaul@rri.res.in

Chetana Jain is an assistant professor at Hans Raj College, University of Delhi.

Contact: chetanajain11@gmail.combinary system


Stars and star cluster heat up their environment even a million years after their birth, say scientists

BimanNath,SiddhartthaGupta-001How does the presence of a star or a gravitationally bound group of stars (star cluster), influence their environment? A previously unclear phenomenon has now been answered, thanks to a recent collaborative work from scientists at the Raman Research Institute (RRI), the Indian Institute of Science (IISc) and P.N Lebedev Physical Institute, Moscow, Russia. They have successfully developed a theoretical model to simulate the interactions between a star cluster and its surroundings, enhancing our understanding about processes that lead to the formation of stars, clusters and galaxies.

The birth of a star begins with gases accumulating under gravity until it gets hot enough to initiate nuclear fusion — a process where lighter atoms merge to form heavier atoms with an enormous outburst of energy. A strong shock wave then pushes the surrounding gas and debris into a bubble, similar to the Oort cloud surrounding our Sun. Following this, radiation from the newborn star bombards the surrounding gas to further push it away. But now, questions have been raised on the mechanism through which the stars, or clusters of stars, transfer their energy to the surrounding gas. Prevalent theories suggest that gaseous winds from the stars, and the explosions at the end of the life of stars, heat up the gas in their vicinity. This hot gas physically pushes the surrounding gas away, transferring mechanical energy in the process. But this idea did not quite explain some of the observations, thus requiring a deeper study.

The current research represents a paradigm shift. According to the new theoretical insight provided by the collaboration, pressure of the radiation dominates during the interaction between stars and the surrounding gases for the first million years of its life following which the interaction continues through the heating of the gases. Indeed, previous observations have recorded the heating of the surrounding gases. The researchers specifically predict that the heating is due to high energy photons emitted by the star cluster, which bombard the surrounding gas particles thus causing their temperature to increase.   

“What we found, something which wasn’t really anticipated, is that radiation from clusters has a two pronged effect in a way. Initially, the cluster exerts radiation pressure where it interacts by pushing against the gas. After about a million years, the radiation pressure decreases as the gas particles move far away from the cluster, but the thermal pressure starts heating the gases, causing it to expand. This thermal pressure wasn’t understood very well”, explains Biman Nath, a Professor of Astronomy and Astrophysics at RRI.

The researchers then built a computer model to simulate the effects of radiation pressure and thermal pressure from a star cluster on its surrounding environment. When they compared their results to observations from the Tarantula Nebula, also known as 30 Doradus, it was found to match closely. “Earlier observations of the nebula showed that the X-ray luminosity from the star cluster, or the brightness of X-rays, was much lower than what was theoretically predicted. Now, after incorporating our new insights, the observations match the predictions very well”, remarks Mr. Siddhartha Gupta, a postgraduate student at RRI and IISc.

The current study advances our knowledge of the formation and evolution of galaxies, but we are far from a complete understanding of the processes. “Galaxies are made of two major building blocks – gasses and stars. So it’s very important to understand the interactions between these building blocks at a micro level, to really understand how galaxies form. People have looked at the macro-scale interactions, but there are severe gaps in our understanding of these micro-level interactions. This work can be thought of as a starting point towards our understanding of the evolution of galaxies”, says Mr. Gupta about the importance of this work.