Toys, Geim and Gupta

Recently I came across an editorial in Nature Physics, titled as Physics is our playground, which emphasized how playfulness has had an important role in some of the major inventions and discoveries in physics.

A particular example of this is the discovery of graphene, and how it has evolved into one of the most important topics in condensed matter science. Nowadays graphene is used as ‘Lego’ blocks to build higher order structures and the so-called ‘Van der Walls’ heterostructures are one of the most exciting applications of 2D materials. What started as a playful project in the lab has now turned out to be an important part of emerging technologies.

Two important inferences can be drawn from the playful attitude towards doing science :

First is that making modular elements and stacking them creatively can lead to emergence of new structures and function. Anyone who has used lego blocks can immediately relate to it.

Second is that toys are powerful research and teaching aids. Please note, that I emphasized research and teaching here. This is because toy-models are ubiquitous in research, and they help us create modular state of a problem in which unnecessary details are discarded and only the essential parts are retained. This way of thinking has been extremely powerful in science and technology (for example : see ball and stick models in chemistry and mega-construction models in civil engineering )

When it comes to toys and education, there is no better example than the remarkable Arvind Gupta (see his TED talk). His philosophy of using toys as thinking aids is very inspiring. Being in Pune, I have had a few opportunities to attend his talks and interact with him (as part of an event at science activity center at IISER-Pune), and I found his approach both refreshing and implementable. Importantly, it also showed me how creativity can emerge from constraints. To re-emphasize this, let me quote APS news article on Andre Geim :

“Geim has said that his predominant research strategy is to use whatever research facilities are available to him and try to do something new with the equipment at hand. He calls this his “Lego doctrine”: “You have all these different pieces and you have to build something based strictly on the pieces you’ve got.””

Now this is an effective research strategy for experiments in India !

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.

Hot Brownian Colloids – talk

On 19th Jan 2023, I gave a ~40 min talk on “Hot Brownian Colloids in Structured Optical Tweezers” in a very interesting conference on Frontiers in Non-Equilibrium Physics (FNEP) held at Institute of Mathematical Sciences, Chennai.

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)

More is not only different, but also wonderful !

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 : https://arxiv.org/abs/2110.10387

Nanowire kink as an antenna for 2D material

https://link.springer.com/article/10.1140/epjs/s11734-022-00511-y

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 : https://arxiv.org/abs/2203.00391

49. Optical Spin in Nanowire Plasmons

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.

preprint version on arxiv : https://arxiv.org/abs/2104.09303

29. hBN crystal growth…..and a poem by Glück

Internet is a funny thing. I started searching for some research paper but I ended up with a totally unrelated news feature from Nature ¹, which ended up as an interesting read. It is about two Japanese crystal growers from Japan : Kenji Watanabe² and Takashi Taniguchi ³ who work at National Institute of Materials Science (NIMS) in Tsukuba. They have come into the limelight for their outstanding skills of growing high quality crystals of hexagonal boron nitride or more commonly called as hBN in the research community. The news article gives a very nice overview of how they go about growing their crystals with an element of human touch ¹ :

“The two researchers have contrasting styles. Taniguchi is known for his parties, blasts the music of Queen through the lab as he runs the press late at night and, even at the age of 60, still plays soccer with his colleagues at lunchtime. Watanabe, three years younger, is soft-spoken, detail-oriented and prefers tennis. But the scientists worked well together and published their first paper on cBN crystals in 2002.”

In scientific research, an important aspect of lab-based experiments is that it critically depends on the quality of the sample that one is interrogating, and more so in research on condensed matter, where quality of the material is paramount. After the emergence of graphene as a remarkable 2D material ⁴, various researchers across the globe (including IISER-Pune) have been intensely studying graphene and other 2D materials. In order to probe graphene in the lab, first you need to place the one atom thick material on a substrate. Generally, silicon (with or without silicon oxide) is used for this purpose. The electronic mobility of graphene, which is to be maximized for its magic work, critically depends on the quality of the substrate (and superstrate) on which it is placed on, and hBN is the best choice for maximum mobility. This is where quality of hBN comes into picture, and Watanabe and Taniguchi are considered the masters of growing high purity hBN crystals, exclusively for this purpose, as the news feature highlights ¹. Of course, hBN is not limited to be just a substrate for graphene based nano-electronic devices. It has also emerged as an interesting nanophotonic material, which can potentially function as a hyper lens. All these applications critically depend on the quality of the sample one can produce, and hence, growth of high quality crystal is so important.

At IISER-Pune, right in front of my lab is the lab of my colleague – Surjeet Singh⁵. In his lab, they grow some fascinating crystals of quantum magnets, superconductors etc. Furthermore, many of my other colleagues, including chemists and biologists, grow and study crystals made of inorganic, organic and biological materials. I have to mention that, in India, there are many researchers across the country who are excellent crystal-growers. In fact, India has a rich history in crystal growth research ( for example: GN Ramachandran school ⁶), and I hope this legacy will continue with the support of academia and funding agencies. After all, high quality materials need high quality skills to grow and characterize them, and I have repeatedly heard that crystal growth is as much as an art as it is a science.

Speaking of science and art, Nobel prizes for 2020 has been announced. Great to see a good representation of women among the laureates this year. It is befitting to end this blog with a poem⁷ by Louise Glück (the literature laureate of 2020):

October (section I) by Louise Glück

Is it winter again, is it cold again,

didn’t Frank just slip on the ice,

didn’t he heal, weren’t the spring seeds planted

didn’t the night end,

didn’t the melting ice

flood the narrow gutters

wasn’t my body

rescued, wasn’t it safe

didn’t the scar form, invisible

above the injury

terror and cold,

didn’t they just end, wasn’t the back garden

harrowed and planted—

I remember how the earth felt, red and dense,

in stiff rows, weren’t the seeds planted,

didn’t vines climb the south wall

I can’t hear your voice

for the wind’s cries, whistling over the bare ground

I no longer care

what sound it makes

when was I silenced, when did it first seem

pointless to describe that sound

what it sounds like can’t change what it is—

didn’t the night end, wasn’t the earth

safe when it was planted

didn’t we plant the seeds,

weren’t we necessary to the earth,

the vines, were they harvested?

References :

1. Zastrow M. Meet the crystal growers who sparked a revolution in graphene electronics. Nature. 2019;572(7770):429-432. https://www.nature.com/articles/d41586-019-02472-0

2. WATANABE, Kenji | SAMURAI – National institute for Materials Science. WATANABE, Kenji | SAMURAI – National Institute for Materials Science. Accessed October 9, 2020. https://samurai.nims.go.jp/profiles/watanabe_kenji_aml

3. TANIGUCHI, Takashi | SAMURAI – National institute for Materials Science. TANIGUCHI, Takashi | SAMURAI – National Institute for Materials Science. Accessed October 9, 2020. https://samurai.nims.go.jp/profiles/taniguchi_takashi

4. The Nobel Prize in Physics 2010. NobelPrize.org. Accessed October 9, 2020. https://www.nobelprize.org/prizes/physics/2010/press-release/

5. Surjeet Singh. Accessed October 9, 2020. http://www.iiserpune.ac.in/~surjeet.singh/

6. G. N. Ramachandran. In: Wikipedia. ; 2020. Accessed October 9, 2020. https://en.wikipedia.org/w/index.php?title=G._N._Ramachandran&oldid=976314830

7. Poets A of A. October (section I) by Louise Glück – Poems | Academy of American Poets. Accessed October 9, 2020. https://poets.org/poem/october-section-i

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.

Colloids in Choreographic Time Crystals

Christmas, Christmas Ornament, Concept, Snowflake

Image courtesy: Pixabay – creative commons license

Crystal to time crystal : Periodic arrangement of atoms in the form of a crystal is well known to us. The periodicity in conventional crystal is with respect to its spatial co-ordinates. An interesting question is : what if the periodicity of a crystal is also considered in temporal co-ordinates, that is with respect to time ? Such crystals, in which atoms (or their equivalents) repeat both in space and time are called time-crystals (specifically space-time crystals).

Origins : Although the term “time crystal” was used in biological context in 1970s, it was a research paper by Alfred Shapere and Frank Wilczek in 2012 which brought this interesting concept into mainstream physics. Wilczek, a Nobel laureate, also postulated the concept of quantum time crystal, which has added great impetus to this exploration. These theoretical concepts were experimentally probed and verified by Zhang’s group at UC Berkeley using ions in a cylindrical arrangement. Since then, there is a lot of research activity in this area. My colleagues at IISER-Pune – Sreejith and Mahesh, have created a new variety of time crystals by subjecting periodic NMR pulses to spins in star-shaped molecules .

I should also mentioned that after Wilczek’s results were published, Patric Bruno criticized the quantum counterpart based on No-Go theorem argument. There are some interesting debates which are still going on regarding the thermodynamic aspects of these crystals. Also many new applications have been proposed and tested based on the initial predications. To know more about the history and current trends in time-crystals, I suggest a recent, comprehensive review article.

Choreographic (time) crystals : Dance is something inherent to humans, and may be to other living beings. As per google dictionary, the term choreography means the sequence of steps and movements in dance or figure skating, especially in a ballet or other staged dance. In a choreographic time crystal, the movement of atoms (or their equivalent) are in a sequence of steps and co-ordinated, just as in a dance sequence. This means the spatial and temporal co-ordinates of this crystal varies in a predictable way, and hence represents a space-time crystal. Such a concept was proposed in a paper in 2016. An interesting issue discussed in this theoretical paper is how Bragg’s diffraction law can be modified and adapted to probe such choreographic crystals. Modification of this law in necessary as atoms in a dancing lattice are in constant motion, and to obtain snapshots of the moving atoms one needs a capture protocol (diffraction in this case).

Colloidal dance : Atoms are tiny objects. If we need to probe the spatial and temporal evolution of atoms in a crystals, then we require sophisticated imaging tools (such as scanning tunneling microscope) to track atoms in space and time in an ultra-high vacuum condition. Is there any alternative, cheaper method to this approach? The answer is yes (with some caveats). One way is to utilize colloids (micron-scale objects floating in a fluid) and treat them as big atoms. This is of course an approximation, as colloids are classical objects, but many of the physical concepts that are applicable to atoms may be scaled up to colloidal size, and this scaling has been verified and harnessed to mimic and study collective behavior of atoms.

Coming back to choreographic time crystal, the obvious question to ask is: can we use colloids to visualize the dance of this crystal ? A recent paper in PRL (arxiv version) addresses this question with numerical simulations. The authors first propose an experimental scheme to create a choreographic optical lattice using light as a tool. They hypothesize optical potential wells that can evolve both in space and time, and numerically study the evolution of colloids in such a choreographic time crystal. An important finding from their study is that they identify three phases of dynamics, in which the interaction between the potential-well and the colloids is weak, medium and strong. In these three phases, they observe chiral looping of colloids, liquid-like behavior and colloidal choreography. I strongly recommend to have a look at the amazing simulation videos for the three simulated regimes of interaction : weak , strong , medium.

Summary : What I have described above is a metaphorical snapshot of how concepts in physics such as time crystals, optical lattice and colloids can come together on a single platform to collectively give something, which is not feasible to obtain by any of the individual entity. The concept of crystal itself is a manifestation of this ‘emergence’ philosophy. In an essence these ideas are both a tribute to, and reinforcement of, the concept: “More is Different”….. adieu Anderson….