Category: History

Why liquid state is amazing ?

One of the research questions I am interested in is how light interacts with the liquid state of matter?

A related question is how is it different from the interaction of light with a solid state?

Specifically, I am interested in understanding how optical forces can influence the structure of liquid matter and if one can control such interaction.

The liquid state of matter is ubiquitous but not as well understood as the solid or gaseous state of matter. This is because the nature of interaction in liquid is intermediate between the other two states, and the way molecules and atoms interact depends on parameters that include interaction in classical and quantum physics.

Jean Perrin – Image courtesy: Nobel Foundation archive.

A connected question to this problem is how liquid behaves under and out of equilibrium. This classification is again non-trivial as it depends on the spatio-temporal scales under which one is making an observation. An important aspect of the liquid state is the local and global fluctuations, which complicate the problem. Jean Perrin, in his 1926 Nobel lecture, alludes to this problem very succinctly.

There is still much to learn about the liquid state, and enormous progress has been made from experimental and theoretical viewpoints. But there are some fascinating ‘landscapes’ yet to be fully explored in this domain.

Sometime in the future, I will write more about how this is done and what the historical context is, especially from an optical force viewpoint.

On a related note :

Here is an illustration of the Brownian motion of gold nanoparticles..imaged using a dark field optical trapping microscope..this is a custom-built system in my lab. We study the effect of laser traps on Brownian dynamics..more on this later.

A Twitter thread on my Japan trip that inspired my podcast

Graviton-like modes – link and some questions

Recently, I wrote a blog on Graviton-like modes in solids.

There is a nice report from the Columbia Quantum Initiative on the Nature paper.

It highlights the role of Aron (a key author in the paper, who passed away in 2022) and his legacy.

Undoubtedly, the graviton-like connection has created a buzz. As the above report mentions, it has also taken a lot of time and effort on the part of the authors to measure it.

Thanks to my colleague Surjeet Singh, I also learnt about the importance of heterostructures in this work and that authors do not oversell the connection to gravitons in the paper.

For me, as an outsider to the field(s), a few questions remain:

  • What lessons can we learn from graviton-like modes in solids that will help us understand the actual undetected graviton?
  • How is it different from the collective excitation of quasiparticles, such as exciton-polaritons in semiconductor heterostructures? Similar experimental methods are used to study them. 
  • Is the prominence only due to spin-2 collective excitation? 
  • Finally – applying Occam’s razor – can there be a simple explanation without evoking graviton-like modes?

It would be great to know the answers, hopefully, in the coming days.

Graviton modes in solids: Old Argentinian wine in new Bottle ?

Recently, there has been a buzz about a Nature paper titled Evidence for chiral graviton modes in fractional quantum Hall liquids. There has been some media reportage on the paper too.

The paper makes interesting claim on observation of ‘chiral graviton modes’ in certain ultra-cooled semiconductors (Gallium Arsenide – famously called GaAs). The cooled temperature is quite low (~50 mK), which is impressive, and the chirality of the mode is unveiled using polarization-resolved Raman scattering. The observation of this so-called ‘Graviton modes’ is essentially a quasiparticle excitation, and has created some buzz. In my opinion, graviton-like behavior is a bit of an exaggeration.

Anyway, this paper has set an interesting discussion among my colleagues (condensed matter and high energy physics) in our department. To add to their discussion, I wrote on 2 points (and an inference) from optics perspective, which I am sharing below :

  1. The measurement scheme used to unveil the chirality of the quasi-particles is a well-known trick in polarization optics. In fact, I teach it to our undergrads. Notice the use of quarter-wave plates (indicated by the arrow in the figure below). This is also the measurement at the heart of unveiling optical anisotropy. Experimentally, what is impressive is the ultra-low energy excitation captured via Raman scattering. This is again thanks to the excellent cooling of the sample (50 mK).
Figure from the Nature paper.

2. The last author of this paper, Aron Pinczuk, was a well-known expert in light scattering in solids. He was an Argentinian-American professor at Columbia University, and passed away in 2022.

Aron Pinczuk

He and the legendary Manuel Cardona were instrumental (pun intended) in laying the foundation for using inelastic light scattering methods in solids. The first edition of the series “Light Scattering in Solids”, written in 1976, has Pincuk discussing the very measurement scheme used in the paper (see picture).  

The first edition (1976) of a great series : Light Scattering in Solids

My initial inference on the paper : This is an old Argentinan wine of quasiparticles in a new GaAs bottle at ultra-low temperature….and NATURE is selling it as champagne de graviton made in China ! 

Asimov on lasers

Isaac Asimov is undoubtedly one of the greatest science fiction writer in English. He also wrote a lot of non-fiction science books that are interesting and accurate in their exposition. I recently came across an interesting book by him on lasers. Written around 1990s, this book discusses the origins of lasers and the basic physics and engineering aspects of lasers. True to his reputation, he weaves interesting history into the science, that makes an engaging read. Some grayscale (charcoal-kind) illustrations in the book are appealing, and makes it a smooth read.

The book also has a short discussion on applications of lasers, and eye surgery is one of them which is explained lucidly.

If you are not from a scientific background, or want to have a light read on lasers, then I would recommend this book. As usual, Asimov does not disappoint you !

Asimov was a biochemistry professor before he became a full time writer. His incliniation towards chemistry is evident when he discusses ‘chemical laser’. I reproduce a couple of paragraphs from his book:

This idea of the so called ‘chemical laser’ is still under exploration, and has not found its full potential. Perhaps there is an interesting research problem here for the interested.

Internet archives has a link to this book, but it is not complete. On the internet, you may find other links to this book.

What was surprising to me is that I found a Marathi translation of this book online. I don’t know how good is the translation, but I will urge you check it out if you know the language well.

Asimov was a unique person, as he blended science, storytelling and writing very efficiently. We need more of his kind in this world.

Raman speaking in German

As I mentioned in the recent essay, Raman was conversant in German and could give a technical talk. Here is a rare audio recording of Raman speaking on crystal physics in German in 1956, Lindau meeting… photographed with another great…

audio :
https://mediatheque.lindau-nobel.org/recordings/31549/physics-of-crystals-german-and-english-presentation-1956

Link to the essay : https://backscattering.wordpress.com/2024/02/28/c-v-raman-as-a-science-communicator-a-historical-perspective/