A droplet of liquid can act as an optical resonator. One can create a droplet of a liquid crystal and utilize its optical and topological properties. In recent times, liquid crystal droplets have emerged as a ‘soft photonic element’ in topological optics and photonics. Studying their optical behaviour in a controlled environment is a contemporary research problem.
In this context, we have an arXiv preprint on liquid crystal droplets and their reversible coupling to a small assembly of nanoparticles on a glass surface (see video).
Specifically, we ask: What happens to the modes of light inside the droplet due to such an interaction?
Thanks to the efforts of Sumant Pandey, we experimentally demonstrate the utility of optical tweezers to proximally couple (and decouple) nematic liquid crystal droplets to gold nanoparticle clusters, and record whispering gallery modes in coupled and decoupled states. We observe tuning of sharp resonant modes. For more details, see the preprint: https://arxiv.org/abs/2509.10126v1
Steven Chu and Arthur Ashkin in 1986, in front of the apparatus shortly after the first optical trapping experiment was completed. Image from Chu’s Nobel lecture.
“In 1986, the world was excited about atom trapping. During this time, Art Ashkin began to use optical tweezers to trap micron sized particles. While experimenting with colloidal tobacco mosaic viruses, he noticed tiny, translucent objects in his sample. Rushing into my lab, he excitedly proclaimed that he had ‘discovered Life’. I went into his lab, half thinking that the excitement of the last few years had finally gotten the better of him. In his lab was a microscope objective focusing an argon laser beam into a petri dish of water. Off to the side was an old Edmund Scientific microscope. Squinting into the microscope, I saw my eye lashes. Squinting harder, I occasionally saw some translucent objects. Many of these objects were ‘floaters’, debris in my vitreous humor that could be moved by blinking my eyes. Art assured me that there were other objects there that would not move when I blinked my eyes. Sure enough, there were objects in the water that could be trapped and would swim away if the light were turned off. Art had discovered bugs in his apparatus, but these were real bugs, bacteria that had eventually grown in his sample beads and water.”
Chu won the physics Nobel in 1997, and Ashkin won the same in 2018. Ashkin was the pioneer of optical trapping and tweezers, and applied it to a variety of problems, including the manipulation of biological matter. Chu harnessed the momentum of light to trap and cool atoms. Both started their work and collaborated at Bell Labs. Chu moved to Stanford, whereas Ashkin stayed back. Bell Labs was a remarkable place in the 1980s, as Chu describes in his lecture :
“Bell Labs was a researcher’s paradise. Our management supplied us with funding, shielded us from bureaucracy, and urged us to do the best science possible. The cramped labs and office cubicles forced us to rub shoulders with each other. Animated discussions frequently interrupted seminars and casual conversations in the cafeteria would sometimes mark the beginning of a new collaboration.”
Can the world afford to have another Bell Labs in 2025? Can it recreate the magic?
This week in my optics class, I have been teaching Kramers-Kronig (KK) relations of electric susceptibility. It is fascinating to see the causality argument emerge from the relationship between the real and imaginary parts of the complex susceptibility. Whereas the time domain explanation is relatively easier to appreciate (that dissipation follows perturbation in time), for me, the frequency domain implication in KK relation is fascinating: the fact that information about the real part of the function at all frequencies can give you insight into the imaginary part at any given frequency (and vice versa) makes it such a powerful mathematical and physical tool. For example, by knowing the absorption spectrum of a medium, you can find out the refractive index of a medium at a particular frequency that is not easily accessible in experiments.
Two inferences I draw:
1) Complex analysis combined with differential calculus is one of the most beautiful and powerful mathematical tools invented, and exploring its application in experimental scenarios has made physics intriguing, useful, and profound.
2) The KK relationship shows how causality and the structure of matter are connected to each other, and by studying them, one will be able to extrapolate the idea beyond the problem at hand and apply it to a different context in physics. It just shows how ideas hop from one domain to another and how mathematics plays a critical role in intellectual arbitrage.
Real is imaginary and vice versa. Complex numbers zindabad!
Learning is a lifelong process, and even the best researchers have to update their knowledge as and when they come across new information. Subrahmanyan Chandrasekhar was undoubtedly one of the most accomplished mathematical astrophysicists in the 20th century, and his range of topics covered almost all aspects of astrophysics. Chandra (as he was known) was a lifelong learner, and took up new topics within astrophysics, researched them deeply, and wrote definitive books on them, which are still of great utility even today. In his research process, Chandra consulted various scholars across the world, irrespective of their age, and learned new things.
In 1967, Chandra, aged 57, wrote a letter to a 25-year-old researcher, Stephan Hawking, to learn more about his work ‘on the occurrence of singularities in cosmology’. In this letter, which is written in a desperate tone, Chandra mentions that he is grappling with some mathematical aspects of Stephen Hawking’s work and is asking him for references that he can consult to understand his papers. Chandra describes reading Hawking’s papers as ‘climbing a staircase moving downwards’. Below, I reproduce the letter (from the University of Chicago archives).
To this letter, Hawking dutifully replies (see below), suggesting specific books on topology and differential geometry. Hawking also suggests some of his published papers. Hawking himself downplays his knowledge of mathematical aspects related to the work, and mentions that it improved after he consulted the mentioned books. Below, I reproduce the handwritten letter (from the University of Chicago archives).
There are two aspects that are interesting to note: one is the fact that even accomplished researchers have to learn and relearn many things as they get exposed to new information, which calls for humility and setting aside egos, and the second aspect is that ideas are built on existing ideas available at that time, and a major part of it is to learn from papers, books and of course communicating with people, as Chandra did in this case.
15th Aug also happens to be birthday of Louis de Broglie, the famous French physicist who played a critical role in understanding wave-particle duality in quantum physics, and laid an important foundation through his formula
λ = h / p ;
where, λ is the wavelength of quantum particle with momentum p and h is the Planck constant.
de Broglie studied and discovered the wave nature of electron, for which he received the Nobel prize in physics in the year 1929. In 1920s, understanding light from a quantum mechanical viewpoint was a challenge. Reconciling light, both as a particle and a wave, was counterintuitive and required a leap of thought that was provided by de Broglie. On 12th Dec 1928, delivered his Nobel lecture and mentions:
“I thus arrived at the following overall concept which guided my studies: for both matter and radiations, light in particular, it is necessary to introduce the corpuscle concept and the wave concept at the same time. In other words the existence of corpuscles accompanied by waves has to be assumed in all cases. However, since corpuscles and waves cannot be independent because, according to Bohr’s expression, they constitute two complementary forces of reality, it must be possible to establish a certain parallelism between the motion of a corpuscle and the propagation of the associated wave.“
This duality still remains, as we try understand the nature of light and harness it for information processing.
Interestingly, de Broglie was one of persons who nominated CV Raman for the Nobel prize in 1930 ! Below snapshot is from the Nobel prize nomination archives.
Every year, I teach an optics course to physics majors (including physics iPhD students and MS Quantum Tech students). In the process of introduction, I discuss how light was discovered to be an electromagnetic wave. One of the thrills of this topic is to quote Maxwell from his legendary 1865 paper1, in which he makes this monumental connection. Every time I teach this, I get an intellectual kick, even after doing this for almost 1.5 decades.
The highlighted text is the famous statement. Before that, Maxwell compares his result with two experimental results and confirms his prediction. I follow this up with Hertz’s experiment.
Note: Electric waves and telegraphy were already known before Maxwell’s paper. There were papers that discussed about velocity of light and its connection to electric waves. See this paper2, for example. However, these interpretations were not as comprehensive as Maxwell’s case, and importantly, the field theory viewpoint needed Faraday’s experiments and Maxwell’s interpretation.
Maxwell, James Clerk. 1865. “VIII. A Dynamical Theory of the Electromagnetic Field.” Philosophical Transactions of the Royal Society of London 155 (January): 459–512. https://doi.org/10.1098/rstl.1865.0008. ↩︎
One of the joys of studying quantum mechanics, at any stage of a career, is to be aware of the fact that there is more scope for interpretations and understanding. This notion has not changed for several decades. A recent survey reinforces this thought.
There are at least 3 interesting points that I infer from the situation:
1) The interpretation of reality at the quantum scale is probabilistic. This has served us well in experiments and has led to the founding of quantum technologies. We are in a situation in the history of science where the philosophical foundations are uncertain, but the technological implications are profound.
2) Having more data is always good, but for a new leap of thought, we may have to pay attention to new connections among the data. Can AI play a role in this?
3) There is more room for exploration in the foundations of quantum physics. Philosophy of physics has a role to play in this exploration. Physics students and researchers with (analytical) philosophical inclination have an opportunity to contribution. This needs a grounding in understanding mathematics and experiments related to quantum physics. I see this as a great opportunity for someone to enter the field.
Conclusion: Good time to explore the foundations of physics*
Ideas in philosophy of science, especially in the 1800s and early 1900s, had their origin in physics. Two philosophers who were deeply influenced by physics were Karl Popper and Thomas Kuhn. Below is a cartoon depiction of the same. Of course, the origins of ideas in philosophy of science have diversified in recent years, and biology and technology (especially AI) dominate the scene nowadays.
Experimental physics is one of the crucial ingredients of physics. There are at least two major tasks within its realm. The first is to examine nature through observation. These observations can then be extrapolated into systematic measurements that can be quantified. The second aspect is that experimental physics serves as a platform to test hypotheses that are already formulated by theory. In this way, it acts as a conduit connecting theory to real-world situations. Additionally, it reveals the limitations of any theory, thereby serving as a valuable test bed.
These two tasks are essentially intertwined: an observation can lead to new hypotheses, and, conversely, a well-formulated hypothesis can lead to systematic measurements.
For example, while hunting for astronomical radio sources, an important discovery was made: the observation of the cosmic microwave background. This finding turned out to be one of the crucial ones in physics, providing vital insights into the Big Bang theory and becoming a foundational aspect of observational cosmology. Another example is the special theory of relativity, where the Michelson-Morley experiment ruled out ether, which enabled Einstein to formulate his theory with greater confidence.
These two examples offer a snapshot of the possibilities within experimental physics and highlight its essential role in the duality between theory and experiment in physics. In a way, experiments and theory complement each other, and are like two sides of a coin.
