Category: physics

More surprises in Optical Momentum…

Electromagnetic momentum is a topic with rich history dating back to Maxwell, Poynting, Minkowski, Abraham, Einstein, and many more1.
It has also led to new questions, and an intriguing controversy in electromagnetism2.

An interesting and contemporary question to ask is: what is the behavior of optical momentum in artificial materials ?

One class of artificial materials is the near zero-refractive index (NZI) materials.

What are NZI materials ? The general definition of refractive index from a material view point is that it is proportional to square root of a product: dielectric permittivity (ε) and magnetic permeability (μ) of the given material.

n = (εμ)½ 

 If either of these material values go to zero at a given wavelength of light, then the refractive index goes to zero or close to zero. Such a situation creates new opportunity for enhanced or supressed light-matter interaction. See this popular review on NZI materials3

A recent theoretical paper4 addresses the consequence of evolution of optical momentum in NZI media.
This analysis has thrown a few fundamental surprises that are fascinating such as : absence of interference in Young’s double slit experiments, and some new opportunities in optical cloaking thanks to quantum nature of light. To quote the authors4 :

being inside an NZI materials would lead to an infinite uncertainty on position and zero uncertainty on momentum. Conceptually, this implies that since the resolution is poor and no correct image can be formed, an object of any shape and material can be “hidden” in a NZI material.

There are a few more interesting prospects, and of course, all of them are yet to be verified with experiments.

If you are interested in this topic, I strongly recommend this recent, popular level article5

1.           M. Buchanan, “Minkowski, Abraham and the photon momentum,” 2, Nature Phys 3(2), 73–73, Nature Publishing Group (2007) [doi:10.1038/nphys519].

2.           S. M. Barnett and R. Loudon, “The enigma of optical momentum in a medium,” Philosophical Transactions of the Royal Society A: Mathematical, Physical and Engineering Sciences 368(1914), 927–939, Royal Society (2010) [doi:10.1098/rsta.2009.0207].

3.           “Optics & Photonics News – Zero-Index Platforms: Where Light Defies Geometry,” <https://www.optica-opn.org/home/articles/volume_27/july_august_2016/features/zero-index_platforms_where_light_defies_geometry/> (accessed 5 May 2022).

4.           M. Lobet et al., “Momentum considerations inside near-zero index materials,” 1, Light Sci Appl 11(1), 110, Nature Publishing Group (2022) [doi:10.1038/s41377-022-00790-z].

5.           “Exotic Materials Through Momentum’s Looking-Glass,” <https://www.optica-opn.org/home/newsroom/2022/may/exotic_materials_through_momentum_s_looking-glass/> (accessed 5 May 2022).

OAM + SAM -New paper from my lab

We have a new paper from our lab to appear in the journal : Laser & Photonics Reviews

on “Simultaneous detection of spin and orbital angular momentum of light through scattering from a single silver nanowire”

preprint version on arxiv : https://arxiv.org/abs/2111.14919

Light can carry orbital angular momentum (OAM) and spin angular momentum (SAM). This momentum can be transferred to an object that is interacting with the light. What we show is the experimental proof of concomitant detection of OAM and SAM in the coherent light scattering signatures from a single, silver nanowire. Essentially, the nanowire acts like a slit, and scatters the light. During this scattering process, the distribution of light in momentum space gets altered according to the spin (polarization) and orbital (topological charge) state illuminating the nanowire.

A notable point is that unlike other (metamaterials) methods, this unambiguous detection scheme does not require sophisticated nanofabrication methods and is mainly founded on fundamental principles of vectorial light scattering in the momentum space.

This experimental work (with a good dose of theoretical optics) was mainly due to the sustained efforts of an outstanding PhD student in my lab : Diptabrata Paul (about to finish PhD !)

He had excellent support and inputs from our PhD alumni Deepak K Sharma (now a postdoc/research scientist at ASTAR, Singapore).

Going further, this study motivates some interesting questions, of which we are interested in exploring the direct transfer of OAM and SAM at sub-wavelength scale to nanoscale objects including (macro)molecules. This will have some interesting manifestation on optical forces and torques at sub-wavelength scale, and we intend to study them in detail. This can be studied in a unique set-up that we have built in our lab that combines nano-optical tweezers with momentum-space imaging microscope. Look out for some studies in this direction from our lab.

We will spend a lot time…in momentum space :)

64. Susskind’s view on philosophical Feynman

Since my student days, I have been studying various things Richard Feynman wrote. His 3 volumes of lectures in physics have been one of the ‘go-to sources’ on basic physics. I have also enjoyed reading his lectures on advanced topics, including nanotechnology and computing. Apart from all this, I have also been impressed by Feynman’s viewpoint on science, society and human living.

In the passing, I have read that he did not like philosophers, but I always felt that Feynman’s thoughts were deeply philosophical without the frills of sophisticated language.

Recently, I came across a wonderful conversation with Leonard Susskind, a close friend of Feynman and an accomplished physicist himself. In this video (around 40.18 min), Susskind highlights that Feynman was deeply philosophical. He emphasizes that Feynman did not like the way philosophical discourse was conducted, especially in the context of the philosophy of science. This point kind of reinforced my impression of Feynman and was heartening.

I admire Feynman for making science interesting, but I am also very well aware that Feynman has been criticized for being sexist. Feynman was a scientific genius, but he had his flaws.

Anyway, the whole conversation with Susskind is educative. The best part is when he talks about why he likes teaching (around 1.06 hours in the video), which is worth watching.

59. Who is a physicist?

“The physicist is most cogently identified, not by the subject studied, but by the way in which a subject is studied and by the nature of the information being sought.”

Above is an interesting quote by Sol Gruner, James Langer, Phil Nelson, and Viola Vogel from a 1995 article in Physics Today titled WHAT FUTURE WILL WE CHOOSE FOR PHYSICS?

Although written more than 25 years ago from the viewpoint of US physics community, many of the issues discussed in this article are pertinent even today. Probably more so in the Indian context.

Nice read :

What Future Will We Choose for Physics?
Sol M. Gruner, James S. Langer, Phil Nelson, and Viola Vogel
Citation: Physics Today 48, 12, 25 (1995); doi: 10.1063/1.881477
View online: https://doi.org/10.1063/1.881477