Category: momentum

Optical Momentum – Lectures

My latest research grant from ANRF is on “Opto-Thermal Binding of Plasmonic Matter”. This is a topic that is at the interface of optical momentum, thermodynamics, statistical physics, and advanced optical microscopy in the real and momentum spaces.

Optical momentum and its measurement have a rich history in understanding electromagnetic waves and their interaction with matter. Over the past century, multiple applications have emerged that harness the transfer of momentum from light to matter. Interestingly, the light that is scattered off this interaction also carries relevant information not only about the interaction but also about certain parameters of light and the participating matter.

These lectures are my attempt to give an overview of the field. My main target audience is my PhD group members and senior undergraduates who are working with me. But these lectures can be followed by anyone who is seriously interested in physics. The discussion involves theoretical optical physics (including elements of statistical and quantum optics), experimental techniques (including advanced microscopy methods) and a few computational techniques connected to the interaction. The goal of the lectures is to reveal the interesting questions in research papers, review articles, monographs and conference papers related to the field and their possible application in industries, including biophotonics and astro and space-photonics. From time to time, I will also discuss our research results from the project.

Black hole image and optical vortex – an analogy

The recent image of the black hole at the center of the milky way has been spectacular. When I teach a course, I generally emphasize analogies across the sub-disciplines of physics. In the below video I draw some analogies between black hole image and an optical vortex.

About the black hole images : https://iopscience.iop.org/journal/2041-8205/page/Focus_on_First_Sgr_A_Results

Some work from our group on optical vortex : ACS Photonics 6, 1, 148–153 (2019) https://doi.org/10.1021/acsphotonics.8b01220

Book on singular optics : https://www.google.co.in/books/edition/Singular_Optics/H-WVDQAAQBAJ?hl=en&gbpv=1&printsec=frontcover

Science paper on optical analog of event horizon https://www.science.org/doi/epdf/10.1126/science.1153625

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).