An optical “tractor beam”: the angular momentum properties of light confined in an optical nanofiber

Cody Leary

Example of a stable solution to Maxwell’s equations (known as a “mode”) for light confined within an optical nanofiber. The arrows denote the direction of the electric field, while the brightness denotes the intensity of the light at various spatial locations. When micron-sized glass particles are present near the surface of the nanofiber (shown in red) where light “leaks” out due to an affect analogous to quantum tunneling, their positions and velocities can manipulated by the intense light present in this region.

Intense laser light can interact with matter, resulting in the transfer of appreciable amounts of energy, momentum and even angular momentum. A newly-discovered way to concentrate and guide intense light is by means of an optical nanofiber, which is similar to a regular optical fiber except that its radius has been reduced to the scale of hundreds of nanometers–smaller than the wavelength of the light propagating inside it! In analogy with quantum tunneling, this causes the light field to penetrate partially out of the fiber, where it can interact with nearby matter. When micron-sized glass particles are present near the surface of the nanofiber, their positions and velocities can manipulated by energy-momentum transfer of the intense light present in this region.

In this project, we will model stable solutions to Maxwell’s equations of light (known as “modes”) which propagate in optical nanofibers, as part of a collaboration with the research group of Dr. Sile Nic Chormaic at the Okinawa Institute of Science and Technology (OIST) in Japan. Dr. Nic Chormaic’s group has experimental results involving the controlled movement of massive dielectric objects near surface of optical nanofibers, but the theory is currently lagging behind experiment. More specifically, the project will focus on understanding the properties of the angular momentum contained by light propagating in optical nanofibers, in order to understand how matter may be caused to spin or move in a curvilinear trajectory as a result of angular momentum transfer from the light modes to the microscale matter.

This modeling project is predominantly theoretical in nature, with the pioneering steps al- ready taken by Wooster summer research student Alishan Premani. The work will involve both pencil-and-paper calculations as well as numerical computational analysis using Mathematica and/or other programming languages. It may also involve analysis of data sent by our collabo- rators at OIST.