Brillouin scattering is a nonlinear optical process via which light waves or photons scatter from density fluctuations or acoustic phonons in a medium to generate new optical fields. At sufficient optical intensities, the incident and scattered optical fields interfere to generate density fields, such that the acoustic and scattered light fields reinforce each other’s growth, thereby resulting in stimulated Brillouin scattering (SBS). The scattered light due to SBS is typically frequency-downshifted or Stokes-shifted in the counter-propagating direction to the incident light, due to the linear momentum and energy conservation requirements between light and the available acoustic phonons in most media.
SBS is of both fundamental and technological importance since on one hand, it sheds light on the light-matter interactions at the quantum level as well as on the mechanical properties of matter, on the other hand, it has found utility in a variety of applications such as distributed sensing, signal processing, beam combination via phase conjugation and optical storage by slowing down light. In addition, SBS is an important consideration for long-haul fiber communication networks as well as high-power single frequency lasers since it can limit the amount of transmitted optical power through a medium.
The primary means to tailor SBS interactions till now have involved designing guided-wave and resonator structures to control and alter the waveguiding properties of both light and sound fields, as well as the overlap between them. Such waveguide designs have led to the demonstration of several distinctive properties such as SBS gain suppression, anti-Stokes cooling and forward SBS, to name a few.
In this thesis, we show that controlling the angular momentum (AM) of light and sound yields an entirely new toolbox with which to tailor SBS. In light fields, angular momentum can result from helical phase or orbital angular momentum (OAM) as well as from circular polarization or spin angular momentum (SAM), whereas in sound fields, only OAM exists. OAM states, in particular, have received tremendous attention in the past couple of decades due to the new degree of freedom afforded by the existence of theoretically infinite number of states encoded in their helicity. More recently, interaction between SAM and OAM, termed spin-orbit coupling, which is observed only in confined geometries such as waveguides, has received enormous interest on account of the remarkable phenomena enabled by such interactions, such as optical super-resolution and spin-Hall effects.
Here, we demonstrate that SBS of AM-carrying light in waveguides such as optical fibers, where light experiences the above-mentioned spin-orbit coupling, results in unique acousto-optic interactions. For light fields with the same helical charge; hence the same effective area that usually controls the strength of the nonlinear interaction, we show that different combinations of the signs of OAM and SAM results in dramatically different behavior due to the requirement to conserve angular momentum. The SBS gain can be lowered by almost a factor of 2 just by the aforementioned choice of OAM and SAM combinations, yielding an alternate means of scaling power of fiber lasers. Alternatively, high-degree spatial phase conjugation is observed for unaberrated coherent optical fields for the first time. This is realized via a fundamentally new mechanism of polarization-mediated angular momentum selectivity, unlocking potential applications in aberration correction in vortex lasers, optical trapping and manipulation of microscopic particles as well as signal processing. Finally, we observe that OAM and SAM control results in the efficient generation of record high (order ~22) OAM acoustic vortices, which, by itself, would usher applications in high-density acoustic communications and acoustic rheology
| Identifer | oai:union.ndltd.org:bu.edu/oai:open.bu.edu:2144/38520 |
| Date | 29 September 2019 |
| Creators | Prabhakar, Gautam |
| Contributors | Ramachandran, Siddharth |
| Source Sets | Boston University |
| Language | en_US |
| Detected Language | English |
| Type | Thesis/Dissertation |
| Rights | Attribution 4.0 International, http://creativecommons.org/licenses/by/4.0/ |
Page generated in 0.0066 seconds