Optical Physics: Brillouin-scattering-induced transparency leads to nonreciprocal optical waveguides

Researchers from the University of Illinois at Urbana-Champaign are using Brillouin-scattering-induced transparency (BSIT) to create one-way optical waveguides; BSIT permits light to travel in the forward direction while light traveling backward is strongly absorbed.¹ The technique can lead to better optical isolators and circulators.

The approach is simple, relying on just a glass microfiber and an adjacent glass sphere. BSIT is a form of electromagnetically induced transparency (EIT), which can be used to produce “slow light” and “fast light.” EIT based on stimulated Brillouin scattering had been considered unfeasible, but the Illinois researchers were able to create phonons that last long enough for BSIT to occur.

For one-way transmission of light to occur, some sort of symmetry-breaking mechanism must be present. In the Illinois device, this mechanism is a propagating acoustic wave, which travels in only one direction, which creates nonsymmetric conditions in the waveguide.

“Light at certain wavelengths can be absorbed out of a thin optical waveguide by a microresonator—which is essentially a tiny glass sphere—when they are brought very close,” explains Gaurav Bahl, an assistant professor of mechanical science and engineering at Illinois. “Through the BSIT phenomenon we showed that we can eliminate this opacity, i.e., we can make this system transparent again by adding another laser at a specially chosen wavelength nearby.”

Previous nonreciprocal optical devices—for example, isolators and circulators—have been exclusively built using the Faraday magneto-optic effect, which uses magnetic fields to break the time-reversal symmetry with certain specialized garnet and ferrite materials. However, these materials are challenging to obtain at the chip scale through conventional foundry processes. Magnetic fields are also sources of interference in many applications such as cold-atom microsystems. These constraints have hindered availability of Faraday effect isolators for on-chip optical systems.

Symmetry broken by phonons

In the BSIT experiment, an optical fiber with a microtapered portion serving as a coupler is brought to within evanescent-wave range of the glass microsphere. Both a control and a pump laser beam, propagating in the same direction down the fiber are coupled into the microsphere resonator’s phase-matched whispering-gallery modes. The control beam is supplied by a 1550 nm laser diode, while the probe signal is created by an electro-optic modulator (EOM), which produces two probe sidebands.

The control laser pumps the lower-frequency Brillouin-phase-matching optical mode, producing anti-Stokes scattering; the resonator itself suppresses Stokes scattering. The probe laser scans through the microsphere’s resonances; the anti-Stokes absorption produced by the control normally would induce loss in the probe at that particular frequency (which can be monitored via a photodiode collecting the fiber’s output light), but interference between Stokes and anti-Stokes scattering pathways induces a transparency at that frequency.

Properly introducing phonons (sound quanta) into the system breaks the symmetry, leading to high loss in one direction and low loss in the other. In the nonreciprocity experiment, two independent probe signals were sent in the forward and backward directions to test transmission. BSIT in a narrow frequency range was seen in the optomechanical system for one direction and was completely absent for the other direction.

“We have demonstrated a method of obtaining linear optical nonreciprocity that requires no magnets, can be implemented in any common optical material system without needing ferrites, and could be implemented today in any commercial optical foundry,” says Bahl. “Brillouin isolators do already exist, but they are nonlinear devices requiring filtering of the scattered light. BSIT, on the other hand, is a linear nonreciprocal mechanism.”

Slow and fast light via BSIT

“While it is already known that slow and fast light can be obtained using Brillouin scattering, our device is far smaller and uses far less power than any other previous demonstration by several orders of magnitude,” says JunHwan Kim, one of the researchers. “However, we must sacrifice bandwidth to obtain such performance.”

In their studies, Bahl’s research group uses the extremely minute forces exerted by light to generate and control mechanical vibrations of microscale and nanoscale devices (a field called optomechanics). In resonant microcavities, these minuscule forces can be enhanced by many orders of magnitude. They are using these phenomena to unearth new physics behind how solids, liquids, and gases interact with light.

REFERENCE
1. J. Kim et al., Nat. Phys. (Jan. 2015); http://dx.doi.org/10.1038/nphys3236.