Physicists in the US have discovered a laser-based “switch” that turns a sample of ions completely transparent at certain frequencies. Working at the California Institute of Technology (Caltech), the researchers found that when they coupled ytterbium ions (Yb3+) to a nanophotonic resonator and strongly excited them with laser light, the ions abruptly stopped reflecting light at frequencies associated with their vibrations. This effect, which the team dubs “collectively induced transparency”, could have applications in quantum optical devices.
“We discovered the phenomenon while trying to develop techniques to control ytterbium atoms coupled to an optical cavity using laser light,” co-team leader Andrei Faraon tells Physics World. The cavity, which measures 20 microns across, contains roughly a million Yb3+ ions. As a group, these ions are vibrating at a broad distribution of frequencies, but Faraon explains that each individual ion only vibrates within a very narrow frequency range.
“When probed with a laser with lower power, the system is opaque,” he continues. “When the laser is tuned at a frequency exactly in the middle of the frequency distribution, however, and its power increased, the system becomes transparent.”
Akin to destructive interference
This selective transparency effect is related to how the ions oscillate with respect to the laser, Faraon says. He compares it to the well-known phenomenon of destructive interference, in which waves from two or more sources cancel each other out. In the system studied in this work, the groups of ions absorb and re-emit light continuously. Normally, this re-emission process means that laser light gets reflected. At the collectively induced transparency frequency, however, something very different happens: the re-emitted light from each of the ions in a group balances, leading to a dramatic decrease in reflection.
As well as collectively induced transparency, Faraon and colleagues also observed that the ensemble of ions can absorb and emit light much faster or slower than a single ion depending on the intensity of the laser. These processes are known as super-radiance and sub-radiance, respectively, and are not well understood. Even so, the researchers say that this highly nonlinear optical emission pattern could be exploited to create more efficient quantum optical technologies. Examples might include quantum memories in which information is stored in an ensemble of strongly coupled ions, as well as solid-state super-radiant lasers for ensemble-based quantum interconnects in quantum information processors.
The research is described in Nature.
