At St. Paul’s Cathedral in London, a section of the dome called the Whispering Gallery makes a whisper audible from the other side of the dome as a result of the way sound waves travel around the curved surface. Researchers at Washington University in St. Louis have used the same phenomenon to build an optical device that may lead to new and more powerful computers that run faster and cooler.
An associate professor of electrical and systems engineering, and her collaborators have developed an essential component of these new computers that would run on light. Their work brings predictions from recently formulated theoretical physics into real world applications.
Yang’s group has created an optical diode by coupling tiny doughnut shaped optical resonators — one with gain and the other with loss — on a silicon chip. “This diode is capable of completely eliminating light transmission in one direction and greatly enhancing light transmission in the other nonreciprocal light transmission,” says Bo Peng, a graduate student in Yang’s group and the paper’s lead author.
An electrical diode prevents electricity from backflow along a wire providing protection to crucial parts of an electronic circuit or processor; an optical diode does the same thing with light.
Simply put, when a “lossy” system is coupled with a “gain” system such that loss of energy exactly equals gain at an equilibrium point, a “phase transition” occurs.
Applying the principles of PT symmetry leads optics to a completely different set of behaviors not predicted by conventional physics with only loss or only gain. The phenomena that occur at the “phase transition” are dramatic and hitherto unexpected, Yang says.
To make their optical diode, Sahin Kaya Ozdemir, PhD, a research scientist in Yang’s group and a key contributor to the paper, and Peng used two micro-resonators positioned so that light can flow from one to the other. One device is the “lossy” silica resonator.
The other incorporates the chemical element erbium into the silica structure for gain. Ozdemir says when erbium interacts with light of wavelength 1450 nm, it emits photons in the wavelength 1550 nm. A transmission detector set for 1550 nm will see a gain from this erbium-containing resonator.
When the rate of gain in one resonator exactly equals that of loss in the other, the phase transition occurs at a critical coupling distance between the resonators.
Most significantly, PT symmetry is broken, and the system shows a strong nonlinear behavior even at very weak input powers- input light gains intensity with a very steep non-linear slope. “As a result, time reversal symmetry is broken and light is able to move in only one direction— forward” says Yang.
This story is reprinted from material from Washington University in St. Louis, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.