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Optical diode paves way for photo-based IC

15 Apr 2014  | Kevin Fogarty

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The device reflects a beam of light between two microresonators, one of which tends to lose energy and the other that increases it. When the gain created by the resonator on one side equals the loss in the other, the PT symmetry of the system is broken, "and the system shows a strong non-linear behaviour even at very weak input powers—input light gains intensity with a very steep linear slope, allowing light to flow in only one direction," according to the article.

The apparent result is that beams of light emerging from a component at far higher levels of energy than they went in. "Time reversal symmetry is a fundamental physical rule that states that if light can travel in one direction, it must be able to travel in the opposite direction too. With this new optical diode, this is no longer the case," according to researcher Kaya Ozdemir, who built the resonators. "Engineers traditionally use magneto-optics and high magnetic fields to break time reversal symmetry, here we do this using strong non-linearity enabled by broken PT symmetry. With an input of only 1µW, we show 17-fold enhancement of light transmission in one direction. There is no transmission in the other direction. Such a performance would not be possible without the use of resonant structures and PT-symmetric concepts."

PT Symmetry explained

The device allows light to dodge around many of the presumed limitations on the manipulation of light and energy but doesn't actually break them. PT Symmetry is a concept pioneered by physicist Carl M. Bender at Washington University, who describes the concept in the 2007 paper: "Making Sense of Non-Hermitian Hamiltonians."

A deeper explanation is found in this series of lecture slides from a 2012 conference held by the International Conference of Numerical Analysis and Applied Mathematics:

PT symmetry requires that the real part of the refraction index (the potential in the language of Schrödinger) be an even function of position, whereas the imaginary part is an odd function. This condition implies that creation and absorption of photons occurs in a balanced manner, so that the net loss or gain is zero. Rüter and colleagues demonstrated these ideas for two coupled PT-symmetric waveguides (each supporting one propagating mode) with one of them providing gain for the guided light, and the other experiencing equal amount of loss.

Studying the light beam propagation in this set-up, these authors recognised that as the gain/loss parameter γ reaches a critical value γPT, a spontaneous PT symmetry breaking occurs. At this point, the total beam power starts growing exponentially, while for γ<γPT power oscillations are observed. The most dramatic effect in the beam evolution is the appearance of non-reciprocal wave propagation. Specifically the beam propagation pattern differs, depending on whether the initial excitation is on the left or right waveguide. This is contrasted with the γ=0 case, where the beam propagation is insensitive to the initial condition.

—explanation of PT Symmetry from the Wave Transport in Complex Systems lab at Wesleyan University.

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