Optical diode intensifies light in one direction
A research yielded a system of optical resonators for computing circuits that utilise beams of light in place of electricity in a compact form factor. The system was developed by Washington University researchers, and works like a diode—allowing the input light, even with weak power, to gain intensity as it travels in the other direction.
Inside a doughnut-shaped component, two microresonators reflect light back and forth. One tends to lose energy, while the other increases it. When the loss equals the gain at a specific wavelength, the system goes through a phase change in which the roles of the resonators are reversed, "their temporal relationships reverse, loss becomes gain and gain becomes loss," according to a paper describing the technique that was published in the April 6 issue of Nature Physics.
In an optical diode, the light input in one direction is transmitted, while the light input in the opposite direction is blocked. The new optical diode, designed by the university researchers, is made from parity time symmetric microresonators in which the loss of one of the resonators is balanced by the gains in the other. Source: Washington University
The result could make it practical to build integrated computing circuits that use beams of light that travel along channels far narrower than would ever be possible using wire and electricity, and at far lower energy levels. The process could still support standard semiconductor circuitry designs.
"We believe that our discovery will benefit many other fields involving electronics, acoustics, plasmonics and meta-materials," according to lab director Lan Yang, who oversaw the study, wrote in a statement from Washington University. "Coupling of so-called loss and gain devices using PT (parity-time)-symmetry could enable such advances as cloaking devices, stronger lasers that need less input power, and perhaps detectors that could 'see' a single atom."
"At present, we built our optical diodes from silica, which has very little material loss at the telecommunication wavelength. The concept can be extended to resonators made from other materials to enable easy CMOS compatibility," according to Bo Peng, a graduate student in Yang's group and lead author of the paper.
Metaphorically, the device works in a way similar to the Whispering Gallery in St. Paul's Cathedral, in which oddities of acoustics make quiet noises audible at one end of the side of the gallery when they are nearly inaudible to those standing nearby.
In theory, the device is more problematic. It takes advantage of the concept of parity-time (PT) symmetry in quantum physics, which describes ways in which the energy coming out of a closed space may not equal the real and potential energy of the particles inside. (There is a more detailed explanation at the end of the story.)
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 1 microwatt, 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 (see Figs. 1bc). 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 (Fig. 1a), 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
- Kevin Fogarty
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