A First of Its Kind Optic Isolator

A micrograph of the experimental setup: Optical polarization is either parallel or perpendicular to the rotation axis of the sphere, compatible with the polarization of the transverse electric or the transverse magnetic mode of the sphere. (Source: Technion)

Technion researchers have constructed a first of its kind optic iso­lator, based on resonance of light waves on a rapidly rota­ting glass sphere. This is the first photonic device in which light advancing in opposite directions moves at different speeds. “Essen­tially, we developed a very effi­cient photonic isolator, which can isolate 99.6% of the light,” said research team leader Tal Carmon of the Faculty of Mecha­nical Engi­neering. “If we sent 1,000 light particles, the device will effec­tively isolate 996 photons and will miss only 4. Such isolation efficiency is necessary for appli­cations that include quantum optics communi­cation devices and building high-powered lasers. The isolator we developed here fulfills several addi­tional require­ments: it also works well when light from both opposing direc­tions is simul­taneously perceived, it is compatible with standard optical-fiber tech­nology, it can be scaled down and it does not change the color of the light.”

Just as swimming downstream is faster than swimming upstream and riding a bicycle with the wind behind you is faster than riding against the wind, light also changes its speed with “tailwinds” or “counter-flow”, in response to the medium in which it is moving. The speed of light in glass, for instance, is slower than its speed in air. Also, two beams of light advancing in opposite direc­tions in glass, or any other material, will advance at the same speed. “At the Technion, I also learned that the speed of light depends on the speed of the medium in which it is moving,” said Carmon. “Precisely like a swimmer in a river – the speed of light against the movement of the medium is slower than its speed with the movement of the medium.”

This effect was already described in 1849 by the French scientist Armond Fizeau, who showed, that like a swimmer in a river, the speed of light down a current is faster than light going up a current. Fizeau’s disco­very had a signi­ficant impact on the develop­ment of Einstein’s theory of Special Rela­tivity. The Fizeau drag may lead to significant applications in optics and computers, as its unique ability to differentiate between the speeds of light for counter-propa­gating beams can generate an optic isolator – a device into which light entering on one side is blocked, while the light entering from another side is trans­mitted. Until now, a device in which opposing light beams advance at different speeds, had not be constructed.

But now, for the first time, Technion researchers have succeeded in construc­ting such a device. The spherical optic device rotates at a high speed. Light beams are deli­vered into it from opposite direc­tions via a nearby tapered fiber. The light ap­proaching from the right moves along the circum­ference of the ball, in the direction of the rotation of the sphere, while the light ap­proaching from the left turns opposite the direction of the rotation and there­fore moves at a slower speed.

The novel device consti­tutes an optic isolator – it transmits light ap­proaching from the left and turns off light coming from the right. Another effect that is relevant here is resonance. Just like a musical instru­ment that reso­nates at a specific frequency, light circum­ferentially circu­lating in the sphere resonantly echoes. Yet, the different speeds for counter-circu­lating light forces these counter-circu­lating light to have different colors. This way, light entering from one side echoes inside the sphere while circu­lating thousands of times in the sphere, until it is absorbed. In contrast, light entering from the opposing side of the isolator is nonresonating and hence passes through the device practi­cally undis­turbed. In other words, the light moving with the device, resonates and is shut off, while the light moving against the device is trans­mitted and continues on.

Carmon noted that the device was con­structed at the Technion glass blowing workshop. It was constructed from a glass rod whose tip was melted to a 1 millimeter-radius ball. The light enters the iso­lator from both sides of a standard optical fiber, tapered at the vicinity of the sphere to a diameter 100-time smaller than that of a hair, and posi­tioned several nano­meters away from the sphere. The sphere, which serves as the resonator, rotates at an ultra-fast speed – the tip of the ball moves at a speed of 300 kph – and the light coming from the fiber rotates within it thousands of times.

One of the engi­neering challenges the research group faced was main­taining the ultra-short distance between the fiber – via which light is provided – and the spherical resonator constant. “Main­taining an accurate distance is a true challenge, even when the device is not moving, and is an enormous challenge when the sphere is rotating at such a high speed,” said Carmon. “There­fore, we sought a means of forcing the fiber to move together with the sphere, despite the fact that the fiber and sphere are not connected. We finally achieved this by designing the fiber to float on the wind gene­rated by the rotation of the sphere. In this way, if the device wobbles – which it does due to the rapid rotation – the fiber will wobble with it and the distance between them will be preserved. In fact, the fiber is actually flying above the rota­ting sphere at a constant and self-alighted nano-ele­vation.”

Carmon hopes this nano-seperated paves a path toward a novel type of mechanical device based on rela­tively unex­plored forces that dominates at nano-scale sepa­ration. “The forces acting at such distances include Casimir and Van der Waals forces – very strong forces originating from quantum effects, which, to date, have barely been exploited in mecha­nical devices, in general, and in mechanical oscil­lators, in particular,” he said. “We recently demon­strated, for the first time, lasers in which water waves mediate laser emission; and also, for the first time, micro-lasers where sound mediates laser emission.”

In the future, the researchers may be able to generate such lasers that are based on vibra­tions where the restoring force is Casimir or Van der Waals. Using their self-aligned nano sepa­ration method might also allow micro electro mecha­nical devices where Casimir and Van der Waals forces will be used. (Source: Technion)

Reference: S. Maayani et al.: Flying couplers above spinning resonators generate irreversible refraction, Nature 558, 569 (2018); DOI: 10.1038/s41586-018-0245-5

Link: Group of Tal Carmon, Faculty of Mechanical Engineering, Technion, Haifa, Israel

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