An optical device that uses mechanical rotation to allow light to propagate in one direction along a fibre, but not in the opposite direction has been built by an international team of researchers. The device could find use in optical circuits, where it is very difficult to prevent light from propagating in unwanted directions. However, practical applications may be difficult to achieve.
The idea of using mechanical rotation to allow waves to travel in one direction, but not in the opposite direction, was first developed in 2014 by Andrea Alú and colleagues at the University of Texas at Austin. They placed a fluid into a circular cavity and stirred it so the fluid rotated. Sound waves travelling around the cavity in one direction were pushed along by the fluid, whereas waves travelling in the opposite direction were held back. As a result, the resonant frequency of the cavity was different for sound moving in opposite directions. By judiciously choosing the rotation speed of the fluid, the researchers could ensure that sound waves at a chosen frequency could only travel in one direction around the cavity.
The same ideas can be applied to light waves, however, the speed of light is much faster than the speed of sound, and consequently the frequencies involved are so much higher – making the technique seemingly impractical. Instead, researchers have looked at other ways of achieving one-way transmission – including the use of strong magnetic fields — but these have also proven to be difficult to adapt for practical applications.
In the new research, Shai Maayani, Raphael Dahan and Tal Carmon at Technion – Israel Institute of Technology and colleagues have returned to rotation. They use a cylindrical, silica-glass resonator that is 4.75 mm in diameter and is rotated on a turbine at speeds up to 6.6 kHz. An optical fibre that is tapered to be 1088 nm in diameter is located 320 nm above the spinning resonator.
Light travelling along the fibre interacts with the nearby resonator via the light’s short-range evanescent field. In analogy to the rotating fluid, light travelling in the same direction as the spinning resonator perceives it to be less dense than does light travelling in the opposite direction. This difference in apparent density means that the index of refraction of the resonator will be different for light moving in opposite directions.
For this reason, the resonant frequency of the system is different for light travelling in opposite directions. This allowed the researchers to pass light of the same frequency down the fibre from both ends and have light be transmitted from one side but blocked from the other: “Wavelengths that are off resonance with the cavity will be transmitted; wavelengths on resonance with the cavity are absorbed,” explains Maayani, now at the Massachusetts Institute of Technology.
The team is now looking at the feasibility of creating practical devices: “A tapered fibre will fail after a few hours because of the humidity in the air,” explains Maayani. “But if you encapsulate it in an inert environment, it can last for years. The other big problem is vibration – at present, this is a delicate experiment that requires an optical table.”
Blocking the symmetry of motion
Alú – now at City University of New York – is impressed by the researchers’ technical achievement, saying that, “from the fundamental side, they’re proving what we already proved for sound, but they’re doing it for light – which is impressive, because the technology required is much more complicated”. He says that the device could have advantages over optomechanical systems in energy efficiency, but the researchers will need to demonstrate its practicability when scaled down: “As you scale things down, typically the quality factors go down and the requirement on speed goes up. At some point, some trade-offs will have to be made,” he says.
Mohammad Hafezi of University of Maryland, College Park agrees: “I find it very cool that we can rotate something and look at the huge spectral shift in transmission between forward and backward,” he says, “but in terms of non-magnetic non-reciprocity, this whole field has been a challenge, and we haven’t seen a real technological solution yet.”
The invention is described in Nature.