Spherical Laser Resonator Achieves 10M Circulations of Light

Technion-Israel Institute of Technology researchers have introduced a laser-resonator design inspired by the work of Arthur Ashkin. Created by graduate student Jacob Kher-Alden under the supervision of Tal Carmon, associate professor and head of the Optomechanics Center at Technion, the floating resonator design achieved 10 million circulations of light.

The resonator is made of a tiny drop of highly transparent oil of about 2 µm in diameter.

Optical resonators are commonly constructed of mirrors, to multiply reflected light. The more mirrors in a system, the closer the resonator’s shape resembles a circle, allowing light to circulate. Creating a spherical structure to allow light to rotate on all planes, passing through the center of the circle regardless of tilt, enabling optimal resonance, has eluded engineers.

In a resonator made of a mirror that can reflect 99.9999% of light, the light will rotate about a million revolutions.

“If we take light that has a power of 1 W, similar to the light of the flash on a cellphone, and we allow it to rotate back and forth between these mirrors, the light power will be amplified to about a million watts — the power equal to the electricity consumption of a large neighborhood in Haifa, Israel. We can use the high light output, for example, to stimulate various light-matter interactions at the region between the mirrors,” Carmon said.

Researchers, including Carmon, have previously developed tiny glass resonators in the shape of a sphere or a ring. However, the stem holding the sphere distorts the sphere’s shape. The factor has repeatedly renewed interest in a floating resonator developed by Ashkin in the 1970s.

Using Ashkin’s Nobel Prize-winning optical tweezing technique, light holds a transparent oil droplet in the air. Because of the drop’s minute dimensions, it is hardly distorted by gravity, allowing it to maintain a high level of spherical integrity. An optical fiber delivers light to the droplet, which then receives returning light after the photons have traveled through the resonator.

The properties of the light returning to the fiber allowed researchers to discern information about the behavior and activity occurring in the droplet. By turning off the light entering the resonator, the duration of a photon’s survival within the resonator can be observed. Based on this information and the speed of light, the researchers can calculate the number of average rotations of the photons.

The research was published in Physical Review X (www.doi.org/10.1103/PhysRevX.10.031049).