Physical examination X recently reported a new optical resonator from Technion – Israel Institute of Technology that is unprecedented in improving resonance. Developed by graduate student Jacob Kher-Alden under the supervision of Professor Tal Carmon, the Technion-born resonator has record-breaking resonance enhancement capabilities.
A resonator is a device that traps waves and enhances or echoes them by reflecting them from wall to wall in a process called resonant enhancement. Today there are complex and sophisticated resonators of all kinds across the world, as well as simple resonators that we are all familiar with. Examples of this include the resonator box of a guitar, which enhances the sound produced by the strings, or the body of a flute, which enhances the sound created in the mouthpiece of the instrument.
The guitar and the flute are acoustic resonators in which sound resonates between the walls of the resonator. In physics, there are also optical resonators, as in laser devices. A resonator is, in fact, one of the most important devices in optics: “It is the transistor of optics,” said Professor Carmon.
Generally speaking, resonators need at least two mirrors to multiply the reflected light (as in the hairdressing salon). But they can also contain more than two mirrors. For example, three mirrors can be used to reflect light in a triangular shape, four in a square, and so on. It is also possible to have many mirrors of almost circular shape so that the light circulates. The more mirrors there are in the ring, the more the structure becomes that of a perfect circle.
But that’s not the end of the story, as the ring restricts the movement of light to a single plane. The solution is a spherical structure, which allows light to rotate on all planes passing through the center of the circle, regardless of their inclination. In other words, in three-dimensional space.
In the transition from physics to engineering, the question arises of how to produce a resonator as close as possible to a sphere that is clean, smooth and gives the maximum number of rotations for optimum resonance. This is a challenge that has engaged many research groups and has given, among other things, a tiny glass resonator in the shape of a sphere or ring, which is held next to a narrow optical fiber. An example of this was presented by Professor Carmon two years ago in Nature.
Here there was still room for improvement, as even the rod that holds the sphere creates a distortion in its spherical shape. Therefore, the desire arose to produce a floating resonator – a resonator which is not held by any material object.
The world’s first micro-resonator was demonstrated in the 1970s by Arthur Ashkin, winner of the 2018 Nobel Prize in Physics, who presented a floating resonator. Despite the achievement, the research direction was soon abandoned. Now, inspired by Ashkin’s pioneering work, the new floating resonator features a resonance improvement of 10,000,000 light flows, compared to about 300 flows in the Ashkin resonator.
The levitating resonator
In a resonator made up of a mirror that reflects 99.9999% of the light, the light will make about a million turns or “back and forth”. According to Professor Carmon, “If we take a light with a power of one watt, similar to the light from a cell phone flash, and let it rotate between these mirrors, the light output will be amplified. at about a million watts – the power is 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 in the region between the mirrors. “
In fact, a million watts are made up of the same particle of light moving back and forth through matter, but matter does not “know” that it is the same particle of light that repeatedly travels through matter. matter, since photons are indistinguishable. He only “feels” the great power. In such an apparatus, it is also important that the million watts pass through a small cross section. Indeed, the device developed by Kher-Alden conducts light in 10 million circular paths, in which the light is focused on a beam area 10,000 times smaller than the cross section of a hair. In doing so, Kher-Alden set a world record in improving the resonance of light.
The resonator developed by Technion researchers is composed of a tiny drop of highly transparent oil about 20 microns in diameter, or a quarter of the thickness of a lock of hair. Using a technique called “optical tweezers,” the drop is held in the air with the help of light. This technique is used to keep the drop in the air without a material support, which can damage its spherical shape or make the drop dirty. According to Professor Carmon, “This ingenious optical invention, the optical clamp, is widely used in life sciences, chemistry, micro-flux devices and more, and it is precisely optical researchers who use it. hardly – much like the shoemaker walking barefoot. In the present study, we show that optical tweezers have enormous potential in the field of optical engineering. It is possible, for example, to build an optical circuit using several optical clamps that hold many resonators and control the position of the resonators and their shape as needed. “
The tiny dimensions of the drop also improve spherical integrity, as gravity hardly deforms it, as it is marginal in these dimensions to the surface tension forces at the liquid interface that give it a spherical shape. In the unique system developed by Technion researchers, the drop of oil is retained by a laser beam and receives light from another fiber, which also receives light after passing through the resonator.
Based on the properties of the light returning to the fiber, researchers can find out what happened inside the drop. For example, they can turn off the light entering the resonator and examine how long a photon will survive in the resonator before fading. Based on this data and the speed of light, they can calculate how many rotations the photon makes (on average) in a drop. The results show a world record for amplification of light: 10,000,000 rotations that cross a cross section of about one square micron, increasing light 10 million times.