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Scientists Free Laser Cavities to Embrace New Shapes

Tuesday, October 17, 2017

From medical technology to cat entertainment, lasers are one of the most revolutionary inventions of the last 75 years. Now, one of the key components of lasers may be in for a revolution. In new research published in the AAAS journal Science, researchers from the University of California, San Diego (UCSD) demonstrate an innovative design for the optical cavity of a laser. This development could help manufacturers pack laser components into less space on a chip, accelerating the development of light-based computing, among other applications.

The newly developed topological cavity is an arbitrarily-shaped closed contour formed between the boundaries of two photonic structures with different topologies. One photonic crystal is enclosed by the cavity (inside the contour). The photonic crystal not enclosed by the contour composes the rest of the system. The laser light leaves the cavity through a line of air holes in the outer photonic crystal.
Image Credit: Science, Bahari, et al.
At its most basic level, a laser works like this: a gas (or other optical gain medium) sits inside of a reflective optical cavity, and an external light source or an electric field excites the molecules of gas into an higher-energy state. When the excited states decay, the molecules emit photons. Some photons are emitted in a direction that aligns with the axis of the optical cavity, while the ones that are emitted in other directions can be reflected by the cavity's walls back into the gain medium, where they'll be re-absorbed and emitted again.

The earliest optical cavities consisted of the space between two carefully placed mirrors. Called a linear optical cavity, the mirrors faced one another so that photons emitted along the axis of the cavity bounced back-and-forth between them. Because of the way light interferes, these photons act as a feedback mechanism, stimulating other atoms to emit coherent photons of the same wavelength. This is called stimulated emission. Some of the coherent light produced in the cavity is siphoned off from the cavity to form the laser beam. This process gives us the term laser, which comes from Light Amplification by the Stimulated Emission of Radiation.

While many lasers rely on linear optical cavities, the standard approach for conventional modern lasers is to use a ring-shaped cavity. Instead of bouncing back-and-forth, the photons make several round trips through a loop. Although the geometry is different, interference again leads to the stimulated emission of coherent photons. Optical cavities haven’t taken on any other shapes because kinks, corners, and bends cause light to scatter and get lost.

In this new work, the researchers demonstrate a completely new approach to constructing optical cavities. Led by Babak Bahari, a graduate student in the lab of Boubacar Kanté at UCSD, the project introduces what the team calls “topological cavities” made from photonic crystals. Photonic crystals aren’t quite as sparkly as well-crafted crystal chandeliers, but they also interact with light in exquisite ways. Photonic crystals are manmade, nanoscale structures that use the repetitive arrangements of materials to manipulate light.

The topological cavities developed by the team consist of two photonic crystals, one surrounding the perimeter of the other. The two crystals are made of the same materials, but they have different topologies. The topology of an object is related to its shape, but it’s more complicated than that; topology considers whether one object can be stretched, deformed, or twisted into the shape of another. If so, the objects are considered to be topologically equivalent. For example, a circle is topologically equivalent to an ellipse, but a bagel is not topologically equivalent to a pretzel—a shape with one hole can’t be stretched, deformed, or twisted into a shape with three holes.

In this arrangement, the “cavity” is the closed contour between the two crystals. Even through it’s called a cavity, it’s not an open space—light traveling through the cavity travels along the boundary between the two photonic crystals. As with a traditional optical cavity, in a topological cavity an optical gain medium is excited by an external source at the interface of the two crystals. Some of the photons circulate along the interface, acting as a feedback mechanism and, as the researchers showed, inducing stimulated emission.

There are a couple of major benefits to this design. First and foremost, the optical cavity doesn’t have to be circular. This arrangement works with any arbitrary shape, including those with sharp corners. This could have a significant impact on manufacturing and performance—just think about how many more square components you can pack onto a chip than circular components. In addition, the light can only travel in one direction around the cavity. In most ring cavities light can travel both ways. This leads to problems, so the ring cavities require an additional component to block light in one direction.

Moving forward, the researchers are continuing to explore the fundamental physics of topological cavities. Their results suggest that these cavities could open the door to integrating light-based components into circuits or other devices in new ways and for new purposes. In addition, the team wants to take a closer look at just how densely topological cavities can be packed on a chip and the opportunities this creates for smaller, more powerful chips and processors.

For more information on this project, check out this video by UCSD's Jacobs School of Engineering: