Lasers can produce a slender beam of light of just one wavelength. For this simple feature, they are everywhere in modern technology, from the internet to medical devices and even in entertainment as holograms. However, traditional lasers have some limitations. For example, to use them in particular applications, engineers currently use ensembles of lenses and mirrors to ‘shape’ the light.
Scientists from Australia and China have reported in Nature a way to produce laser light in specific shapes. A laser is a device that produces intense, focused beams of light. The new device, called a metalaser, produces such light using a metasurface: ultra-thin layers made up of artificial atoms that control light in specific ways.
By designing the metasurface carefully, the scientists could control the colour and brightness of the laser as well as the precise shape and direction of the light waves it emitted. This meant the laser could create images, focus to a point or line, and even produce special patterns like spirals or holograms, all without the need for bulky optical elements.
The scientists created a flat surface covered with a grid of microscopic discs made of silicon nitride. Each disc had a small hole that could be rotated to a specific angle. By changing the angle of each hole, the scientists could control the phase (the position in the light wave’s cycle) of the light coming from each disc. This phase is called the geometric phase. When the laser emitted light, these various geometric phases combined to closely shape the overall wavefront.
By the way, the geometric phase is called the Pancharatnam-Berry phase after S. Pancharatnam and Michael Berry. Pancharatnam independently discovered the geometric phase in 1956. His promising career in physics was cut disappointingly short when he died in 1969 aged only 35. Berry discovered the geometric phase in 1986. Eight years later, the British physicist George Series wrote in a special issue of Current Science dedicated to Pancharatnam that “his work received renewed attention and acclaim only after the recognition, in the eighties, that he had derived and used the concept of geometric phases in his studies of the interference of polarised light.” More here.
Back to the study: the laser light was trapped on the metasurface by an effect called a bound state in the continuum (BIC). A BIC is a special kind of wave that, despite having enough energy to escape and spread out, remains perfectly trapped and doesn’t leak away. It’s akin to a wave that should be able to travel freely but due to conditions like interference is confined and doesn’t interact with the surrounding ‘free’ waves. BICs are also unique because they persist rather than dissipate away.
The BIC helped keep the light focused and intense. When it was time for the metalaser to emit a laser beam, the team introduced a controlled perturbation in the structure, such as by adjusting the shape, position or orientation of the meta-atoms on the metasurface. This disturbance allowed the light to ‘leak’ into the outside world through a well-defined channel. At this point, the disks acted like small antennas, emitting light whose direction and phase depended on the holes’ rotation.
By carefully setting the phase at each point, the scientists could make the laser beam form any pattern they wanted even as it travelled away from the surface.
The scientists started with a glass base and added a patina of silicon nitride. Using electron-beam lithography — which is a technique like 3D printing but with electrons — they created the tiny discs and holes with precise angles. The discs were finally covered with a thin layer of dye-doped plastic that helped to amplify the emitted light. The discs were called ‘meta-atoms’ because they were essentially artificial atoms.
First, the team pumped the metalaser to make it emit light. Pumping is an essential step in any laser. In a regular laser, a population of atoms called the gain medium is ‘pumped’ with energy to push the atoms’ electrons into an excited state. Then, when some electrons spontaneously lose the energy they absorbed earlier and become de-excited, they release photons. The presence of these photons stimulates other excited atoms to emit identical photons as they return to a lower energy state. as well. As this process repeats itself, the gain medium releases more and more photons of a fixed energy that reflect back and forth in the cavity before entering the outside world as laser light.
The metalaser had one extra step: a separate laser pumped the metalaser’s gain medium. According to the paper, using a laser as the pump source was more efficient because its light was tuned to exactly match the energy levels required to excite the gain medium.
By rotating the holes in the discs, the scientists could create metalasers of almost any shape: focused spots, lines, spirals, even complex images like holograms. Traditional lasers often create speckle noise — the reason holograms have that grainy look — but the metalasers could produce holograms with almost none of this noise. The results were much clearer and sharper images.
The team also reported that the metalaser achieved a quality factor — a measure of how well it stores energy — high enough to mean the laser was producing a pure colour. The design could also be changed to produce different types of beams, including those with special polarisation (the direction in which the light waves vibrated) or orbital angular momenta (twisting light). In fact, the team wrote in its paper that their approach could be used to make programmable lasers that switch between patterns on demand.
The metalaser setup in the new study is microscopic, per the study, and can be integrated into small devices. Since the laser itself shapes the beam, there is no need for additional mirrors, lenses or filters.
