Supremely Small BICSEL Laser Traps Light in Open Air

Bound states in the continuum lead to more-compact laser

2 min read
schematic of new BICSEL laser
Illustration: University of California San Diego/Nature

 Tapping into an idea from quantum mechanics that dates back to the Jazz Age, researchers have created a new type of laser that could be much tinier than conventional lasers, potentially leading to faster optical communications and more powerful computers.

The laser relies on a phenomenon known as bound states in the continuum (BICs), which allows researchers to build a laser cavity in open air. “It’s not every day that you have the possibility to make a new type of laser,” says Boubacar Kante, a professor of electrical and computer engineering at the University of California, San Diego, who with his colleagues described their laser in this week’s issue of Nature.

In most traditional lasers, the laser cavity consists of some material in which light waves oscillate back and forth between two mirrors, exciting electrons in the material and causing them to emit more photons. Eventually some of the light passes through one of the mirrors, which is not perfectly reflecting, producing the laser beam.

The BICs laser cavity, by contrast, is a type of photonic crystal, shaped to limit and control how light waves can propagate. The researchers built a membrane of a dielectric material, indium gallium arsenide phosphide. The membrane consists of small discs of the material, 300 nm thick and 528.4 nm in radius. The discs, which the researchers call “particles,” are where the lasing takes place, and they’re spaced 1200 nm apart, joined by tiny bridges of material to give the membrane structural stability. The team built arrays ranging in size from 8 x 8 to 20 x 20 particles. The periodic arrangement of the particles creates bandgaps, frequencies at which the light waves can’t exist. Those bandgaps act as insubstantial mirrors, confining the light between them. Slight flaws in the design make the mirrors imperfect, allow the laser to produce a light beam.  

The upshot is that they can build a laser cavity with a much higher “quality factor,” a dimensionless parameter that describes how much energy a cavity can hold. That means they can achieve lasing with a much lower energy input, which in turn allows them to build smaller lasers than ever, or build higher power lasers that build up less heat.

The team calls their lasers BICSELs, by analogy to the long used vertical-cavity surface-emitting laser (VCSEL), and Kante says they can build a laser 100 times smaller than a VCSEL. That could mean packing them more densely on a chip without having to worry about heat dissipation, leading to higher-rate optical communications inside a computer. The lasers can also be designed to emit light at different angles, making them potentially useful as beam-steering optical antennas.

The team built its laser to work at 1550 nm, a popular telecommunications wavelength, but using different materials should allow them to build any other usual wavelength. The demonstration device was powered with light from another laser, but Kante has a three-year grant from the National Science Foundation to create an electrically powered device, which is generally required to make a laser worthwhile.

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3D-Stacked CMOS Takes Moore’s Law to New Heights

When transistors can’t get any smaller, the only direction is up

10 min read
An image of stacked squares with yellow flat bars through them.
Emily Cooper

Perhaps the most far-reaching technological achievement over the last 50 years has been the steady march toward ever smaller transistors, fitting them more tightly together, and reducing their power consumption. And yet, ever since the two of us started our careers at Intel more than 20 years ago, we’ve been hearing the alarms that the descent into the infinitesimal was about to end. Yet year after year, brilliant new innovations continue to propel the semiconductor industry further.

Along this journey, we engineers had to change the transistor’s architecture as we continued to scale down area and power consumption while boosting performance. The “planar” transistor designs that took us through the last half of the 20th century gave way to 3D fin-shaped devices by the first half of the 2010s. Now, these too have an end date in sight, with a new gate-all-around (GAA) structure rolling into production soon. But we have to look even further ahead because our ability to scale down even this new transistor architecture, which we call RibbonFET, has its limits.

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