Bubble lasers are sturdy and sensitive

An ordinary soap bubble, it turns out, makes a pretty good laser. As Zala Korenjak and Matjaž Humar , of the Jožef Stefan Institute in Ljubljana, Slovenia, report in a new paper, all you have to do is dissolve a few specks of laser dye in the soap solution and shine a pump laser on the resulting bubbles. The bubbles lase at wavelengths that correspond to whispering-gallery modes (WGMs): roughly, those for which a whole number of light waves fit around the bubble’s circumference, as illustrated by the orange dots in the figure.

That picture is a bit simplified. Although soap bubbles look smooth, their thickness is nonuniform, as evidenced by the swirling iridescent colors they display when light catches them just right. For WGMs, which depend intricately on how light bounces off a bubble’s inner and outer surfaces, such imperfections matter a lot. So a soap bubble’s spectrum of laser modes shifts erratically from moment to moment—and it quickly vanishes altogether when the bubble pops.

But the researchers demonstrate another type of bubble that makes an even better laser. Smectic liquid crystals—those whose molecules arrange themselves into orderly layers—can form freestanding liquid films that can be blown into bubbles. Because the number of molecular layers is the same everywhere, a smectic bubble’s thickness is extraordinarily uniform. So when Korenjak and Humar tinged a smectic bubble with laser dye, it lased with a regimented spectrum of equally spaced WGMs.

Smectic bubbles last a lot longer than soap bubbles, too, because their liquid doesn’t inevitably drain from the top of the bubble to the bottom. In some of their experiments, Korenjak and Humar studied the same bubble for as long as 30 minutes. That’s plenty of time to watch how the WGM spectrum responds to tiny changes in bubble size.

Annoyingly, because smectic films are so thin—just tens of nanometers, compared with hundreds of nanometers for soap films—smectic bubbles steadily leak air over time. But when that effect is accounted for, shifts in the WGM spectrum can be used to detect tiny changes in ambient pressure. And since sound waves are just pressure oscillations, Korenjak and Humar are now exploring whether smectic-bubble lasers can work as microphones. They haven’t yet tried recording complicated waveforms like people’s voices, but they can distinguish low-volume tones of different frequencies. (Z. Korenjak, M. Humar, Phys. Rev. X 14, 011002, 2024 .)