Behind the Cover: March 2024

Each month, Physics Today editors explore the research and design choices that inspired the latest cover of the magazine.

A few dozen soap bubbles hover in front of a black background on the cover of Physics Today’s March issue. The bubbles’ familiar and delightful rainbow of shimmering colors arises from thin-film interference: A soap film’s thickness is about on par with visible wavelengths of light, so light waves reflecting off a bubble’s inner and outer surfaces combine to enhance some colors and diminish others.

As detailed in a March issue news story , a recent study published in Physical Review X shows that the wonders of soap-bubble optics go further: When doped with a little fluorescent dye, the bubbles can act as lasers. The video below captures one of the soap-bubble lasers in action.

No soap bubble lasts forever. As water evaporates from the soap film and drains from it under gravity, the film loses thickness, eventually becomes too thin to sustain itself, and bursts. The changing film thickness makes a soap-bubble laser’s output hard to control. But the study describes another type of bubble, made of liquid-crystal molecules rather than soap and water, that lacks that disadvantage. Such smectic bubbles are thinner than optical wavelengths by an order of magnitude, so they don’t display the same iridescent colors that soap bubbles do. But when made into lasers, smectic bubbles have outputs so precise and uniform that they possibly can be used as microphones.
Physics Today art director Freddie Pagani found a photo for the cover that strikingly conveys the optical properties of soap bubbles. She picked the font Acme Gothic Extrawide because of how its widened look complements the bubbles; the yellow color from among the bubble highlights contrasts with the dark background.