(Featured image from Physics.)
Soap bubbles are marvelously playful. A cascade of bubbles blown into the air can send children running in circles to pop them before they hit the ground. And if you know how to look, soap bubbles are just as playful on much smaller scales, sending scientists running in circles to understand their fascinating physics. By blowing a soap bubble on the end of a thin capillary tube and trapping light inside its surface (a technique shown in the featured image above), scientists can even transform an ordinary soap bubble into—wait for it—a FLOATING LASER!
If you've ever noticed the beautiful rainbows formed by bubbles like the one in Fig. 1, you already know there's something special about the way light interacts with soap bubbles. Because the soap film enclosing a bubble is so thin — as thin as 100 nanometers, where visible light has wavelengths between 400 and 700 nanometers and the width of a human hair is around 40,000 nanometers — the light waves entering and exiting the soap film intercept each other. Physicists call this interference, and slight differences in the thickness of the soap film create interference patterns which show up as bands of different colors (Fig. 2). (To learn more about how interference works and what we can learn from the patterns it produces, see Dude, Where My Atoms At?)
By adding a fluorescent dye to the soap bubble solution, scientists are able to make bubbles that glow like fireflies when illuminated with a specific wavelength of light, as shown in Fig. 3. Sometimes, light that enters the soap film gets trapped and bounces around inside the film. This trapped light, visible as bright rings, travels around and around the bubble’s circumference. A similar phenomenon was observed for sound waves in the so-called "whispering gallery" of St. Paul's Cathedral in London, where a person whispering on one side of the cathedral could be heard on the other side as sound waves traveled across the surface of the dome. For this reason, any waves traveling across the surface of a circular cavity are know as whispering-gallery waves. And it turns out that whispering-gallery waves are the key to transforming a soap bubble into a laser!
To understand how soap bubble lasers work, it's helpful to know what makes laser light different from regular light in the first place. As shown in Fig. 4, most light sources emit waves with a variety of different wavelengths traveling in different directions and in different phases (we call this incoherent light). Lasers, on the other hand, produce a single wavelength of light with all waves traveling in the same direction and in the same phase (coherent light). You can think of incoherent light as a crowd of people chanting similar phrases but each starting and stopping at different times; coherent light is like a crowd chanting exactly the same phrase in perfect unison. Even if the volume of the individual voices is the same in both cases, the effect of the coherent crowd is much more powerful, and coherent light is the same way.
To produce coherent light, a laser needs two things: an energy source and a medium. When electrons in the medium are stimulated by the energy source, they release the extra energy as light. That light, in turn, becomes an energy source for more electrons, which release more light of the same wavelength. As the process continues, more and more light waves join the the dance in perfect rhythm, amplifying the light. In just this way, light trapped inside a soap bubble film stimulates the emission of more light waves, the light is amplified, and—voilà!—a laser is born! (Bonus fun fact: The word "laser" is actually an acronym that stands for Light Amplification by Stimulated Emission of Radiation.) And it’s not just one laser. Just as airplanes can take different paths around the globe, light can take different paths around the soap bubble, producing many simultaneous whispering modes with slightly different colors.
This effect is very sensitive to the molecules that make up the soap film. In your kitchen at home, you use simple dish detergent, but we can also make bubbles from a special kind of substance called a liquid crystal. Liquid crystals are made of long molecules that move and jumble around like the molecules in a liquid but can also stack into layers like the atoms in solid crystals. When laser light is pumped into a liquid crystal bubble, the regular spacing of the layers produces a specific color pattern (Fig. 5). This pattern depends on the size and shape of the bubble, changing as the bubble inflates or deflates slightly. It is incredibly difficult to change the color of a regular laser, but a liquid crystal bubble laser does it with a simple puff of air! Because their colors are so sensitive to the bubble's size and shape, these lasers also make amazing sensors. Scientists can monitor changes in the colors to calculate forces that distort the bubble, including mechanical pressure but also electric forces. In fact, bubble lasers may be the world’s best electric sensors!
So, next time you blow a soap bubble . . . remember that this funsize object contains king-size physics!