A Glowing Capsule That Bursts on Command ✛
A tiny capsule, like a soap bubble but with a shell of glowing crystals, behaves like a laser. While its wall is intact, light runs in circles, amplifies, and escapes as a narrow, bright beam. Once the wall bursts, the beam turns into a dim spot.
The capsule's shell was assembled from quantum dots quantum dots. Inside is a drop of oil. While the shell is continuous, light that enters reflects off the walls and runs in circles, like sound in a circular gallery. If the wall cracks, light no longer runs in circles but scatters in all directions.
You can destroy the capsule in three ways: dry it, heat it, or shine an infrared laser on it. When heated, the rupture temperature depends on the oil molecule's length: the longer the molecule, the lower the temperature. For oil with 18 carbon atoms, the shell bursts at just 23 °C, slightly warmer than room temperature.
Before this work, it was unclear whether a thin shell could confine light well enough for lasing if the inside is liquid rather than solid. It turns out it can: an intact capsule emits a narrow laser beam. After rupture, the laser glow disappears, replaced by a broad band. Such switchable microlasers could be useful in bioimaging, sensing, and labeling.
The colloidosome shell occupies only 17% of the volume of an equivalent solid sphere, and requires 83% fewer quantum dots. Yet lasing is preserved because light circulates precisely along the shell.
The idea of a microcontainer that releases its contents on command evokes drug capsules from science fiction, such as those in the series 'Star Trek'. Here, instead of a drug, we have quantum dots, and the signal is an infrared laser beam.