A light beam inscribes a pattern on a thin crystal, and heat runs along that pattern in the desired direction, like water along a dug channel. This is how the new method works to control energy flows over distances of tens of nanometers — hundreds of times thinner than a human hair.

Inside the molybdenum oxide crystal, special waves arise — hyperbolic phonon polaritons. They resemble elastic vibrations of the crystal lattice that carry heat, but they move not in all directions, only along certain directions. It's as if sound in a room propagated not as a sphere but as a narrow beam.

Previously, to launch such waves, metal disks or strips cut with nanometer precision were deposited on the crystal. That was slow and expensive. Now, instead of metal, a laser is used: it locally heats the bottom layer made of In₃SbTe₂, which changes its properties, turning into "rails" for polaritons. The pattern can be erased and redrawn, like chalk on a board.

The scientists tested the method on various structures. For example, a laser-drawn disk focused heat just as well as a gold disk made by the conventional method. And from two disks, they assembled a cavity where polaritons are trapped, like light between two mirrors. By changing the distance between the disks, one can tune how strongly heat is concentrated at the center.

This approach saves time and money, and crucially, it allows rapid reconfiguration of the device for a new task. Perhaps soon microchips will be cooled or transmit signals not by electricity but by directed heat flows controlled by light.