A light wave is like a rope that has been shaken: it oscillates in a specific plane. This property, polarization, reveals the shape and composition of objects. But capturing it is not easy, especially when you need to cover a wide range of colors. Usually, this requires bulky systems of lenses and filters.
Physicists have proposed using just a thin flake of the magnetic material CrCl3. The crystal itself, without any external tricks, senses the direction of light oscillations. Its working range is enormous — from 300 to 1700 nanometers, from ultraviolet to infrared heat. Inside the material, long-lived excitations, excitons, are born, living for microseconds. This is like an echo that lingers in the crystal and multiplies the electrical signal many times over: the amplification exceeds 45,000 times.
The most interesting part begins when the color of the incident light changes. Different excitons in the crystal are oriented differently: their dipoles (sensitivity axes) point in different directions. Depending on the photon energy, the detector's response rotates in the plane. If you shine light at an angle, vertical dipoles wake up. Competition between different excitonic transitions causes the polarization sensitivity not only to rotate but also to change sign. As a result, the degree of polarization can be smoothly tuned from −90% to +75%.
The result is a tiny crystal that acts as a tunable polarization filter, controlled by the color and angle of illumination. This promises cameras and sensors that see details invisible to the naked eye: stress in glass, tissue structure, cloud composition.
Chromium chloride (CrCl3) is a layered magnetic material that can be exfoliated down to a single atom thick, like graphite in a pencil. It retains its magnetic properties at low temperatures.
In Stanislaw Lem's novel "Solaris," the planet's ocean reacts to polarized X-ray radiation — perhaps alien intelligence uses similar principles for detection.