In a thin plate of a layered crystal, light can get stuck, like sound in an empty room. At low temperatures, samples of (PEA)₂PbI₄ with thicknesses ranging from tens to hundreds of nanometers almost stop transmitting light at certain wavelengths. Instead of the usual dip in the spectrum, a whole band appears where light is reflected back, as if an invisible mirror had appeared in its path.
The secret lies in how light interacts with excitons — bound pairs of electrons and holes that form inside the crystal lattice. When there are many such pairs and they are strongly bound, they can alter the material's optical properties so that its dielectric permittivity (the ability to transmit an electric field) becomes negative. In this regime, light cannot penetrate inside: it is reflected as if from a metal, but without free electrons. The authors observed this transition by gradually increasing the sample thickness: from a transparent "window" to almost complete opacity.
Calculations showed that in this state, light and matter mix especially strongly — almost like in ultrastrong coupling, where the interaction energy is comparable to the energy of the transition itself. This paves the way for creating ultra-compact optical elements without complex mirror coatings: the material's structure itself becomes a mirror at the desired wavelength.
Excitons in layered perovskites are so stable that they can exist even at room temperature — unlike in many other semiconductors, where they decay almost instantly.
In Stanisław Lem's novel "Solaris," the planet's ocean creates optical mirages by changing its properties. Here, the material itself controls light reflection at the molecular level — without external energy.