They used a new metamaterial made of a single structured layer. few hundred identical atoms. Interfering laser beams hold two layers of atoms in arrays. This has come from an emerging new field of subwavelength quantum optics with ordered atoms. The mirror is the only one of its kind.
The new mirror is only several tens of nanometers thin but the reflection can be seen by the human eye. The mirror has a diameter of around seven microns.
The machinery has a thousand single optical components and weighs about two tons.
Metamaterials properties are not from the materials they are made of but from the specific structures they are designed with. The characteristics – the regular pattern and the subwavelength spacing – and their interplay are the two crucial workings behind this novel kind of optical mirror.