Optical Mirror Thinner Than Hair, Still Has Visible Reflection

This optical mirror uses only a few hundred atoms, and may deepen our knowledge of quantum theory.

Physicists have engineered the lightest optical mirror ever — consisting of one singular layer of a few hundred atoms — according to a new study published in the journal Nature.


Lightest optical mirror only hundreds of atoms

Composed of a novel metamaterial and consisting of a singular structured layer of a few hundred identical atoms, this lighter-than-imagination optical mirror was developed by physicists at the Max Planck Institute of Quantum Optics (MPQ).

The atoms were organized into the two-dimensional array of an optical lattice created with interfering laser beams. This research provided the first experimental observations of their kind in the nascent field of subwavelength quantum optics with ordered atoms, according to

Typically, mirrors use highly-polished metal surfaces — or optical glasses with a special coating to enhance performance for ultra-low weights. But the physicists at MPQ have now proved possible for the first time that a single structured layer of a few hundred atoms is sufficient to form a working optical mirror.

The new mirror is merely a few tens of nanometers thin — 1,000 times thinner than a human hair. But it's size is deceptive because its reflection is so strong that we can see it with the naked eye.

Mechanisms, not magic, make the quantum mirror

The new optical mirror uses identical atoms organized into a two-dimensional array. They're arranged in a normal pattern with atom-to-atom spacing lower than the optical transition wavelength of the atom — a feature both necessary and typical of metamaterials.

Metamaterials are synthetic — or artificially designed — structures with extremely specific properties found rarely in nature. Their properties stem not from composition but from the structures into which they're formed.

Their characteristics — that regular pattern with subwavelength spacing — along with their interplay are the two critical features that make the novel optical mirror work its magic.

Firstly, the subwavelength spacing of the atoms along with the regular pattern together suppresses a diffuse scattering of light, bunching up the reflection into a one-directional and very steady beam of light.

The second critical feature at work is the relatively close and discrete distances between atoms — which allow the incoming photon to bounce back and forth between atoms several times before it's reflected out into the stream of light.

Both of these effects — which suppress the bouncing of photons along with the tendency of light to scatter, lead to an "enhanced cooperative response to the external field" — in other words, a strong and steady reflection we can see with our eyes.

Jun Rui David Wei MIT
Jun Rui and David Wei with their machine that houses an optical mirror invisible to the naked eye. Source: Max Planck Institute of Quantum Optics

Continued advancements, more efficient quantum devices

While the reflection is visible, at a diameter of roughly 7 microns, the mirror itself is much too small to see with the naked eye. But the machine that houses it is, however, gigantic. Akin to other quantum optical experiments, it contains more than a thousand single optical components and weighs roughly 2 tons. This is why the novel metamaterial will likely have no impact whatsoever on the commodity mirrors people use on the daily. But the scientific repercussions of the development might be, however, far-reaching.

"The results are very exciting for us. As in typical dilute bulk ensembles, photon-mediated correlations between atoms, which play a vital role in our system, are typically neglected in traditional quantum optics theories. On the other hand, ordered arrays of atoms made by loading ultracold atoms into optical lattices were mainly exploited to study quantum simulations of condensed models. But it now turns out to be a powerful platform to study the new quantum optical phenomena as well," said Jun Rui, first author of the paper and researcher.

"Many new exciting opportunities have been opened, such as an intriguing approach to study quantum optomechanics, which is a growing field of studying the quantum nature of light with mechanical devices. Or, our work could also help to create better quantum memories or even build a quantum switchable optical mirror," added Doctoral Researcher and Second Author David Wei. "Both of which are interesting advancements for quantum information processing."

While these are all exciting, for now, more research in this field might deepen our fundamental understanding of quantum theories about light-matter interaction, optical photons' many-body physics, and also help engineers create more efficient quantum devices.

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