A team of researchers from Peking University in China has fabricated an optical analogue of “magic-angle” graphene bilayers in a photonic nanocrystal. They have used the structure to create a completely new type of highly-efficient nanolaser.
Graphene is a flat crystal of carbon just one atom thick. When two such sheets are placed on top of each other with a small angle misalignment, they form a Moiré superlattice. At a twist angle of 1.08°, the material becomes highly correlated and begins to show properties such as superconductivity at low temperatures.
At this so-called magic angle, the way in which electrons move in the two coupled sheets changes because they are now forced to organize themselves at the same energy. This leads to “flat” electronic bands, in which electron states have exactly the same energy despite having different velocities.
This flat band state makes an electron dispersionless – that is, its kinetic energy is completely suppressed and it cannot move in the Moiré lattice. The result is that electrons slow down almost to a halt and become localized at specific positions along the coupled sheets, where they can strongly interact with one another. This is the effect that gives rise to the abovementioned superconductivity, as well as producing many exotic and unexpected phenomena such as correlated insulator states and orbital magnetism.
Stopped-light nanolasing
Nanolasers are key to developing integrated photonics. They work by confining light in a nanocavity and emitting coherent light within a very narrow spectral range through optical amplification, after passing through the cavity multiple times to increase its gain.
Researchers have designed many nanolaser schemes over the years, with very different optical cavity designs, including nanodisc lasers, nanowire lasers, plasmonic nanolasers and photonic crystal nanolasers. However, the cavities of these nanolasers require materials with highly differing properties or disorder/defects to localize a light field.
The laser developed by Ren-Min Ma and colleagues works in a very different way. The new device makes use of dispersionless light stopping in an optical magic-angle graphene-like lattice of nanoholes in a semiconductor membrane. The membrane consists of InGaAsP multi-quantum wells, which act as the active gain medium.
No need for a “cavity”
The researchers introduced two graphene-like photonic crystals into the same semiconductor membrane. When they twisted one with respect to the other at an angle of 2.65°, they found that the coupling between the two photonic crystals created a completely flat-band dispersion, which stops and localizes light – just like magic-angle twisted graphene. This effect does away with the need for a conventional laser cavity.
Ma and colleagues optically pumped their laser to induce gain in the structure. They say they unequivocally observed lasing at around 1.5 µm, a wavelength that is important for telecommunications applications.
A completely new design for nanolasers
As well as representing a completely new design of nanolaser, the device also has many advantageous properties. For one, it has a threshold of only 0.037 mW in pump power. This is much lower than nanodisc lasers, nanowire lasers or plasmonic nanolasers and is on a par with state-of-the-art photonic crystal defect nanolasers made from the same gain materials. It also boosts a higher “quality factor over mode volume” (a figure of merit usually used to characterize laser cavity quality) compared with the types of lasers mentioned above. Indeed, at more than 400 000, its quality factor is among the highest of all kinds of nanolaser cavities, says Ma.
‘Magic-angle’ graphene doubles up
“Our scheme provides a novel flexible and robust platform to construct high-quality nanocavities for lasers, nanoLEDs, nonlinear optics and cavity quantum electrodynamics,” he adds. “It could be used in applications in integrated photonics, near-field spectroscopy and sensing.”
The researchers say that they will now be studying the exotic light–matter interaction in their new platform. “We will also be pushing forward applications for the device,” Ma tells Physics World.
The new laser is detailed in Nature Nanotechnology.
