Realising chiral emission with topological LEDs

Applying strong magnetic fields during crystal growth enables the fabrication of LEDs that transfer chirality from electrons to photons.

BY JUNYONG KANG FROM XIAMEN UNIVERSITY

The revolution in the sharing of information is underpinned by the highly efficient processing, storage and transmission of data. For the latter, information is sent and received over optical networks, with data encoded through effective modulation of the amplitude or phase of the transmitted light. However, despite the ubiquity of this approach, it is not without weakness. One major drawback is that external perturbations and attenuations compromise the quality of this form of optical communication. So significant is this issue that the technology of today will fail to meet the challenges associated with tomorrow’s wide-band, high-speed data transmission.

Offering a solution to this important problem is chirality, an intrinsic feature of the photon. By encoding photons with different chirality, namely either the left-handed or the right-handed form, networks will be able to efficiently transfer binary information while offering a much stronger resistance to background noise. That’s not the only merit, however: encoding light with phase and chirality will also enable highly stable quantum key distribution, crucial to secret communication.

The chirality of a photon is closely related to the spin of an electron. To ensure simplicity and elegance, rather than using polarised optical elements to realise chirality, existing chiral photon sources employ spin-polarised materials. Such structures enable the transfer of electron spin to photon chirality via spin injection. However, chiral selectivity is far from ideal, due to constraints in spin polarisation and spin injection efficiency. Additional impediments to producing high-performance, practical systems are the need to use external magnetic fields or low temperatures to suppress electromagnetic or thermal perturbations. Substantial strides have to be made in stability and polarisation, in order to develop high-performance chiral photon sources.

Figure 1. Large-scale meron lattices produced by high magnetic field MBE and observed by magnetic force microscopy.

Turning to topology
A very promising way forward involves the use of a relatively new class of materials. For this particular family, characteristics depend not only on what the material is made from, but how it is arranged. Due to this rather striking behaviour, such materials are known as topological structures, named after a branch of mathematics known as topology.

Researchers discovered the first topological structures, which were insulators, as far back as 2007. In such materials, electrons zip along edges or surfaces in a manner that does not change under deformation of the material. Over the intervening years, the range of topological materials has expended to include photonic topological insulators – structures with photons zipping around the edges and surfaces – and topological superconductors and semimetals.

Within this growing family are those that involve the spin of the electron. Within this subset are skyrmions and merons – they feature vortex-like spin textures in magnetic thin films that exist out-of-plane and in-plane, respectively. Due to their unique topological protection features, skyrmions and merons have a higher stability than conventional electronic materials.

Our team at Xiamen University is trying to seize on this asset, which promises to address the stability bottleneck in polarised materials. Breaking new ground, we are pioneering the introduction of topological spin structures for chiral photon sources.