Researchers have demonstrated lasing from a perovskite-based photonic topological insulator―implying condensation into a single coherent state [Image: Rensselaer Polytechnic Institute]
The part-light, part-matter quasiparticles known as exciton–polaritons can flow unimpeded around obstacles and condense into a single coherent state―both characteristics of a topological insulator. Or so say researchers from the United States and China, who have developed a new technique to fabricate microcavities from chloride-based halide perovskites (Nat. Nanotechnol., doi: 10.1038/s41565-024-01674-6). They foresee this work leading to a new type of laser based on topological polaritons.
Probing with exciton–polaritons
Materials known as topological insulators have the remarkable property of being conductors on the outside but insulators within, thanks to what can be thought of as a knot in electrons' wavefunctions. This characteristic ensures that electrons flow around the edge of such materials without losing energy or being scattered off non-magnetic impurities. First observed in condensed-matter systems in 2007, the phenomenon has since been seen in acoustic, cold-atom and photonic systems.
In the latest work, Wei Bao, Rensselaer Polytechnic Institute, and colleagues have probed the physics of topological insulators using exciton–polaritons. Excitons are bound states of electrons and holes and can form polaritons by coupling to photons inside a semiconductor microcavity. Such particles are interactive bosons with a small effective mass, possessing strong nonlinearity. These combined properties enable the polaritons to form a Bose–Einstein condensate at much higher temperatures than is possible with systems of cold atoms.
Other groups have studied topology using polaritons but have been unable to fully condense topological edge states. Bao and colleagues show how to pull off the feat by fabricating micro-cavities from a crystal made up of cesium, lead and chlorine atoms. Unlike previous research that used perovskites featuring bromine rather than chlorine atoms, this crystal has an isotropic refractive index―crucial for enabling electromagnetic modes that propagate freely around the material's edge.
Zigzag topological waveguides
As the researchers explain, growing large, thin single crystals of CsPbCl3 is difficult because the necessary precursors do not dissolve well in suitable solvents. They got around this problem by using toluene vapor as an anti-solvent to promote perovskite nucleation in solution-based growth or adopting a long two-step cooling process in chemical vapor deposition―showing it is possible to produce single-crystalline perovskites that are both broad and thin.
The cavities consist of the perovskite crystal sandwiched between distributed Bragg reflectors, with a layer of polymer between the crystal and the upper reflector. The researchers used standard lithography to shape the polymer, yielding an array of asymmetrical hexagonal holes―each hexagon having three sides 0.73 µm long and three 0.27 µm long. By carving two distinct regions in the array, one containing holes with the longer side as their base and the other featuring holes having the shorter base, they were able to create a zigzag-like interface that served as a “topological waveguide.”
To demonstrate the characteristics of their waveguide, Bao and colleagues directed a pumping laser at one spot on the interface and then measured the photoluminescence given off using a camera on a homemade microscope. Detecting photoluminescence along the length of the interface, both upstream and downstream of the pumping spot, they showed that the exciton–polariton edge states propagate in both directions along the waveguide. Given that the interface contained a number of 120° turns, they concluded that the edge states were able to propagate even in the presence of formidable obstacles―the hallmark of topological insulators.
Detecting photoluminescence along the length of the interface, both upstream and downstream of the pumping spot, they showed that the exciton–polariton edge states propagate in both directions along the waveguide.
Toward the lasing state
Even more importantly, the researchers say, they observed nonlinear condensation of the polariton edge states. To do so they made cavities with a different array of hexagonal holes―ones in which the interface between the two different regions marked out an equilateral triangle rather than a zigzag line. Pumping the triangle's edges by directing laser pulses through a spatial light modulator, again they detected photoluminescence from the interface. They found that with the pulse energy below a certain threshold―about 10 µJ for each cm2 of crystal―the detected light was fairly weak and diffuse. Once over the threshold, however, the output became much more intense and focused―having entered the lasing state.
According to the team, this nonlinearity demonstrates polariton condensation. What's more, they saw the same behavior when pumping more complex waveguide geometries―they recorded the tell-tale step up in intensity from interfaces resembling fish with their mouths either open or closed.
These results establish “a room-temperature polariton platform capable of constructing large-scale condensation lattices with arbitrary potential landscapes for emulating topological physics and exploring potential new phases of quantum matter,” reported Bao and colleagues.
They also argue that the fairly low energy threshold needed for condensation could enable energy-efficient polariton lasers. Better still, they say, polariton lasers could be hooked up into arrays to yield high powers―given that their phases, unlike those of conventional lasers, would all be locked to one another. Before such devices become a reality, however, the researchers must first demonstrate how to electrically pump them, which requires identifying a suitable material as a contact electrode and also a perovskite better able to conduct heat.