Polarization polariton topology pointing towards a new type of laser

Semi light, partially matter quasi particles, known as excitons polaritons, can easily bypass obstacles and condense into a single coherent state - both of which are characteristics of topological insulators. Researchers from the United States and China have developed a new technology to manufacture microcavities from chloride based halide perovskites. They expect this work to lead to a new type of laser based on topological polaritons.

Using exciton polariton detection
The material known as topological insulator has significant characteristics, that is, it is a conductor on the outside but an insulator on the inside, thanks to the junction that can be considered as an electron wave function. This characteristic ensures that electrons flow at the edges of this material without losing energy or being scattered by non-magnetic impurities. This phenomenon was first observed in condensed matter systems in 2007, and has since appeared in acoustics, cold atoms, and photon systems.

In the latest research, Wei Bao and colleagues from Rensselaer Institute of Technology explored the physical properties of topological insulators using exciton polaritons. Excitons are bound states of electrons and holes, which can form polaritons by coupling photons into semiconductor microcavities. This type of particle is an interacting boson with a small effective mass and exhibits strong nonlinearity. These comprehensive characteristics enable polaritons to form Bose Einstein condensates at much higher temperatures than cold atomic systems.

Other groups used polaritons to study topological structures, but were unable to fully aggregate topological edge states. Bao and his colleagues demonstrated how to achieve this feat by creating microcavities from crystals composed of cesium, lead, and chlorine atoms. Unlike previous studies on perovskites using bromine instead of chlorine atoms, this crystal has an isotropic refractive index, which is crucial for achieving electromagnetic modes that propagate freely at the edges of the material.

Serrated topological waveguide
As explained by the researchers, growing large and thin CsPbCl single crystals 3 is difficult because the necessary precursors cannot dissolve well in suitable solvents. They solved this problem by using toluene vapor as an antisolvent to promote the nucleation of perovskite in solution growth, or by using a long two-step cooling process in chemical vapor deposition, indicating the ability to produce both wide and thin single crystal perovskites.

The cavity is composed of perovskite crystals sandwiched between distributed Bragg reflectors, with a layer of polymer between the crystals and the upper reflector. Researchers used standard photolithography techniques to shape polymers, resulting in a series of asymmetric hexagonal holes - each hexagon has three sides with a length of 0.73 μ m and three sides with a length of 0.27 μ m. By carving two different regions in the array, one containing holes with longer edges as the base and the other containing holes with shorter bases as the base, they can create a serrated interface that serves as a "topological waveguide".

To demonstrate the characteristics of waveguides, Bao and his colleagues aimed the pump laser at a point on the interface and measured photoluminescence using a camera on a self-made microscope. They detected photoluminescence along the interface length upstream and downstream of the pump point, indicating that exciton polariton edge states propagate along the waveguide in both directions. Given that the interface contains many turns of 120 °, they concluded that even in the presence of strong obstacles, edge states can propagate - a hallmark of topological insulators.

Facing the laser state
More importantly, researchers have observed nonlinear condensation of polariton edge states. For this purpose, they created cavities with different hexagonal hole arrays, where the interface between two different regions is marked with equilateral triangles instead of serrated lines. By guiding laser pulses through spatial light modulators to pump the edges of the triangle, they once again detected photoluminescence from the interface. They found that when the pulse energy was below a certain threshold, a crystal of about 10 μ J2 per centimeter - detected light was quite weak and diffuse. However, once the threshold is exceeded, the output becomes stronger and more concentrated - it has already entered the laser state.

According to the team, this nonlinearity demonstrates polariton condensation. More importantly, when pumping more complex waveguide geometries, they saw the same behavior - they recorded an increase in strength at interfaces similar to fish, with their mouths either open or closed.

"These results establish a" room temperature polariton platform capable of constructing large-scale condensed lattices with arbitrary potential landscapes for simulating topological physics and exploring potential new phases of quantum matter, "reported Bao and colleagues.

They also believe that a relatively low energy threshold required for condensation can make energy-saving polarized polariton lasers possible. They said that even better, polarizer lasers can be connected to the array to generate high power - because their phase is different from that of traditional lasers, they will lock in each other. However, before such devices become a reality, researchers must first demonstrate how to electrically pump them, which requires determining a suitable material as the contact electrode and a perovskite that can better conduct heat.

Source: Laser Net