Trapping polaritons in a quantum box for perfect conduction

Wednesday 9 November 2022

Physicists have trapped exotic hybrid particles in a quantum box a single atom thick, potentially promoting perfect conductivity.

The team from the Quantum Science and Technology Department made the box from a tiny rectangular layer of tungsten disulfide and successfully trapped exciton polaritons, particles that are a hybrid of a photon, and an electron and a hole. The resulting increase in population density in the trap might be a crucial stepping stone to perfect conductivity.

“It’s exciting how simply quantum confinement can be achieved for polaritons in two-dimensional materials  said Dr Matthias Wurdack, the lead author of the team’s paper, in Physical Review Letters. Dr Wurdack is also part of the FLEET ARC Centre of Excellence. 

“The higher confinement substantially increased their macroscopic coherence and redistributed the population towards lower energy states.”

Exciton-polaritons are a promising platform for future ultra-low energy electronics, because they can flow without dissipation of energy. However, exciton-polaritons in conventional semiconductors are often not stable enough to exist without cryogenic conditions, which consume significant energy to maintain.

To address this, the group decided to work towards dissipationless behaviour in two dimensional (2D) materials, in which exciton-polaritons can be created at room temperature.

“However, this dissipationless transport requires a phase transition to a fully macroscopically coherent quantum state, which only occurs at very large particle densities that are hard to access in two-dimensional semiconductors,” said group leader Professor Elena Ostrovskaya, who is also a FLEET member.

A way forward appeared as the group experimented in a semiconductor made from a single layer of transition-metal dichalcogenide (in this work, tungsten disulfide). They noticed that exciton polaritons were trapped in flaws of such 2D polariton devices, and at cryogenic temperatures, the transition to a coherent quantum state was achieved within such a trap.

They reasoned that the trapping occurred when the flaw was of a similar size to the particle wavelength of the exciton-polariton and set about deliberately engineering a structure with the same properties, to allow careful investigation of the physics of the trapped polaritons.

However, controlled building of a perfectly clean trap for polaritons in 2D materials is difficult, because these materials are extremely fragile and easily damaged using conventional nanofabrication machines that involve hot and abrasive particles. 

Instead, the team used a method for building the trap gently involving stacking of the 2D materials. Here, the 2D layers were initially prepared on polymer films, from where they can easily be transferred onto microscopically flat target areas. When stacking these layers from the polymer films on top of each other step by step, strong van-der Waals interactions between the layers make them stick together.

They found an effective trap structure to be a small square of tungsten disulfide placed on the larger layer, separated by an insulating barrier made of gallium oxide. The stronger coupling between the exciton and the light on the double layer robs the particles of potential energy, creating the trapping potential.

As hoped, the exciton-polaritons accumulated and stayed confined within the box trap, showing a greatly increased density within the box.

“We have been able to demonstrate that polaritons which form anywhere outside the quantum box can travel for many micrometres and be trapped and accumulate inside the box”, explained Dr Wurdack.

They found that the trapping leads to energy redistribution towards lower energy states, signalling an advance towards the desired quantum states of Bose-Einstein condensation (BEC) and superfluidity.

The researchers also found that trapping significantly enhances the macroscopic coherence of the polaritons, even before the BEC phase is reached.

This is because the confined light is much longer-lived than the tungsten disulfide excitons, and trapping strongly reduces the phase fluctuations of the polariton gas.

“It’s an exciting demonstration of how a very effective trap can be built for exciton polaritons in 2D materials,” said Professor Ostrovskaya.

“Trapping is key to proposed applications, such as ultra-low threshold lasers, and enhances useful properties of the exciton polaritons, such as coherence.”

 

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