A laser based on exciton-polaritons, hybrid particles of light and matter, may boost precision laser technology to another level.

A team from the Quantum Science and Technology (QST) Department and FLEET found polariton lasers have unexpectedly high spectral purity. This quality, combined with their extremely low power requirements and very small footprint, means they could potentially have an important role in reducing the energy consumption of lasers used in optical communications, PhD scholar Bianca Rae Fabricante said.

“I thought I was just making a commonplace measurement of the laser linewidth, but to then discover the spectral purity was on par with existing technology was exciting,” said Ms Fabricante, who is a member of QST and FLEET, the ARC Centre of Excellence for Future Low-Energy Electronics Technologies.

Ms Fabricante is the lead author in the publication of the research, in the journal Optica. High spectral purity means that a laser emits a very narrow band of frequencies, which indicates exceptional stability – a sort-after quality for applications.

Polariton lasers were first demonstrated over two decades ago, and their low power characteristics were quickly recognised.

This is because polariton laser light originates from particles that are already in a coherent quantum state, known as a Bose-Einstein condensate (BEC). In contrast, conventional lasers rely on the coherent triggering of a collection of individual atoms that have been excited to a high energy state – requiring start-up energy that polariton lasers don’t need.

Polariton laser spectral purity was not expected to be remarkable: scientists were aware that alongside the BEC, there was always a collection of other particles that were not behaving coherently, and it was assumed these incoherent particles would introduce significant noise to the polariton laser and degrade the purity of its emission.

However, other published results suggesting that polariton lasers might have good spectral purity prompted the group to investigate more closely.

Using a polariton laser made from a high-quality sample of ultrathin semiconductor sandwiched between two mirrors, they chose a different method from previous work to measure the spectral purity (as quantified in the related properties coherence time and linewidth).

Rather than a Michelson interferometer, which hinges on averaging that can wash out precision, the team used a Fabry-Perot interferometer.

To their surprise the new method revealed that the line width was 56 MHz or 0.24 µeV, ten times smaller than previously published results.

This places polariton lasers on par with the current leading technology, vertical cavity surface emitting lasers (VCSELS).

 “Polariton lasers are potentially better than VCSELS for low-energy applications since they can operate at lower powers,” Dr Mateusz Król said.

A narrow linewidth means a long coherence time, in this case at least 5.7 nanoseconds.

This is enough time to perform, in principle, thousands of successive operations on the source of the laser, a macroscopic quantum state of condensed exciton-polaritons. This is a critical step for quantum information processing, the leader of the research Dr Eliezer Estrecho said.

"Our work not only pushes the boundaries of exciton-polariton laser technology but also opens up new avenues for utilising exciton-polaritons for classical and quantum computing."

Polariton lasers may hold more surprises. The current measurement pushes the linewidth measurement to its resolution limit – finer measurement techniques could reveal an even narrower linewidth.

“Polariton lasers are predicted to exhibit interesting properties because the underlying particles interact with each other,” said Dr Estrecho.

“For example, the polariton BEC itself is predicted to be in a squeezed state, a quantum state with noise lower than the standard quantum limit, which is useful for many light-based quantum technologies.

“Measuring the linewidth is one way of measuring phase noise, so this research is really one of the first steps towards understanding the state.”

As researchers gain a deeper understanding of polariton lasers, their unique characteristics will define the uses they can be put to, group leader Professor Elena Ostrovskaya said.

"Ever since their discovery, low-threshold polariton lasers that do not require a population inversion, have been waiting for practical applications.

“Our study suggests that such applications can be broader than previously thought."