The demonstration of Bose-Einstein condensation in semiconductor lasers has paved the way for a new understanding of lasers via thermodynamics.

When a dense gas of bosons, one of two types of quantum particle, is cooled beyond a critical temperature, another state of matter (like gas, liquid, solid or plasma) occurs – known as a Bose-Einstein Condensate (BEC). Although photons are bosons, photon BEC was only shown relatively recently, for light emitted by organic dyes. Now results published in Nature Photonics by a team at Imperial College London, led by Professor Rupert Oulton and Dr Ross Schofield, have demonstrated that semiconductor lasers, under the correct conditions, are Bose-Einstein Condensates of photons. This unlocks a new way of thinking about lasers through the lens of thermodynamics that could lead to smaller, higher-powered devices.

A new way of thinking

Lasers are complex systems, and as they become larger and operate at higher powers, that complexity grows significantly; to understand the physics behind them, one needs to identify how the billions of photons they emit are divided among hundreds and thousands of different oscillating modes.

When faced with complex problems in systems in thermal equilibrium, physicists often turn to statistical mechanics to understand them. Before now, it was believed that the conditions required by a semiconductor to lase prevented the photons they produced from being in thermal equilibrium, a necessary condition for Bose-Einstein condensation. Professor Oulton and Dr Schofield have now identified that thermal equilibrium can occur under certain conditions, which encompass a wide variety of common semiconductor lasers.

The light inside most semiconductor lasers can now be described as a gas of particles. Their behaviour follows the same laws that have been used to understand gasses for over 100 years, and they can be assigned properties as a gas would, like temperature. Crucially, therefore, understood now as a system in thermal equilibrium, semiconductor lasers can be analysed via the powerful theory of statistical mechanics.

Finding a BEC needle in a haystack

The condensation phenomenon at this heart of this discovery has been demonstrated before in dye-based lasers. At Imperial, this work on condensates of light first began in 2015 under Dr Robert Nyman. Professor Oulton, building on his research in semiconductor lasers, sought the same behaviours as Dr Nyman had seen in dye-based laser. Together, in 2018, they began a project, funded by the Engineering and Physical Sciences Research Council (EPSRC) to investigate this, with experiments carried out by Dr Schofield, and assistance from Professor Florian Mintert and Dr Ming Fu.

In their experiment, the goal was to search for a spectrum of photons that matches a thermodynamic distribution at room temperature. But this requires precise control of the laser properties. Commercially available semiconductor lasers can by chance produce this distribution (as evidenced by a complementary study by a Polish team out of Wrocław University of Science and Technology also published in Nature Photonics), however, this requires analysing many devices to find one that possess the correct properties.

Instead, Professor Oulton’s team designed a bespoke semiconductor laser sourced from the UK’s National III-V Facility in Sheffield. Through the minute manipulation of the mirrors that form the laser, the conditions for thermal equilibrium could be tweaked. When the photons can thermalise and their density is high enough, a BEC is formed, as shown in the phase diagram figure. Conversely, if the photons do not reach thermal equilibrium, at high photon density, the system behaves like a conventional semiconductor laser.

A challenge for high powered lasers

Given their capacity to generate bright, coherent and directional radiation from electricity, semiconductor lasers can be found everywhere in modern day life – fibre telecommunications, facial recognition devices in phones, lidar, engraving and machining – but they tend to be relatively low power devices.

Generating high powers requires complex multistage laser systems, and so there is much work being done to make them smaller, more compact and more efficient. Using thermodynamics to tackle these problems, as unlocked by this new research, scientists now have a crucial, pre-existing tool with which to work on those miniaturisation ambitions. As condensates naturally seek out the minimum energy configuration of a system, this discovery could lead to more stable high-power lasers as well as new computing capabilities.

This experiment also gives physicists data with which to begin investigating the rich physics of interactions within BECs, such as superfluid light.

‘Bose–Einstein condensation of light in a semiconductor quantum well microcavity’ by Ross C. Schofield et al. is published in Nature Photonics.