Quantum Chaos Improves Laser Performance

High-powered semi­conductor lasers are used in materials processing, bio­medical imaging and industrial research, but the emitted light they produce is affected by insta­bilities, making it incoherent. The insta­bilities in the laser are caused by optical filaments; light structures that move randomly and change with time, causing chaos. Removing these insta­bilities has long been a goal in physics, but previous strategies to reduce fila­ments have usually involved reducing the power of the laser.

The D-shaped cavity producing quantum chaos within the cavity, and a more stable laser resulting. (Source: S. Bittner at al.)

Now, a research team from Imperial College London, Yale Uni­versity, Nanyang Techno­logical Univer­sity and Cardiff Uni­versity have come up with a new solution. Their technique uses quantum chaos to prevent the laser filaments, which lead to the insta­bilities, from forming in the first place. By creating quantum wave chaos in the cavity used to create the laser, the laser itself remains steady. Ortwin Hess, from the Depart­ment of Physics at Imperial, contri­buted much of the theory, simulation and interpretation of the new system. He said: “The way the optical filaments, which cause the laser instabi­lities, grow and resist control is for the laser a bit like the unruly behaviour of tornadoes. Once they form, they move about chaotically, causing destruc­tion in their wake.”

“However, tornadoes are more likely to form and move about over flat country. For example, in America they form frequently in beautiful Okla­homa but not as often in hilly West Virginia. The hills appear to be a key dif­ference – they prevent tornadoes from being able to form or move around. In the same way, by creating a hilly optical landscape right inside our lasers using quantum chaos, we don’t allow the fila­ments – optical tornados – to form or grow out of control.”

The laser system, manufactured at the Nanyang Techno­logical University in Singapore, has been proven experi­mentally at Yale Uni­versity. The team are now working to further explore and tailor the light emission, such as improving the direc­tionality of the laser. They say however that the break­through should already allow semi­conductor lasers to work at higher power with high emission quality, and that the same idea could be applied to other types of lasers.

When large semi­conductor lasers are switched on, this bouncing back and forth creates filaments – sections of the light that swiftly begin to act chaotically. To create a different kind of chaos – the quantum chaotic landscape . the team designed a new shape of cavity for the laser. Most cavities are cuboid in shape, but by using a D-shaped cavity, the team were able to induce quantum chaos in the light bouncing around. This quantum chaos acts on a smaller scale than the wave­length of the light, creating the optical hills that help to dispel the optical tornadoes.

Hui Cao from Yale Univer­sity said: “We use wave-chaotic or disordered cavities to disrupt the forma­tion of self-organized structures such as filaments that lead to insta­bilities.” The team gained insight into the processes and cavity shapes likely to create this kind of quantum chaos from theories and experiments in nano­photonics and nano­plasmonics. (Source: ICL)

Reference: S. Bittner et al.: Suppressing spatiotemporal lasing instabilities with wave-chaotic microcavities, Science, online 16 August 2018; DOI: 10.1126/science.aas9437

Link: Centre for Plasmonics and Metamaterials, Dept. of Physics, Imperial College London, London, UK

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