Loss becomes win for laser performance

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Scientists from the School of Engineering and Applied Science at Washington University in St. Louis have devised a way to improve laser performance by introducing losses into the system. The work was published in the journal Science.

In a series of three experiments, the researchers demonstrated that by varying the distance between two microresonators and introducing controllable loss to one of them, the light intensity increased as the loss increased.

‘The loss added beyond a critical value increased the total light intensity and its distribution between the resonators,’ said Bo Peng, a graduate student working on the project at Washington University.

The researchers also reported achieving two nonlinear phenomena, the thermal effect and a Raman gain in silica despite increasing loss.

‘Light intensity is a very important parameter in optical systems, and here we have provided a new route to increase light intensity by modulating loss in the system,’ commented Dr Lan Yang, the Das Family Career Development Associate Professor in Electrical and Systems Engineering at Washington University. ‘Instead of the standard method of adding more energy into the system, we’re offering a more energy-efficient method.’

‘Too much of something can be really detrimental,’ said Dr Sahin Kaya Ozdemir, a research scientist working on the project. ‘If you pump in more energy to get more laser intensity, and it’s too strong, you can get a sudden drop in the laser performance.

‘It is counterintuitive and paradoxical to see that the lasing starts and its output power becomes higher and higher when more loss is introduced, that is, less pumping is used,’ Ozdemir said. ‘This turns the conventional textbook understanding of lasers upside down.’

The experimental system consisted of two tiny directly coupled silica microtoroid (doughnut-shaped) resonators, each coupled to a different fibre-taper coupler that aids in guiding light from a laser diode to photodetectors; the fibre is tapered in the middle so that light can pass between the fibres and the resonators. Loss is delivered to one of the microresonators by a tiny device, a chromium-coated silica nanotip. Yang said the concept will work in any coupled physical system.

‘When we steer the system through the exceptional point, the symmetric distribution of the fields between two resonators become asymmetric,’ Ozdemir said. ‘Asymmetric distribution leads to field localisation, increasing the light intensity in one of the resonators, in this case the resonator with less loss. As a result, all nonlinear processes, which depend on the intensity of light, in that subsystem become affected.’

Yang said that in addition to lasing improvements, their findings could lead to new schemes and techniques for controlling and reversing the effects of loss in various other physical systems, such as in photonic crystal activities, plasmonic structures and metamaterials.

‘The beauty of this work is in how we came to provide new schemes and techniques to engineer a physical system by controlling loss,’ Yang said. ‘Normally, loss is considered bad, but we actually take advantage of this and reverse the bad effect. We used the laser to show it.’

Further information:

School of Engineering and Applied Science, Washington University