New loss mitigation technique paves the way for perfect optical resonators

Optical resonators are used in everything from familiar laser pointers to cutting-edge photonic quantum computers. But they all suffer from losses that degrade their performance. Researchers at Aalto University have now developed a way to prevent those losses, boosting the resonators’ performance.

The role of an optical resonator is to trap and concentrate photons for the longest time possible. Their main losses are due to light escaping the system by propagation away from the resonator (radiation loss) or by being absorbed into the material and converted into heat (absorption loss).

Aalto University Academy Research Fellow Radoslaw Kolkowski and Senior Lecturer Andriy Shevchenko have shown how to cancel out both the radiation and absorption losses.

‘By mitigating both the radiation and absorption losses, we can theoretically trap photons in a closed system indefinitely, despite using an absorbing material to build the system,’ Kolkowski says. ‘Boosting the resonance quality factors allows us to greatly enhance the interaction of light with matter, which can be used in a multitude of applications, for example, in laser technology, spectroscopy, metrology, and nonlinear optics.’

In a paper published in August in the journal Nanophotonics, Kolkowski simulated a metasurface made of an array of loss-prone gold nanoparticles and a planar waveguide supporting two guided modes. There, he was able to create a hybrid resonance in which there was neither radiation nor absorption loss. The demonstrated loss cancellation mechanism is universal and has the potential to radically improve all kinds of resonators beyond the realm of optics.

‘Think of any oscillators: pendulums, acoustic or seismic vibrations, quantum excitations. This approach can be applied to any of them, which can lead to a variety of useful applications and creation of new devices,’ says Shevchenko, who leads the Optics and Photonics research group.

While the theory is airtight, there is still room for inefficiencies in real-world use. This could be due to fabrication imperfections and finite size of absorbing resonant structures. Even still, the quality improvement of photonic systems made evident in the research carves the path forward for making superior devices with untold new functionalities.