Magnetically Controlled Nanolasers Protect Optical Signals

A lasing control mechanism, introduced by a research team at Aalto University, enables users to control plasmonic nanolasers remotely, using a magnetic field. Until now, the only way to switch plasmonic nanolasers on and off has been through direct manipulation.

The ability to control nanolasers with magnets supports more robust optical signaling through the emerging field of topological photonics, which aims to produce light signals that are immune to external disruptions.

Where most plasmonic lasers are based on noble metals such as gold or silver, which render the optical mode structure of the laser inert to external fields, the Aalto researchers used a periodic array of cobalt-platinum multilayered nanodots, patterned on a continuous layer of gold and insulating silicon dioxide.

Using this design, the researchers exploited the magnetic nature of the nanoparticles to control the lasing action and demonstrated active magnetic-field control over lasing in the periodic array. Analysis showed that the material used to build the nanolaser and the arrangement of the nanodots in periodic arrays were critical to achieving magnetic control.

The research demonstrated that magnetization can be used to externally control plasmonic nanolasers in a manner that complements their modulation by excitation, gain medium, or substrate. Nanolasers can be more power-efficient than traditional lasers, and have been used to advantage in many fields, including biophotonics, where they have increased the sensitivity of biosensors used in medical diagnostics.

By enabling robust signal processing, the findings of the Aalto team could also influence the field of topological photonics. Up to now, strong magnetic fields have been required to create topologically protected optical signals using magnetic materials. The current work showed that the effect of magnetism in the context of on-and-off lasing can be unexpectedly amplified using a nanoparticle array of a particular symmetry.

The researchers believe that their work could lead to new, nanoscale, topologically protected signals.

“The idea is that you can create specific optical modes that are topological, that have certain characteristics which allow them to be transported and protected against any disturbance,” professor Sebastiaan van Dijken said. “That means if there are defects in the device or if the material is rough, the light can just pass them by without being disturbed, because it is topologically protected.

“Normally, magnetic materials can cause a very minor change in the absorption and polarization of light. In these experiments, we produced very significant changes in the optical response — up to 20%. This has never been seen before,” he said.

A plasmonic laser is turned on (top) and off (bottom) by switching the magnetization of a nanodot array. The zoomed insets show the magnetic field around a single nanodot. Courtesy of Jenna Rantala.

Professor Päivi Törmä, who collaborated with van Dijken on the research, said the results could lead to the realization of topological photonic structures in which magnetization effects can be amplified by choosing the appropriate nanoparticle array geometry.

The research was published in Nature Photonics (www.doi.org/10.1038/s41566-021-00922-8).