Ultrafast, Tunable Nanoparticle-Array Laser Could Speed Optical Switching

Scientists combined organic dye material with metallic nanostructures (gold nanoparticles) to demonstrate ultrafast lasing with short, rapidly appearing laser pulses. Such ultrafast modulation speeds could be utilized in sensors and optical switches.

The organic dye nanoparticle-array laser demonstrated more than a 100-GHz modulation bandwidth. Researchers showed that the lasing modulation speed could be tuned by the array parameters.

For its experiments, the team from Aalto University placed gold nanoparticles on glass and immersed the samples in an organic, light-emitting material. The nanoparticles were arranged very close to each other in a square array. Researchers said that the arrangement of the nanoparticles (in an array) improved the directionality of the nanolaser.

Nanoparticle-array laser generating laser pulses one-trillionth of a second long (picosecond). Courtesy of Konstantinos Daskalakis.

Electric fields localized around the gold nanoparticles, causing high field strengths that accelerated the molecular dynamics in the organic dye. The electromagnetic fields and the conducting gold nanoparticles interacted with each other and with the organic dye, generating a directional laser pulse that, according to researchers, was one-trillionth of a second long (picosecond). Accelerated dynamics were observed for plasmonic lasing modes at the blue side of the dye emission.

The laser light was squeezed by the gold nanoparticles into subwavelength dimensions; then it escaped from the surface lattice resonance modes as a picosecond-fast, concentrated laser pulse.

The pulse generated from the nanoparticle-array laser was so fast that no conventional electronic cameras could capture its dynamics. Researchers used another laser as a “camera,” taking very fast pictures of the tiny laser — a method known as pump-probe spectroscopy.

Because of the tremendous speed, measuring the properties of the pulses was a significant challenge.

“The key achievement here is that we have succeeded in experimentally demonstrating that the laser pulses are indeed ultrafast," said professor Päivi Törmä. "The lasing occurs in optical modes that are hybrids of light and the motion of electrons in metal. These modes are called surface lattice resonances.”

A laser pulse of this kind could potentially be used to improve the speed of optical telecommunications and the performance of devices that use light to process information, such as cameras and transistors. Very small nanolasers have been created in several laboratories around the world. But according to researchers, the ultrafast potential of these nanolasers has not been demonstrated until the Aalto team’s experiments.

“We wanted to find out how fast we can turn our laser device on and off," said researcher Konstantinos Daskalakis. "Generating laser pulses quickly can be very useful in information processing and can improve the response of some optoelectronic devices.”

The research was published in Nano Letters (doi:10.1021/acs.nanolett.8b00531).