The lasers used to light up the world's optical-communications networks are generally made from erbium-doped fibers or III–V semiconductors, since these can emit at the infrared wavelengths transmittable through optical fiber. At the same time, however, such materials are not so easy to integrate with conventional silicon electronics.
In new research, scientists in Spain say it might be possible to make infrared lasers that can be either coated along optical fibers or deposited directly on silicon as part of the CMOS manufacturing process. They have shown that colloidal quantum dots integrated within a specially designed optical cavity can generate laser light across the optical telecommunications window at room temperature (Nat. Photon., doi: 10.1038/s41566-021-00878-9).
Quantum dots’ infrared problem
Quantum dots are nanometer-sized pieces of semiconductor containing electrons that exist in well-defined energy levels similar to those in real atoms. They are often fabricated by heating colloids containing chemical precursors of the quantum-dot crystals, and have optoelectronic properties that can be tailored by varying their size and shape. To date, they have been incorporated into a wide range of devices, including photovoltaic cells, light-emitting diodes and photon detectors.
Colloidal quantum dots have also been used to make visible-light lasers, thanks to the high gain of cadmium selenide, one of the most commonly used quantum dot materials, at these wavelengths. However, demonstrating lasing in the infrared has proved trickier, given the limited gain of the material best suited to that task—lead sulfide.
In 2006, a group at the University of Toronto, Canada, demonstrated infrared lasing using lead-sulfide colloidal quantum dots but had to do so at cryogenic temperatures to avoid the Auger recombination of thermally excited electrons and holes. Last year, researchers in Nanjing, China, reported infrared lasing from dots made with silver selenide, but their resonator was fairly impractical and hard to tune.
Combining quantum dots with gratings
In the latest work, Gerasimos Konstantatos and colleagues at the Barcelona Institute of Science and Technology, Spain, rely on a so-called distributed-feedback cavity to achieve infrared lasing at room temperature. This employs a grating to confine a very narrow band of wavelengths and thereby produce a single lasing mode.
To make their gratings, the researchers etched patterns on a sapphire substrate using electron-beam lithography. They chose sapphire because its high thermal conductivity can carry away much of the heat generated by optical pumping—heat that would otherwise prompt recombination and also destabilize the laser’s output.
Konstantatos and colleagues then deposited a lead-sulfide quantum-dot colloid onto nine different gratings with varying spacings, ranging from 850 nm to 920 nm. They also employed three different-sized quantum dots, with diameters of 5.4 nm, 5.7 nm and 6.0 nm.
In room-temperature tests, the team showed that it could generate laser light across the telecommunications C-band, L-band and U-band—from 1553 nm to 1649 nm—achieving full widths at half maximum as low as 0.9 meV. They also found that they could lower the pumping intensity by around 40% as a result of n-doping the lead sulfide. This reduction, according to Konstantatos, paves the way to more practical, lower-power pump lasers and perhaps even electrical pumping.
Toward better lasing sources for communications
As for potential applications, Konstantatos says the quantum-dot scheme could lead to new CMOS-integrated lasing sources, enabling cheap, efficient and rapid communication within or between integrated circuits. It might also improve lidar, he adds, given that infrared lasers are considered harmless to human vision.
Before the laser can be put to use, however, the researchers must first optimize their material to demonstrate lasing with either continuous-wave or longer-pulse pumping sources. “The reason is to avoid the use of expensive and bulky sub-picosecond lasers,” says Konstantatos. “Nanosecond pulses or continuous-wave would allow us to use diode lasers, making it a more practical setting.”