Semiconductor nanowire lasers are promising ultracompact light sources for miniaturized optical processing and sensing, but their efficiency is limited by the difficulty of confining light in a structure much smaller than its wavelength. By using a silicon photonic crystal to trap light in a semiconductor nanowire, researchers at NTT Basic Research Laboratories in Japan have now turned the chip itself to their advantage. They have shown that a photonic crystal/nanowire hybrid can sustain telecom-band lasing stable enough to transmit a high-frequency data signal (APL Photonics 2 46106), and believe that the platform’s advantages for component integration could enable them to build an on-chip photonic network.
Light confinement, which is crucial to laser oscillation, becomes less effective when the nanowire diameter is smaller than half the light wavelength. Masato Takiguchi and colleagues needed lasing at infrared wavelengths (here 1342 nm), but wanted to keep the nanowire diameter less than a tenth of that for compact integration (around 100 nm). In most nanowire lasers to date, which have operated around the sub-micron visible wavelength range (400–700 nm), the confinement challenge has not been so extreme.
The researchers tackled the problem using the hybrid cavity they first presented in 2014 (Nat. Mater. 13 279), in which a single InAsP/InP nanowire is carefully placed in a groove on a silicon photonic crystal. Such photonic crystals contain periodic holes that slow down and trap light, guiding it into the nanowire and enabling lasing in the infrared. This is only possible because these longer wavelengths match the transparency window of silicon – which also explains why the 1260–1625 nm band is the mainstay of current silica optical fibre communications.
To achieve high-frequency data transmission, the NTT researchers needed to show that they could sustain stable continuous-wave lasing, which is crucial for subsequent modulation to represent binary information. When pumped with another laser in a pseudorandom bit sequence, tests showed that the team’s nanolaser responds fast enough to transmit 1s and 0s that could be distinguished at 10 billion bits per second, a typical fibre-optic communication speed.
Unlike the pulsed lasing demonstrated by the NTT research group earlier this year (ACS Photonics 4 355), continuous-wave lasing requires relentless dumping of power into the small nanowire volume, worsening the effects of problematic heating. To minimize this, they kept measurements at temperatures as low as 4 K. They also employed single-photon sensitive techniques from their earlier work, instead of conventional telecom signal detection, to combat low signal amplitude. Future work will improve laser gain and heat management by the photonic chip, with first author Takiguchi commenting to AIP News that they’ll “aim for room-temperature current-driven lasing as well”.
The photonic crystal platform offers exciting advantages for coupling other components to the nanolaser. Having proven its data-transmission capabilities, the next target is connecting the nanowire to input/output waveguides, en route to an on-chip photonic network. “We want to demonstrate that we’re able to integrate a number of photonic devices by having different functionalities on a single chip,” concludes Takiguchi.
