Micron-scale nanolasers have properties remarkably different from those of macroscopic lasers. However, it is almost impossible to determine the lasing threshold of the nanolaser. For practical applications, it is important to distinguish between the two regimes of the nanolaser: the true lasing action with a coherent output at high currents and the LED-like regime with incoherent output at low currents. Researchers from the Moscow Institute of Physics and Technology have developed a method that allows to find under what circumstances nanolasers qualify as true lasers.1
Nanolasers have the potential to be incorporated into integrated optical circuits, where they are required for the new generation of high-speed interconnects based on photonic waveguides. In addition, researchers are already developing laser-based micron-sized chemical and biological sensors and mechanical stress sensors as small as several nanometers. Nanolasers are also expected to be used for controlling neuron activity in living organisms, including humans. At a laser's lasing threshold, the emission spectrum of a conventional macroscopic laser narrows down and its output power spikes. The latter property provides for an easy way to determine the lasing threshold—namely, by investigating how output power varies with pump current.
Many nanolasers behave the way their conventional macroscopic counterparts do: they exhibit a threshold current. However, for some devices, a lasing threshold cannot be pinpointed by analyzing the output power versus pump current curve, since it has no special features and is just a straight line on the log-log scale. Such nanolasers are known as "thresholdless." This begs the question: At what current does their radiation become coherent? The obvious way to answer this is by measuring the coherence. However, unlike the emission spectrum and output power, coherence is very hard to measure in the case of nanolasers, since this requires equipment capable of registering intensity fluctuations at the picosecond time scale, which is the scale on which the internal processes in a nanolaser occur.
"Thresholdless" nanolasers actually have a threshold
Andrey Vyshnevyy and Dmitry Fedyanin from the Moscow Institute of Physics and Technology have found a way to bypass the technically challenging direct coherence measurements. They developed a method that uses the main laser parameters to quantify the coherence of nanolaser radiation. The researchers claim that their technique allows to determine the threshold current for any nanolaser. They found that even a thresholdless nanolaser does in fact have a distinct threshold current separating the LED and lasing regimes. Surprisingly, the threshold current of a nanolaser turned out to be not related in any way to the features of the output characteristic or the narrowing of the emission spectrum, which are telltale signs of the lasing threshold in macroscopic lasers. Even if a well-pronounced kink is seen in the output characteristic, the transition to the lasing regime in a nanolaser occurs at higher currents. This is what laser scientists could not expect from nanolasers.
"Our calculations show that in most papers on nanolasers, the lasing regime was not achieved," Fedyanin says. "Despite researches performing measurements above the kink in the output characteristic, the nanolaser emission was incoherent, since the actual lasing threshold was orders of magnitude above the kink value." "Very often, it was simply impossible to achieve coherent output due to self-heating of the nanolaser," adds Vyshnevyy. Therefore, it is highly important to distinguish the illusive lasing threshold from the actual one. While both the coherence measurements and the calculations are difficult, Vyshnevyy and Fedyanin came up with a simple formula that can be applied to any nanolaser. Using this formula and the output characteristic, nanolaser engineers can now rapidly gauge the threshold current of the structures they create.
The findings reported by Vyshnevyy and Fedyanin enable predicting in advance the point at which the radiation of a nanolaser, regardless of its design, becomes coherent. This will allow engineers to deterministically develop nanoscale lasers with predetermined properties and guaranteed coherence.
1. A.A. Vyshnevyy and D.Yu. Fedyanin, Optics Express (2018); https://doi.org/10.1364/OE.26.033473.