Analyzing the Lasing Threshold of Nanolasers

Nanolasers have recently emerged as a new class of light sources that have a size of only a few micro­meter and unique properties remarkably different from those of macro­scopic lasers. However, it is almost impossible to determine at what current the output radiation of the nanolaser becomes coherent, while 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 developed a method that allows to find under what circumstances nanolasers qualify as true lasers.

Illustration of a systematic study of the second-order coherence properties of metal-semiconductor nanolasers at both low- and high-pump rates. (Source: MIPT)

The design of nanolasers is similar to that of the conventional semi­conductor lasers based on hetero­structures, which have been known for several decades. The difference is that the cavities of nanolasers are exceedingly small, on the order of the wavelength of the light emitted by these light sources. Since they mostly generate visible and infrared light, the size is on the order of one millionth of a meter. In the near future, nanolasers will be incor­porated into integrated optical circuits, where they are required for the new generation of high-speed inter­connects based on photonic waveguides, which would boost the per­formance of CPUs and GPUs by several orders of magnitude. In a similar way, the advent of fiber optic internet has enhanced connection speeds, while also boosting energy effi­ciency.

And this is by far not the only possible appli­cation of nanolasers. Researchers are already developing chemical and biological sensors, mere millionths of a meter large, and mechanical stress sensors as tiny as several billionths of a meter. Nanolasers are also expected to be used for control­ling neuron activity in living organisms, including humans. For a radiation source to qualify as a laser, it needs to fulfill a number of require­ments, the main one being that it has to emit coherent radiation. One of the distinctive properties of a laser, which is closely associated with coherence, is the presence of a lasing threshold. At pump currents below this threshold value, the output radiation is mostly spon­taneous and it is no different in its properties from the output of conventional light emitting diodes (LEDs). But once the threshold current is reached, the radiation becomes coherent. 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, that is, 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, or laserlike? 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 trillionths of a second.

Andrey Vyshnevyy and Dmitry Fedyanin 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. The emitted radiation is incoherent below this threshold current and coherent above it. 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.

“Our calcu­lations show that in most papers on nanolasers, the lasing regime was not achieved. 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,” Dmitry Fedyanin says. “Very often, it was simply impossible to achieve coherent output due to self-heating of the nanolaser,” Andrey Vyshnevyy adds. 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. (Source: MIPT)

Reference: A. A. Vyshnevyy & D. Y. Fedyanin: Lasing threshold of thresholdless and non-thresholdless metal-semiconductor nanolasers, Opt. Exp. 26, 33473 (2018); DOI: 10.1364/OE.26.033473

Link: Laboratory of Nanooptics and Plasmonics, Moscow Institute of Physics and Technology, Dolgoprudny, Russia

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