Physicists Take Big Step in Nanolaser Design

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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 (figure 1B). 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. Figure 1B clearly shows that even if a well-pronounced kink is seen in the output characteristic, the transition to the lasing regime 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. 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 (see figure 2). 

 
Figure 2. Nanolaser threshold current versus device temperature. The blue and green curves give a very good approximation of the exact value shown by the red line. Image courtesy of the researchers 

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.

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