Recently, nanolasers have evolved as a new category of light sources with a size of just a few millionths of a meter and with special properties strikingly different compared to those of macroscopic lasers.
Yet, it is nearly impossible to ascertain at what value of current the nanolaser’s output radiation turns coherent, while in the case of practical applications, it is crucial to differentiate between the two regimes of the nanolaser: the true lasing action that leads to a coherent output at high currents and the LED-like regime that leads to incoherent output at low currents. Scientists at the Moscow Institute of Physics and Technology devised a technique that enables finding under what conditions nanolasers qualify as true lasers. The study has been reported in Optics Express.
Lasers are extensively used in industry, household appliances, telecommunications, medicine, and more. Many years ago, a new type of lasers, known as nanolasers, was developed. Their design is analogous to that of the traditional semiconductor lasers based on heterostructures, which have been familiar for many decades. The difference is that the nanolasers’ cavities are extremely small, of the order of the wavelength of the light radiated by these light sources. As they primarily produce visible and infrared light, the size is of the order of one-millionth of 1 m.
Very soon, integrated optical circuits will be equipped with nanolasers, where they are necessary for the future generation of high-speed interconnects based on photonic waveguides, which would considerably enhance the performance of GPUs and CPUs by a number of orders of magnitude. In the same way, the emergence of fiber-optic Internet has improved connection speeds, apart from improving energy efficiency.
Undoubtedly, this is not the only possible application of nanolasers. Already, scientists are developing biological and chemical sensors, with a size of just millionths of a meter, and mechanical stress sensors with a size of several billionths of a meter. It is also anticipated that nanolasers would be used for controlling neuron activity in living organisms, including humans.
A radiation source can qualify as a laser only if it meets several requirements, of which the principal one is that it must emit coherent radiation. One of the unique properties of a laser, closely related to coherence, is the presence of a purported lasing threshold. When the pump currents are below this threshold value, the output radiation is usually spontaneous and its properties are not different from the output of traditional light emitting diodes (LEDs). However, upon reaching the threshold current, the radiation turns coherent. At this point, the emission spectrum of a traditional macroscopic laser gets narrowed down and there is a sudden increase in its output power. The latter property acts as a means to find the lasing threshold—that is, by analyzing how output power changes with a change in pump current.
The behavior of many nanolasers is the same as their traditional macroscopic counterparts; in other words, they display a threshold current. Yet, for certain devices, it is hard to pinpoint a lasing threshold by analyzing the output power versus pump current curve, as it does not have any special features and is merely a straight line on the log-log scale. Nanolasers such as these are called “thresholdless.” This leads to the question: At what value of current does their radiation turn coherent, or laserlike?
The apparent way to give a solution to this is through the measurement of the coherence. Yet, in contrast to the emission spectrum and output power, coherence is extremely difficult to measure for nanolasers as this necessitates the use of equipment with the ability to register intensity fluctuations at one-trillionth of a second—the timescale on which a nanolaser’s internal processes take place.
Andrey Vyshnevyy and Dmitry Fedyanin from the Moscow Institute of Physics and Technology have discovered a technique to do away with the technically difficult direct measurements of coherence. They devised an approach in which the main laser parameters are used to quantify the nanolaser radiation’s coherence. According to the researchers, their approach enables determination of the threshold current for any nanolaser. They discovered that even a “thresholdless” nanolaser actually has a unique threshold current differentiating the lasing and LED regimes. Below this threshold current, the emitted radiation is incoherent, and above that, it turns coherent.
Fascinatingly, it was found that a nanolaser’s threshold current was not related in any way to the narrowing of the emission spectrum or the output characteristic features, which are evident signs of the lasing threshold in macroscopic lasers. Even if a noticeable kink is observed in the output characteristic, the shift to the lasing regime takes place at higher currents. This is precisely 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, Moscow Institute of Physics and Technology
Very often, it was simply impossible to achieve coherent output due to self-heating of the nanolaser.
Andrey Vyshnevyy, Moscow Institute of Physics and Technology
Hence, it is very crucial to differentiate the illusive lasing threshold from the original one. Although the coherence measurements as well as the calculations are challenging, Vyshnevyy and Fedyanin devised a simple formula that can be applied to any nanolaser. Nanolaser engineers can now use this formula and the output characteristic to rapidly gauge the threshold current of the structures developed by them.
The outcomes reported by Vyshnevyy and Fedyanin allow estimating in advance the point at which a nanolaser’s radiation—irrespective of its design—turns coherent. This will enable engineers to definitively build nanoscale lasers with preset properties and assured coherence.
The Russian Science Foundation supported the study.