Defining True Lasers: When Nanolasers Meet Laser Criteria

• Coherent light generated by nanolasers is difficult to measure. 
• Researchers discover a method to carry out such challenging measurements.
• They develop a simple formula to figure out under what conditions nanolasers qualify as true lasers. 

In a recent couple of years, nanolasers have emerged as a new kind of light source that has a size of about a billionth of a meter. Its unique properties set it apart from macroscopic lasers.

The design of nanolasers is very much similar to that of traditional heterostructure-based semiconductor lasers. However, their cavities are extremely small: on the magnitude of light wavelength (infrared and visible light).

In the coming years, nanolasers will be used in integrated optical circuits to increase the performance of GPUs and CPUs, and boost internet connection speeds via fiber optics by multiple orders of magnitude. It may also be incorporated with techniques that are used to control neuron activity in living organisms.

Achieving Coherence In Nanolasers

There is one major problem with nanolasers: we don’t know at what point (current) its output radiation becomes coherent. For real-world applications, it’s crucial to differentiate between 2 phases of the nanolaser: the LED-like phase with incoherent output at low currents and the true lasing action with coherent output at high currents.

To qualify as a real laser, the radiation source must fulfill a few requirements. The most important one is it has to eject coherent radiation. Coherence is achieved above a point called the lasing threshold. Below this point, the source emits spontaneous radiation which is no different than outputs of traditional LEDs.

This lasing threshold can be determined by analyzing the relationship between pump current and output power (figure 1A). However, some equipment exhibit no special features (red line in figure 1b), which makes it very difficult to pinpoint the threshold value in the pump current versus output power curve. This type of nanolasers is called ‘thresholdless’.

New Formula For Measuring Coherence Directly

Coherent light generated by nanolasers is difficult to measure because it requires delicate instruments to capture intensity fluctuations at billionths of a millisecond.

Scientists at the Moscow Institute of Physics and Technology have discovered a method to carry out such difficult measurements. It involves quantifying the coherence of nanolaser radiation using the main laser parameters.

Reference: Optics Express | doi:10.1364/OE.26.033473 | MIPT

The method can be used to pinpoint the threshold current for almost all nanolasers, including ‘thresholdless’ nanolasers, which have a distinct threshold value isolating the lasing and LED phases. The radiation ejected by nanolasers is coherent above this threshold current and incoherent below it.

Previously, it was almost impossible to obtain coherent radiation due to self-heating nature of nanolasers. Thus, distinguishing the actual lasing threshold from the illusive one is quite crucial.

To do this, researchers developed a simple formula that can be universally employed to all nanolasers. Utilizing this formula along with input-output parameters (figure 2), physicists can quickly gauge the threshold value of any structure they build.

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This new study makes it easy to predict (in advance) at what pump current the nanolaser will yield coherent radiation irrespective of its design. This will help physicists design and build nanolasers with predetermined characteristics and guaranteed coherence.