A frequency comb is a light source whose lines are evenly spaced. Their regularity and ability to make precise measurements have revolutionized many fields, and for that reason the Nobel Prize in Physics was awarded for their development. Until recently, frequency combs were primarily based on mode-locked lasers, and could not be used in applications requiring compactness. However, this changed with the introduction of semiconductor frequency combs, which use a technology similar to what you might find in a laser pointer.
Although it is possible to naturally form combs in semiconductor lasers, this mode of operation is difficult to produce reliably or predictably over a wide dynamic range, since the dispersion could not be controlled by material growth to the necessary precision. We demonstrated that the concept of dispersion engineering, which had previously been well-established in ultrafast optics, was also valuable for semiconductor quantum cascade lasers. In doing this, we were able to demonstrate the first laser-based terahertz quantum cascade laser combs. We also showed how the temporal profile of these combs could be directly measured, solving a problem that had been standing in the field for decades. This led to the discovery that many different combs can produce frequency-modulated (FM) states, which we showed was an emergent property of many lasers.