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. This changed with the introduction of semiconductor frequency combs, which use technology similar to what one 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 cannot be controlled by material growth to the necessary precision. We demonstrated that the concept of dispersion engineering—well-established in ultrafast optics—was also valuable for 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.
As our work progressed, this effort broadened into a larger program that we now call phase-active nonlinear optics: nonlinear optics whose dynamics are governed primarily by the optical phase rather than the amplitude. This area emerged from widespread adoption of our SWIFTS technique, which revealed that many different lasers naturally enter self–frequency-modulated (FM) regimes in which the instantaneous frequency sweeps linearly across the gain bandwidth before periodically resetting. These FM combs appear in quantum cascade lasers, quantum well lasers, quantum dot lasers, and many other semiconductor platforms.
To understand this unexpectedly universal behavior, we developed a general mean-field theory of active cavities and showed that FM combs constitute a previously unrecognized fundamental comb state—extendons—whose bandwidth arises from phase modulation rather than amplitude modulation. Building on this theoretical framework, we have created the first active ring-resonator combs pumped by an on-chip laser, broadened FM combs to cover nearly the full gain bandwidth using intracavity gain shaping, and recently identified a new phase of light that preserves equidistance without temporal stability. Together, these efforts form the basis for our ongoing program in phase-active nonlinear optics and chip-scale active-cavity combs.
