Solid State & Optics Seminar: Dr. David Burghoff, University of Notre Dame, “Frequency combs in active cavities: broadband sources in the terahertz, mid-infrared, and beyond”

Frequency combs are light sources whose lines are perfectly evenly-spaced, and recent years have seen the development of a number of chip-scale comb sources. Historically, such sources have been based on either classical mode-locking or on the Kerr nonlinearity, but over the last few years, a new class of combs has been developed that instead rely on nonlinearities inherent in materials with gain. Combs based on active cavities require neither slow gain media nor ultra-low loss cavities, and this makes them particularly valuable for longer wavelengths, which lack compact broadband sources.
In this seminar, I will discuss some of our work on combs based on active cavities, with aspects ranging from fundamental physics to new applications. In the first part of the seminar, I will focus on combs based on quantum cascade lasers, active devices that offer unique opportunities in long-wavelength photonics. Dispersion- and bandstructure-engineering were combined to generate the first broadband terahertz frequency combs, which were used to perform the first terahertz dual-comb spectroscopy. I will also discuss how they can be engineered to generate as much as a half-octave of bandwidth and how they can be used to produce isolated pulses. The second part of the seminar will focus on the physics of active cavity combs, particularly on the recent discovery that frequency-modulated combs not only exist, but are ubiquitous. These combs were discovered through the widespread adoption of an experimental technique we developed, and they manifest in many laser systems spanning the electromagnetic spectrum. Recently, we showed that they represent a new fundamental comb state that can be regarded as an inverse of the classical soliton. Finally, I will discuss applications of these combs, emphasizing our recent discovery of frequency comb ptychoscopy. This is a new sensing technique that combines the resolution and speed of classical heterodyne spectroscopy with the bandwidth of comb spectroscopy, and we anticipate that this approach will allow combs to transform remote sensing in much the same way that dual-comb techniques have revolutionized local sensing.
Host: Hui Cao (hui.cao@yale.edu)