The longwave infrared and terahertz bands present unique obstacles for the generation of compact broadband frequency combs. I will discuss some of our work that aims to address this grand challenge, focusing on two types of combs. First, I will discuss our work on quantum cascade laser-based frequency combs, light sources that are able to directly generate broadband combs through an active cavity nonlinearity. In particular, I will discuss how our work on terahertz combs led to the discovery of frequency-modulated combs, a fundamental comb state that can manifest in any laser at any wavelength. This mode of operation is well-suited for efficient and broadband comb generation in semiconductor lasers. Following this, I will discuss some of our recent work on low-loss passive photonic platforms in the longwave infrared based on hybrid photonic integration. This approach allowed us to create optical microresonators in the longwave infrared with quality factors two orders of magnitude better than the state-of-the-art, offering a promising direction for the production of broadband microresonator-based solitons.

Inverse design is used to spectrally shape Kerr microcombs via dispersion

optimization. We experimentally demonstrate flexible ‘meta’ dispersion control using

selective multimode hybridization in photonic crystal ring resonators, and present initial

comb shaping results.

Frequency combs (FC) generated by quantum cascade lasers (QCLs) are a promising tool for precision spectroscopy and gas sensing. Recently, ring QCLs have emerged as a new platform for generating FC with unique advantages over Fabry-Perot geometry. While the bandwidth of such Fabry-Perot devices is determined by the device geometry and dispersion, radio-frequency injected devices with circular geometry enable the exploitation of the full gain bandwidth in a controlled manner. Together with this platform, a predictive analytical model that shows excellent agreement with the experimental data was developed. Our results pave the way for a new approach for frequency comb generation based on fast-gain saturation.

Locking multiple modes into a frequency comb is key for multiple metrological applications, and a great effort has been therefore invested in this challenge over the last decade. The most common techniques are based on either nonlinearities or modulation of the cavity, while the latter is considered the more controllable method to produce frequency combs. The modulation couples cavity modes and creates a lattice in a synthetic dimension with coherent walk dynamics, but typically these dynamics are overthrown by the dissipative processes, leading to a spectrum that is narrow relatively to the full frequency ladder potential. Here we propose and demonstrate that by using fast-gain we preserve the full potential of the coherent walk and lock the frequency comb at its maximum possible bandwidth. Moreover, we find in our system a unique regime of dissipative fast-gain Bloch oscillations. We demonstrate these dynamics in RF-modulated quantum cascade laser ring devices.

We present a dual-comb spectrometer based on quantum cascade lasers operating at 7.7 µm with a stabilization scheme that enables coherent averaging. We show that by illuminating a low cost near-infrared light source of the front facet of the quantum cascade laser, we can tightly lock one comb line of the dual-comb spectrum, resulting in narrow linewidth with sub-radian integrated phase noise for all RF comb lines.