We demonstrate tunable GTI-enhanced dispersion compensation in a 500-nm-thick active region QCL Fabry–Pérot cavity, mitigating dispersion and stabilizing frequency-comb operation, enabling higher-temperature and energy-efficient mid-IR combs suited for compact sensing applications.

Broadband light sources with equidistant modes are critical for spectroscopy, imaging, and communications. Semiconductor lasers engineered for broadband coherent multimode operation frequently exhibit states that appear to have low coherence. These states, typically identified by a broad RF spectrum with a full width at half maximum in the MHz range, have therefore received substantially less attention. Here, for the first time, we demonstrate that these states, despite lacking long-term stability, exhibit high mutual coherence and preserve mode equidistance. We refer to these states as liquid combs. We introduce a frequency-resolved characterization technique that shows that the temporal phase differences between the modes of a liquid comb vary identically. Because liquid combs often exhibit broader bandwidths than conventional frequency combs, they are promising for spectroscopic applications and may enable more flexible broadband source designs.

Quantum cascade lasers (QCLs) are unipolar semiconductor lasers first demonstrated in 1994. Since then, they have played a central role in advancing mid-infrared and terahertz photonics, becoming among the most reliable light sources in these regions of the electromagnetic spectrum. Their importance is further reinforced by their ability to generate self-starting optical frequency combs, whose investigation is motivated both by fundamental physics and by a wide range of applications, including molecular spectroscopy and free-space optical communications. This Roadmap provides a unified overview of current advances and emerging directions in QCL research. The chapters are organized into three main sections: device design and technology; frequency combs and pulse formation; and applications of QCLs. Each chapter reviews the relevant background, summarizes the current state of the art, and identifies key challenges and future directions within its specific research area.

The gain-loss coupling in optical cavities induces exceptional points (EPs), where two optical modes coalesce. The large modal overlap near an EP intensifies gain competition, favoring single-mode lasing. Recent studies further revealed self-modulation closer to the EP that transforms the lasing mode into a frequency comb. Such EP-enabled comb formation suggests a previously unaccounted-for mechanism that overcomes the strong gain competition and drives a second mode to threshold. Here, using a Bloch coupled-mode theory derived from first principles, we show that the second threshold arises from dynamical couplings among the population inversion, the lasing field, and a dark cavity mode. The lasing-inversion coupling produces extra EPs, whose spectral structure governs switching among single-mode lasing and frequency combs with different repetition rates. This above-threshold mode-switching mechanism enables new opportunities for tunable photonic systems, including adaptive optical communication links and dual-comb spectroscopy.

Engineered intersubband structures like quantum cascade lasers are ideal for mid-infrared and terahertz applications. While the theoretical intersubband gain and absorption predictions are well-developed, performance often deviates due to reasons such as variations in molecular beam epitaxy (MBE) machines, requiring experimental investigation. Existing methods either produce inaccurate gain profiles or are limited in their ability to assess dependence on frequency, bias, and temperature. In this work, we present a delay-resolved absolute-referencing method for accurate gain evaluation without these limitations. Our findings show that gain profiles match theoretical predictions at low-temperature biases below the threshold, with gain and dispersion clamping after lasing. A faster-than-expected degradation in gain performance was also observed at higher temperatures. These results provide a precise experimental evaluation of temperature-dependent gain performance, including high-temperature conditions that differ from theoretical predictions, and offer insights for optimizing high-temperature performance and future quantum cascade laser-based frequency comb design.

We show that the comb bandwidth scales linearly with the gain bandwidth for frequency-modulated combs in fast-gain media. Furthermore, we demonstrate that loss shaping can increase the effective gain bandwidth and generate significantly broader (~40%) combs.

Mid-infrared (mid-IR) fiber sensors offer highly specific and sensitive detection and analysis of various chemical species due to many molecular vibrations and fundamental absorption bands in this range. In this paper, we present a compact transflection-based mid-IR fiber probe, designed to be potentially suitable for in vivo sensing, with an optical pathlength that can be controlled during fabrication. The optical fiber probe was fabricated using a silver halide polycrystalline fiber positioned in front of a gold-coated short fiber to act as a mirror, and a connector to hold the two parts facing each other at a predetermined distance. The optical fiber probe was tested with a quantum cascade laser (QCL) in the mid-IR region. To investigate the impact of optical pathlength, the reflected signal was recorded at various distances between the cleaved fiber and the mirror in air and water. The influence of gold layer thickness was also examined to optimize the optical fiber probe's performance. To validate its sensing capability, the probe was employed to detect glucose solutions at physiological concentrations, achieving a detection limit of 8.91 mmol/L. The results highlight the potential of the proposed optical fiber probe sensor for molecular detection and analysis, offering a promising platform for in situ monitoring for chemical and biomedical applications.

Dispersion engineering is critical for the creation of integrated broadband laser frequency combs. In the long wavelength infrared range (LWIR, 8-13 µm), frequency combs based on quantum cascade lasers are attractive since they are monolithic, fundamental oscillators with high power levels and efficiencies. One effective approach for expanding quantum cascade laser gain bandwidth is by stacking multiple gain media with different center lasing frequencies, as this leads to flatter broadband gain spectra. However, as the gain bandwidth is increased, dispersion becomes the main limiting factor for comb bandwidth. Therefore, achieving broadband combs requires schemes that can flexibly engineer the dispersion over broad bandwidths. Here, we demonstrate the ultimate nanophotonic dispersion compensation scheme: an air-dielectric slab double-chirped mirror, which we fully integrate with the quantum cascade laser gain section. This scheme relies on the highest possible index contrast and therefore provides the maximum correction per unit length over a very broad bandwidth. With this approach, we report the successful demonstration of a broadband room-temperature LWIR laser frequency comb on a gain medium that normally does not form combs without deliberate dispersion compensations. Our air-dielectric mirrors are versatile and can be extended to other integrated laser frequency combs in different material platforms and frequency bands. Using an on-chip dispersion measurement platform, we designed an air–dielectric slab double-chirped mirror integrated into a quantum-cascade laser, achieving room-temperature LWIR combs spanning >100 cm−1 at 9.6 µm.

Quantum cascade laser (QCL) frequency combs are promising for ultra-compact, broadband mid-infrared sources used in spectroscopy, sensing, and communication. A smaller active region not only improves beam quality and spectral stability but also improves heat dissipation, enabling higher continuous-wave operating temperatures, enhanced frequency comb coherence, and compact integration. However, a thin active region could introduce stronger normal dispersion, increasing the likelihood of single-mode operation in a Fabry-Perot cavity rather than the formation of a broadband frequency comb. Balancing thermal resistance, dispersion control, and modal competition remains challenging. Here, we introduce Gires-Tournois interferometer (GTI) dispersion compensation(1) in a QCL Fabry-Perot cavity with a 500 nm-thick active region. A GTI mirror on the rear facet stabilizes the comb regime by mitigating dispersion-related limitations. This approach establishes a foundation for the development of high-temperature, broadband, energy-efficient, continuous wave-operating mid-IR QCL combs, paving the way for broader spectroscopic applications

Integrated frequency combs in active cavities are appealing for a broad spectrum of applications. A powerful framework for describing these cavities is mean‐field theory, which captures the averaged effect of internal dynamics over a round trip. Lasers based on media with slow gain dynamics can be described by solving the population over many round trips, while lasers based on fast gain media can be described by adiabatic elimination. However, most gain media actually have both fast and slow components, and effects often ascribed to fast gain media can also arise in slower gain media. An operator‐based mean‐field theory is developed that non‐adiabatically describes the dynamics of bidirectional active cavities, both fast and slow. This first‐principles formalism provides a semi‐exact replacement for the Maxwell–Bloch equations and accommodates non‐trivial gain lineshapes and population dynamics. As an example, this formalism is used to establish an additional constraint on the formation of frequency‐modulated combs. These results are broadly applicable to bidirectional and unidirectional active cavities alike (including both Fabry–Pérot and ring cavities), and they extend naturally to nearly any chip‐scale laser system.

Photonic integrated circuits incorporating intersubband transitions are ideal for mid-infrared and terahertz nanophotonics. However, the design of epitaxies has long been inhibited by two factors: the modest predictivity of ab initio theory and the absence of absolute intersubband gain and loss measurements under operating conditions. Existing measurements either yield inaccurate gain profiles or do not accurately assess dependence on frequency, bias, and temperature. Here, we present a delay-resolved absolute-referencing method for accurate gain evaluation without these limitations, addressing a long-standing challenge. By creating a photonic circuit that allows broadband pulses to traverse different lengths of a gain medium, we measure the absolute transmission of intersubband structures. Gain profiles match theoretical predictions at lower temperatures, with gain and dispersion clamping after lasing, and faster-than-expected degradation occurs at higher temperatures. Our approach provides a precise experimental evaluation of temperature-dependent gain performance and gives insight into optimizing temperature performance and frequency comb designs.

We present a delay-resolved spectroscopy method enabling accurate absolute gain measurements of terahertz QCLs, showing theoretical alignment below threshold at low temperatures, clamping post-lasing, and faster-than-expected gain degradation at higher temperatures. © 2024 The Author(s)

Broadband light sources with equidistant modes represent a key capability in advancing the fields of spectroscopy, imaging, and communications. Semiconductor lasers designed for broadband coherent multimode operation often exhibit states that appear to have low coherence. These states, identified by a broad RF spectrum with a full width at half maximum typically in the MHz range, have thus received considerably less attention. Here, for the first time, we show that these states, despite lacking long-term stability, possess high mutual coherence and maintain equidistance. We refer to these states as liquid combs. We develop a frequency-resolved characterization technique that proves that the temporal phase differences between the modes of a liquid comb vary identically. Since liquid combs often possess a wider bandwidth than typical frequency combs, they are attractive for spectroscopic applications and may enable more flexible designs of broadband sources.

Topological phases of matter commonly feature protected states at their boundaries. Transferring this protection to time-metamaterials is extremely challenging, as it requires the generation of an abrupt interface between two topologically distinct bulks. Here, we realize and measure an ultrafast topological non-Hermitian skin mode bound to an interface circulating within the cavity of a fast-gain semiconductor laser. The nonlinear stationary state generated in such devices features a jump in the instantaneous frequency. We show that this discontinuity gives rise to a topological interface for the field fluctuations in the system. Using direct intensity sampling, we experimentally measure the skin modes and their positioning at the frequency jump of the stationary state. Analysis of these isolated modes reveals an ultrashort full-width at half-maximum of 583 ± 16 fs. Furthermore, we show that we can tune the shape and relative timing shift of the skin modes via external bias modulation. Finally, both numerical and experimental analysis of the noise in the system reveal that field fluctuations are funneled into the topological interface. Our findings reveal a new way to generate topologically protected states of light in time, which paves the way for novel time-varying physics as well as metrological applications.

Bandpass filtering techniques are widely used in spectroscopy. However, conventional symmetric-padding filtering methods introduce boundary artifacts that distort the signal at the edges. We present a rubber band filter: a robust method for achieving band-limited filtering without these detrimental edge artifacts. The technique applies an optimal padding scheme during the filtering process, thereby overcoming longstanding challenges in achieving artifact-free filtering. Importantly, it is iterative and requires only a few extra Fourier transforms over conventional approaches. We demonstrate its superiority and versatility by applying it to three spectroscopic examples-time-domain spectroscopy, Fourier-transform spectroscopy, and dual-comb spectroscopy-as well as a two-dimensional imaging example.

Topological bound states offer robust and defect-immune confinement of energy, enabling precise control over light waves even in the presence of imperfections or disorder in the system. Initially studied only in Hermitian systems, Topological states were recently explored in non-Hermitian platforms [1], which through gain and loss engineering allow for robust states, topological skin-effect [2] and even topological lasing. Interestingly, such states result from models in space, while suggestions to study topology in time varying frames [3] are experimentally challenging, resulting only in artificial lattices mimicking the time dimension [4].

Mid-infrared (mid-IR) optical fiber sensors offer highly specific and sensitive detection and analysis of various chemical species due to many molecular vibrations and fundamental absorption bands in this range. In this paper, we present a novel optical fiber probe design allowing for controlled optical pathlength. The optical fiber probe was fabricated using a silver halide polycrystalline fiber, a gold-coated short fiber as a mirror, and a connector to align the two parts to face each other. The outer diameter of the connector, 1.59 mm, dictates the overall probe diameter. To demonstrate the sensing performance, a quantum cascade laser (QCL) was coupled to the optical fiber probe to measure glucose solutions at physiological concentration levels by monitoring the C-O stretching vibration at 1,035 cm(-1). A detection limit of 8.91 mM for glucose was achieved. The results highlight the potential of the proposed optical fiber probe for molecular detection and analysis, offering a promising solution for chemical and biomedical applications.

Integrated frequency combs based on active cavities are of interest for sensing, but they are typically described very differently depending on the gain dynamics. I will discuss our recent demonstration of an operator-based mean-field theory that non-adiabatically describes the dynamics of bidirectional active cavities, both fast and slow. It is based on first principles and semi-exactly replaces the Maxwell-Bloch equations, but is flexible enough to accomodate non-trivial lineshapes and population dynamics. Our results are general and apply to any bidirectional or unidirectional active cavity, and as a result, generalize to essentially any chip-scale laser.

We demonstrate a silicon-free germanium-on-zinc selenide (GOZ) platform for integrated longwave infrared photonics, achieving transparency from 2 µm to 14 µm and optical losses of just 1 cm−1 (at 7.8 µm).