Frequency-Modulated (FM) combs based on active cavities, such as quantum cascade lasers, have recently shown potential as light sources across various spectral regions. Unlike passive mode-locking, which generates amplitude modulation from the field’s amplitude, FM comb formation relies on phase modulation from the field’s phase, essentially acting as a phase-domain version of passive mode-locking. While the fundamental scaling laws of passive mode-locking have been well-established since Haus’s 1975 work showing that the bandwidth of pulses mode-locked by a fast saturable absorber is proportional to the effective gain bandwidth, the limits of FM combs have been less clear. This talk will discuss our recent findings, demonstrating that FM combs based on fast gain media adhere to the same fundamental limits, resulting in combs with bandwidths linear in the effective gain bandwidth. Theoretically, I will show that the diffusive effect of gain curvature constrains comb bandwidth and experimentally how this limit can be increased, particularly focusing on terahertz quantum cascade lasers.
Integrated optical frequency combs based on active cavities like Quantum Cascade Lasers (QCLs) have emerged as promising light sources in the mid-infrared and terahertz (THz) spectral regions. Their bandwidths are limited by two fundamental parameters: dispersion, which originates from variation in the group velocity, and diffusion, which originates from variation in the gain. However, while dispersion has been extensively engineered, diffusion shaping has been elusive. In this work, we show that the addition of carefully engineered anti-diffusive loss can enhance the bandwidth of QCL combs. We demonstrate theoretically and experimentally that adding resonant loss to the cavity of a THz QCL can counteract the diffusive effect of the gain and allow broader bandwidth combs to form, fully exploiting the bandwidth and dynamic range of the gain medium. Our results give a new degree of freedom for creating active chip-scale combs and can be applied to a wide array of cavity geometries and comb systems.
Terahertz Quantum Cascade Lasers (QCLs) are crucial for advancing research due to their high-power output, compact size, and efficiency. These lasers, designed through precise intersubband structure engineering, often show experimental outcomes that diverge from theoretical prediction. This discrepancy highlights the need for experimental gain characterization of QCLs. Using terahertz time-domain spectroscopy and a uniquely structured QCL with two beam paths of identical curvature but different lengths, self-referenced gain measurements were conducted across various temperatures and biasing voltages. A dispersion correction technique was used to distinguish signals from the two paths. The gain profile was extracted by analyzing the spectra of pulses through these different paths, providing an accurate gain profile by negating the zero-bias loss profile’s impact. At 23 K, an absorption peak was identified at 2.37 THz under low bias, shifting to lower frequencies with increased bias, aligning with density matrix simulations. Beyond the lasing threshold, the peak gain at 3.2 THz was consistently around 0 cm−1 for all bias levels. These findings underscore the self-referenced method’s significance in extracting absolute gain and dispersion information, enhancing device performance understanding.
Optical frequency combs based on quantum cascade lasers (QCLs) are promising broadband light sources in the mid-infrared and terahertz spectral regions. Their bandwidths are limited by two main parameters: dispersion, which originates from variation in the group velocity, and diffusion, which originates from variation in the gain. While dispersion has been extensively engineered, diffusion shaping has been elusive. We show that the addition of carefully engineered diffusive loss can enhance the bandwidth of QCL combs, demonstrating theoretically and experimentally that adding resonant loss to the cavity of a terahertz QCL can counteract the diffusive effect of the gain medium and allow broader bandwidth combs to form. Our results give a new degree of freedom for the creation of active chip-scale combs and can be applied to a wide array of cavity geometries and comb systems.
Quantum Cascade Lasers (QCLs) have immense potential for generating chip-scale frequency combs in the mid-infrared and terahertz spectral regions. In this work, we demonstrate the formation of frequency combs within ring terahertz QCLs using optical injection from a Distributed Feedback (DFB) laser. By carefully selecting a DFB design frequency that aligns with the ring cavity modes (around 3.3 THz) and employing a bus waveguide for light injection, we show that combs can be selectively formed and controlled within the ring cavity. Numerical modeling suggests that the observed comb formation is frequency-modulated in nature, with the optical injection acting as a trigger. Furthermore, we demonstrate the ring cavity's ability to function as a filter, a feature that could hold significant value for terahertz photonic integrated circuits. Our findings highlight the promise of waveguide couplers as a robust approach for injecting and extracting radiation from ring terahertz comb and offer exciting possibilities for generation of new comb states in the terahertz domain, including frequency-modulated waves, solitons, and more.
Topological crystalline insulators—topological insulators whose properties are guaranteed by crystalline symmetry—
can potentially provide a promising platform for terahertz optoelectronic devices, as their properties can be tuned
on demand when layered in heterostructures. We perform the first optical-pump terahertz-probe spectroscopy
of topological crystalline insulators, using them to study the dynamics of Pb1−xSnxSe as a function of temperature. At low temperatures, excitation of Dirac fermions leads to an increase in terahertz transmission; from this negative photoconductivity, the intrasubband relaxation rate of 6 ps is extracted. At high temperatures where only massive fermions exist, the free-carrier losses induced by the pump reduce the terahertz transmission for the duration of the 27 ps interband lifetime. Both effects are present at temperatures near the topological-to-trivial transition. Our experimental observations provide critical details for potential applications of Pb1−xSnxSe and provide a direct measurement of the topological character of Pb1−xSnxSe heterostructures.
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