Quantum cascade frequency combs as key technology for quantum sensing and metrology

Frequency comb synthesizers (FC) are optical sources emitting a set of discrete, equally spaced and coherent optical modes in the frequency domain, sharing a fixed phase relation. Being inherently narrow linewidth, these light sources allow for absolute frequency measurements throughout the electromagnetic spectrum, and have proved an appealing technology in quantum science [1]. Spontaneous FC generation in the terahertz (THz) frequency range has been observed in quantum cascade lasers (QCLs), attracting considerable research effort. Unfortunately, the FC operation is self-starting only in regimes where the chromatic dispersion is sufficiently low to allow the cavity modes to be injection-locked by the modes generated by the intracavity four-wave-mixing process (FWM) [2]. Dispersion compensation scheme has then been adopted to reduce the dispersion, achieving FC operation over a broad bandwidth on 29%-55% of the laser operation range [3]. In contrast with this research track, it was recently shown [4] that comb spectra covering the entire available gain bandwidth or across the whole lasing regime do not necessarily arise from the absence of dispersion, and that the Kerr effect, together with the intracavity light intensity profile plays a dominant role in the formation of frequency-modulated combs [5]. In this work, we lithographically cover the back-facet of a 0.3 THz-bandwidth heterogeneous THz QCL FC with 7 layers of CVD graphene, reducing its reflectivity and therefore altering its light intensity intracavity profile. A proliferation of the comb modes over a 1.3 THz bandwidth, with 12 mW optical power, across 60% of the laser operational range is achieved, in particular over the entire negative differential resistance regime (NDR) where QCLs are usually intrinsically unstable due to high field domain formation. We then reconstruct the QCL FC intensity emission profile, instantaneous frequency and electric field, and assess its phase coherence by retrieving the modal phases for each mode. Finally, the demonstrated long-term stability of this FC, which rates the presented device as a metrological-grade source, opens up the possibility for scalable incorporation into integrated quantum networks at terahertz and millimeter wave frequencies.

The seminar is realized in the framework of the funded project "SPRINT ERC Consolidator Grant n.681379".


References:
[1] M.S. Vitiello and P. De Natale, “Terahertz Quantum Cascade Lasers as Enabling Quantum Technology”, Advanced Quantum Technologies, 2100082, (2022).
[2] Khurgin, J. B., Dikmelik, Y., Hugi, A. & Faist, J.,”Coherent frequency combs produced by self frequency modulation in quantum cascade lasers” Appl. Phys. Lett. 104, 081118 (2014).
[3] Mezzapesa, F. P., Garrasi K., Schmidt J., Salemi L., Pistore V., Li L., Davies A. G., Linfield E. H., Riesh M., Jirauschek C., Carey T., Torrisi F., FerrariA. C., Vitiello M. S. , “Terahertz Frequency Combs Exploiting an ON-chip Solution Processed Graphene Quantum Cascade Laser Coupled Cavity” ACS Photonics 7, 3489–3498 (2020).
[4] Beiser, M., Opačak, N., Hillbrand, J., Strasser, G. & Schwarz, B., “Engineering the spectral bandwidth of quantum cascade laser frequency combs” Opt. Lett. 46, 3416–3419 (2021).
[5] Burghoff, D. , “Unraveling the origin of frequency modulated combs using active cavity mean-field theory” Optica 7, 1781–1787 (2020).