Among semiconductor devices, Quantum Cascade Lasers (QCLs) and Interband Cascade Lasers (ICLs) are very promising candidates for developing novel quantum technologies operating in the so far much less explored mid-infrared (MIR) and THz spectral ranges as compared to the well-known telecom wavelength range.
QCLs are unipolar, chip-scale current-driven semiconductor-heterostructure lasers, based on intersubband transitions in quantum wells engineerable for emitting in mid-to-far infrared spectral region. Furthermore, they can be engineered to work at room temperature, and due to the very short laser transition lifetime (τ < 1 ps), they can be modulated at high frequencies (GHz and above).
Currently, they are the best-performing lasers in the mid-infrared (> 4 μm) and terahertz wavelength region, in terms of reliability and emission power combined with tunability and wide spectral coverage for multimode devices. Recently, the possibility of engineering QCLs able to directly generate and emit high-power infrared frequency combs (QCL-combs) was proved, demonstrating that the phenomenon is triggered by a Four-Wave Mixing (FWM) non-linear process.
At the contrary, ICLs are quantum-engineered bipolar lasers, with a hybrid structure exploiting elements of QCLs and diode lasers. Transitions occur between interband levels with a type-II band alignment, leading to much longer lifetimes of the lasing levels (∼1 ns) compared to QCLs. Consequently, this leads to a higher optical emission efficiency, with much reduced threshold current densities (few hundreds of mA/cm2) and, in general, lower power consumptions (hundreds of mW under lasing operation, only). The interband emission allows ICLs to cover the 2.5 - 4 μm wavelength region as their sweetspot, thus filling the spectral gap between diode lasers and QCLs. Ring configurations with vertical emission have been recently designed and developed opening the doors to efficient source-receiver integrated devices. Moreover, ICLs have also demonstrated frequency comb operation, and recent theoretical studies point out the crucial role of FWM in ICL combs, too.
In recent years, our group activities regarding frequency comb characterization focused on the research of high spectral purity and controllability [Cappelli16] leading to the development of a new characterization technique named FACE [Cappelli19,Consolino19,DeRegis21]. This has opened new frontiers in technological applications such as trace-gas detection, high-resolution spectroscopy, and infrared frequency metrology [Consolino20,Corrias22]. In parallel, within our group, low intrinsic linewidths (10 kHz) [Borri20] have been measured for single-mode ICLs, which are important features for sensing and communication applications.
Furthermore, since 2018, our group has been part of the H2020 Project Qombs [Qombs]. Within this project, possible nonclassical correlations among the modes emitted by QCL combs have started to be investigated. In multimodal QCLs-combs the presence of a third-order parametric non-linear process (FWM), represents a new motivation for the investigation of the quantum properties of these sources. In principle, this process is capable of entangling different spectral modes of the comb. In particular, FWM is able to provide two-mode squeezing and bi-partite entangled states in polarisation, frequency, and quadratures, as demonstrated already in many photonic platforms such as in microring resonators. Recently, a mid-IR detection system able to unveil sub-shot noise signals has been realized [Gabbrielli21] and the presence of classical intensity correlations in harmonic combs emitted by MIR QCLs has been confirmed [Gabbrielli22]. The demonstration of non-classicality is our next target.
REFERENCES:
[Borri20] Borri, S., Siciliani de Cumis, M., Viciani, S., D’Amato, F., & De Natale, P. (2020). Unveiling quantum-limited operation of interband cascade lasers. APL Photonics, 5(3), 036101.
[Cappelli16] Cappelli, Francesco, Giulio Campo, Iacopo Galli, Giovanni Giusfredi, Saverio Bartalini, Davide Mazzotti, Pablo Cancio et al. "Frequency stability characterization of a quantum cascade laser frequency comb." Laser & Photonics Reviews 10, no. 4 (2016): 623-630.
[Cappelli19] Cappelli, Francesco, Luigi Consolino, Giulio Campo, Iacopo Galli, Davide Mazzotti, Annamaria Campa, Mario Siciliani de Cumis et al. "Retrieval of phase relation and emission profile of quantum cascade laser frequency combs." Nature Photonics 13, no. 8 (2019): 562-568.
[Consolino19] Consolino, L., Nafa, M., Cappelli, F. et al. Fully phase-stabilized quantum cascade laser frequency comb. Nat Commun 10, 2938 (2019). https://doi.org/10.1038/s41467-019-10913-7
[Consolino20] Consolino, Luigi, Malik Nafa, Michele De Regis, Francesco Cappelli, Katia Garrasi, Francesco P. Mezzapesa, Lianhe Li et al. "Quantum cascade laser based hybrid dual comb spectrometer." Communications Physics 3, no. 1 (2020): 1-9.
[Corrias22] Nicola Corrias, Tecla Gabbrielli, Paolo De Natale, Luigi Consolino, and Francesco Cappelli, "Analog FM free-space optical communication based on a mid-infrared quantum cascade laser frequency comb," Opt. Express 30, 10217-10228 (2022).
[DeRegis21] DeRegis, M., F. Cappelli, L. Consolino, P. De Natale, and R. Eramo. "Theoretical study of the Fourier-transform analysis of heterodyne comb-emission measurements." Physical Review A 104, no. 6 (2021): 063515.
[Gabbrielli21] Gabbrielli, T., Cappelli, F., Bruno, N., Corrias, N., Borri, S., De Natale, P., & Zavatta, A. (2021). Mid-infrared homodyne balanced detector for quantum light characterization. Optics Express, 29(10), 14536-14547.
[Gabbrielli22] Gabbrielli, T., Bruno, N., Corrias, N., Borri, S., Consolino, L., Bertrand, M., Shahmohammadi, M., Franckié, M., Beck, M., Faist, J., Zavatta, A., Cappelli, F. and De Natale, P. (2022), Intensity Correlations in Quantum Cascade Laser Harmonic Frequency Combs. Adv. Photonics Res., 3: 2200162. https://doi.org/10.1002/adpr.202200162
[Qombs] https://www.qombs-project.eu/index.php/Home . The Qombs Project started on October 1, 2018 and will end on July 31, 2022. The Qombs Project is funded by the European Union’s Horizon 2020 research and innovation programme under grant agreement number 820419.