CC BY
20THz-I-2
Time and spectrally resolved gain dynamics in THz quantum cascade lasers
J. Darmo1, G. Derntl1, D. Theiner1, G. Scalari2, M. Beck2, J. Faist2, K. Unterrainer1 1Photonics Institute, TU Wien, Campus Gusshausstrasse, 1040 Vienna, Austria 2Institute of Quantum Electronics, ETH Zürich, Auguste-Piccard-Hof 1, Zurich, 8093, Switzerland
In this contribution we present our investigations of the gain recovery dynamics of a heterogeneous terahertz quantum cascade laser and their role in the stable operation of a broadband active region. We employ THz-pump/THz-probe time domain spectroscopy (TDS). The measurements reveal a strong coupling between cavity photon field and the carrier transport in the quantum cascades as well as between individual sections of the QCL active region.
A broadband terahertz (THz) gain medium is a basic component for future THz sources. Remarkable progress in this field was achieved using heterogeneous THz quantum cascade heterostructures (QCH), which stacks different quantum cascades. However, the coherent and stable operation of those devices depends crucially on the interaction between the individual stacks. The individual stacks can interact optically due to the overlapping spectral gain curves of the individual stacks and due to the strong feedback from the photon field to the electron transport through the heterostructure. Therefore, the gain profile and its dynamics in the heterogeneous quantum cascade laser (QCL) can differ significantly from a QCL with a standard design. The investigated heterogeneous THz QCH [1] is composed of three individual stacks centered at 2.3 THz, 2.6 THz and 2.9 THz, respectively. It was fabricated in a double metal waveguide cavity and biased above lasing threshold. We have measured gain recovery times between 34 ps and 50 ps, where the shorter times were measured for low temperature and high bias current [2]. Furthermore, our time domain technique allows also spectrally resolve the gain dynamics and thus to reveal the interactions between the individual sections of the heterogeneous QCH.
We observed that the gain of the low (transition at <2.3 THz) and the high frequency sections (transition at >2.7 THz) recover similarly, while the recovery of the central gain section (transition at 2.3-2.5 THz) exhibits completely different development in time within the first 24 ps after gain perturbation. For longer delay times the three gain sections show the same recovery time for low temperature and high bias current operation and is matching the global GRT values presented in [2]. The abnormal recovery behavior for small pump-probe delay times, we believe, is connected to a re-absorption or an absorption and re-emission process of photons shared by two stacks. The experimental results are compared with a rate equation model and we analyze the connection of the observed interactions to the stability of a heterogeneous THz QCL and discuss the results with respect to a comb formation.
[1] D. Turcinkova et al., Appl. Phys. Lett., 99, 191104, 2011.
[2] C. G. Derntl at al., Appl. Phys. Lett., 113, 181102, 2018.
