Real-time multimode dynamics of terahertz quantum cascade lasers via intracavity self-detection: observation of self mode-locked population pulsations

Abstract

Mode-locking operation and multimode instabilities in Terahertz (THz) quantum cascade lasers (QCLs) have been intensively investigated during the last decade. These studies have unveiled a rich phenomenology, owing to the unique properties of these lasers, in particular their ultrafast gain medium. Thanks to this, in QCLs a modulation of the intracavity field intensity gives rise to a strong modulation of the population inversion, directly affecting the laser current. In this work we show that this property can be used to study in real-time the dynamics of multimode THz QCLs, using a self-detection technique combined with a 60GHz real-time oscilloscope. To demonstrate the potential of this technique we investigate a 4.2THz QCL operating in free-running, and observe a self-starting periodic modulation of the laser current, producing trains of regularly spaced, ∼100ps-long pulses. Depending on the drive current we find two distinct regimes of oscillation with dramatically different properties: a first regime at the fundamental cavity repetition rate, characterised by large amplitude and phase noise, with coherence times of a few tens of periods; a much more regular second-harmonic-comb regime, with typical coherence times of ∼105 oscillation periods. We interpret these measurements using a set of effective semiconductor Maxwell-Bloch equations that qualitatively reproduce the fundamental features of the laser dynamics, indicating that the observed carrier-density and optical pulses are in antiphase, and appear as a rather shallow modulation on top of a continuous wave background. Thanks to its simple implementation and versatility, the demonstrated broadband self-detection technique is a powerful tool for the study of ultrafast dynamics in THz QCLs.

Broadband Terahertz Quantum Cascade Laser Dual‐Comb Sources under Off‐Resonant Microwave Injection

“…The other microwave signal is the intermode beatnote signal generated by the beating of the terahertz modes. As reported in our recent paper, [39] indeed, the microwave transmission S 12 measurement shows that in a 15 mm-long single-plasmon terahertz QCL, the attenuation in the microwave frequency range from 0 to 40 GHz along the laser ridge is measured to be %2.6 dB mm À1 . This attenuation is attributed to the combined effect of the device conductance and free carrier absorption in the doped contact layers and metal contacts.…”

“…In this situation, the four‐wave mixing locking can be stronger and therefore, much better comb operation is obtained. Furthermore, in our recent work, [ 39 ] we experimentally show that a 15 mm‐long cavity terahertz QCL with a same active region is able to produce self‐started optical pulses and a basic solitonic character of the long cavity QCL comb is observed. Although we have not yet finely tuned the cavity length to study the length effect on the frequency comb and dual‐comb operation, the experimental results obtained from many devices with cavity lengths ranging from 2 to 15 mm show an empirical conclusion that the short cavity is not suitable for the QCL frequency comb operation for the given active region design.…”

Theory of Hybrid Microwave–Photonic Quantum Devices

“…While microstrips in the form of metal-metal waveguides have been identified as the most suitable type for hybrid QCL devices, [17] it has been pointed out that the microwave loss may still be a limiting factor. [29] Furthermore, it has been argued that effective index matching and moderate microwave loss are more difficult to realize for MIR than for THz QCL structures, [21,26] although beneficial effects of microstrip-like waveguides have also been observed for MIR devices. [30][31][32][33] Thus, there is an ongoing debate if the observed phenomena are really due to microwave-optical co-propagation or can be explained by conventional modulation effects, with the measured microwave beat-notes along the waveguide arising purely from local stimulation by the laser radiation.…”

THz frequency combs form dispersion-compensated antenna-coupled ring quantum cascade lasers

“…Quantum cascade laser frequency combs in free-running standard Fabry-Perot QCLs are indeed typically characterized by a quasi-continuous waveform combined with a linear 16,17 or more complex 18 frequency modulation in the mid-IR and THz domain respectively. The rea-son behind this behaviour is the extremely fast dynamics (∼ ps timescale) of the saturable gain combined with the spatial hole burning (SHB) arising from the presence of standing waves [19][20][21][22] and the presence of non linearities 23 . This issue can be overcome by exploiting travelling-wave resonators, such as ring cavities which can sustain whispering gallery modes exhibiting a well defined direction of propagation.…”