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arXiv 2103.13102physics.optics
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太赫兹量子级联激光器腔内自探测实时多模动力学:自锁模粒子数脉动观测

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

H. Li, W. Wan, Z. Li, J. C. Cao, S. Lepillet, J-F. Lampin, K. Froberger, L. Columbo, M. Brambilla, S. Barbieri

摘要:

太赫兹(THz)量子级联激光器(QCL)的锁模操作和多模不稳定性在过去十年中得到了广泛的研究。由于这些激光器的独特性质,特别是它们的超快增益介质,这些研究揭示了丰富的现象学。由于这一点,在QCL中,腔内场强度的调制引起粒子数反转的强烈调制,直接影响激光电流。在这项工作中,我们表明,这种属性可以用来研究多模太赫兹QCL的实时动态,使用自检测技术结合60 GHz的实时示波器。为了证明这种技术的潜力,我们研究了自由运行的4.2THz QCL,并观察到激光电流的自启动周期性调制,产生规则间隔的,~ 100 ps长的脉冲序列。取决于驱动电流,我们发现两个制度的振荡具有显着不同的属性:第一制度的基本重复率,其特征在于大幅度和相位噪声,具有几十个周期的相干时间;一个更有规律的二次谐波梳状制度,具有典型的相干时间约105个振荡周期。我们解释这些测量使用一组有效的半导体麦克斯韦-布洛赫方程,定性地再现激光动力学的基本特征,表明所观察到的载流子密度和光脉冲是反相的,并出现作为一个相当浅的调制顶部的连续波背景。由于其简单性和多功能性,演示的技术是一个强大的工具,在太赫兹QCL超快动力学的研究。

英文摘要:

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 the real-time 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 free-running 4.2THz QCL, 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 regimes of oscillation with dramatically different properties: a first regime at the fundamental 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 simplicity and versatility, the demonstrated technique is a powerful tool for the study of ultrafast dynamics in THz QCLs.

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