The physics of plasmonic gain in low-dimensional electronic systems

最近，包括我们自己在内的许多理论研究表明，等离子体激元的性质可以受到支持它们的材料中流动的直流电流的强烈影响。原则上，这导致了通过从直流电流转移能量来获得等离子体激元的可能性，等离子激元和电流之间的相互作用的强度与直流电子漂移速度成正比，由于电子碰撞要频繁得多，直流电子漂移速度在半导体中可能非常高，但在金属中只有很小的漂移速度。不幸的是，只有少数几个直接实验观测到了等离子体与直流电流之间的相互作用。然而，这些都证实了基本的预测--等离子激元波矢取决于直流电子漂移速度的强度和方向。目前该领域的最新进展可概括如下：1)一些理论工作预测了低维系统(LDSS)中等离子体与直流电流相互作用产生的太赫兹振荡；2)LDSS已经获得了低功率的THz辐射，但等离子体在这些实验中的作用还没有完全了解；3)电流驱动的等离子体激元增益从未被直接测量过。现在，进一步的进展需要对等离子体激元和直流电流之间的相互作用进行基础实验研究，并从理论上解释等离子体激元的获得机制。以前的排放实验不适合这一目的。当等离子体激元增益(由于与直流电流的相互作用)超过损耗时，相干太赫兹辐射应该出现在阈值处，例如在激光器中。然而，在亚阈值区域，当增益较弱时，不能观察到发射，因此，以前的实验不能量化增益。此外，微弱的等离子体发射可能会被其他机制所遮挡，特别是热辐射。同样，许多广泛使用的基于光热等离子激元检测的方法是不合适的，因为它们不允许探测与直流电流的相互作用。我们的实验技术使我们能够解决这些问题，并研究等离子体在激光二极管中的传播，并恢复其完整的太赫兹透射谱。非常适合于研究等离子体激元增益，它提供了一种独特的能力，可以在无源(无直流电流)、亚阈值和阈值以上区域表征相同的等离子体激元设备。同时，我们的新理论模型将允许我们分析实验结果并设计优化的结构。这些平行的进展对于第一次理解和证明等离子激元增益将是至关重要的。循序渐进的改进最终将导致太赫兹辐射--强大、廉价和可调(100 GHz-10太赫兹)源是国际社会的长期目标，但尽管在许多领域取得了进展，仍不存在CW MW功率输出在1至4太赫兹之间的紧凑型室温源。我们的颠覆性技术提供了解决这一问题的潜力，因为半导体等离子体的共振频率在受限于LDSS时落在太赫兹范围内。它还将有助于实现激光二极管中用于太赫兹探测器和传感器的等离子激元的潜力。

A number of theoretical studies, including our own, have recently shown that the properties of plasmons can be influenced strongly by dc currents flowing in the materials that support them. This, in principle, leads to the possibility of plasmon gain by a transfer of power from the dc current, with the strength of the interaction between the plasmons and the current being proportional to the dc electron drift velocity, which can be very high in semiconductors but is only small in metals, due to much more frequent electron collisions. Unfortunately, there have been only a few direct experimental observations of the interaction between plasmons and dc currents. These have, though, confirmed the basic prediction - that the plasmon wavevector depends on the strength and direction of the dc electron drift velocity. The current state-of-the-art in the field can be summarised as follows: 1) Several theoretical works predict THz oscillations by interaction of plasmons with dc currents in low-dimensional systems (LDSs); 2) Low-power THz emission from LDSs has been obtained, but the role of plasmons in these experiments has not been fully understood; 3) Current-driven plasmon gain has never been directly measured. Further progress now requires basic experimental studies of the interaction between plasmons and dc currents, supported by theoretical interpretation of the mechanisms for plasmon gain. Previous emission experiments were ill-suited for this purpose. Coherent THz emission should appear at a threshold when plasmon gain (due to the interaction with a dc current) exceeds loss, as in a laser. However, in the sub-threshold regime when the gain is weak, no emission can be observed and, therefore, the gain could not be quantified in previous experiments. Moreover, weak plasmon emission may be obscured by other mechanisms, notably by thermal radiation. Likewise, many widely used methodologies based on photothermal plasmon detection are unsuitable since they do not permit interactions with dc currents to be probed.Our experimental technique allows us to address these issues, and to study the propagation of plasmons through LDSs, and recover their full THz transmission spectra. Suited ideally for studying plasmon gain, it gives an unique ability to characterise the same plasmon device in the passive (no dc current), sub-threshold, and above-threshold regimes. At the same time, our new theoretical models will allow us both to analyse experimental results and to design optimized structures. These parallel advances will be crucial to understand and demonstrate plasmon gain for the first time. Step-by-step improvements will ultimately lead to THz emission - powerful, cheap, and tuneable (100 GHz - 10 THz) sources are a long-standing goal for the international community, but despite progress in many areas, no compact, room-temperature source exists with CW mW power output between 1 and 4 THz. Our disruptive technology offers the potential to solve this problem since semiconductor plasmons have resonant frequencies that fall in the THz range when confined to LDSs. It will also contribute to realising the potential of plasmons in LDS for THz detectors and sensors.