The physics of plasmonic gain in low-dimensional electronic systems

包括我们自己在内的许多理论研究最近都表明，等离子体激元的性质会受到支撑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功率来获得等离子体激元增益的可能性，其中等离子体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比例，由于更频繁的电子碰撞，直流电子漂移速度在半导体中可以非常高，但在金属中仅很小。不幸的是，只有少数直接的实验观察等离子体激元和直流电流之间的相互作用。不过，这些都证实了基本的预测-等离子体波矢量取决于直流电子漂移速度的强度和方向。目前该领域的最新技术可以总结如下：1）一些理论工作预测了低维系统（LDSs）中等离子体激元与直流电流相互作用的THz振荡; 2）已经获得了LDSs的低功率THz发射，但等离子体激元在这些实验中的作用尚未完全理解; 3）从未直接测量过电流驱动的等离子体激元增益。进一步的进展，现在需要等离子体激元和直流电流之间的相互作用的基础实验研究，支持等离子体激元增益的机制的理论解释。之前的排放实验不适合此目的。当等离子体增益（由于与直流电流的相互作用）超过损耗时，相干太赫兹发射应该出现在阈值处，就像在激光器中一样。然而，在亚阈值制度时，增益是弱的，没有发射可以观察到，因此，增益不能量化在以前的实验。此外，弱的等离子体发射可能被其他机制，特别是热辐射所掩盖。同样，许多广泛使用的光热等离子体激元检测的基础上的方法是不合适的，因为他们不允许与直流电流的相互作用被probing.Our实验技术使我们能够解决这些问题，并研究通过LDS等离子体激元的传播，并恢复其完整的THz透射光谱。理想地适合于研究等离子体增益，它提供了一个独特的能力，使相同的等离子体器件在被动（无直流电流），亚阈值，和阈值以上的制度。同时，我们的新理论模型将使我们能够分析实验结果并设计优化结构。这些平行的进展将是至关重要的理解和证明等离子体增益的第一次。逐步改进将最终导致太赫兹发射-强大，廉价和可调谐（100 GHz - 10 THz）的源是国际社会的长期目标，但尽管在许多领域取得了进展，但没有紧凑的室温源存在CW mW功率输出在1和4 THz之间。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激元在局限于LDS时具有落入THz范围内的谐振频率。它还将有助于实现LDS中等离子体激元用于THz探测器和传感器的潜力。

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.