The physics of plasmonic gain in low-dimensional electronic systems

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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.