Abstract
Metallic structures with nanogap features have proven highly effective as building blocks for plasmonic systems, as they can provide a wide tuning range of operating frequencies and large near-field enhancements. Recent work has shown that quantum mechanical effects such as electron tunnelling and nonlocal screening become important as the gap distances approach the subnanometre length-scale. Such quantum effects challenge the classical picture of nanogap plasmons and have stimulated a number of theoretical and experimental studies. This review outlines the findings of many groups into quantum mechanical effects in nanogap plasmons, and discusses outstanding challenges and future directions.
Highlights
Metallic structures with nanogap features have proven highly effective as building blocks for plasmonic systems, as they can provide a wide tuning range of operating frequencies and large near-field enhancements
The spatial localization of the surface charges can be characterized by the frequency-dependent distance parameter dF, the Feibelman parameter[41], which is in the angstrom (Å) range. dF is defined as the position of the centroid of the induced surface charge density with respect to the geometrical boundaries[36,37,38,39,40]
As the gap distance continues to decrease and the geometry is characterized by gaps narrower than a ‘threshold tunnel-distance’ dth, the electron tunnelling effect completely modifies the behaviour of the plasmonic response: the red-shifting gap plasmon modes progressively disappear and the blue-shifting charge-transfer plasmons (CTPs) gradually emerge
Summary
Wenqi Zhu[1,2], Ruben Esteban[3], Andrei G. Recent work has shown that quantum mechanical effects such as electron tunnelling and nonlocal screening become important as the gap distances approach the subnanometre length-scale Such quantum effects challenge the classical picture of nanogap plasmons and have stimulated a number of theoretical and experimental studies. Recent theoretical[28,29,30,31] and experimental[32,33] advances show that as the gap distance enters the nanometre and subnanometre scale, the quantum nature of the electrons and the nonlocal screening[34,35] associated with them significantly alter the plasmonic response In this quantum regime, the classical descriptions fail to account for the actual localization of the surface charges induced by an incident electromagnetic field[36,37,38,39,40]. To correctly quantify the nonlocal screening effects and associated changes in plasmonic response, the surface
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