Electrical Excitation of Surface Plasmons
In the paper "Electrical Excitation of Surface Plasmons" by Palash Bharadwaj, Alexandre Bouhelier, and Lukas Novotny, the authors investigate a novel mechanism to convert low-energy tunneling electrons into propagating photons via surface plasmon polaritons (SPPs). This research entails exploiting a two-step momentum downconversion scheme enabled by plasmonic excitations, facilitating an alternative to conventional photonic coupling strategies.
The paper is grounded in the conceptual framework established by Ritchie in 1957, which has evolved through subsequent explorations of high-energy electron excitation of plasmons. This work distinguishes itself by utilizing inelastic electron tunneling to initiate plasmon generation—a concept initially observed in scanning tunneling microscopy (STM) photoemission. The novelty lies in demonstrating that tunneling electrons excite localized gap plasmons that can couple to propagating surface plasmons, thereby providing a non-optical, voltage-controlled method for launching SPPs in nanostructures such as plasmonic waveguides.
The experimental setup integrates an STM with an inverted optical microscope, employing a gold tip to establish a tunneling current in contact with an extended gold nanowire. The emitted light, peaking around 700 nm, is detected via an electron-multiplying charge-coupled device (EMCCD) camera. Notably, the photon emission suggests an electron-photon conversion efficiency of approximately 10−5, contingent on biases above 1.9V.
Key numerical findings from this research include:
- Photon count rates of 5–10 kcps were achieved with a 20 nm gold film.
- The spatial distribution of emitted photons aligns with theoretical predictions, underscoring the 'localized gap plasmon' hypothesis.
- A coupling efficiency of about 7% was found for SPP propagation along a gold nanowire of specific dimensions.
The results provide robust experimental support for the theoretical underpinnings laid out in the paper, specifically addressing the momentum mismatch between photons and electrons through plasmonic intermediaries. The experiment with the monocrystalline gold nanowire corroborates the ability to spatially separate excitation and emission regions, which is vital for practical applications in nanoscale photonic sources.
Theoretical implications are profound, suggesting a new pathway for coupling electrons to photons through electrical means rather than optical or high-energy mechanistic routes. Practically, this research advances the integration of plasmonics with existing electronic infrastructures, paving the way for electron-photon signal transduction in nanodevices. The potential seamless integration with CMOS technology further highlights the applicability and value of this research in developing integrated optoelectronic systems.
Future research could focus on optimizing tunneling junctions to enhance coupling efficiencies further, engineering advanced plasmonic structures, and exploring this excitation mechanism's possible integration with diverse nanoscale applications. The exploration of various nanostructures like optical antennas may contribute to improved electron-photon conversion efficiencies, thus expanding the horizon for practical nanophotonic devices and enhancing signal transduction technologies.