Optical Rectification and Field Enhancement in a Plasmonic Nanogap (1104.0035v1)

Abstract: Metal nanostructures act as powerful optical antennas[1, 2] because collective modes of the electron fluid in the metal are excited when light strikes the surface of the nanostructure. These excitations, known as plasmons, can have evanescent electromagnetic fields that are orders of magnitude larger than the incident electromagnetic field. The largest field enhancements often occur in nanogaps between plasmonically active nanostructures[3, 4], but it is extremely challenging to measure the fields in such gaps directly. These enhanced fields have applications in surface-enhanced spectroscopies[5-7], nonlinear optics[1, 8-10], and nanophotonics[11-15]. Here we show that nonlinear tunnelling conduction between gold electrodes separated by a subnanometre gap leads to optical rectification, producing a DC photocurrent when the gap is illuminated. Comparing this photocurrent with low frequency conduction measurements, we determine the optical frequency voltage across the tunnelling region of the nanogap, and also the enhancement of the electric field in the tunnelling region, as a function of gap size. The measured field enhancements exceed 1000, consistent with estimates from surface-enhanced Raman measurements[16-18]. Our results highlight the need for more realistic theoretical approaches that are able to model the electromagnetic response of metal nanostructures on scales ranging from the free space wavelength, $\lambda$, down to $\sim \lambda/1000$, and for experiments with new materials, different wavelengths, and different incident polarizations.

• The paper shows that nonlinear tunnelling between gold electrodes in a subnanometer gap induces optical rectification, generating a measurable DC photocurrent.
• It quantifies field enhancement in the nanogap, observing over 1000x increase consistent with surface-enhanced Raman metrics.
• The paper emphasizes the need for refined theoretical models and experimental techniques to capture quantum effects in nanoscale plasmonic systems.

Optical Rectification and Field Enhancement in a Plasmonic Nanogap

This paper explores the phenomena of optical rectification and field enhancement within a sub-nanometer plasmonic nanogap. The research addresses the challenge of directly measuring the enhanced fields in nanogaps—a critical requirement for advancing applications in surface-enhanced spectroscopies, nonlinear optics, and nanophotonics.

Summary of Key Findings

1. Optical Rectification in Nanogaps: The paper reveals that nonlinear tunnelling conduction between gold electrodes, separated by a subnanometer gap, leads to optical rectification. This process results in the generation of a DC photocurrent when the nanogap is illuminated. By comparing this photocurrent with low-frequency conduction measurements, the researchers could determine the optical frequency voltage across the tunnelling region of the nanogap.
2. Field Enhancement Measurement: The research successfully quantifies the enhancement of the electric field in the tunnelling region as a function of gap size. Field enhancements exceeding 1000 times were observed, corroborating estimates from surface-enhanced Raman measurements. These results support the validity of theoretical models suggesting large enhancements due to plasmonic activity.
3. Implications for Theoretical Models: The findings underscore the necessity for more realistic theoretical frameworks capable of modeling electromagnetic responses of metal nanostructures across scales ranging from the free-space wavelength to down to about λ/1000. The paper calls for experiments utilizing new materials, wavelengths, and incident polarizations.

Theoretical and Practical Implications

The paper provides pivotal insights at both theoretical and practical levels. From a theoretical standpoint, the data highlights a need for improved models that account for the complex electromagnetic interactions and quantum effects that govern nanoscopic systems. Practically, demonstrating optical rectification in such nanostructures opens pathways for developing advanced photodetectors and sensors, leveraging the tunable characteristics of plasmon-enhanced fields.

Future Directions

Several directions for future research emerge from this work:

• Material and Structural Variability: Investigating different materials and structural configurations could help optimize the field enhancement for various applications. Expanding studies to include alternative plasmonic materials like silver could provide additional insights.
• Quantum Effects: Further refinement of quantum mechanical models could help elucidate the impact of tunneling and plasmonic interactions at atomic scales, potentially improving design strategies for nanoplasmonic devices.
• Experimental Techniques: The integration of new experimental techniques, such as advanced microscopy and spectroscopy, may offer more detailed characterizations of nanogaps, aiding in the validation of theoretical predictions.

The paper positions itself as a critical pivot point toward practical implementations in nanoscale optics and photonics, providing substantial evidence for the high potential of plasmonic nanogaps in enhancing electromagnetic fields dramatically. As the field expands, integrating such findings with ongoing advancements in computational methods and material sciences will be essential in comprehensively harnessing these nanoscale phenomena.