Optical rectification and field enhancement in a plasmonic nanogap

Area of Science:

• Plasmonics
• Nanophotonics
• Nonlinear Optics

Background:

• Metal nanostructures function as optical antennas by exciting collective electron modes (plasmons) upon light illumination.
• Plasmons generate intense evanescent electromagnetic fields, particularly in nanogaps, which are difficult to measure directly.
• These enhanced fields are vital for applications like surface-enhanced spectroscopies and nonlinear optics.

Purpose of the Study:

• To quantify optical frequency electric field enhancements in subnanometre gaps of metal nanostructures.
• To establish a method for measuring electromagnetic fields at the nanoscale using nonlinear optical effects.

Main Methods:

• Utilized nonlinear tunneling conduction in gold electrodes with subnanometre gaps.
• Observed optical rectification, generating a d.c. photocurrent under illumination.
• Compared photocurrent with low-frequency conduction measurements to determine optical frequency voltage and field enhancement.

Main Results:

• Demonstrated optical rectification in subnanometre gaps, producing a measurable d.c. photocurrent.
• Determined optical frequency voltage and electric field enhancement as a function of gap size.
• Achieved electric field enhancements exceeding 1,000, consistent with surface-enhanced Raman spectroscopy estimates.

Conclusions:

• Nonlinear tunneling conduction provides a viable method for measuring nanoscale electric fields and plasmonic field enhancements.
• The findings necessitate advanced theoretical models for metal nanostructures across multiple length scales.
• Suggests future experiments exploring new materials, wavelengths, and polarizations for enhanced optical phenomena.