Optical rectification and field enhancement in a plasmonic nanogap

Abstract

Metal nanostructures act as powerful optical antennas 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, but it is extremely challenging to measure the fields in such gaps directly. These enhanced fields have applications in surface-enhanced spectroscopies, nonlinear optics and nanophotonics. Here we show that nonlinear tunnelling conduction between gold electrodes separated by a subnanometre gap leads to optical rectification, producing a d.c. 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 1,000, consistent with estimates from surface-enhanced Raman measurements. 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, λ, down to ∼λ/1,000, and for experiments with new materials, different wavelengths and different incident polarizations.

Thermoplasmonics: Quantifying Plasmonic Heating in Single Nanowires

“…This polarization dependence is consistent with previous plasmonic studies and calculations, 35 and the magnitude of the heating as a function of wire geometry is consistent with finite element modeling of the optical interactions and the thermal transport. Under illumination conditions like those used in previous work, [33][34][35] the optically driven temperature increase is approximately 2 K at a substrate temperature of 80 K, which is comparable to results seen in other thermoplasmonic studies. 28 The dependence of the optically induced temperature elevation on the substrate's structure and the wire geometry confirm that the dominant path for heat flow out of the wire is via electronic thermal conduction in the metal, and then into the substrate from the large contacts via phonon thermal conduction.…”

A classical treatment of optical tunneling in plasmonic gaps: extending the quantum corrected model to practical situations

“…We put particular emphasis in the extension of the QCM to incorporate the hydrodynamical treatment of nonlocal effects, and discuss the effect of non-zero temperatures on the optical properties. While this paper focuses on the linear response, the QCM can be extended to nonlinear phenomena 34,[127][128][129][130] by taking into account the dependence of tunneling across the gap on the strength of the local eld.…”

Harvesting the loss: surface plasmon-based hot electron photodetection

“…Similar to the three-step model [56,57], the photoemission of carriers across a MIM diode can be described in a series of five steps [103] as depicted in the band diagram shown in Figure 3A. This is similar to the junctions in plasmonic nanogap antennas [110,113], where electrons can tunnel through the nanogap leading to photocurrent ( Figure 3B). In the MIM diode, the hot electrons can be generated in both metal contacts; therefore, the key here is to create asymmetric hot electron generation or transport in the top and bottom metal layers.…”

“…MIM diodes have been used as "rectennas" [108] at infrared and lower frequencies for power conversion and transmission. At optical frequencies, the operation of MIM diodes as rectification antennas has proven to be difficult [109], with efficiencies remaining low [110] due to the limitation on the RC time constant of the MIM diodes. However, the generation of hot electrons circumvents this limitation: hot electrons generated in one metal can tunnel through the thin insulating layer and be collected in the other metal contact, leading to a measurable photocurrent.…”