Probing the Radiative Electromagnetic Local Density of States in Nanostructures with a Scanning Tunneling Microscope

Abstract

A novel technique for the investigation of the radiative contribution to the electromagnetic local density of states is presented. The inelastic tunneling current from a scanning tunneling microscope (STM) is used to locally and electrically excite the plasmonic modes of a triangular gold platelet. The radiative decay of these modes is detected through the transparent substrate in the far field. Emission spectra, which depend on the position of the STM excitation, as well as energy-filtered emission maps for particular spectral windows are acquired using this technique. The STM-nanosource spectroscopy and microscopy results are compared to those obtained from spatially resolved electron energy loss spectroscopy (EELS) maps on similar platelets. While EELS is known to be related to the total projected electromagnetic local density of states, the light emission from the STM-nanosource is shown here to select the radiative contribution. Full electromagnetic calculations are carried out to explain the experimental STM data, and provide valuable insight into the radiative nature of the different contributions of the breathing and edge plasmon modes of the nanoparticles. Our results introduce the STM-nanosource as a tool to investigate and engineer light emission at the nanoscale.

Selectively Exciting and Probing Radiative Plasmon Modes on Short Gold Nanorods by Scanning Tunneling Microscope-Induced Light Emission

“…We conduct both numerical calculations and STM-LE experiments to study the radiative plasmon modes of Au nanorods on an indium tin oxide (ITO) substrate. A tungsten tip is used to avoid exciting the tip–sample gap plasmons, thanks to the nonplasmonic optical behavior of tungsten in the measured spectral range. , Figure a shows scanning electron microscopy (SEM) images of Au nanorods on a silicon substrate. The nanorods have identical diameters ( D ) of 11 ± 1 nm and lengths ( L ) that range from 55 to 155 nm, as determined by SEM.…”

“…In the following, we study the excitation mechanisms of the plasmon modes. In CL and STM-LE, the measured far-field light is generated from the radiative decay of the near-field plasmon modes excited by a broad-band electron source. , Different from CL electrons, which are in an energy range of several kiloelectron volts, the energy of STM electrons can be finely tuned by the bias voltage to gain further insight into the excitation mechanisms of the plasmon modes. STM-LE spectra with different tip biases and a constant tunneling current are measured when placing the tip at the middle of the nanorod or 24 nm from the middle.…”

Scanning Tunneling Microscopy-Induced Light Emission and I(V) Study of Optical Near-Field Properties of Single Plasmonic Nanoantennas

“…The detailed full electromagnetic calculations were carried out to explain the obtained experimental STM data. As shown in ref , EELS provides information about the total electromagnetic local density of states, while the light emission under STM-tip shows the radiative contribution.…”

“…The STM junction can be also considered as a highly localized source of light, which can be used to probe local optoelectronic properties, in particular for the studies of electromagnetic field distribution in the vicinity of plasmonic structures with high spatial resolution. 7 Recently, 8 the radiative and nonradiative electromagnetic local density of states (denoted as EM-LDOS) were studied by STML and electron energy loss spectroscopy (EELS) implemented in a transmission electron microscope. The detailed full electromagnetic calculations were carried out to explain the obtained experimental STM data.…”

Boosting Optical Nanocavity Coupling by Retardation Matching to Dark Modes

“…While plasmonic nanogaps support a few bright nanocavity modes, many modes are dark and only accessible via the near field. − Making these bright and accessible at near normal incidence (θ = 0) would greatly improve optical access as it provides more operational frequencies and scattering angles, but how to do so is poorly understood and difficult to achieve. Plasmon resonances tune with the surrounding refractive index n d , − although the antenna size, metal, and shape are more commonly employed to tune plasmon resonances instead as these effects have been well characterized and documented.…”