Tip-Induced and Electrical Control of the Photoluminescence Yield of Monolayer WS2.

The photoluminescence (PL) of monolayer tungsten disulfide (WS2) is locally and electrically controlled using the nonplasmonic tip and tunneling current of a scanning tunneling microscope (STM). The spatial and spectral distribution of the emitted light is determined using an optical microscope. When the STM tip is engaged, short-range PL quenching due to near-field electromagnetic effects is present, independent of the sign and value of the bias voltage applied to the tip-sample tunneling junction. In addition, a bias-voltage-dependent long-range PL quenching is measured when the sample is positively biased. We explain these observations by considering the native n-doping of monolayer WS2 and the charge carrier density gradients induced by electron tunneling in micrometer-scale areas around the tip position. The combination of wide-field PL microscopy and charge carrier injection using an STM opens up new ways to explore the interplay between excitons and charge carriers in two-dimensional semiconductors.

An electrically induced probe of the modes of a plasmonic multilayer stack.

A new single-image acquisition technique for the determination of the dispersion relation of the propagating modes of a plasmonic multilayer stack is introduced. This technique is based on an electrically-driven, spectrally broad excitation source which is nanoscale in size: the inelastic electron tunnel current between the tip of a scanning tunneling microscope (STM) and the sample. The resulting light from the excited modes of the system is collected in transmission using a microscope objective. The energy-momentum dispersion relation of the excited optical modes is then determined from the angle-resolved optical spectrum of the collected light. Experimental and theoretical results are obtained for metal-insulator-metal (MIM) stacks consisting of a silicon oxide layer (70, 190 or 310 nm thick) between two gold films (each with a thickness of 30 nm). The broadband characterization of hybrid plasmonic-photonic transverse magnetic (TM) modes involved in an avoided crossing is demonstrated and the advantages of this new technique over optical reflectivity measurements are evaluated.

Scanning Tunneling Microscope-Induced Excitonic Luminescence of a Two-Dimensional Semiconductor.

The long sought-after goal of locally and spectroscopically probing the excitons of two-dimensional (2D) semiconductors is attained using a scanning tunneling microscope (STM). Excitonic luminescence from monolayer molybdenum diselenide (MoSe_{2}) on a transparent conducting substrate is electrically excited in the tunnel junction of an STM under ambient conditions. By comparing the results with photoluminescence measurements, the emission mechanism is identified as the radiative recombination of bright A excitons. STM-induced luminescence is observed at bias voltages as low as those that correspond to the energy of the optical band gap of MoSe_{2}. The proposed excitation mechanism is resonance energy transfer from the tunneling current to the excitons in the semiconductor, i.e., through virtual photon coupling. Additional mechanisms (e.g., charge injection) may come into play at bias voltages that are higher than the electronic band gap. Photon emission quantum efficiencies of up to 10^{-7} photons per electron are obtained, despite the lack of any participating plasmons. Our results demonstrate a new technique for investigating the excitonic and optoelectronic properties of 2D semiconductors and their heterostructures at the nanometer scale.

Directional light beams by design from electrically driven elliptical slit antennas.

We report on the low-energy, electrical generation of light beams in specific directions from planar elliptical microstructures. The emission direction of the beam is determined by the microstructure eccentricity. A very simple, broadband, optical antenna design is used, which consists of a single elliptical slit etched into a gold film. The light beam source is driven by an electrical nanosource of surface plasmon polaritons (SPP) that is located at one focus of the ellipse. In this study, SPPs are generated through inelastic electron tunneling between a gold surface and the tip of a scanning tunneling microscope.

Surface plasmon polariton beams from an electrically excited plasmonic crystal.

Surface plasmon polariton (SPP) beams with an in-plane angular spread of 8° are produced by electrically exciting a 2D plasmonic crystal using a scanning tunneling microscope (STM). The plasmonic crystal consists of a gold nanoparticle (NP) array on a thin gold film on a glass substrate and it is the inelastic tunnel electrons (IET) from the STM that provide a localized and spectrally broadband SPP source. Surface waves on the gold film are shown to be essential for the coupling of the local, electrical excitation to the extended NP array, thus leading to the creation of SPP beams. A simple model of the scattering of SPPs by the array is used to explain the origin and direction of the generated SPP beams under certain conditions. In order to take into account the broadband spectrum of the source, calculations realized using finite-difference time-domain (FDTD) methods are obtained, showing that bandgaps for SPP propagation exist for certain wavelengths and indicating how changing the pitch of the NP array may enhance the SPP beaming effect.

Temporal coherence of propagating surface plasmons.

The temporal coherence of propagating surface plasmons is investigated using a local, broadband plasmon source consisting of a scanning tunneling microscope. A variant of Young's experiment is performed using a sample consisting of a 200-nm-thick gold film perforated by two 1-µm-diameter holes (separated by 4 or 6 µm). The resulting interference fringes are studied as a function of hole separation and source bandwidth. From these experiments, we conclude that apart from plasmon decay in the metal, there is no further loss of plasmon coherence from propagation, scattering at holes, or other dephasing processes. As a result, the plasmon coherence time may be estimated from its spectral bandwidth.

An electrically excited nanoscale light source with active angular control of the emitted light.

We report on the angular distribution, polarization, and spectrum of the light emitted from an electrically controlled nanoscale light source. This nanosource of light arises from the local, low-energy, electrical excitation of localized surface plasmons (LSP) on individual gold nanoparticles using a scanning tunneling microscope (STM). The gold nanoparticles (NP) are chemically synthesized truncated bitetrahedrons. The emitted light is collected through the transparent substrate and the emission characteristics (angular distribution, polarization, and spectrum) are analyzed. These three observables are found to strongly depend on the lateral position of the STM tip with respect to the triangular upper face of the gold NP. In particular, the resulting light emission changes orientation when the electrical excitation via the STM tip is moved from the base to the vertex of the triangular face. On the basis of the comparison of the experimental observations with an analytical dipole model and finite-difference time-domain (FDTD) calculations, we show that this behavior is linked to the selective excitation of the out-of-plane and in-plane dipolar LSP modes of the NP. This selective excitation is achieved through the lateral position of the tip with respect to the symmetry center of the NP.

Edge scattering of surface plasmons excited by scanning tunneling microscopy.

The scattering of electrically excited surface plasmon polaritons (SPPs) into photons at the edges of gold metal stripes is investigated. The SPPs are locally generated by the inelastic tunneling current of a scanning tunneling microscope (STM). The majority of the collected light arising from the scattering of SPPs at the stripe edges is emitted in the forward direction and is collected at large angle (close to the air-glass critical angle, θ(c)). A much weaker isotropic component of the scattered light gives rise to an interference pattern in the Fourier plane images, demonstrating that plasmons may be scattered coherently. An analysis of the interference pattern as a function of excitation position on the stripe is used to determine a value of 1.42 ± 0.18 for the relative plasmon wave vector (kSPP/k0) of the corresponding SPPs. From these results, we interpret the directional, large angle (θ~θ(c)) scattering to be mainly from plasmons on the air-gold interface, and the diffuse scattering forming interference fringes to be dominantly from plasmons on the gold-substrate interface.