Upconversion electroluminescence in 2D semiconductors integrated with plasmonic tunnel junctions.

Plasmonic tunnel junctions are a unique electroluminescent system in which light emission occurs via an interplay between tunnelling electrons and plasmonic fields instead of electron-hole recombination as in conventional light-emitting diodes. It was previously shown that placing luminescent molecules in the tunneling pathway of nanoscopic tunnel junctions results in peculiar upconversion electroluminescence where the energy of emitted photons exceeds that of excitation electrons. Here we report the observation of upconversion electroluminescence in macroscopic van der Waals plasmonic tunnel junctions comprising gold and few-layer graphene electrodes separated by a ~2-nm-thick hexagonal boron nitride tunnel barrier and a monolayer semiconductor. We find that the semiconductor ground exciton emission is triggered at excitation electron energies lower than the semiconductor optical gap. Interestingly, this upconversion is reached in devices operating at a low conductance (<10-6 S) and low power density regime (<102 W cm-2), defying explanation through existing proposed mechanisms. By examining the scaling relationship between plasmonic and excitonic emission intensities, we elucidate the role of inelastic electron tunnelling dipoles that induce optically forbidden transitions in the few-layer graphene electrode and ultrafast hot carrier transfer across the van der Waals interface.

Engineering the Outcoupling Pathways in Plasmonic Tunnel Junctions via Photonic Mode Dispersion for Low-Loss Waveguiding.

Outcoupling of plasmonic modes excited by inelastic electron tunneling (IET) across plasmonic tunnel junctions (TJs) has attracted significant attention due to low operating voltages and fast excitation rates. Achieving selectivity among various outcoupling channels, however, remains a challenging task. Employing nanoscale antennas to enhance the local density of optical states (LDOS) associated with specific outcoupling channels partially addressed the problem, along with the integration of conducting 2D materials into TJs, improving the outcoupling to guided modes with particular momentum. The disadvantage of such methods is that they often involve complex fabrication steps and lack fine-tuning options. Here, we propose an alternative approach by modifying the dielectric medium surrounding TJs. By employing a simple multilayer substrate with a specific permittivity combination for the TJs under study, we show that it is possible to optimize mode selectivity in outcoupling to a plasmonic or a photonic-like mode characterized by distinct cutoff behaviors and propagation length. Theoretical and experimental results obtained with a SiO2-SiN-glass multilayer substrate demonstrate high relative coupling efficiencies of (62.77 ± 1.74)% and (29.07 ± 0.72)% for plasmonic and photonic-like modes, respectively. The figure-of-merit, which quantifies the tradeoff between mode outcoupling and propagation lengths (tens of µm) for both modes, can reach values as high as 180 and 140. The demonstrated approach allows LDOS engineering and customized TJ device performance, which are seamlessly integrated with standard thin film fabrication protocols. Our experimental device is well-suited for integration with silicon nitride photonics platforms.

Phase Matching via Plasmonic Modal Dispersion for Third Harmonic Generation.

The overall effectiveness of nonlinear optical processes along extended nonlinear media highly depends on the fulfillment of the phase-matching condition for pump and generated fields. This is traditionally accomplished by exploiting the birefringence of nonlinear crystals requiring long interaction lengths (cm-scale). For nonbirefringent media and integrated photonic devices, modal phase matching can compensate the index mismatch. Here, the various interacting waves propagate in transverse modes with appropriate phase velocities, but they suffer from a low refractive index contrast and cm-scale interaction lengths. This work harnesses modal phase matching for third-harmonic generation (THG) in plasmonic waveguides using an organic polymer (poly[3-hexylthiophene-2,5-diyl]) as the nonlinear medium. One demonstrates experimentally an effective interaction area as small as ≈ 0.11 µm2 and the phase-matched modal dispersion results in THG efficiency as high as ≈ 10-3 W-2 within an effective length scale of ≈ 4.3 µm. THG also shows a strong correlation with the polarization of the incident laser beam, corresponding to the excitation of the antisymmetric plasmonic modes, corroborating that plasmonic modal phase matching is achieved. This large reduction in device area of orders of magnitude is interesting for various applications where space is critical (e.g., device integration or on-chip applications).

Optical Anisotropy in van der Waals materials: Impact on Direct Excitation of Plasmons and Photons by Quantum Tunneling.

Inelastic quantum mechanical tunneling of electrons across plasmonic tunnel junctions can lead to surface plasmon polariton (SPP) and photon emission. So far, the optical properties of such junctions have been controlled by changing the shape, or the type of the material, of the electrodes, primarily with the aim to improve SPP or photon emission efficiencies. Here we show that by tuning the tunneling barrier itself, the efficiency of the inelastic tunneling rates can be improved by a factor of 3. We exploit the anisotropic nature of hexagonal boron nitride (hBN) as the tunneling barrier material in Au//hBN//graphene tunnel junctions where the Au electrode also serves as a plasmonic strip waveguide. As this junction constitutes an optically transparent hBN-graphene heterostructure on a glass substrate, it forms an open plasmonic system where the SPPs are directly coupled to the dedicated strip waveguide and photons outcouple to the far field. We experimentally and analytically show that the photon emission rate per tunneling electron is significantly improved (~ ×3) in Au//hBN//graphene tunnel junction due to the enhancement in the local density of optical states (LDOS) arising from the hBN anisotropy. With the dedicated strip waveguide, SPP outcoupling efficiency is quantified and is found to be ∼ 80% stronger than the radiative outcoupling in Au//hBN//graphene due to the high LDOS of the SPP decay channel associated with the inelastic tunneling. The new insights elucidated here deepen our understanding of plasmonic tunnel junctions beyond the isotropic models with enhanced LDOS.

Geometric control over surface plasmon polariton out-coupling pathways in metal-insulator-metal tunnel junctions.

Metal-insulator-metal tunnel junctions (MIM-TJs) can electrically excite surface plasmon polaritons (SPPs) well below the diffraction limit. When inelastically tunneling electrons traverse the tunnel barrier under applied external voltage, a highly confined cavity mode (MIM-SPP) is excited, which further out-couples from the MIM-TJ to photons and single-interface SPPs via multiple pathways. In this work we control the out-coupling pathways of the MIM-SPP mode by engineering the geometry of the MIM-TJ. We fabricated MIM-TJs with tunneling directions oriented vertical or lateral with respect to the directly integrated plasmonic strip waveguides. With control over the tunneling direction, preferential out-coupling of the MIM-SPP mode to SPPs or photons is achieved. Based on the wavevector distribution of the single-interface SPPs or photons in the far-field emission intensity obtained from back focal plane (BFP) imaging, we estimate the out-coupling efficiency of the MIM-SPP mode to multiple out-coupling pathways. We show that in the vertical-MIM-TJs the MIM-SPP mode preferentially out-couples to single-interface SPPs along the strip waveguides while in the lateral-MIM-TJs photon out-coupling to the far-field is more efficient.

Efficient Surface Plasmon Polariton Excitation and Control over Outcoupling Mechanisms in Metal-Insulator-Metal Tunneling Junctions.

Surface plasmon polaritons (SPPs) are viable candidates for integration into on-chip nano-circuitry that allow access to high data bandwidths and low energy consumption. Metal-insulator-metal tunneling junctions (MIM-TJs) have recently been shown to excite and detect SPPs electrically; however, experimentally measured efficiencies and outcoupling mechanisms are not fully understood. It is shown that the MIM-TJ cavity SPP mode (MIM-SPP) can outcouple via three pathways to i) photons via scattering of MIM-SPP at the MIM-TJ interfaces, ii) SPPs at the metal-dielectric interfaces (bound-SPPs) by mode coupling through the electrodes, and iii) photons and bound-SPP modes by mode coupling at the MIM-TJ edges. It is also shown that, for Al-AlO x -Cr-Au MIM-TJs on glass, the MIM-SPP mode outcouples efficiently to bound-SPPs through either electrode (pathway 2); this outcoupling pathway can be selectively turned on and off by changing the respective electrode thickness. Outcoupling at the MIM-TJ edges (pathway 3) is efficient and sensitive to the edge topography, whereas most light emission originates from roughness-induced scattering of the MIM-SPP mode (pathway 1). Using an arbitrary roughness profile, it is demonstrated that various roughness facets can raise MIM-SPP outcoupling efficiencies to 0.62%. These results pave the way for understanding the topographical parameters needed to develop CMOS-compatible plasmonic circuitry elements.

Negative photoresponse in ZnO-PEDOT:PSS nanocomposites and photogating effects.

We report negative photoresponse or increase of resistance in nanocomposites of n-type ZnO nanoparticles dispersed in a p-type polymer (PEDOT:PSS) under UV and visible light excitation, contrary to that of planar heterojunctions of the constituents. The underlying mechanism of charge transport, specifically negative photoresponse, is explored using spectroscopic and opto-electrical characterisation. Systemic variability in conductance, photoresponse sensitivity and rate with fractional nanoparticle loading in the nanocomposite is demonstrated. Here, photogenerated electrons in ZnO nanoparticles, trapped by the unbiased interfacial barrier, are understood to localize holes in the PEDOT:PSS conduction channel thereby increasing the overall nanocomposite resistance. Reversibility of the negative PR although with a slow decay rate bears testament to the proposed photogating mechanism as opposed to photocatalytic activity. Replacement of the p-type polymer with an electron transport matrix turns the negative photoresponse positive accentuating the role of the interfacial barrier in tuning the optoelectronic response of the composites. These hybrid materials and their unusual behaviour provide alternative strategies for building devices with novel photogating effects, exploiting the properties of their nanostructured forms.

Scanning tunnelling microscope light emission: Finite temperature current noise and over cut-off emission.

The spectral distribution of light emitted from a scanning tunnelling microscope junction not only bears its intrinsic plasmonic signature but is also imprinted with the characteristics of optical frequency fluc- tuations of the tunnel current. Experimental spectra from gold-gold tunnel junctions are presented that show a strong bias (V b ) dependence, curiously with emission at energies higher than the quantum cut-off (eV b ); a component that decays monotonically with increasing bias. The spectral evolution is explained by developing a theoretical model for the power spectral density of tunnel current fluctuations, incorporating finite temperature contribution through consideration of the quantum transport in the system. Notably, the observed decay of the over cut-off emission is found to be critically associated with, and well explained in terms of the variation in junction conductance with V b . The investigation highlights the scope of plasmon-mediated light emission as a unique probe of high frequency fluctuations in electronic systems that are fundamental to the electrical generation and control of plasmons.