Electron-induced nonlinear dynamics in atomic chains

We study the nonlinear response of collective optical resonances in linear atomic chains with metallic, semiconducting, and topologically insulating character to low-energy free electrons. The nonlinearity, which manifests in the amplitude and frequency of resonant features in cathodoluminescence and electron energy-loss spectra, is shown to depend on the speed and trajectory of the excitation as well as the length and electronic structure of the chain. Time-domain analysis of charge carrier dynamics within the atomic chain reveals that the Fermi velocity sets the threshold speed for triggering an electron-induced nonlinear response, a phenomenon which can elucidate nonlinear light-matter interactions on the nanoscale.</p

Spontaneous breaking of time-reversal symmetry at the edges of 1T' monolayer transition metal dichalcogenides

Using density functional theory calculations and the Greens's function formalism, we report the existence of magnetic edge states with a non-collinear spin texture present on different edges of the 1T' phase of the three monolayer transition metal dichalcogenides (TMDs): MoS$_2$, MoTe$_2$ and WTe$_2$. The magnetic states are gapless and accompanied by a spontaneous breaking of the time-reversal symmetry. This may have an impact on the prospects of utilizing WTe$_2$ as a quantum spin Hall insulator. It has previously been suggested that the topologically protected edge states of the 1T' TMDs could be switched off by applying a perpendicular electric field. We confirm with fully self-consistent DFT calculations, that the topological edge states can be switched off. The investigated magnetic edge states are seen to be robust and remains gapless when applying a field.Comment: 7 pages, 7 figure

Schottky barrier lowering due to interface states in 2D heterophase devices

The Schottky barrier of a metal-semiconductor junction is one of the key quantities affecting the charge transport in a transistor. The Schottky barrier height depends on several factors, such as work function difference, local atomic configuration in the interface, and impurity doping. We show that also the presence of interface states at 2D metal-semiconductor junctions can give rise to a large renormalization of the effective Schottky barrier determined from the temperature dependence of the current. We investigate the charge transport in n- and p-doped monolayer MoTe$_2$ 1T'-1H junctions using ab-initio quantum transport calculations. The Schottky barriers are extracted both from the projected density of states and the transmission spectrum, and by simulating the IT-characteristic and applying the thermionic emission model. We find interface states originating from the metallic 1T' phase rather than the semiconducting 1H phase in contrast to the phenomenon of Fermi level pinning. Furthermore, we find that these interface states mediate large tunneling currents which dominates the charge transport and can lower the effective barrier to a value of only 55 meV.Comment: 6 figure

Nonlinear thermoplasmonics in photoexcited graphene nanoribbons

Nonlinear optical phenomena enable the control of light by light both temporally and spectrally, and therefore represent a crucial resource for emerging integrated photonic and quantum optical technologies [1,2]. Graphene is an excellent material in this context due to its large intrinsic nonlinear optical response. Additionally, the combination of graphene's linear electronic bands and atomic thickness results in plasmon resonances with high sensitivity to charge carrier doping, enabling the electrical tunability of plasmon resonances at terahertz (THz) or infrared (IR) frequencies. However, pushing graphene plasmon resonances to the near-IR and visible spectral regime requires high charge doping levels that are difficult to achieve via electrostatic gating, or patterning on ~10 nm length scales, where quantum finite-size effects play an important role [3,4].</p

Nonlinear thermoplasmonics in graphene nanostructures

The linear electronic dispersion relation of graphene endows the atomically thin carbon layer with a large intrinsic optical nonlinearity, with regard to both parametric and photothermal processes. While plasmons in graphene nanostructures can further enhance nonlinear optical phenomena, boosting resonances to the technologically relevant mid- and near-infrared (IR) spectral regimes necessitates patterning on ∼10 nm length scales, for which quantum finite-size effects play a crucial role. Here we show that thermoplasmons in narrow graphene nanoribbons can be activated at mid- and near-IR frequencies with moderate absorbed energy density, and furthermore can drive substantial third-harmonic generation and optical Kerr nonlinearities. Our findings suggest that photothermal excitation by ultrashort optical pulses offers a promising approach to enable nonlinear plasmonic phenomena in nanostructured graphene that avoids potentially invasive electrical gating schemes and excessive charge carrier doping levels.</p

Plasmons in phosphorene nanoribbons

Phosphorene has emerged as an atomically-thin platform for optoelectronics and nanophotonics due to its excellent nonlinear optical properties and the possibility of actively tuning light-matter interactions through electrical doping. While phosphorene is a two-dimensional semiconductor, plasmon resonances characterized by pronounced anisotropy and strong optical confinement are anticipated to emerge in highly-doped samples. Here we show that the localized plasmons supported by phosphorene nanoribbons (PNRs) exhibit high tunability in relation to both edge termination and doping charge polarity, and can trigger an intense nonlinear optical response at moderate doping levels. Our explorations are based on a second-principles theoretical framework, employing maximally localized Wannier functions constructed from ab-inito electronic structure calculations, which we introduce here to describe the linear and nonlinear optical response of PNRs on mesoscopic length scales. Atomistic simulations reveal the high tunability of plasmons in doped PNRs at near-infrared frequencies, which can facilitate synergy between electronic band structure and plasmonic field confinement in doped PNRs to drive efficient high-harmonic generation. Our findings establish phosphorene nanoribbons as a versatile atomically-thin material candidate for nonlinear plasmonics.Comment: 12 pages, 7 figure