Tuning and Persistent Switching of Graphene Plasmons on a Ferroelectric Substrate.

We characterized plasmon propagation in graphene on thin films of the high-κ dielectric PbZr0.3Ti0.7O3 (PZT). Significant modulation (up to ±75%) of the plasmon wavelength was achieved with application of ultrasmall voltages (< ±1 V) across PZT. Analysis of the observed plasmonic fringes at the graphene edge indicates that carriers in graphene on PZT behave as noninteracting Dirac Fermions approximated by a semiclassical Drude response, which may be attributed to strong dielectric screening at the graphene/PZT interface. Additionally, significant plasmon scattering occurs at the grain boundaries of PZT from topographic and/or polarization induced graphene conductivity variation in the interior of graphene, reducing the overall plasmon propagation length. Lastly, through application of 2 V across PZT, we demonstrate the capability to persistently modify the plasmonic response of graphene through transient voltage application.

Colossal Ultraviolet Photoresponsivity of Few-Layer Black Phosphorus.

Black phosphorus has an orthorhombic layered structure with a layer-dependent direct band gap from monolayer to bulk, making this material an emerging material for photodetection. Inspired by this and the recent excitement over this material, we studied the optoelectronics characteristics of high-quality, few-layer black phosphorus-based photodetectors over a wide spectrum ranging from near-ultraviolet (UV) to near-infrared (NIR). It is demonstrated for the first time that black phosphorus can be configured as an excellent UV photodetector with a specific detectivity ∼3 × 10(13) Jones. More critically, we found that the UV photoresponsivity can be significantly enhanced to ∼9 × 10(4) A W(-1) by applying a source-drain bias (VSD) of 3 V, which is the highest ever measured in any 2D material and 10(7) times higher than the previously reported value for black phosphorus. We attribute such a colossal UV photoresponsivity to the resonant-interband transition between two specially nested valence and conduction bands. These nested bands provide an unusually high density of states for highly efficient UV absorption due to the singularity of their nature.

Unusual angular dependence of the Raman response in black phosphorus.

Anisotropic materials are characterized by a unique optical response, which is highly polarization-dependent. Of particular interest are layered materials formed by the stacking of two-dimensional (2D) crystals that are naturally anisotropic in the direction perpendicular to the 2D planes. Black phosphorus (BP) is a stack of 2D phosphorene crystals and a highly anisotropic semiconductor with a direct band gap. We show that the angular dependence of polarized Raman spectra of BP is rather unusual and can be explained only by considering complex values for the Raman tensor elements. This result can be traced back to the electron-photon and electron-phonon interactions in this material.

Ultrafast and nanoscale plasmonic phenomena in exfoliated graphene revealed by infrared pump-probe nanoscopy.

Pump-probe spectroscopy is central for exploring ultrafast dynamics of fundamental excitations, collective modes, and energy transfer processes. Typically carried out using conventional diffraction-limited optics, pump-probe experiments inherently average over local chemical, compositional, and electronic inhomogeneities. Here, we circumvent this deficiency and introduce pump-probe infrared spectroscopy with ∼ 20 nm spatial resolution, far below the diffraction limit, which is accomplished using a scattering scanning near-field optical microscope (s-SNOM). This technique allows us to investigate exfoliated graphene single-layers on SiO2 at technologically significant mid-infrared (MIR) frequencies where the local optical conductivity becomes experimentally accessible through the excitation of surface plasmons via the s-SNOM tip. Optical pumping at near-infrared (NIR) frequencies prompts distinct changes in the plasmonic behavior on 200 fs time scales. The origin of the pump-induced, enhanced plasmonic response is identified as an increase in the effective electron temperature up to several thousand Kelvin, as deduced directly from the Drude weight associated with the plasmonic resonances.