Moiré patterns are periodic superlattice structures that appear when two crystals with a minor lattice mismatch are superimposed. A prominent recent example is that of monolayer graphene placed on a ...crystal of hexagonal boron nitride. As a result of the moiré pattern superlattice created by this stacking, the electronic band structure of graphene is radically altered, acquiring satellite sub-Dirac cones at the superlattice zone boundaries. To probe the dynamical response of the moiré graphene, we use infrared (IR) nano-imaging to explore propagation of surface plasmons, collective oscillations of electrons coupled to IR light. We show that interband transitions associated with the superlattice mini-bands in concert with free electrons in the Dirac bands produce two additive contributions to composite IR plasmons in graphene moiré superstructures. This novel form of collective modes is likely to be generic to other forms of moiré-forming superlattices, including van der Waals heterostructures.
Optical phase contrast has for the first time been observed on a nanometer scale, with a near-field microscope of scattering type that maps the complete optical field of amplitude and phase. Backed ...by quasielectrostatic theory, we demonstrate the significance and experimental accessibility of even complex optical constants on a subwavelength scale. Further, our method can separate the near-field response from background artifacts and thus is expected to enable nanoscale optical mapping of even topography-rich objects such as resonant clusters and macromolecules.
Electrons in correlated insulators are prevented from conducting by Coulomb repulsion between them. When an insulator-to-metal transition is induced in a correlated insulator by doping or heating, ...the resulting conducting state can be radically different from that characterized by free electrons in conventional metals. We report on the electronic properties of a prototypical correlated insulator vanadium dioxide in which the metallic state can be induced by increasing temperature. Scanning near-field infrared microscopy allows us to directly image nanoscale metallic puddles that appear at the onset of the insulator-to-metal transition. In combination with far-field infrared spectroscopy, the data reveal the Mott transition with divergent quasi-particle mass in the metallic puddles. The experimental approach used sets the stage for investigations of charge dynamics on the nanoscale in other inhomogeneous correlated electron systems.
We present combined experimental and numerical work on light-matter interactions at nanometer length scales. We report numerical simulations of near-field infrared nanospectroscopy that consider ...detailed tip geometry and have no free parameters. Our results match published spectral shapes of amplitude and phase measurements even for strongly resonant surface phonon-polariton (SPhP) modes. They also verify published absolute scattering amplitudes. A unique, ultrabroadband light source enables near-field amplitude and phase acquisition into the far-infrared spectral range. This allowed us to discover a strong SPhP resonance in the polar dielectric SrTiO_{3} (STO) at an ∼24-μm wavelength of incident light.
We report on the first implementation of ultrafast near field measurements carried out with the transient pseudoheterodyne detection method (Tr-pHD). This method is well suited for efficient and ...artifact free pump-probe scattering-type near-field optical microscopy with nanometer scale resolution. The Tr-pHD technique is critically compared to other data acquisition methods and found to offer significant advantages. Experimental evidence for the advantages of Tr-pHD is provided in the near-IR frequency range. Crucial factors involved in achieving proper performance of the Tr-pHD method with pulsed laser sources are analyzed and detailed in this work. We applied this novel method to femtosecond time-resolved and nanometer spatially resolved studies of the photo-induced effects in the insulator-to-metal transition system vanadium dioxide.
We report experimental signatures of plasmonic effects due to electron tunneling between adjacent graphene layers. At subnanometer separation, such layers can form either a strongly coupled bilayer ...graphene with a Bernal stacking or a weakly coupled double-layer graphene with a random stacking order. Effects due to interlayer tunneling dominate in the former case but are negligible in the latter. We found through infrared nanoimaging that bilayer graphene supports plasmons with a higher degree of confinement compared to single- and double-layer graphene, a direct consequence of interlayer tunneling. Moreover, we were able to shut off plasmons in bilayer graphene through gating within a wide voltage range. Theoretical modeling indicates that such a plasmon-off region is directly linked to a gapped insulating state of bilayer graphene, yet another implication of interlayer tunneling. Our work uncovers essential plasmonic properties in bilayer graphene and suggests a possibility to achieve novel plasmonic functionalities in graphene few-layers.
IR scattering‐type near‐field microscopy is applied to simultaneously map material composition and conduction properties in cross‐sectional samples of industrial bipolar and metal‐oxide‐ ...semiconductor devices with nanoscale spatial resolution. Within a single mid‐IR image, all relevant materials such as metals, Si, Si3N4, and oxides can be identified by material‐specific amplitude and phase contrasts.
Summary
We describe the principles of two scattering‐type near‐field optical microscopes (s‐SNOMs), one operating at 633 nm wavelength, the other at selectable wavelengths in the range 7.3–11.3 µm, ...and compare the measurement experience. Both use interferometric detection of scattered radiation, and are therefore capable of amplitude and phase‐contrast imaging. In this study both instruments use the same or even identical commercial probe tips, and measure a single, three‐component, test sample. Our results show that the imaging process of s‐SNOM is wavelength‐independent, namely, that the resolution is determined by the properties of the tip only, and that the contrast is given by the complex refractive index of the sample, predictable from a simple, analytical model of tip–sample interaction. A novel, ‘edge‐darkening’ artefact is described which may appear in s‐SNOM and that is wavelength‐independent.
We present time-resolved cyclotron resonance spectra of holes in p-Ge measured during single magnetic field pulses by using a rapid-scanning, fiber-coupled terahertz time-domain spectroscopy system. ...The key component of the system is a rotating monolithic delay line featuring four helicoid mirror surfaces. It allows measurements of THz spectra at up to 250 Hz repetition rate. Here we show results taken at 150 Hz. In a single 900 ms measurement 135 cyclotron resonance spectra were recorded that fully agree with what is expected from literature.
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