Abstract:We investigated the excitation of surface plasmon polaritons on gold films with the metallized probe tip of a scattering-type scanning near-field optical microscope (s-SNOM). The emission of the polaritons from the tip, illuminated by near-infrared laser radiation, was found to be anisotropic and not circularly symmetric as expected on the basis of literature data. We furthermore identified an additional excitation channel via light that was reflected off the tip and excited the plasmon polaritons at the edge of the metal film. Our results, while obtained for a non-rotationally-symmetric type of probe tip and thus specific for this situation, indicate that when an s-SNOM is employed for the investigation of plasmonic structures, the unintentional excitation of surface waves and anisotropic surface wave propagation must be considered in order to correctly interpret the signatures of plasmon polariton generation and propagation.

Plasma waves play an important role in many solid-state phenomena and devices. They also become significant in electronic device structures as the operation frequencies of these devices increase. A prominent example is field-effect transistors (FETs), that witness increased attention for application as rectifying detectors and mixers of electromagnetic waves at gigahertz and terahertz frequencies, where they exhibit very good sensitivity even high above the cutoff frequency defined by the carrier transit time. Transport theory predicts that the coupling of radiation at THz frequencies into the channel of an antenna-coupled FET leads to the development of a gated plasma wave, collectively involving the charge carriers of both the two-dimensional electron gas and the gate electrode. In this paper, we present the first direct visualization of these waves. Employing graphene FETs containing a buried gate electrode, we utilize near-field THz nanoscopy at room temperature to directly probe the envelope function of the electric field amplitude on the exposed graphene sheet and the neighboring antenna regions. Mapping of the field distribution documents that wave injection is unidirectional from the source side since the oscillating electrical potentials on the gate and drain are equalized by capacitive shunting. The plasma waves, excited at 2 THz, are overdamped, and their decay time lies in the range of 25-70 fs. Despite this short decay time, the decay length is rather long, i.e., 0.3-0.5 μm, because of the rather large propagation speed of the plasma waves, which is found to lie in the range of 3.5-7 × 10 6 m/s, in good agreement with theory. The propagation speed depends only weakly on the gate voltage swing and is consistent with the theoretically predicted 1 4 power law.

Light-matter interaction with two-dimensional materials gained significant attention in recent years leading to the reporting of weak and strong coupling regimes, and effective nano-laser operation with various structures. Particularly, future applications involving monolayer materials in waveguide-coupled on-chip integrated circuitry and valleytronic nanophotonics require controlling, directing and optimizing photoluminescence. In this context, photoluminescence enhancement from monolayer transition-metal dichalcogenides on patterned semiconducting substrates becomes attractive. It is demonstrated in our work using focussed-ion-beam-etched GaP and monolayer WS 2 suspended on hexagonal-BN buffer sheets. We present a unique optical microcavity approach capable of both efficient in-plane and out-ofplane confinement of light, which results in a WS 2 photoluminescence enhancement by a factor of 10 compared to the unstructured substrate at room temperature. The key concept is the combination of interference effects in both the horizontal direction using a bull's-eye-shaped circular Bragg grating and in vertical direction by means of a multiple reflection model with optimized etch depth of circular air-GaP structures for maximum constructive interference effects of the applied pump and expected emission light.

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Crowded with the fundamental signatures of many popular molecules, the mid-infrared range of the electromagnetic spectrum is particularly attractive for applications ranging from identification of transient phenomena to sensing of trace gases. Dual-comb spectroscopy is a technique that unveils the potential to access this region with a pair of phase-locked optical frequency combs on a high-resolution, real-time basis without the mechanical limitations of traditional spectrometers. As the ideal characteristics of an optical frequency comb are strongly influenced by the target application, electro-optic dual-comb systems are one of the most promising solutions with full capabilities to neatly fit to the application of interest beyond laboratory environments. Parameters such as resolution, measurement speed, or central wavelength are easily adjustable by means of compact, low-cost arrangements based on commercial off-the-shelf components. To fully exploit their potential for molecular spectroscopy, we present here a modular instrument designed to perform ultrafast dual-comb spectroscopy in the mid-infrared region. The architecture comprises a fiberized near-infrared electro-optic dual-comb scheme and a single-crystal difference frequency generation module to generate mid-infrared combs, thus significantly alleviating the complexity of the free-space setup while preserving absolute independence between the instrument and the sample of study. The feasibility of the instrument is successfully validated by recovering the absorption profile of methane at 2896.98 cm −1 within tens of microseconds.

We study laser-induced guided-mode resonances on an ultrathin anisotropic metallic metasurface using direct near-field mapping. Coupling of the incident light to the guided-mode resonances is provided by a resonant laser-printed silicon nanosphere placed on the metasurface using nanomanipulation under electron-beam supervision. To visualize the propagation of the guided modes, we employ scattering-type scanning near-field optical microscopy in the near-infrared region (850 nm). The guided-mode resonances excited by the nanosphere demonstrate vanishing anisotropy due to weak confinement at the metasurface/air interface. We also extract the wavenumber of the observed modes through Fourier transformation of the measured near-field maps and find it to be in good agreement with numerical calculations.

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