A set of biocompatible, biodegradable, and biofunctionalizable diffractive optical elements (DOEs) using silk proteins as the building materials is reported. The diffraction pattern of a DOE is ...highly sensitive to the surrounding environment and the structural integrity, offering numerous opportunities for biosensing applications.
Most van der Waals crystals present highly anisotropic optical responses due to their strong in-plane covalent bonding and weak out-of-plane interactions. However, the determination of the ...polarization-dependent dielectric constants of van der Waals crystals remains a nontrivial task, since the size and dimension of the samples are often below or close to the diffraction limit of the probe light. In this work, we apply an optical nano-imaging technique to determine the anisotropic dielectric constants in representative van der Waals crystals. Through the study of both ordinary and extraordinary waveguide modes in real space, we are able to quantitatively determine the full dielectric tensors of nanometer-thin molybdenum disulfide and hexagonal boron nitride microcrystals, the most-promising van der Waals semiconductor and dielectric. Unlike traditional reflection-based methods, our measurements are reliable below the length scale of the free-space wavelength and reveal a universal route for characterizing low-dimensional crystals with high anisotropies.
The scattering-type scanning near-field optical microscope (s-SNOM) has emerged as a powerful tool for resolving nanoscale inhomogeneities in laterally heterogeneous samples. However, most analytical ...models used to predict the scattering near-field signals are assuming homogenous landscapes (bulk materials), resulting in inconsistencies when applied to samples with more complex configurations. In this work, we combine the point-dipole model (PDM) to the finite-element method (FEM) to account for the lateral and vertical heterogeneities while keeping the computation time manageable. Full images, spectra, or hyperspectral line profiles can be simulated by calculating the self-consistent dipole radiation demodulated at higher harmonics of the tip oscillation, mimicking real experimental procedures. Using this formalism, we clarify several important yet puzzling experimental observations in near-field images on samples with rich typography and complex material compositions, heterostructures of two-dimensional material flakes, and plasmonic antennas. The developed method serves as a basis for future investigations of nano-systems with nontrivial topography.
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.
We investigated phonon–polaritons in hexagonal boron nitridea naturally hyperbolic van der Waals materialby means of the scattering-type scanning near-field optical microscopy. Real-space ...nanoimages we have obtained detail how the polaritons are launched when the light incident on a thin hexagonal boron nitride slab is scattered by various intrinsic and extrinsic inhomogeneities, including sample edges, metallic nanodisks deposited on its top surface, random defects, and surface impurities. The scanned tip of the near-field microscope is itself a polariton launcher whose efficiency proves to be superior to all the other types of polariton launchers we studied. Our work may inform future development of polaritonic nanodevices as well as fundamental studies of collective modes in van der Waals materials.
An array of passive metamaterial antennas fabricated on all protein‐based silk substrates were conformally transferred and adhered to the surface of an apple. This process allows the opportunity for ...intimate contact of micro‐ and nanostructures that can probe, and accordingly monitor changes in, their surrounding environment. This provides in situ monitoring of food quality. It is to be noted that this type of sensor consists of all edible and biodegradable components, holding utility and potential relevance for healthcare and food/consumer products and markets.
Infrared nano-spectroscopy based on scattering-type scanning near-field optical microscopy (s-SNOM) is commonly employed to probe the vibrational fingerprints of materials at the nanometer length ...scale. However, due to the elongated and axisymmetric tip shank, s-SNOM is less sensitive to the in-plane sample anisotropy in general. In this article, we report an easy-to-implement method to probe the in-plane dielectric responses of materials with the assistance of a metallic disk micro-antenna. As a proof-of-concept demonstration, we investigate here the in-plane phonon responses of two prototypical samples, i.e. in (100) sapphire and x-cut lithium niobate (LiNbO
). In particular, the sapphire in-plane vibrations between 350 cm
to 800 cm
that correspond to LO phonon modes along the crystal b- and c-axis are determined with a spatial resolution of < λ/10, without needing any fitting parameters. In LiNbO
, we identify the in-plane orientation of its optical axis via the phonon modes, demonstrating that our method can be applied without prior knowledge of the crystal orientation. Our method can be elegantly adapted to retrieve the in-plane anisotropic response of a broad range of materials, i.e. subwavelength microcrystals, van-der-Waals materials, or topological insulators.
THz scattering-type scanning near-field microscopy (s-SNOM) has become a powerful technique for measuring carrier dynamics in nanoscale materials and structures. Changes in a material's local THz ...reflection or transmission can be correlated to changes in electrical conductivity. Here, we perform tip-based THz nano-imaging of subwavelength gold nanostructures and demonstrate image contrast unrelated to any spatially varying material properties. We show that the specific physical configuration of the gold structures can have a strong influence on local excitations which can obscure the sample's true dielectric response, even in cases where the relevant structures are far outside of the spatial region probed by the AFM tip.
We report on time-resolved mid-infrared (mid-IR) near-field spectroscopy of the narrow bandgap semiconductor InAs. The dominant effect we observed pertains to the dynamics of photoexcited carriers ...and associated surface plasmons. A novel combination of pump–probe techniques and near-field nanospectroscopy accesses high momentum plasmons and demonstrates efficient, subpicosecond photomodulation of the surface plasmon dispersion with subsequent tens of picoseconds decay under ambient conditions. The photoinduced change of the probe intensity due to plasmons in InAs is found to exceed that of other mid-IR or near-IR media by 1–2 orders of magnitude. Remarkably, the required control pulse fluence is as low as 60 μJ/cm2, much smaller than fluences of ∼1–10 mJ/cm2 previously utilized in ultrafast control of near-IR plasmonics. These low excitation densities are easily attained with a standard 1.56 μm fiber laser. Thus, InAsa common semiconductor with favorable plasmonic properties such as a low effective masshas the potential to become an important building block of optically controlled plasmonic devices operating at infrared frequencies.
Silk protein fibres produced by silkworms and spiders are renowned for their unparalleled mechanical strength and extensibility arising from their high-β-sheet crystal contents as natural materials. ...Investigation of β-sheet-oriented conformational transitions in silk proteins at the nanoscale remains a challenge using conventional imaging techniques given their limitations in chemical sensitivity or limited spatial resolution. Here, we report on electron-regulated nanoscale polymorphic transitions in silk proteins revealed by near-field infrared imaging and nano-spectroscopy at resolutions approaching the molecular level. The ability to locally probe nanoscale protein structural transitions combined with nanometre-precision electron-beam lithography offers us the capability to finely control the structure of silk proteins in two and three dimensions. Our work paves the way for unlocking essential nanoscopic protein structures and critical conditions for electron-induced conformational transitions, offering new rules to design protein-based nanoarchitectures.
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