Electrodynamic resonance of surface conduction and THz transmission through arrays of rectangular apertures in opaque metallic thin films.

A modal method is developed analytically to investigate the THz optical transmission and reflection of a metallic thin film perforated by a 2D array of rectangular apertures. For subwavelength apertures, this optical model is interpreted in terms of passive electrical circuits, with interface admittances accounting for the THz surface conduction properties of the metallic film. The reactive component of the admittance of the evanescent diffraction cloud is shown to exhibit resonant behavior governed by the shape factors of the array. Interaction of such an electrodynamic resonance with the Rayleigh diffraction orders may alter their standard Fano profiles. Experimental evidence of the resonance is obtained owing to lineshape analysis of transmittance measurements in the THz range on metallic thin films deposited on a dielectric substrate, both above and below the first Wood-Rayleigh anomaly.

Electrochemical Imaging of Dense Molecular Nanoarrays.

The aim of the present work is to explore the combination of atomic force electrochemical microscopy, operated in molecule touching mode (Mt/AFM-SECM), and of dense nanodot arrays, for designing an electrochemically addressable molecular nanoarray platform. A high density nanoarray of single grained gold nanodots (∼15 nm-diameter nanoparticles, 100 nm pitch) is decorated by a model molecular system, consisting of ferrocene (Fc) labeled polyethylene glycol (PEG) disulfide chains. We show that the high resolution of Mt/AFM-SECM enables the electrochemical interrogation of several hundreds of individual nanodots in a single image acquisition. As a result, the statistical dispersion of the nanodot molecular occupancy by Fc-PEG chains can be reliably quantified, evidencing that as little as a few tens of copies of redox-labeled macromolecules immobilized on individual nanodots can be detected. The electrochemical reactivity of individual nanodots can also be reliably sampled over a large population of nanodots. We evidence that the heterogeneous rate constant characterizing the electron transfer between the nanodots and the Fc heads displays some quantifiable variability but that the electron transfer remains in any case in the quasi-reversible regime. Overall, we demonstrate that Mt/AFM-SECM enables high throughput reading of dense nanoarrays, with a sensitivity and a read-out speed considerably higher than previously reported for scanning electrochemical microscopy (SECM) imaging of molecular microarrays.

Quantized Vortices and Four-Component Superfluidity of Semiconductor Excitons.

We study spatially indirect excitons of GaAs quantum wells, confined in a 10 µm electrostatic trap. Below a critical temperature of about 1 K, we detect macroscopic spatial coherence and quantized vortices in the weak photoluminescence emitted from the trap. These quantum signatures are restricted to a narrow range of density, in a dilute regime. They manifest the formation of a four-component superfluid, made by a low population of optically bright excitons coherently coupled to a dominant fraction of optically dark excitons.

Publisher's Note: Quantized Vortices and Four-Component Superfluidity of Semiconductor Excitons [Phys. Rev. Lett. 118, 127402 (2017)].

This corrects the article DOI: 10.1103/PhysRevLett.118.127402.

On the Mechanism of In Nanoparticle Formation by Exposing ITO Thin Films to Hydrogen Plasmas.

We present our systematic work on the in situ generation of In nanoparticles (NPs) from the reduction of ITO thin films by hydrogen (H2) plasma exposure. In contrast to NP deposition from the vapor phase (i.e., evaporation), the ITO surface can be considered to be a solid reservoir of In atoms thanks to H2 plasma reduction. On one hand, below the In melting temperature, solid In NP formation is governed by the island-growth mode, which is a self-limiting process because the H2 plasma/ITO interaction will be gradually eliminated by the growing In NPs that cover the ITO surface. On the other hand, we show that above the melting temperature In droplets prefer to grow along the grain boundaries on the ITO surface and dramatic coalescence occurs when the growing NPs connect with each other. This growth-connection-coalescence behavior is even strengthened on In/ITO bilayers, where In particles larger than 10 µm can be formed, which are made of evaporated In atoms and in situ released ones. Thanks to this understanding, we manage to disperse dense evaporated In NPs under H2 plasma exposure when inserting an ITO layer between them and substrate like c-Si wafer or glass by modifying the substrate surface chemistry. Further studies are needed for more precise control of this self-assembling method. We expect that our findings are not limited to ITO thin films but could be applicable to various metal NPs generation from the corresponding metal oxide thin films.

Touching surface-attached molecules with a microelectrode: mapping the distribution of redox-labeled macromolecules by electrochemical-atomic force microscopy.

We report on the development of a mediator-free electrochemical-atomic force microscopy (AFM-SECM) technique designed for high-resolution imaging of molecular layers of nanometer-sized redox-labeled (macro)molecules immobilized onto electrode surfaces. This new AFM-SECM imaging technique, we call molecule touching atomic force electrochemical microscopy (Mt/AFM-SECM), is based on the direct contact between surface-anchored molecules and an incoming microelectrode (tip). To validate the working-principle of this microscopy, we consider a model system consisting of a monolayer of nanometer long, flexible, polyethylene glycol (PEG) chains covalently attached by one extremity to a gold surface and bearing at their free end a ferrocene (Fc) redox tag. Using Mt/AFM-SECM in tapping mode, i.e., by oscillating the tip so that it comes in intermittent contact with the grafted chains, we show that the substrate topography and the distribution of the redox-tagged PEG chains immobilized on the gold surface can be simultaneously and independently imaged at the sub-100 nm scale. This novel type of SECM imaging may be found useful for characterizing the surface of advanced biosensors which use electrode-grafted, redox-tagged, linear biochains, such as peptides or DNA chains, as sensing elements. In principle, Mt/AFM-SECM should also permit in situ imaging of the distribution of any kind of macromolecules immobilized on electrode surfaces or simply conducting surfaces, provided they are labeled by a suitable redox tag.

Metasurface-based total internal reflection microscopy.

Recent years have seen a tremendous progress in the development of dielectric metasurfaces for visible light applications. Such metasurfaces are ultra-thin optical devices that can manipulate optical wavefronts in an arbitrary manner. Here, we present a newly developed metasurface which allows for coupling light into a microscopy coverslip to achieve total internal reflection (TIR) excitation. TIR fluorescence microscopy (TIRFM) is an important bioimaging technique used specifically to image cellular membranes or surface-localized molecules with high contrast and low background. Its most commonly used modality is objective-type TIRFM where one couples a focused excitation laser beam at the edge of the back focal aperture of an oil-immersion objective with high numerical aperture (N.A.) to realize a high incident-angle plane wave excitation above the critical TIR angle in sample space. However, this requires bulky and expensive objectives with a limited field-of-view (FOV). The metasurface which we describe here represents a low cost and easy-to-use alternative for TIRFM. It consists of periodic 2D arrays of asymmetric structures fabricated in TiO2 on borosilicate glass. It couples up to 70% of the incident non-reflected light into the first diffraction order at an angle of 65° in glass, which is above the critical TIR angle for a glass-water interface. Only ∼7% of the light leaks into propagating modes traversing the glass surface, thus minimizing any spurious background fluorescence originating far outside the glass substrate. We describe in detail design and fabrication of the metasurface, and validate is applicability for TIRFM by imaging immunostained human mesenchymal stem cells over a FOV of 200 µm x 200 µm. We envision that these kinds of metasurfaces can become a valuable tool for low-cost and TIRFM, offering high contrast, low photodamage, and high surface selectivity in fluorescence excitation and detection.

Optimized hand fabricated AFM probes for simultaneous topographical and electrochemical tapping mode imaging.

In this work hybrid AFM-electrochemical (SECM) probes to be used in dynamic atomic force microscopy are presented. These nanosensors are hand fabricated from gold microwires using a simple benchtop method. They display proportions close to commercially available silicon and silicon nitride cantilevers giving comparable performance in terms of resolution and imaging stability. The remarkable characteristic of these hybrid nanosensors is that they allow the coupling of 3D imaging ability and versatility of atomic force microscopy with the power of electrochemical methods. Local measurement of electrochemical-activity of a test sample consisting of gold bands functionalized by redox-labeled nanometer-sized polyethylene glycol chains has been achieved with simultaneous imaging of the 3D surface topography at high resolution. These hybrid AFM-SECM tips are capable of sensing local electrochemical currents down to ∼ 10 fA emphasizing the sensitivity and resolution of this technique.

Electrochemical atomic force microscopy using a tip-attached redox mediator for topographic and functional imaging of nanosystems.

We describe the development of a new type of high-resolution atomic force electrochemical microscopy (AFM-SECM), labeled Tarm (for tip-attached redox mediator)/AFM-SECM, where the redox mediator, a ferrocene (Fc), is tethered to the AFM-SECM probe via nanometer long, flexible polyethylene glycol (PEG) chains. It is demonstrated that the tip-attached ferrocene-labeled PEG chains effectively shuttle electrons between the tip and substrate, thus acting as molecular sensors probing the local electrochemical reactivity of a planar substrate. Moreover the Fc-PEGylated AFM-SECM probes can be used for tapping mode imaging, allowing simultaneous recording of electrochemical feedback current and of topography, with a vertical and a lateral resolution in the nanometer range. By imaging the naturally nanostructured surface of HOPG, we demonstrate that Tarm/AFM-SECM microscopy can be used to probe the reactivity of nanometer-sized active sites on surfaces. This new type of SECM microscopy, being, by design, free of the diffusional constraints of classical SECM, is expected to, in principle, enable functional imaging of redox nanosystems such as individual redox enzyme molecules.