Nanoscale layering of antiferromagnetic and superconducting phases in Rb(2)Fe(4)Se(5) single crystals

We studied phase separation in the single-crystalline antiferromagnetic superconductor Rb(2)Fe(4)Se(5) (RFS) using a combination of scattering-type scanning near-field optical microscopy and low-energy muon spin rotation (LE-μSR). We demonstrate that the antiferromagnetic and superconducting phases segregate into nanometer-thick layers perpendicular to the iron-selenide planes, while the characteristic in-plane size of the metallic domains reaches 10  μm. By means of LE-μSR we further show that in a 40-nm thick surface layer the ordered antiferromagnetic moment is drastically reduced, while the volume fraction of the paramagnetic phase is significantly enhanced over its bulk value. Self-organization into a quasiregular heterostructure indicates an intimate connection between the modulated superconducting and antiferromagnetic phases.

Infrared-spectroscopic nanoimaging with a thermal source

Fourier-transform infrared (FTIR) spectroscopy is a widely used analytical tool for chemical identification of inorganic, organic and biomedical materials, as well as for exploring conduction phenomena. Because of the diffraction limit, however, conventional FTIR cannot be applied for nanoscale imaging. Here we demonstrate a novel FTIR system that allows for infrared-spectroscopic nanoimaging of dielectric properties (nano-FTIR). Based on superfocusing of thermal radiation with an infrared antenna, detection of the scattered light, and strong signal enhancement employing an asymmetric FTIR spectrometer, we improve the spatial resolution of conventional infrared spectroscopy by more than two orders of magnitude. By mapping a semiconductor device, we demonstrate spectroscopic identification of silicon oxides and quantification of the free-carrier concentration in doped Si regions with a spatial resolution better than 100  nm. We envisage nano-FTIR becoming a powerful tool for chemical identification of nanomaterials, as well as for quantitative and contact-free measurement of the local free-carrier concentration and mobility in doped nanostructures.

Material-specific infrared recognition of single sub-10 nm particles by substrate-enhanced scattering-type near-field microscopy

We study the optical material contrast of single nanoparticles in infrared scattering-type near-field optical microscopy (IR s-SNOM) in the presence of strong probe-substrate coupling. It is shown theoretically and experimentally that the contrast depends on both the dielectric properties of the nanoparticles and on their size. We can separate the two dependencies by correlating the simultaneously acquired topography and near-field images pixel-by-pixel. This allows us to establish material-specific mapping of polydisperse nanoparticle mixtures with nanoscale spatial resolution. We experimentally demonstrate the differentiation between sub-10 nm gold and polymer particles adsorbed on a Si substrate. Possible applications of our method range from the material-specific mapping of nanoparticle assemblies to the measurement of the doping concentration in single semiconductor nanoparticles.

Analytical model for quantitative prediction of material contrasts in scattering-type near-field optical microscopy

Nanometer-scale mapping of complex optical constants by scattering-type near-field microscopy has been suffering from quantitative discrepancies between the theory and experiments. To resolve this problem, a novel analytical model is presented here. The comparison with experimental data demonstrates that the model quantitatively reproduces approach curves on a Au surface and yields an unprecedented agreement with amplitude and phase spectra recorded on a phonon-polariton resonant SiC sample. The simple closed-form solution derived here should enable the determination of the local complex dielectric function on an unknown sample, thereby identifying its nanoscale chemical composition, crystal structure and conductivity.

Infrared imaging of single nanoparticles via strong field enhancement in a scanning nanogap

We demonstrate nanoscale resolved infrared imaging of single nanoparticles employing near-field coupling in the nanoscopic gap between the metal tip of a scattering-type near-field optical microscope and the substrate supporting the particles. Experimental and theoretical evidence is provided that highly reflecting or polariton-resonant substrates strongly enhance the near-field optical particle contrast. Using Si substrates we succeeded in detecting Au particles as small as 8 nm (<lambda/1000) at midinfrared wavelengths of about lambda=10 microm. Our results open the door to infrared spectroscopy of individual nanoparticles, nanocrystals, or macromolecules.

Nanoscale resolved infrared probing of crystal structure and of plasmon-phonon coupling

We show that slight variations of a crystal lattice cause significant spectral modifications of phonon-polariton resonant near-field interaction between polar semiconductor crystals and a scanning metal tip. Exploiting the effect for near-field imaging a SiC polytype boundary, we establish infrared mapping of crystal structure and crystal defects at 20 nm spatial resolution (lambda/500). By spectroscopic probing of doped SiC polytypes, we find that phonon-polariton resonant near-field interaction is also sensitive to electronic properties due to plasmon-phonon coupling in the crystals.

Subwavelength-scale tailoring of surface phonon polaritons by focused ion-beam implantation

Recent advances in optical nanotechnologies by controlling surface plasmon polaritons in metallic nanostructures demonstrate high potential for subwavelength-scale waveguiding of light, data storage, microscopy or biophotonics. Surprisingly, surface phonon polaritons-infrared counterparts to surface plasmon polaritons-have not been widely explored for nanophotonic applications. As they rely on the infrared or terahertz excitation of lattice vibrations in polar crystals they offer totally different material classes for nanophotonic applications, such as semiconductors and insulators. In an initial step towards nanoscale surface phonon photonics we show evidence that the local properties of surface phonon polaritons can be tailored at a subwavelength-scale by focused ion-beam modification of the crystal structure, even without significant alteration of the surface topography. Such single-step-fabricated, monolithic structures could be used for controlling electromagnetic energy transport by surface phonon polaritons in miniaturized integrated devices operating at infrared or terahertz frequencies. We verify the polaritonic properties of an ion-beam-patterned SiC surface by infrared near-field microscopy. The near-field images also demonstrate nanometre-scale resolved infrared mapping of crystal quality useful in semiconductor processing or crystal growth.