Plasmonic superlensing in doped GaAs.

We demonstrate a semiconductor based broadband near-field superlens in the mid-infrared regime. Here, the Drude response of a highly doped n-GaAs layer induces a resonant enhancement of evanescent waves accompanied by a significantly improved spatial resolution at radiation wavelengths around λ = 20 µm, adjustable by changing the doping concentration. In our experiments, gold stripes below the GaAs superlens are imaged with a λ/6 subwavelength resolution by an apertureless near-field optical microscope utilizing infrared radiation from a free-electron laser. The resonant behavior of the observed superlensing effect is in excellent agreement with simulations based on the Drude-Lorentz model. Our results demonstrate a rather simple superlens implementation for infrared nanospectroscopy.

Low-temperature nanospectroscopy of the structural ferroelectric phases in single-crystalline barium titanate.

We optically investigate the local-scale ferroelectric domain structure of tetragonal, orthorhombic, and rhombohedral barium titanate (BTO) single crystals using scattering-type scanning near-field infrared (IR) optical microscopy (s-SNIM) at temperatures down to 150 K. Thanks to the precisely tunable narrow-band free-electron laser FELBE, we are able to explore the spectral fingerprints and IR resonances of these three phases and their domain orientations in the optical IR near-field. More clearly, every structural phase is analyzed with respect to its near-field resonances close to a wavelength of 17 µm when exploring the (111)-oriented BTO sample surface. Furthermore, near-field imaging at these resonances is performed, that clearly allows for the unambiguous optical identification of different domain orientations. Since our s-SNIM is based on a non-contact scanning force microscope, our s-SNIM findings are backed up by sample-topography and piezoresponse force microscopy (PFM) imaging, providing complementary information in an excellent match to the s-SNIM results.

Infrared nanoscopy down to liquid helium temperatures.

We introduce a scattering-type scanning near-field infrared microscope (s-SNIM) for the local scale near-field sample analysis and spectroscopy from room temperature down to liquid helium (LHe) temperature. The extension of s-SNIM down to T = 5 K is in particular crucial for low-temperature phase transitions, e.g., for the examination of superconductors, as well as low energy excitations. The low temperature (LT) s-SNIM performance is tested with CO2-IR excitation at T = 7 K using a bare Au reference and a structured Si/SiO2-sample. Furthermore, we quantify the impact of local laser heating under the s-SNIM tip apex by monitoring the light-induced ferroelectric-to-paraelectric phase transition of the skyrmion-hosting multiferroic material GaV4S8 at Tc = 42 K. We apply LT s-SNIM to study the spectral response of GaV4S8 and its lateral domain structure in the ferroelectric phase by the mid-IR to THz free-electron laser-light source FELBE at the Helmholtz-Zentrum Dresden-Rossendorf, Germany. Notably, our s-SNIM is based on a non-contact atomic force microscope (AFM) and thus can be complemented in situ by various other AFM techniques, such as topography profiling, piezo-response force microscopy (PFM), and/or Kelvin-probe force microscopy (KPFM). The combination of these methods supports the comprehensive study of the mutual interplay in the topographic, electronic, and optical properties of surfaces from room temperature down to 5 K.

Characteristics of ferroelectric-ferroelastic domains in Néel-type skyrmion host GaV4S8.

GaV4S8 is a multiferroic semiconductor hosting Néel-type magnetic skyrmions dressed with electric polarization. At Ts = 42 K, the compound undergoes a structural phase transition of weakly first-order, from a non-centrosymmetric cubic phase at high temperatures to a polar rhombohedral structure at low temperatures. Below Ts, ferroelectric domains are formed with the electric polarization pointing along any of the four 〈111〉 axes. Although in this material the size and the shape of the ferroelectric-ferroelastic domains may act as important limiting factors in the formation of the Néel-type skyrmion lattice emerging below TC = 13 K, the characteristics of polar domains in GaV4S8 have not been studied yet. Here, we report on the inspection of the local-scale ferroelectric domain distribution in rhombohedral GaV4S8 using low-temperature piezoresponse force microscopy. We observed mechanically and electrically compatible lamellar domain patterns, where the lamellae are aligned parallel to the (100)-type planes with a typical spacing between 100 nm-1.2 µm. Since the magnetic pattern, imaged by atomic force microscopy using a magnetically coated tip, abruptly changes at the domain boundaries, we expect that the control of ferroelectric domain size in polar skyrmion hosts can be exploited for the spatial confinement and manipulation of Néel-type skyrmions.