Nanotechnology and modern materials science demand reliable local probing techniques on the nanoscopic length scale. Most commonly, scanning probe microscopy methods are applied in numerous variants and shades, for probing the different sample properties. Scattering scanning near-field optical microscopy (s-SNOM), in particular, is sensitive to the local optical response of a sample, by scattering light off an atomic force microscopy (AFM) tip, yielding a wavelength-independent lateral resolution in the order of ∼10 nm. However, local electric potential variations on the sample surface may severely affect the probe–sample interaction, thereby introducing artifacts into both the optical near-field signal and the AFM topography. On the other hand, Kelvin-probe force microscopy (KPFM) is capable of both probing and compensating such local electric potentials by applying a combination of ac and dc-voltages to the AFM tip. Here, we propose to combine s-SNOM with KPFM in order to compensate for undesirable electrostatic interaction, enabling the in situ probing of local electric potentials along with pristine optical responses and topography of sample surfaces. We demonstrate the suitability of this method for different types of materials, namely, metals (Au), semiconductors (Si), dielectrics (SiO2), and ferroelectrics (BaTiO3), by exploring the influence of charges in the systems as well as the capability of KPFM to compensate for the resulting electric force interactions.

We report nonlinear charge-carrier response in GaAs/InGaAs core/shell nanowires that are driven by intense THz pulses. In the first experiment, half-cycle THz pulses emitted from an organic DSTMS crystal lead to a red-shift of the plasmon peak indicating intervalley transfer of the electrons. In the second experiment, a single, highly electron doped nanowire is investigated by scattering near-field infrared microscopy using intense free-electron laser (FEL) pulses. Here the observed red shift of the mid-infrared plasma resonance depends on the pulse energy and can be explained by heating the electron system in the nonparabolic conduction band.

This talk advertises scattering-type scanning near-field. [...]

Lukas Wehmeier,1,* Denny Lang,2 Yongmin Liu,3,4,5 Xiang Zhang,5 Stephan Winnerl,2 Lukas M. Eng,1,6 and Susanne C. Kehr1,† 1Institute of Applied Physics, Technische Universität Dresden, 01062 Dresden, Germany 2Institute of Ion Beam Physics and Materials Research, Helmholtz-Zentrum Dresden-Rossendorf, 01328 Dresden, Germany 3Department of Mechanical and Industrial Engineering, Northeastern University, Boston, Massachusetts 02115, USA 4Department of Electrical and Computer Engineering, Northeastern University, Boston, Massachusetts 02115, USA 5Department of Mechanical Engineering, University of California, Berkeley, California 94720, USA 6ct.qmat, Dresden-Würzburg Cluster of Excellence EXC 2147, TU Dresden, 01062 Dresden, Germany

Resonant infrared near-field optical spectroscopy provides a highly material-specific response with sub-wavelength lateral resolution of similar to 10 nm. Here, we report on the study of the near-field response of selected paraelectric and ferroelectric materials, i.e., SrTiO3, LiNbO3, and PbZr0.2Ti0.8O3, showing resonances in the wavelength range from 13.0 to 15.8 mu m. We investigate these materials using scattering scanning near-field optical microscopy in combination with a tunable mid-infrared free-electron laser. Fundamentally, we demonstrate that phonon-induced resonant near-field excitation is possible for both p- and s-polarized incident light, a fact that is of particular interest for the nanoscopic investigation of anisotropic and hyperbolic materials. Moreover, we exploit that near-field spectroscopy, as compared to far-field techniques, bears substantial advantages such as lower penetration depths, stronger confinement, and a high spatial resolution. The latter permits the investigation of minute material volumes, e.g., with nanoscale changes in crystallographic structure, which we prove here via near-field imaging of ferroelectric domain structures in PbZr0.2Ti0.8O3 thin films.

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.

This talk advertises scattering-type scanning near-field infrared micro-spectroscopy (s-SNIM) in the spectral range of 75 to 1.3 THz [1], as provided by the free-electron laser FELBE, the narrow-band laser-light source at Helmholtz-Zentrum Dresden-Rossendorf (HZDR), Germany. We demonstrate the -independent s-SNIM resolution of a few 10 nm by exploring structured Au samples, Graphene-transistors, meta-materials [2], and local ferroelectric phase-transitions down to LHe [3]. s-SNIM secondly was integrated into a THz pump-probe experiment for the inspection of excited states in structured SiGe samples. We developed a novel demodulation technique with high temporal resolution [4] hence achieving an excellent Signal-to-Noise Ratio. Thirdly using the super-radiant TELBE light source [5], HZDR recently extended the wavelength range down to 100 GHz radiation. We adapted our s-SNIM to this TELBE photon-source as well, achieving an equally high spatial resolution as with FELBE. Moreover, the superb 30-fs temporal resolution of TELBE will allow us to study a multitude of physical phenomena with sub-cycle resolution [5,6], such as spin-structures, magnons and phonon polaritons. [1] F. Kuschewski et al., Appl. Phys. Lett. 108 (2016) 113102. [2] S.C. Kehr et al., ACS Photonics 3 (2016) 20. [3] J. Döring et al., Appl. Phys. Lett. 105 (2014) 053109. [4] F. Kuschewski et al., Sci. Rep. 5 (2015) 12582. [5] B. Green et al., Sci. Rep. 6 (2016) 22256. [6] S. Kovalev et al., Struct. Dyn. 4 (2017) 024301.

We present nanoscopic infrared-optical investigations on highly n-type doped GaAs-based nanowires, revealing interesting nonlinear phenomena such as a pronounced redshift of the plasma resonance by the strong THz fields of a free-electron laser.

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.

We report a strong shift of the plasma resonance in highly-doped GaAs/InGaAs core/shell nanowires (NWs) for intense infrared excitation observed by scattering-type scanning near-field infrared microscopy. The studied NWs show a sharp plasma resonance at a photon energy of about 125 meV in the case of continuous wave excitation by a CO2 laser. Probing the same NWs with the pulsed free-electron laser with peak electric field strengths up to several 10 kV cm-1 reveals a power-dependent redshift to about 95 meV and broadening of the plasmonic resonance. We assign this effect to a substantial heating of the electrons in the conduction band and subsequent increase of the effective mass in the nonparabolic Γ-valley.

A key milestone for the next generation of high-performance multifunctional microelectronic devices is the monolithic integration of high-mobility materials with Si technology. The use of Ge instead of Si as a basic material in nanoelectronics would need homogeneous p - and n -type doping with high carrier densities. Here we use ion implantation followed by rear side flash-lamp annealing (r-FLA) for the fabrication of heavily doped n -type Ge with high mobility. This approach, in contrast to conventional annealing procedures, leads to the full recrystallization of Ge films and high P activation. In this way single crystalline Ge thin films free of defects with maximum attained carrier concentrations of 2.20 ± 0.11 × 10 20 cm −3 and carrier mobilities above 260 cm 2 /(V·s) were obtained. The obtained ultra-doped Ge films display a room-temperature plasma frequency above 1,850 cm −1 , which enables to exploit the plasmonic properties of Ge for sensing in the mid-infrared spectral range.

Superlenses enable near-field imaging beyond the optical diffraction limit. However, their widespread implementation in optical imaging technology so far has been limited by large-scale fabrication, fixed lens position, and specific object materials. Here we demonstrate that a dielectric lamella of subwavelength size in all three spatial dimensions behaves as a compact superlens that operates at infrared wavelengths and can be positioned to image any local microscopic area of interest on the sample. In particular, the lamella superlens may be placed in contact with any type of object and therefore enables examination of hard-to-scan samples, for example, with high topography or in liquids, without altering the specimen design. This lamella-based local superlens design is directly applicable to subwavelength light-based technology, such as integrated optics.