Infrared nanoscopy down to liquid helium temperatures

Abstract

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 CO-IR excitation at T = 7 K using a bare Au reference and a structured Si/SiO-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 GaVS at T = 42 K. We apply LT s-SNIM to study the spectral response of GaVS 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.

Polarization-dependent near-field phonon nanoscopy of oxides: SrTiO3, LiNbO3 , and PbZr0.2Ti0.8Wehmeier1, Lang2, Liu3 et al. 2019Phys. Rev. BSelf Cite235230View full textAdd to dashboardCite

“…[12][13][14][15] Wavelength-independent spatial resolution in the order of ∼ 10 nm has been demonstrated via s-SNOM and nano-FTIR for different material systems, such as metal/nonmetal structures, [16][17][18][19] organic 16,20 and biological materials, 1,21,22 semiconductors, 18,23 and ferroelectric domain structures. [24][25][26][27] Close to material resonances such as plasmon and phonon modes, signal strength and material contrast in s-SNOM can be strongly enhanced. 17,[24][25][26][27][28][29][30][31] At infrared wavelengths, this mechanism is highly sensitive to the material properties and may be applied to polar materials, 28,31 metals, semiconductors, 18,23 and biological samples.…”

Internal strain tunes electronic correlations on the nanoscale

“…Although electronic charge ordering in this material is seemingly at odds with a second-order scenario, no distinct evidence of phase segregation with a phase boundary separating the metallic and insulating regions was reported so far, despite several attempts to spatially map the transition ( 20 , 21 ). The recent advances in nanoscopic measurement techniques open up a completely new view that was not possible before ( 4 , 5 , 22 ). Here, we want to address the question how the metal-insulator phase transition of an electronically correlated solid proceeds on the nanometer scale and eventually resolve the controversially discussed nature of the phase transition.…”

Phonon-induced near-field resonances in multiferroic BiFeO3 thin films at infrared and THz wavelengths

“…13,[17][18][19][20][21][22] In addition to the spatial resolution, the probing depth of s-SNOM of about 100 nm 21,23-26 presents a major advantage for the investigation of small volumes or thin film samples, allowing for IR thin film spectroscopy with negligible direct substrate contribution to the optical signal. 27 The signal strength of s-SNOM is greatly enhanced via polaritoninduced resonant tip-sample interaction, 12,17,27,[30][31][32][34][35][36][37][38][39][40][41][42][43][44][45] which is of special advantage when exploring technically challenging wavelength regimes, 17,45 such as the "THz gap" (30-300 lm, i.e., 1-10 THz). 46 Particularly, sample-resonant s-SNOM provides enhanced sensitivity to the smallest material variations such as doping level and charge carrier concentration, 22,36,39,47 optical anisotropy tensor orientation in ferroelectric materials, 30,31,40,48 polymorphism, 36 and local stress distribution.…”

“…The wavelength regions of interest are investigated with a homebuilt s-SNOM that implements demodulation at higher harmonics of the mechanical cantilever oscillation frequency 53,54 and a selfhomodyne detection scheme, with the latter leading to a combined response of near-field amplitude and phase. 17,53 For illumination, we use the tunable narrow-band free-electron laser (FEL) at the Helmholtz-Zentrum Dresden-Rossendorf (HZDR), Germany, 27,31,40,41,55 which is a linearly-polarized, pulsed laser source at 13 MHz repetition rate covering the wavelength range of 5-250 lm, i.e., 1.2-60.0 THz. 56 The experimental setup is described in detail elsewhere.…”