Terahertz Nanoimaging and Nanospectroscopy of Chalcogenide Phase-Change Materials

Abstract

Chalcogenide phase-change materials (PCMs) exhibit optical phonons at terahertz (THz) frequencies, which can be used for studying basic properties of the phase transition and which lead to a strong dielectric contrast that could be exploited for THz photonics applications. Here, we demonstrate that the phonons of PCMs can be studied by frequency-tunable THz scattering-type scanning near-field optical microscopy (s-SNOM). Specifically, we perform spectroscopic THz nanoimaging of a PCM sample comprising amorphous and crystalline phases. We observe phonon signatures, yielding strong s-SNOM signals and, most important, clear spectral differences between the amorphous and crystalline PCM, which allows for distinguishing the PCM phases with high confidence on the nanoscale. We also found that the spectral signature can be enhanced, regarding both signal strength and spectral contrast, by increasing the radius of the probing tip. From a general perspective, our results establish THz s-SNOM for nanoscale structural and chemical mapping based on local phonon spectroscopy.

Fabrication of Epitaxial W-Doped VO2 Nanostructured Films for Terahertz Modulation Using the Solvothermal Process

“…Here, phase change materials, which are widely discussed as key components of THz manipulation devices, play a special role. 7,8 Vanadium dioxide (VO 2 ) undergoes a reversible metal-toinsulator phase transition (MIT) at a critical temperature (T MIT ) of 68 °C between insulating monoclinic VO 2 (M 1 ) with high THz transparency and metallic rutile VO 2 (R) with strong THz absorbance and reflection. 9,10 This ultrafast (<80 fs) 11 phase transition also involves sharp changes in electrical resistance and can be triggered by various types of stimuli, including thermal heating, electrical current excitation, light irradiation, and mechanical strain.…”

Terahertz Nano-Imaging with s-SNOM

“…Another aspect is the tip apex geometry. Surprisingly, blunt tips show a better THz s-SNOM signal than sharp tips [33,59]. This is explained by the increased interaction volume of the near-field zone to dominate over the reduction of the field enhancement.…”

“…• New approaches such as synthetic optical holography (SOH) [59,60], which relies on data post-processing, have been demonstrated successfully as well.…”

“…Another phase-sensing study was reported recently in Ref. [59]. It investigated the chalcogenide Ge 1 Sb 2 Te 4 , a so-called phase-change material Àa class of materials which is employed for nonvolatile rewritable data storage.…”

Near-Field Terahertz Nanoscopy of Coplanar Microwave Resonators

“…30 Notable uses of THz SNOM include probing surface waves, 31 resolving femtosecond interlayer tunnelling processes, 32 mapping of carrier concentrations in integrated circuits, 21,33 and tracking phase transitions in correlated materials at the nanoscale. 34,35 Here, we use THz SNOM to investigate the material properties and imperfections of wetetched aluminum resonators on silicon in a non-invasive way after different fabrication steps.…”