Abstract-Terahertz Nanoimaging and Nanospectroscopy of Chalcogenide Phase-Change Materials

Chao Chen, Shu Chen, Ricardo P.S.M. Lobo, Carlos Maciel-Escudero, Martin Lewin, Thomas Taubner, Wei Xiong, Ming Xu, Xinliang Zhang, Xiangshui Miao, Peining Li,  Rainer Hillenbrand

https://pubs.acs.org/doi/10.1021/acsphotonics.0c01541

 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.

Abstract-Active Chiral Plasmonics

Xinghui Yin^*†§, Martin Schäferling^†, Ann-Katrin U. Michel^‡, Andreas Tittl^†, Matthias Wuttig^‡, Thomas Taubner^‡ and Harald Giessen^†

*E-mail: x.yin@pi4.uni-stuttgart.de.

†4th Physics Institute and Research Center SCoPE, University of Stuttgart, 70550, Stuttgart, Germany‡I. Institute of Physics (IA), RWTH Aachen University, 52056, Aachen, Germany§Max Planck Institute for Solid State Research, Heisenbergstrasse 1, D-70569 Stuttgart, Germany

Nano Letters

http://pubs.acs.org/doi/abs/10.1021/nl5042325?journalCode=nalefd

DOI: 10.1021/nl5042325

Active control over the handedness of a chiral metamaterial has the potential to serve as key element for highly integrated polarization engineering approaches, polarization sensitive imaging devices, and stereo display technologies. However, this is hard to achieve as it seemingly involves the reconfiguration of the metamolecule from a left-handed into a right-handed enantiomer and vice versa. This type of mechanical actuation is intricate and usually neither monolithically realizable nor viable for high-speed applications. Here, enabled by the phase change material Ge₃Sb₂Te₆(GST-326), we demonstrate a tunable and switchable mid-infrared plasmonic chiral metamaterial in a proof-of-concept experiment. A large tunability range of the circular dichroism response from λ = 4.15 to 4.90 μm is achieved, and we experimentally demonstrate that the combination of a passive bias-type chiral layer with the active chiral metamaterial allows for switchable chirality, that is, the reversal of the circular dichroism sign, in a fully planar, layered design without the need for geometrical reconfiguration. Because phase change materials can be electrically and optically switched, our designs may open up a path for highly integrated mid-IR polarization engineering devices that can be modulated on ultrafast time scales.