Terahertz Radiation Imaging of Ferroelectric Domain Topography in Room-Temperature Hydrogen-Bonded Supramolecular Ferroelectrics

Abstract

Organic ferroelectrics with lightweight, flexible, low-cost, and environmentally benign characteristics are recently receiving great attention for new electric and optical devices. Since the propagation of ferroelectric domain walls and the subsequent reformation of ferroelectric domains are the basis for these devices, detection of the ferroelectric domain topology is crucial. Here, we demonstrate a new method to detect ferroelectric domains in inside and surface regions of organic ferroelectrics by mapping out two orthogonally polarized terahertz waves radiated from the crystal upon the irradiation of near-infrared femtosecond laser pulses. We used polarization dependence of the effective depths radiating the terahertz waves, which originate from the optical anisotropy in the terahertz frequency region. This allows us to distinguish ferroelectric domains in the inside and surface regions of the crystals. We applied this method to a room-temperature organic supramolecular ferroelectric crystal, 1:1 salt of 5,5'-dimethyl-2,2'-bipyridine and deuterated iodanilic acid. A single domain covering almost all the area of an as-grown crystal (600 m  800 m) is discerned in the inside region, while complicated multi-domain in size of 200 m is observed in the surface 2 region. By applying external electric field along the 2c-b axis (ferroelectric polarization direction), the polarization switching proceeds with successive propagations of uncharged (neutral) and quasi-one-dimensional 180 domain walls (DWs) along the b-axis ( 2c-b axis). This results in the formation of another uncharged and two-dimensional 180 DW parallel to the (100) plane, which covers all the area of the crystal. We discuss the usefulness of the present terahertz radiation imaging technique and ferroelectric DW dynamics in terms of anisotropic stacking of hydrogen-bonded chains. PACS 78.30.Jw, 77.80.-e, 42.30.-d, 42.65.Re DWs perpendicular to P. Since uncharged DWs are electrically stable (divP = 0), they are observed in various ferroelectrics [2]. On the other hand, charged DWs such as head-to-head (tail-to-tail) DWs have net bound charge (divP  0), which should be compensated by charged objects such as free carriers, defects, and impurities. Charged DWs frequently appear in as-grown crystals [7-10] and as-grown thin films [11-13].Therefore, real-space imaging of ferroelectric domain topology is not only indispensable to understand the nature of ferroelectricity but also crucial for the applications of ferroelectric materials.Recently, new series of room-temperature hydrogen-bonded ferroelectrics with large ferroelectric polarization have been developed in a family of low-dimensional

Terahertz radiation by subpicosecond spin-polarized photocurrent originating from Dirac electrons in a Rashba-type polar semiconductor

“…Unless otherwise stated, we detected the emitted terahertz wave by a standard electro-optic (EO) sampling with an [110]-oriented 1mm-thick ZnTe crystal. By using terahertz radiation imaging[22][23][24][25][26], we confirmed that the BiTeBr crystal used in the present study has a polar single-domain structure. The result of the domain imaging is reported in the Supplemental Material[27].…”

Ultrafast Photoinduced Electric-Polarization Switching in a Hydrogen-Bonded Ferroelectric Crystal

“…Comparing the two types of extensions, the one-dimensional extension seems to be more easily realized. Such tendency is also found in the microscope images of ferroelectric domains in the croconic acid crystal [28] and other materials [35,36], as a prevalence of one-dimensional domains, that are, line-or stripe-type domains running along the polarization direction. We also estimate a maximum domain size for the case of the one-dimensional domain as follows.…”

Imaging of Chemical Reactions Using a Terahertz Chemical Microscope

“…Laser-excited THz microscopy (LTEM) has also introduced THz pulses to various scientific and industrial fields. THz-TDS measures the optical properties of materials, whereas LTEM evaluates the dynamics of carriers or electric dipoles in materials and has already analyzed high-temperature superconductors (HTS) [8,9], ferroelectrics [10] and semiconductor devices [11][12][13][14]. LTEM irradiates materials by femtosecond laser pulses, generating THz radiation.…”