Milan Berta

Lead zirconate titanate ceramics Pb(Zr_1–x Ti _x )O₃ with spatially graded composition from X = 46 to 49 were studied by Raman scanning microscopy and the results were compared with Raman spectra in PZT ceramics of different compositions around the morphotropic phase boundary (MPB). An E-symmetry mode located near 200 cm⁻¹ was found to be a measure of the tetragonality of PZT in both standard graded ceramics. The singular value decomposition method was applied to decompose the experimental spectra of the graded ceramics. Two main contributions were found: one is related to the intensity of the main transverse modes – especially the E(TO2) mode – and it gives evidence of the change between tetragonal and rhombohedral symmetries in the sample; the second one shows changes just through the spatial MPB and it is related to the intensities of A₁(TO2) and E(LO2) modes. All the phonons involved in the main changes correspond to Zr and Ti vibrations.

Novel lead-free piezoelectric single-crystal with a morphotropic boundary composition (0.94Na_1/2Bi_1/2TiO₃–0.06BaTiO₃) was characterized by dielectric spectroscopy, far-IR reflectivity and Raman spectroscopy. Analysis of polarized Raman spectra suggests that the three principal Raman bands are associated with polar optic modes.

We report on results of a near-field imaging experiment with broadband terahertz pulses applied to a BaTiO₃ multidomain single crystal. Data with a spatial resolution below 15 µm were collected by our scanning time-domain terahertz imaging setup. We performed the scanning by an asymmetric skewed sapphire probe metallised from two sides ended with a 60 µm × 140 µm facet. The imaging scans were performed with various angles of the polarisation of the probing field with respect to the direction of the domain walls in order to identify the orientations with optimal sensitivity. The raw experimental data were prewhitened and processed by the singular value decomposition. In this way, independent components belonging to experimental features (polarisation, noise and external perturbations) were found. The spatial resolution of the sensing probe was found significantly higher than that allowed by the outer dimensions of the probe end facet. This result was also confirmed by electromagnetic-field simulations.

Phonons in both ceramic samples KNbO₃ (KN) and K_0.5Na_0.5NbO₃ (KNN-50) were investigated from 10 to 900 K by means of Raman, infrared, and THz spectroscopy. First-order transitions from cubic to tetragonal phases were detected at about 755 K (in KN) and 710 K (in KNN-50), where the first component of the polarization appears. Transitions from the tetragonal to the orthorhombic phases take place around 510 and 475 K, respectively. The last transitions from orthorhombic to rhombohedral phases are strongly first-order type. T_C is shifted from 200 K in KN to about 90 K in KNN-50. All Raman active modes below 200 cm^-1. disappear in KN but not in KNN-50. The overdamped soft mode present at high temperatures in the THz range changes its dielectric strength at each phase transition (when the corresponding component jumps to higher frequencies) and abruptly disappears from THz spectra in the rhombohedral phase, because it stiffens up to 200 cm^-1. This mode has lower frequency and higher dielectric strength in KNN-50.

We report on a study of the near-field sensitivity of a metal-dielectric near-field terahertz probe by time-domain terahertz spectroscopy. The obtained experimental data were analysed using principal component analysis. Principal components corresponding to the changes in the output terahertz pulse upon varying the probe-sample distance and reflecting the local anisotropy in a ferroelectric BaTiO3 crystal were extracted and identified. Simulations of the pulse propagation within a model of the probe revealed very similar independent components.

Terahertz (THz) time-domain spectroscopy was used in vivo to measure the body reflectance with the aim of determining experimentally the influence of haemodialysis on the content of water in the human skin. For this purpose, an original methodology of determining the skin properties at THz frequencies from the reflectance was developed. A series of measurements were performed before and after dialysis with ten subjects. The results strongly indicate that the surface body hydration is not the main parameter determining the skin conductivity in the THz range.

We present a high performance near-field microwave and terahertz imaging method suitable for broadband applications. The approach is based on a near-field antenna which consists of a rectangular-shaped block of low-loss dielectric material sharpened to a pyramidal tip which is partially metallized and terminated by a micrometer-sized plane facet.

In this paper we present a high performance near-field imaging method suitable for broadband applications. Recent advances in preparing micrometer and submicrometer resolution near-field probes are reported, along with examples of applications of this method both for frequency and time domain measurements. For the latter, pulsed terahertz radiation has been used to probe a broadband (300 GHz-2 THz) spectroscopic response of substances with different dielectric properties. For frequency domain measurements a continuous wave 80 GHz radiation has been applied to metallic, semiconducting, ferroelectric and biological objects. Frequency independent high spatial resolution and high sensitivity have been demonstrated for all measurements.