Uwe Voelker

Domain size effects in a uniaxial ferroelectric relaxor system: The case of SrxBa1−xNb2O6

We present investigations of the domain dynamics and of the evolution of the critical exponent β in a ferroelectric relaxor system, exemplarily in the up-to-date controversial strontium barium niobate (SBN). k-space spectroscopy at the phase-transition and when applying an electric field reveals a size-dependent response of the domains. This is supported by pyroelectric measurements that show, by analysis in terms of criticality, the critical exponent β is not only dependent on the level of poling but also on the manner in which the poling was achieved. It must be concluded that the crystals undergo a phase transition not—as commonly assumed—in a uniform way with homogeneous polarization throughout the crystal. Instead they behave as a set of more or less independent domains with size-dependent stability. Therefore, one should be very careful with critical exponents—especially those derived from experiments that explicitly or implicitly—assume a uniform polarization behavior.

The ferroelectric phase transition of calcium barium niobate: experimental evidence of Smolenskii's model for diffuse phase transitions?

We present investigations on temperature-dependent changes in the size distribution of ferroelectric domains in single crystals of the novel tungsten bronze type calcium barium niobate (CBN). Since its congruently melting composition has a relatively high ferroelectric phase transition temperature of about 265 °C, CBN can be considered as an interesting material for various future applications. Using k-space spectroscopy, both unpoled polydomain crystals and crystals poled at room temperature have been investigated in the vicinity of the ferroelectric phase transition. In unpoled CBN, an intermixture of domain-size dependent phase transitions has been observed, which can be described with the model for diffuse phase transitions established by Smolenskii.

Laser-Based Space Debris Monitoring

The extended use of outer space left remarkable amounts of objects uncontrollably orbiting the earth. These objects, hereafter denoted as “space debris”, impose reasonable risk on present space missions, as they hold relative velocities on the order of 10 km/s. Due to their high kinetic energies, objects up from 1 cm size have to be monitored in order to prevent collisions with satellites. For the purpose of accuracy and effectiveness, we present a concept for a laser‐based space debris monitoring system. The chosen approach is primarily based on laser ranging of optically non‐cooperative targets. The involved physical processes such as optical turbulence, absorption, and scattering are modeled and the results will define the design of a future system. In addition, short range on‐ground tracking and ranging will be conducted where downscaling of photon densities simulates the strong signal attenuation and system feasibility will be proven.

Investigating the Domain-Size Dependent Ferroelectric Switching Behaviour with K-Space Spectroscopy

K-space spectroscopy has emerged as a powerful tool for measuring ferroelectric domain structures. We report on the ferroelectric switching behaviour of calcium barium niobate, which has been investigated for the first time using this technique.

k-Spectra of the Uniaxial Relaxor Ferroelectric Sr x Ba 1-x Nb 2 O 6 (SBN)

We present k-spectra of the ferroelectric SBN showing that domains react size-dependent at the phase transition and on external electric fields. Thus, all models assuming domains in SBN to behave uniquely have to be reconsidered.

k-Space Spectroscopy: Investigation of a Size Dependent Phase Transition Behavior of Ferroelectric Domains

We report on a technique for the measuring of ferroelectric domain structuring (k-space spectroscopy). Momentum conservation for optical second harmonic generation is achieved by the Fourier representation of the domain structuring (random quasiphase matching).