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Priv.-Doz. Dr. Elisabeth Soergel
Physikalisches Institut
Nußallee 12
D-53115 Bonn
Tel.  #49 - 228 - 73 3240
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Bureau (050) in the souterrain of the Wegelerstrasse 10.

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 A ferroelectric material in general is defined by the existence of a permanent spontaneous polarization Ps at temperatures below the Curie temperature whereby the direction of Ps can be reversed by the application of an electric field exceeding the coercive field Ec. The value of Ec is defined using the hysteresis loop which can be recorded during a poling cycle ramping up and down the applied electric field. Theoretical calculations of the value of Ec failed up to now. A ferroelectric material is by nature always piezoelectric and also pyroelectric.

Ferroelectric domains
A ferroelectric domain is an area of oriented spontaneous polarization. Local poling, i.e. the controlled formation of domains, makes of ferroelectrics very important materials for applications such as data storage devices or optical frequency converters. The controlled formation of domains is therefore of major importance. In general, local poling is performed by locally applying an electric field surpassing Ec using structured electrodes, named electric field poling (EFP).  We investigated a new technique for controlled domain formation thereby defining the domain pattern by UV-laser light irradiation. Another technique under investigation for local poling consists in the application of an electric field with the help of a scanning probe microscope tip, named tip-based domain formation.

Domain formation by UV-laser irradiation
In close collaboration with Sakellaris Mailis from the ORC in Southampton, GB.
This method for domain revesal uses a tightly focused, strongly absorbed UV-laser beam to define the area of domain reversal. Scanning the laser beam across the LiNbO3 surface results either in poling inhibition (+z) or in direct writing (-z and the non-polar x- and y-faces). This method for domain reversal allow for the formation of domain patterns irrespective of the crystallographic preferences (a domain growth along the y-axis) and further more avoids the cumbersome and expensive clean-room processing required for EFP.


Tip-based domain formation in He-implanted LiNbO3
In close collaboration with Richard Osgood from Columbia University, USA.
Driven by the progess of smart-cut technology which allows to fabricate lithium niobate thin film single crystals we investigated the possibility of domain formation in He-implanted crystals. In view of their further applicability for photonic crystal devices, we focused on large-area nano-domain patterning.
Tip-based anomalous domain formation
Domain reversal with the help of the tip in ultra-thin single crystals was proven to allow for storage-devices of high density. This technique relies on the reproducible fomratin of domains, all of identical shape. However, it has been observed that in some cases tip-based domain formation yields doughnut-shaped domains, calles anomalous domain formation. This effect is caused by charge injection into the sample during the poling pulse. We optimized the parameters for domain formation in order to reliably allow for full-domains to be generated.


Tip-based determination of the coercive field
The determination of the coercive field requires in general the application of an electric field across a sample slab. This is technically realised by evaporating conductive electrodes or using a liquid-electrode setup. In particular for small samples, this technique fails. But also for slightly conducting materials, the coercive field can not be determined in this was. Scannig probe microscopy allows for a different method for determining Ec by analyzing the temporal evolution of the size of tip-based domains.


Fabrication of photonic microstructures
In close collaboration with Sakellaris Mailis from the ORC in Southampton, GB.
Microstructures of nonlinear optical crystalline material are in general fabricated by a cumbersome cutting and polishing procedure. Highly desirable for applications such as whispering mode galleryl resonatores the surface quality of sich components, however, is of major importance. A very different approach for the fabrication of topographical microstructures takes advantage of the domain selective etching. This allows to transfer a domain pattern into a topographical structure. In order to smoothen the surfaces which became rough during the etching process, the samples are submitted to a thermal annealing procedure. This technique yields ultra-smooth microstructures as it can be seen in the figure below.

FD ultra smooth

SEM images of a) the initial structure, comprising an undercut, produced by inhibition of poling followed by deep chemical etching using HF acid, b) corresponding annealed structure showing a quasi-oblate spheroid top. In both images the sample is tilted by 45°.
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