Polarization driven conductance variations at charged ferroelectric domain walls.

Conducting domain walls (CDWs) in ferroelectric materials are promising candidates for applications in a manifold of nanoscale, optoelectronic devices. Characterization of their microscopic properties, however, remains challenging due to their small dimension and highly insulating environment. Here, we inspect individual CDWs in single-crystalline LiNbO3 by the combination of photoemission electron microscopy (PEEM) and second harmonic generation (SHG) microscopy. While SHG unveils the overall domain wall inclination angle α, PEEM is sensitive to local conductance variations, both at and away from the domain wall. Thus, the two imaging techniques deliver complementary information over a large field of view. In agreement with earlier theoretical predictions we find that the local conductance is dictated by α and reveal a quantitative connection between them. Our results help to elucidate the electronic structure of CDWs and underline the value of PEEM as a non-contact characterization tool for mapping local conductance variations in highly resistive environments.

Bloch oscillations in plasmonic waveguide arrays.

The combination of modern nanofabrication techniques and advanced computational tools has opened unprecedented opportunities to mold the flow of light. In particular, discrete photonic structures can be designed such that the resulting light dynamics mimics quantum mechanical condensed matter phenomena. By mapping the time-dependent probability distribution of an electronic wave packet to the spatial light intensity distribution in the corresponding photonic structure, the quantum mechanical evolution can be visualized directly in a coherent, yet classical wave environment. On the basis of this approach, several groups have recently observed discrete diffraction, Bloch oscillations and Zener tunnelling in different dielectric structures. Here we report the experimental observation of discrete diffraction and Bloch oscillations of surface plasmon polaritons in evanescently coupled plasmonic waveguide arrays. The effective external potential is tailored by introducing an appropriate transverse index gradient during nanofabrication of the arrays. Our experimental results are in excellent agreement with numerical calculations.

Piezoresponse force microscopy at sub-room temperatures.

Piezoresponse force microscopy is demonstrated at temperatures between -80 °C and +120 °C using a commercial room temperature atomic force microscope upgraded with a home-built cooling/heating-stage. We applied temperature-ramp-synchronized piezoresponse force microscope (PFM) for tracing the temperature dependence of the formation of ferroelectric domains. The potential of our sub-room temperature PFM is demonstrated by investigating the formation and evolution of ferroelectric domains in RbHSO4 as a function of temperature and time, respectively.

Large-area regular nanodomain patterning in He-irradiated lithium niobate crystals.

Large-area ferroelectric nanodomain patterns, which are desirable for nonlinear optical applications, were generated in previously He-implanted lithium niobate crystals by applying voltage pulses to the tip of a scanning force microscope. The individual nanodomains were found to be of uniform size, which depended only on the inter-domain spacing and the pulse amplitude. We explain this behavior by the electrostatic repulsion of poling-induced buried charges between adjacent domains. The domain patterns were imaged by piezoresponse force microscopy and investigated by domain-selective etching in conjunction with focused ion beam etching followed by scanning electron microscopy imaging. In order to optimize the He-irradiation parameters for easy and reliable nanodomain patterning a series of samples subjected to various irradiation fluences and energies was prepared. The different samples were characterized by investigating nanodomains generated with a wide range of pulse parameters (amplitude and duration). In addition, these experiments clarified the physical mechanism behind the facile poling measured in He-irradiated lithium niobate crystals: the damage caused by the energy loss that takes place via electronic excitations appears to act to stabilize the domains, whereas the nuclear-collision damage degrades the crystal quality, and thus impedes reliable nanodomain generation.

Spectral and electro-optic response of UV-written waveguides in LiNbO3 single crystals.

An experimental study of the spectral and electro-optic response of direct UV-written waveguides in LiNbO3 is reported. The waveguides were written using c.w. laser radiation at 275, 300.3, 302, and 305 nm wavelengths with various writing powers (35-60 mW) and scan speeds (0.1-1.0 mm/sec). Spectral analysis was used to determine the multimode and single mode wavelength regions and, the cut-off point of the fabricated waveguides. Electro-optic characterization of these waveguides reveals that the electro-optic coefficient (r33) decreases for longer writing wavelengths, with a maximum of 31 pm/V for 275 nm and, is reduced to 14 pm/V for waveguides written with 305 nm.

Direct-writing of inverted domains in lithium niobate using a continuous wave ultra violet laser.

The inversion of ferroelectric domains in lithium niobate by a scanning focused ultra-violet laser beam (lambda = 244 nm) is demonstrated. The resulting domain patterns are interrogated using piezoresponse force microscopy and by chemical etching in hydrofluoric acid. Direct ultra-violet laser poling was observed in un-doped congruent, iron doped congruent and titanium in-diffused congruent lithium niobate single crystals. A model is proposed to explain the mechanism of domain inversion.

Consequences of the background in piezoresponse force microscopy on the imaging of ferroelectric domain structures.

The interpretation of ferroelectric domain images obtained with a piezoresponse force microscope (PFM) is discussed. The influence of an inherent experimental background on the domain contrast in PFM images (enhancement, nulling, inversion) as well as on the shape and the location of the domain boundaries are described. We present experimental results to evidence our analysis of the influence of the background on the domain contrast in PFM images.

Cross-talk correction in atomic force microscopy.

Commercial atomic force microscopes usually use a position-sensitive photodiode to detect the motion of the cantilever via laser beam deflection. This readout technique makes it possible to measure bending and torsion of the cantilever separately. A slight angle between the orientation of the photodiode and the plane of the readout laser beam, however, causes false signals in both readout channels. This cross-talk may lead to misinterpretation of the acquired data. We demonstrate this fault with images recorded in contact mode on periodically poled ferroelectric crystals and present a simple electronic circuit to compensate for it. This circuit can correct for cross-talk with a bandwidth of approximately 1 MHz suppressing the the false signal to <<1%.