F. Koch

Giant birefringence in anisotropically nanostructured silicon

We performed a study of the in-plane birefringence of anisotropically nanostructured Si layers, which exhibit a greater difference in the main value of the anisotropic refractive index than that of natural birefringent crystals. The anisotropy parameters were found to be strongly dependent on the typical size of the Si nanowires used to assemble the layers. This finding opens the possibility of an application of birefringent Si retarders to a wide spectral range for control of the polarization state of light.

Dichroic Bragg reflectors based on birefringent porous silicon

Multilayers of anisotropically nanostructured silicon (Si) have been fabricated and studied by polarization-resolved reflection measurements. Alternating layers having different refractive indices exhibit additionally a strong in-plane anisotropy of their refractive index (birefringence). Therefore, a stack of layers, acting as a distributed Bragg reflector, has two distinct reflection bands, depending on the polarization of the incident linearly polarized light. This effect is governed by a three-dimensional (in-plane and in-depth) variation of the refractive index. These structures can yield optical effects which are difficult to achieve with conventional Bragg reflectors.

Defects in porous silicon investigated by optically detected and by electron paramagnetic resonance techniques

The defect properties of as‐etched and annealed porous silicon are studied by electron paramagnetic resonance (EPR) and optically detected magnetic resonance (ODMR). The paramagnetic defect observed is closely related to the Pb0 center at the Si/SiO2 interface. In EPR a minimum defect density of 1016 cm−3 is observed for the as‐etched silicon, which reaches a maximum of 8×1018 cm−3 for samples annealed at about 400 °C. In the ODMR experiments, the same dangling bond center is observed on the 1.5 eV luminescence band enhancing the luminescence—but with increased sensitivity and as a decrease of the emission intensity in the infrared emission band at 1 eV of porous silicon.

Highly sensitive recognition element based on birefringent porous silicon layers

Anisotropically nanostructured silicon layers exhibit a strong in-plane birefringence. Their optical anisotropy parameters are found to be extremely sensitive to the presence of dielectric substances inside of the pores. Polarization-resolved transmittance measurements provide an extremely sensitive tool to analyze the adsorption of various atoms and molecules in negligible quantities. A variation of the transmitted linearly polarized light intensity up to two orders of magnitude combined with a fast optical response in the range of seconds make these layers a good candidate for sensor applications.

Investigation of interface and bulk fatigue scenarios in sol-gel derived Pb(Zr0.5Ti0.5)O3 films by asymmetric field driving

A method to distinguish the bulk and interface scenarios of fatigue in sol-gel derived Pb(Zr0.5Ti0.5)O3 (PZT) thin films on Pt and LaNiO3 (LNO) substrates is proposed based on the asymmetric electric field driving of capacitors. The “hard” and “soft” failures of polarization, which are typical for sputtering and sol-gel deposited PZT, respectively, are realized in sol-gel derived Pt/PZT/Pt capacitors by changing the asymmetricity of driving pulses, indicating that the interface plays an important role in the polarization breakdown. For Pt/PZT/LNO capacitors, it is shown that the bulk pinning of domains appears to be more evident than the interface pinning. A model based on the electromigration and entrapment of charged defects subjected to the asymmetric field is also given, and the plausible approximation of space charge field versus charge density is discussed.

Band-gap states and ferroelectric restoration in strontium bismuth tantalate

By means of photoluminescence (PL) and sub-band-gap (sub-Eg) optical illumination, the degradation and restoration of ferroelectric properties in strontium bismuth tantalate thin films have been investigated, and the existence of band-gap states is demonstrated. It is shown that the suppression and recovery of ferroelectricity are closely correlated with the change of PL intensity, since both switchable polarization and PL are related to Ta5+ ions in the TaO6 octahedron. Furthermore, the electric-field-induced restoration increases dramatically by the aid of sub-band-gap light (2.5 eV⩽hν⩽Eg) illumination absorbed in band-gap states.

Strongly opalescent liquid network formed in a porous silicon matrix

We report on a medium exhibiting extremely efficient light scattering properties: a liquid network formed in a porous silicon matrix. We find that the scattering efficiency depends strongly on the filling factor of the liquid in the pores, its dielectric constant, and the type of termination of the internal surface of the layer. The spectral dependence of the scattering length of photons evidences the fact that the phenomenon is governed by a Mie-type scattering mechanism. The mean free path of photons in this medium is found to be in the micrometer range.

Electroabsorption in ZnSe/ZnSSe heterostructures: N-type negative differential resistance for self-electro-optic-effect-device applications.

We compute the spectrally dependent electroabsorption α(ωo, F) on an undoped ZnSe layer considered as part of a p-i-n heterostructure self-electro-optic-effect device (SEED). It is shown that efficient SEED operation, with a reflectivity contrast ratio of ∼4, can be expected for a 0.3-μm-thick ZnSe layer in a reflection geometry with an applied voltage of 4 V.