Biomedical subjects

Karsten Buse

Publications and source records attributed to Karsten Buse.

11 recordsLinked to original sources

PubMed2006

Holographic grating formation in a colloidal suspension of silver nanoparticles.

Holographic gratings are recorded in colloidal suspensions of silver nanoparticles by utilizing interfering nanosecond pulses. The diffraction efficiency is measured with continuous-wave light. An instantaneous response together with a transient grating are observed: the nanoparticles absorb the pump light and heat up. Heat is transferred to the solvent, and a delayed thermal grating appears. The final decay time constant of this grating depends quadratically on the period length and has a typical value of 1 micros for grating spacings of several micrometers.

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PubMed2006

Conductivity of oriented bis-azo polymer films.

The conductivity properties of electro-optic, photoaddressable, dense bis-azo chromophore polymer films are investigated by using samples corona poled at various temperatures. A dielectric spectrometer is applied to measure the frequency dependence of the conductivity at different temperatures before and after heating the material to above the glass transition temperature. The results show that the orientation of the chromophores changes the charge-carrier mobility. Ionic conductivity dominates in a more disordered configuration of the material, while the competing process of hole hopping takes over as a transition to a liquid-crystalline phase occurs when the material is heated to much higher than the glass transition temperature. Such microcrystallization strongly enhances the conductivity.

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PubMed2005

Large-area Fabry-Perot modulator based on electro-optic polymers.

We present a large-area electro-optic Fabry-Perot modulator utilizing a photoaddressable bis-azo polymer placed between two dielectric mirrors with an open aperture of 2 cm. A modulation efficiency of 1% at an effective modulation voltage of 20 V for a wavelength of 1.55 microm is demonstrated. By comparing distance tuning of the cavity with wavelength tuning, an effective electro-optic coefficient of -7 pm/V is measured.

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PubMed2005

Femtosecond holography in lithium niobate crystals.

Spatial gratings are recorded holographically by two femtosecond pump pulses at 388 nm in lithium niobate (LiNbO3) crystals and read out by a Bragg-matched, temporally delayed probe pulse at 776 nm. We claim, to our knowledge, the first holographic pump-probe experiments with subpicosecond temporal resolution for LiNbO3. An instantaneous grating that is due mostly to the Kerr effect as well as a long-lasting grating that results mainly from the absorption caused by photoexcited carriers was observed. The Kerr coefficient of LiNbO3 for our experimental conditions, i.e., pumped and probed at different wavelengths, was approximately 1.0 x 10(-5) cm2/GW.

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PubMed2005

Modeling of X-ray-induced refractive index changes in poly(methyl methacrylate).

The refractive index of bulk poly(methyl methacrylate) samples is modified by X-ray exposure. The intensity of the radiation is distributed over an energy range of 1 to 20 keV with a maximum at 3 keV. For an absorbed dose of about 1 kJ cm(-3), an increase of the refractive index of up to 3x10(-4) is observed. However, for much lower doses of about 10 J cm(-3) a decrease of 4x10(-4) appears. These refractive index changes, as well as the simultaneously arising radiation-induced thickness changes, are detected interferometrically. Based on these observations, on dose calculations, and on literature data, a model is proposed which describes quantitatively the observed refractive index changes. The refractive index increase is caused by a radiation-induced chemical alteration of the polymer composition, while the refractive index decrease is correlated with a decrease of mass density.

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PubMed2005

Holography in commercially available photoetchable glasses.

Volume holographic gratings are recorded and retrieved in two commercially available glasses: Schott Foturan and Hoya PEG3. These materials are photoetchable, which describes their major application, but they also allow storage of volume holograms without any chemical etching. The samples are illuminated with ultraviolet light at a wavelength of 325 nm and thermally processed to achieve a maximum diffraction efficiency of approximately 9% for a 1-mm-thick sample. The two glasses show similar behavior; the diffraction efficiencies in Foturan tend to be slightly larger, whereas PEG3 tends to have weaker light scattering.

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PubMed2004

Light deflection from ferroelectric domain structures in congruent lithium tantalate crystals.

Congruently grown lithium tantalate crystals containing ferroelectric domain structures have been illuminated with ultraviolet light along the crystallographic c axis with simultaneous application of an external electric field. This gives rise to a star-shaped light diffraction pattern in the transmitted beam. The characteristics of this phenomenon are investigated and compared with those observed in lithium niobate crystals.

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PubMed2004

Electrical fixing in near-stoichiometric lithium niobate crystals.

Photorefractive gratings are recorded in lithium niobate crystals in the presence of externally applied electric fields close to the coercive field. After optical erasure a voltage pulse above the poling threshold is applied, and by these means a photorefractive grating reappears that can be erased optically. The results point to electrical fixing, i.e., ferroelectric domain reversal induced by a space-charge field.

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PubMed2004

Increased thermal stability of a poled electro-optic polymer using high-molar-mass fractions.

Polymer fractionation yields materials with higher average molar masses and thus substantially increased glass transition temperatures. We apply this technique to a photoaddressable polymer that exhibits high electro-optic coefficients, in order to enhance the thermal stability of the poled material. The degree of orientation of the dipolar chromophores is investigated with both pyroelectric and electro-optic measurements. The pyroelectric measurements at elevated temperatures show no significant contribution of beta relaxation as compared to alpha relaxation.

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PubMed2003

Visualization of ferroelectric domains with coherent light.

Illumination of a lithium niobate or lithium tantalate crystal along the crystallographic c axis with coherent light while simultaneously applying an external electrical field through transparent electrodes allows real-time, in situ, nondestructive monitoring of ferroelectric domain patterns. Imaging of the optical near-field through a lens directly visualizes the domain walls, whereas the far-field yields averaged information about the spatial orientation of the domain boundaries.

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PubMed2003

Holographic recording of Bragg gratings for wavelength division multiplexing in doped and partially polymerized poly(methyl methacrylate).

Bragg gratings are recorded in doped and partially polymerized poly(methyl methacrylate) with green light (wavelength, 532 nm) in transmission geometry, and the gratings are read in reflection geometry with infrared light (wavelength, approximately 1550 nm). Diffraction efficiencies of more than 99% with a wavelength bandwidth of approximately 1 nm are obtained for single gratings with a typical length of 15 mm. Superposition of four gratings in a volume sample has been demonstrated as well. The material is promising for use in the fabrication of add-drop filters, attenuators, switches, and multiplexers-demultiplexers for optical networks that use wavelength division multiplexing.

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