Volker Dieckmann

Transition Metal Compounds Towards Holography

We have successfully proposed the application of transition metal compounds in holographic recording media. Such compounds feature an ultra-fast light-induced linkage isomerization of the transition-metal–ligand bond with switching times in the sub-picosecond regime and lifetimes from microseconds up to hours at room temperature. This article highlights the photofunctionality of two of the most promising transition metal compounds and the photophysical mechanisms that are underlying the hologram recording. We present the latest progress with respect to the key measures of holographic media assembled from transition metal compounds, the molecular embedding in a dielectric matrix and their impressive potential for modern holographic applications.

Pronounced photosensitivity of molecular [Ru(bpy) 2 (OSO)] + solutions based on two photoinduced linkage isomers

Photosensitive properties of [Ru(bpy)(2)(OSO)] PF(6) dissolved in propylene carbonate originating from photoinduced linkage isomerism have been studied by temperature and exposure dependent transmission and UV/Vis absorption spectroscopy. An exceeding photochromic photosensitivity of S = (63 +/- 10) x 10(3) cm l J(-1) mol(-1) is determined. It is attributed to a maximum population of 100% molecules in the photoinduced isomers, a unique absorption cross section per molecule and a very low light exposure of Q(0) = (0.25 +/- 0.03) Ws cm(-2) for isomerism. Relaxation studies of O-bonded to S-bonded isomers at different temperatures verify the existence of two distinct structures of linkage isomers determined by the activation energies of E(A,1) = (0.76 +/- 0.08) eV and E(A,2) = (1.00 +/- 0.14) eV.

Tin hypothiodiphosphate: nonlinear response in the sub-100 fs time domain

The interaction of sub-100 fs light pulses (τp ≲ 75 fs) with single crystals of nominally undoped tin hypothiodiphosphate, Sn2P2S6, is studied in the near-infrared spectral range (590 – 1630 nm). A predominant contribution of the two-photon absorption (TPA) is verified in the measurements of the sample transmission as a function of pulse intensity and of the time delay between pump and probe pulses. Scans over the photon energy show that the two-photon absorption coefficient β increases in a superlinear way for photon energies hω exceeding Eg/2; for any quantum energy it is nearly independent of propagation direction and polarization of the incident beam. Such a behavior is qualitatively similar to that predicted by perturbation theory within models with allowed-forbidden transitions. The TPA coefficient saturates at a maximum value of β ≈ 8 cm GW−1 at hω ≈ 1.80 eV. It drops when reaching the bandgap Eg. Using pump-probe measurements at 626 nm, a transient absorption is verified that persists for probe pulse delays much longer than the pump pulse duration, up to 2.5 ns. We discuss our results in the framework of the microscopic structure of Sn2P2S6 with emphasis on the optical generation of S− small hole polarons.

Self-diffraction from two-photon absorption gratings in Sn 2 P 2 S 6

Self-diffraction with the appearance of higher diffraction orders is discovered when writing a grating with a single sub-100 fs pulse in a nominally undoped Sn(2)P(2)S(6) sample. The short time of grating development, dependence of diffraction efficiency on the recording light intensity, correlation of wavelength dependence of efficiency with the spectrum of the two-photon absorption (TPA) constant, and a π phase shift of the diffracted beam allow for attributing the recorded grating to a dynamic amplitude grating of TPA.

Small-polaron based holograms in LiNbO3 in the visible spectrum

Diffraction efficiency, relaxation behavior and dependence on pump-beam intensity of small-polaron based holograms are studied in thermally reduced, nominally undoped lithium niobate in the visible spectrum (λ = 488 nm). The pronounced phase gratings with diffraction efficiency up to η = (10.8 ± 1.0)% appeared upon irradiation by single ns-laser pulses (λ = 532 nm) and are comprehensively assigned to the optical formation of spatially modulated densities of small bound NbLi4+ electron polarons, NbLi4+:NbNb4+ electron bipolarons, and O⁻ hole polarons. A remarkable quadratic dependence on the pump-beam intensity is discovered for the recording configuration K || c-axis and can be explained by the electro-optic contribution of the optically generated small bound polarons. We discuss the build-up of local space-charge fields via small-polaron based bulk photovoltaic currents.

Scanning nonlinear absorption in lithium niobate over the time regime of small polaron formation

Nonlinear absorption is studied in presence of small polaron formation in lithium niobate using the z-scan technique and ultrashort laser pulses with pulse durations of 70 – 1,000 fs. A model for the analysis of the transmission loss as a function of pulse duration is introduced that considers (i) the individual contributions of two-photon and small polaron absorption, (ii) the small polaron formation time and (iii) an offset time between the optical excitation of free carriers by two-photon absorption and the appearance of small polarons. It is shown that the model allows for the analysis of the experimentally determined z-scan data with high precision over the entire range of pulse durations using a two-photon absorption coefficient of β = (5.6 ± 0.8) mm/GW. A significant contribution by small polaron absorption to the nonlinear absorption is uncovered for pulse durations exceeding the characteristic small polaron formation time of ≈ 100fs. It can be concluded that the small polaron formation time is as short as (70 – 110) fs and the appearance of small polaron formation is delayed with respect to two-photon absorption by an offset of about 80 fs.

High-contrast, high-resolution photochromic silicone polymer based on photoswitchable [Ru(bpy) 2 OSO]PF 6 building blocks

The implementation of photoinduced linkage isomerism in molecular-based optical materials represents a promising approach for the synthesis of high-contrast, high-resolution photosensitive materials that are necessary for high-density (holographic) data storage and/or real-three-dimensional (holographic) displays. The unsolved task of embedding a photofunctional coordination complex into a matrix like polymer polydimethylsiloxane (PDMS) with photoinduced isomerism of a SO-bond in the sulfoxide compound [Ru(bpy)2OSO]PF6 is addressed. This approach allows to preserve the spectral properties within the solid dielectric environment, with an impact of PDMS on population and relaxation dynamics. All data are discussed in the framework of photofunctionality, storage, and display applications.

Fixed index gratings in LiNbO 3 :Fe upon long-term exposure to an intense laser beam

Permanently fixed noisily recorded refractive index gratings are discovered in single crystals of LiNbO(3):Fe upon long-term exposure to a single laser beam with high intensities of I=100 kW/m(2) and ambient temperature. The fixing process is explained by considering the effect of laser-induced heating of the sample and the well-known simultaneous thermal fixing procedure.

Parametric hybrid scattering from light-induced ferroelectric and photorefractive structures.

Processes of photo-induced light scattering are studied in single crystals of LiNbO(3):Fe to uncover the origin of a new part of the entire scattering pattern which can be observed on a viewing screen. The new scattering manifests itself as two arcs enclosing the directly transmitting pump beam. It is shown that this type of scattering is due to a parametric wave-mixing process of coherent optical noise and a pump beam on a combination of photorefractively recorded phase-gratings and photo-induced ferroelectric structures. Phase-matching conditions corresponding to the new scattering are introduced. All photo-induced scattering phenomena contributing to the total scattering pattern are discussed, compared and classified.

Interplay of Photorefractively Recorded Parasitic Gratings and Photo-Induced Ferroelectric Structures

The interplay of photorefractively recorded parasitic gratings and photo-induced ferroelectric structures is uncovered in Fe-doped LiNbO3. The phenomenon results in polarization-anisotropic light scattering appearing as segments of two cones upon exposure to a single pump-beam.