Lothar Kador

Stark effect of polar and unpolar dye molecules in amorphous hosts, studied via persistent spectral hole burning

A simple analytical model is presented describing the shape of a persistent spectral hole in a homogeneous electric field for random orientation of the dye molecules in the host matrix. We consider the two cases that the absorbers have either one fixed value or a distribution of values of the electric dipole moment difference Δμ between the ground and excited electronic states. Comparison with experiment yields the Δμ values for chlorin (Δμ=0.214 D) and a substituted Zn‐tetrabenzoporphin (∼(Δμ) =0.174 D) in poly(vinylbutyral). In the latter case the isolated dye molecule has inversion symmetry, yet there is nevertheless a dipole moment through the interaction with the polar matrix of low local symmetry.

Stochastic Theory of Inhomogeneous Spectroscopic Line Shapes Reinvestigated

The inhomogeneous distribution of an ensemble of absorption or fluorescence lines in a disordered matrix can be described by a stochastic theory whose fundamental ideas have been known for many decades. Due to its very general principles, it can be applied to inhomogeneous effects of many different types, including inhomogeneous broadening in optical spectra and spectral diffusion in magnetic resonance and line‐narrowed optical experiments. In the case of absorption and luminescence bands, it is often convenient to perform the so‐called Gaussian approximation, which is valid in the limit that the density of the matrix molecules is high compared to the reciprocal volume of the cavity containing the absorbing or fluorescing center; this leads to the analytical result that the line shapes of the optical bands are Gaussian. Numerical calculations beyond the Gaussian approximation help to clarify its physical meaning and to interpret inhomogeneous bandwidths from a statistical point of view. Three types of int...

Pressure effects on single molecules of terrylene in p-terphenyl

Abstract The hydrostatic-pressure shift of the zero-phonon lines of single terrylene molecules in a p -terphenyl host crystal was measured at 1.6 K. We found four inhomogeneous absorption bands of the terrylene molecules in p -terphenyl with inhomogeneous widths between 15 and 33 GHz, corresponding to four crystal sites, and we measured the pressure shift of single molecules in the sites X 2 , X 3 , and X 4 . All the molecules investigated (35 in total) exhibited a linear and reversible red-shift with increasing pressure, with magnitudes between −0.92 and −1.48 MHz/hPa. The pressure shift parameter was found to be independent of the spectral position of the molecules.

Levy Statistics for Random Single-Molecule Line Shapes in a Glass

We demonstrate that the statistical behavior of random line shapes of single tetra-tert-butylterrylene chromophores embedded in an amorphous polyisobutylene matrix at T=2 K is described by Lévy statistics as predicted theoretically by Barkai, Silbey, and Zumofen [Phys. Rev. Lett. 84, 5339 (2000)]. This behavior is a manifestation of the long-range interaction between two-level systems in the glass and the single molecule. A universal amplitude ratio is investigated, which shows that the standard tunneling model assumptions are compatible with the experimental data.

Dipole moment differences of nonpolar dyes in polymeric matrices: Stark effect and photochemical hole burning. I

The difference of the induced dipole moments Δμind between the ground and excited state of (centro‐)symmetric dye molecules (tetraphenylporphin and tetrapropylporphycene) embedded in different nonpolar and polar polymers is investigated. The technique of photochemical hole burning combined with Stark effect measurements was used to determine the values as a function of the burning frequency. For almost all systems an increase of Δμind from higher to lower optical frequencies was found. A plot of the <Δμind≳ values measured at the absorption band maximum vs the low‐temperature values of the dielectric constant e shows a clear correlation between the dipole moment difference and the low‐temperature e values over the whole investigated range which is accessible with commercially available polymers. In order to evaluate the data the dielectric constant e of each sample was determined in a series of separate experiments as a function of frequency and temperature.

Absorption Spectroscopy on Single Molecules in Solids

Absorption signals of single terrylene molecules in n-hexadecane and naphthalene crystals were recorded at liquid-helium temperatures. The method is based upon rf Stark effect modulation in the megahertz range. The electric rf field strength was applied by means of interdigitating electrodes with 18 μm spacing. Signal-to-noise ratios better than 10 were obtained with approximately 300 ms integration time. The measured line shapes depend on the relative contributions of the linear and the quadratic Stark shift.

Polarization Dependence of the Formation of Surface Relief Gratings in Azobenzene-Containing Molecular Glasses

This paper presents a comprehensive study of the formation of surface relief gratings in a series of photoresponsive molecular glasses. Holographic experiments were performed on films of the azobenzene-containing molecular glasses. Seven relevant polarization configurations of the writing beams were systematically applied, and simultaneously the diffraction efficiency was monitored during the process of inscription. The temporal evolution of the diffraction efficiency can be precisely simulated with a model which takes both the surface relief and the phase grating in the volume into account. From the measured diffraction efficiencies, the modulation heights can be directly calculated and they were independently confirmed by atomic force microscopy. We found that all experimental results can be explained with the gradient force model, and we suggest that the grating heights generated with different writing polarizations can be ascribed to the varying strengths of the gradient force. For materials with different substituents at the azobenzene chromophore, the optical susceptibility at the writing laser wavelength and, therefore, the gradient force varies. By applying the most efficient polarization configuration in combination with the best material, we were able to reach modulation heights of up to 600 nm, which is a factor of 2 higher than modulations usually reported for azobenzene-containing polymers.

Correlation between Strain-Field and Electric-Field Effects in Hole-Burning Spectra

The effect of external hydrostatic pressure and electric fields on the absorption lines of dye molecules in amorphous host matrices is discussed in a common framework. For the regime of low pressure and field strength, where only lowest-order terms are important, symmetric expressions for the line shifts are derived, which are connected with the corresponding solvent shift values. In this context the variation of both effects as a function of the position within the inhomogeneous absorption band is discussed. The results of hole-burning experiments on pentacene in PMMA and on a substituted Zn-tetrabenzoporphin in poly(vinyl butyral) show qualitative agreement with the predictions of the solvent shift model.

Photoluminescence frequency up-conversion in GaSe single crystals as studied by confocal microscopy

The photoluminescence spectrum of melt-grown GaSe single crystals was investigated with a confocal Raman microscope equipped with a HeNe laser. Three luminescence bands of different intensity were observed, which are mainly located to the blue of the laser line. The luminescence signals show a quadratic dependence on excitation intensity. The effect is interpreted as second-harmonic generation in the strongly optically nonlinear material followed by the excitation of electrons into the conduction band and luminescence emission from direct-gap Wannier excitons. The relative intensities of the three luminescence peaks exhibit a spatial variation on the crystal surface, which was mapped with the confocal microscope. Possible explanations of this effect are discussed. In an external electric field the luminescence shows a strong increase and a quadratic redshift, whose magnitude is consistent with the Franz–Keldysh mechanism.

Holographic Gratings and Data Storage in Azobenzene-Containing Block Copolymers and Molecular Glasses

This review covers synthesis, materials development, and photophysics of azobenzene-containing block copolymers as potential media for reversible volume holographic data storage. For high-density holographic data storage, volume gratings must be inscribed in millimeter-thick samples to achieve efficient angle multiplexing. It is demonstrated that block copolymers with azobenzene side-groups in the minority block develop no detrimental surface relief structures and exhibit superior performance regarding volume gratings, compared to homopolymers and statistical copolymers. Several material concepts for optimizing the refractive index modulation and the stability of volume gratings are presented. Stabilities of more than 2 years were achieved. Most important is the development of polymer blends comprising the azobenzene-containing block copolymer and an optically transparent homopolymer. This enables the preparation of millimeter-thick samples with the required optical density of ∼ 0. 7 at the writing wavelength by conventional injection molding techniques. The inscription of up to 200 holograms at the same lateral position was demonstrated. In addition, more than 1,000 write/erase cycles can be performed. This is the first time that the inscription and erasure of the long-term stable angle-multiplexed volume gratings in a rewritable polymeric medium have been achieved by purely optical means. A second important application for azobenzene-containing materials is the controlled preparation of surface relief structures. It is demonstrated that azobenzene-containing molecular glasses are an ideal class for efficient formation of surface relief gratings (SRGs) with amplitude heights of more than 600 nm. Clear relationships can be established between the chemical structure of the molecules and the behavior of SRG formation. All results are in agreement with the gradient force model by Kumar et al. The surface patterns are stable enough to be transferred to a polymer surface via replica molding.