Per Michael Johansen

Characterization of azobenzene chromophores for reversible optical data storage: molecular quantum calculations

Azobenzene chromophores are promising as molecularly engineered materials for reversible optical data storage based on molecular reorientation. In this paper, the optical properties of several different azobenzene chromophores are studied using molecular quantum calculations. Special emphasis is put on molecular anisotropy since a high degree of anisotropy is essential for the storage performance. The trans isomers are all found to be practically one-dimensional whereas the anisotropy of the cis isomers is highly dependent on substituents. Molecular reorientation of chromophores in liquid-crystalline polymers is simulated in order to study the influence of lacking cis anisotropy. It is demonstrated that photoinduced birefringence is significantly reduced in materials characterized by a low degree of cis anisotropy.

Single-mode operation of a laser-diode array with frequency-selective phase-conjugate feedback.

A new technique for single-mode operation of laser-diode arrays is presented. A gain-guided GaAlAs laser diode array is coupled to an external frequency-selective phase-conjugate feedback system that contains a photorefractive barium titanate crystal, a Fabry-Perot etalon, and a spatial filter. The etalon is the key component, and it forces the array, which has low spatial and spectral coherence when it runs freely, to oscillate in a single spatial and a single longitudinal mode. At a drive current of two times the threshold current, the far-field pattern is reduced to only 1.4 times the diffraction limit, the spectral bandwidth is less than 0.02 nm, and the coherence length is increased by a factor of 70. The technique has general validity and can be applied to various other multimode laser systems.

Limitations of the stretched exponential function for describing dynamics in disordered solid materials

Around the glass transition temperature, relaxation dynamics in glass-forming materials follows a strong nonexponential behavior. It is widely accepted that an empirically based stretched exponential function, known as the Kohlrausch–Williams–Watts (KWW) function, ϕ(t)=e−(t∕τ)β, describes universally a broad variety of experimental data. Using intuitive pictures and ellipsometric measurements, we show that (1) in order to describe the dynamics in disordered materials such as in polymers using a KWW function, the response has to be considered over a specific region of time, (2) a single KWW function is not sufficient for correctly describing more than one relaxation processes, and (3) in certain cases, stretching exponents depending on temperature do not cover the ranges previously suggested (from 0 to 1, e.g., as a sigmoid function). As an example, we show that the temperature dependence of the stretching exponent β(T) depends highly on how the curve fits with the KWW function are performed.Around the glass transition temperature, relaxation dynamics in glass-forming materials follows a strong nonexponential behavior. It is widely accepted that an empirically based stretched exponential function, known as the Kohlrausch–Williams–Watts (KWW) function, ϕ(t)=e−(t∕τ)β, describes universally a broad variety of experimental data. Using intuitive pictures and ellipsometric measurements, we show that (1) in order to describe the dynamics in disordered materials such as in polymers using a KWW function, the response has to be considered over a specific region of time, (2) a single KWW function is not sufficient for correctly describing more than one relaxation processes, and (3) in certain cases, stretching exponents depending on temperature do not cover the ranges previously suggested (from 0 to 1, e.g., as a sigmoid function). As an example, we show that the temperature dependence of the stretching exponent β(T) depends highly on how the curve fits with the KWW function are performed.

Photorefractive grating formation in piezoelectric La3Ga5SiO14:Pr3+ crystals

Photorefractive grating formation and erasure in piezoelectric crystals of La3Ga5SiO14:Pr3+ are presented. The specific photoconductivity and the photorefractive sensitivity are determined. The polarization dependence of the grating formation due to the bulk photovoltaic effect is shown and compared favorably with the theoretical expression. This photorefractive material provides a possibility for separate investigations of the charge migration processes responsible for the photorefractive effect.

Suppressing self-induced frequency scanning of a phase conjugate diode laser array using counterbalance dispersion

Experimental results show that angular dispersion strongly influences the self-induced frequency scanning of a multimode broad-area diode laser array coupled to a photorefractive self-pumped phase conjugate mirror. Prisms or a dispersive grating placed in the external cavity opposing the material frequency dispersion of the phase conjugate BaTiO3 crystal suppress the frequency scanning and stabilize the center wavelength and the output power. We show that the dispersion of the crystal is crucial for the mechanism of the frequency scanning.

Nonlinear self-defocusing in doped silica sono-gels

Experiments with nonlinear self-refraction of Gaussian laser beams in silica sono-gels doped with copper tetrasulfonated phthalocyanine are reported. The propagation of laser beams inside nonlinear sol-gel samples with different Cu-phthalocyanine concentrations has been monitored by measuring the spatial beam profile in the near field and in the far field behind the sample. The experimental results are analyzed by a new simple theoretical approach, in which we assume that the incident Gaussian beam induces a phase shift that varies as a Gaussian function of the beam radius. The beam propagation behind the sample is determined by the Huygens–Fresnel integral formalism. By solving the Huygens–Fresnel integral, analytical expressions for the spatial beam profile in both the near field and the far field after the nonlinear sample are obtained. Experiments are carried out with a diode pumped frequency doubled Nd–YAG laser at 532 nm. We obtain very large third-order nonlinearities in these doped sol-gel samples...

Incoherent enhancement of the photorefractive response inBi 12 SiO 20 by subharmonic interaction

A new mechanism for enhancing the photorefractive grating amplitude in Bi(12)SiO(20) is presented. We show experimentally that, by recording two photorefractive running gratings induced by two pairs of mutually incoherent laser beams, it is possible for the two gratings to transfer energy from one to the other. As might be expected, maximum energy transfer is obtained when the phase velocities of the two gratings are equal.

Electro-optic response of chromophores in a viscoelastic polymer matrix to a combined dc and ac poling field

We present a generalized model for the electro-optic response of chromophores in a viscoelastic polymer matrix to a combined dc and ac applied poling field. The model includes a local molecular field of random orientation at each chromophore site to take into account the influence of the polymer matrix on the chromophore reorientation. Hence the model relies on a physically intuitive picture of chromophore dynamics in a viscoelastic polymer matrix. The dynamics is described by the rotational diffusion equation, where the local molecular field is inferred and a solution is presented with a variational approach. We obtain an analytical expression for the electro-optic response both at the modulating frequency and at two times the modulating frequency, having explicit frequency and molecular-field dependence. The model is successfully compared with frequency-resolved ellipsometric measurements in an azo-dye containing polymer guest–host system that is poled in a combined dc and ac electric field. The experimental setup is a modified Teng–Man ellipsometer in a balanced detection scheme, and, as a model system for investigating the electro-optic response, we use the chromophore Disperse Red 1 in a polymer matrix of poly(methyl methacrylate). Finally, the model is compared with independent experiments on a 2,5-dimethyl-4-(p-nitrophenylazo)-anisole containing photorefractive polymer composite, and good qualitative agreement is found.

Observation of angularly tilted subharmonic gratings in photorefractive bismuth silicon oxide.

A new subharmonic regime in photorefractive bismuth silicon oxide has been discovered by the use of the running-grating technique. In this regime the subharmonic grating splits into two gratings that are angularly tilted with respect to the fundamental grating. These observations necessitate an extension of previously proposed theories.

Orientational dynamics in dye-doped organic electro-optic materials

The time dependent birefringence of polymer-based electro-optic materials is investigated using ellipsometry. We show that the birefringence after switching off the poling field does not depend only on the induced refractive index, but also on how that level was reached. The role of the poling voltage and poling time is discussed in turn-on and turn-off experiments and an original curve-fit function is introduced. We also propose a schematic model of the polymer dynamics in the system, which is consistent with complementary dielectric measurements.