Beam-size effect in time-delayed laser-induced double gratings

We have studied theoretically the effect of the probe-beam size on time-delayed laser-induced double gratings by assuming that the probe beam is described as a Gaussian beam. LettingE _s1 andE _s2 be the Four-Wave Mixing (FWM) signals originating from the diffraction of the probe beam by the gratings, the condition for the occurrence of the FWM signal modulation is that the divergence angle ofE _s1 andE _s2 is larger than half of the intersection angle between the propagation directions ofE _s1 andE _s2. We have also proposed methods to increase the modulation contrast.     

Theoretical and experimental studies on laser-induced transient gratings in laser dyes

Laser-induced transient grating technique has been used to measure the diffraction efficiency (η) and calculate the third-order nonlinear susceptibility (χ⁽³⁾) of some laser dyes. Theoretical simulations have been carried out on η and χ⁽³⁾ as a function of wavelength covering the spectral range corresponding to the first excited singlet state of the dyes. Theoretically simulated values have been found in agreement to those observed experimentally. The decay profiles for these dyes have been measured by using diffraction of a delayed probe laser pulse to estimate the relaxation times in the excited state.     

Surface relief gratings in photoaddressable polymers generated by cw holography

Surface relief gratings (SRGs) are a widely observed phenomenon in holographic experiments with photoaddressable polymers. This article proposes a model which is able to explain the observed effects in cw experiments. It is based on forces in an electrical field which are caused by the inhomogeneous spatial distribution of the refractive index and the permittivity in the illuminated sample. Received: 11 September 2000 / Revised version: 5 December 2000 / Published online: 21 February 2001     

Charge carrier injection and transport in polymer blend films

The steady current-voltage characteristics of single layer organic devices based on MEH-PPV and N,N′-diphenyl-N,N′-bis(4′-[N,N-bis(naphth-1-yl)-amino]-biphenyl-4-yl)-benzidine (TPTE) blend with different TPTE concentrations was investigated. The thickness dependence of the current-voltage relationship clearly demonstrates that the current at low voltage and at high voltage are all space charge limited. The current density-electric field characteristic proves the blend polymer LEDs to operate in the tunneling-controlled model. The effective hole mobility is directly determined by space charge limited current at high voltage and increases with increasing TPTE content in the blend. The EL efficiency shows concentration dependence, which is attributed to the change of the transport of holes in the blend film.     

All-Optical Switching Based on Azo Polymer Material

Conventional all-optical switches based on azo polymer films and the all-optical switches based on the attenuated total reflection （ATR） geometry are investigated. A conventional switch system, including a pump beam of 532 nm and a probe beam of 650 nm, is based on the photoinduced birefringence effect of azo polymer. An ATR switch in a prism-multilayer configuration is achieved by changing the reflectance of the probe beam with an external pump beam. The ATR method provides the substantial improvement of the speed and the efficiency of the modulation over the conventional method. Although the azo polymer response still remains relatively slow, an enhanced nonlinear refractive index of the azo polymer film can effectively increase the modulation.     

Photothermal deflection spectroscopy of polymer thin films

Photothermal Deflection Spectroscopy (PDS) is known to be one of the most sensitive techniques for measuring the absorption of weakly absorbing materials. We have applied PDS for measuring the optical absorption of a few polymer thin-film samples over the wavelength region from 0.4–2.0 m. The results are useful for optical evaluation of these polymers.     

Polarized-light-controlled holographic recording in an azobenzene-doped polymer film

A new polarized-light-controlled holographic recording based on the optical enhancement/restraint of self-diffraction has been demonstrated in an azobenzene-doped polymer film. It is found that a continuous variation of the polarization status as well as of the intensity of the pumping light results in a continuous variation of self-diffraction efficiency. The mechanism originates from the photo-induced anisotropy and polarization-dependent absorption. Both positive and negative replicas of an incident image were presented in real time by means of this incoherent–coherent optical conversion technique. Received: 30 March 2000 / Revised version: 28 September 2000 / Published online: 27 April 2001     

Determinations of the transient thermal lensing effect in metal cluster Polymer {WS4Cu4I2(bpe)3}n solution by the use of the Z-scan

The nonlinear optical (NLO) properties of a novel cluster Polymer {WS₄Cu₄I₂(bpe)₃}_n solution are studied by using Z-scan technique with laser pulses of 4.5 ns pulse-width at a wavelength of 532 nm. The results show that the cluster solution possesses strong nonlinear absorption and refraction. Nonlinear refraction of the cluster is composed of third-order nonlinear refraction and transient thermal effect. The thermal effect is mainly due to the strong nonlinear absorption. Numerical simulations obtained by solving simultaneously photo-acoustic and electromagnetic wave equations, agrees basically with experimental results.     

Nonlinear optical and optical power limiting studies on a new thiophene-based conjugated polymer in solution and solid PMMA matrix

An experimental investigation of the third-order nonlinear optical properties of new poly{2-{5-[3,4-ditetradecyloxy-5-(1,3,4-oxadiazol-2-yl)thiophen-2-yl]-1,3,4-oxadiazol-2-yl}pyridine} (P) in tetrahydrofuran (THF) solution and in solid poly(methylmethacrylate) (PMMA) matrix, by Z-scan technique is reported. The Z-scan traces reveal that the composite films exhibit large negative nonlinear refractive index of the order 10⁻¹⁰ esu. The excited-state absorption cross-section was found to be larger than the ground-state absorption indicating that the operating nonlinear process is reverse saturable absorption (RSA). The new polymer P exhibits good optical power limiting properties in the nanosecond regime in solution and as well in solid PMMA matrix.     

Sound-velocity measurements in gases by laser-induced electrostrictive gratings

Laser-induced electrostrictive gratings have been applied to measure the adiabatic sound velocity in various gases on a single-pulse basis. The gratings are generated by the interference of two parallel polarized, crossed excitation beams arising from a pulsed Nd: YAG laser at 532.1 nm, and are detected by diffracting a probe beam originating from a cw Ar⁺ laser operating single-line at 514.5 nm. Measurements were performed in the overlap volume of unfocused and focused excitation beams. Using unfocused beams, the sound velocities in various gases at pressures of 5 bar were measured with a statistical error for single-pulse measurements of about 0.3%. With focused beams, the accuracy of the measurements is reduced because of the propagation of the sound waves out of the smaller overlap volume of 0.2 mm diameter and 4 mm length. Measurements with focused beams were performed in air and CO₂, with an error of about 1%.     

Proton-irradiation effect on the luminescence of the MEH-PPV conjugated polymer

We report proton-irradiation effect on luminescence in the MEH-PPV conjugated polymer. While the luminescence was severely weakened by the low-energy irradiation with a high dosage, it was affected only weakly by the high-energy irradiation with a low dosage. The electroluminescence was affected by the irradiation more severely than the photoluminescence was, preserving the external quantum efficiency. While the luminescence spectrum of the severely damaged polymer was broadened with a blue-shift, a weak damage resulted only in a decrease in the luminescence intensity apparently preserving the spectral shape. The change in the luminescence spectra may be explained by chain conformational disorders as well as chain scission induced by the irradiation.     

Photoluminescence spectroscopy study on tris(8-hydroxyquinoline) aluminum film

In situ photoluminescence spectroscopy (PL) measurements of tris(8-hydroxyquinoline) aluminum (Alq₃) film were carried out. Upon deposition of Alq₃ on the glass substrate, the PL intensity changes dramatically, while the peak position of Alq₃ emission shows a sharp red-shift from 524 nm at the initial deposition of Alq₃, and tends to a saturation value of 536 nm for the film thickness range from 2 to 500 nm. This red-shift is associated with the change from the 2D to 3D exciton state with increasing Alq₃ film thickness. Temperature dependent PL spectra of Alq₃ films showed, besides the changes in the PL intensity, clearly a blue-shift of Alq₃ emission about 9 nm for the film annealing up to 150 °C, while no any shift of Alq₃ emission was observed for the film annealing below 130 °C. Both changes in PL intensity, and especially in the peak position of Alq₃ emission were attributed to crystallization (thermal) effect of Alq₃ film upon annealing.     

Reverse saturable absorption in chlorophyll A solutions

The dynamic nonlinear absorption of a chloroform solution of chlorophyll A was investigated using the Z-scan technique with picosecond pulses at 532 nm. The nonlinear absorption exhibits a reverse saturation, indicating a strong intersystem crossing (singlet–triplet) process. The time evolution of the optical nonlinearity, modeled by means of a five-level energy diagram, allows the determination of excited-state cross sections and the lifetime of the intersystem crossing based on its absorption characteristics and efficient formation of triplet states. Chlorophyll A was found to be a good candidate for a sensitizer in photodynamic therapy. Received: 14 February 2002 / Published online: 2 May 2002     

Temperature measurements in gases using laser-induced electrostrictive gratings

Received: 1 December 1997 / Revised version: 27 January 1998     

Thermo-optical investigation of clamped ferroelectric polymer films

Attenuated total reflection leaky-mode spectroscopy is used for a thermo-optical investigation of a clamped film of the ferroelectric vinylidene fluoridetrifluoroethylene copolymer. A hysteresis loop for the optical parameters refractive index and film thickness is found for a full temperature cycle between 25° C and 60° C. In the vicinity of the ferroelectric/paraelectric phase transition increased optical losses are found.     

Gas-phase diagnostics by laser-induced gratings II. Experiments

In this article we review the results achieved in the past ten years at the Paul Scherrer Institute on the topic of diagnostics in the gas phase by laser-induced gratings (LIGs). The technique has been applied for thermometry in air and in flames at different pressures, for flow velocimetry, for concentration measurements, and for imaging purposes. The influence of collisional energy-transfer and relaxation processes in molecules on the temporal evolution of the LIG signals has also been investigated. It has been demonstrated that, for molecules with a low fluorescence quantum yield, excitation of laser-induced thermal gratings can be used as a sensitive spectroscopic tool. For the quantitative interpretation of the experiments shown in this work, the findings presented in the companion paper [1] have been used.     

Quasi-persistent photocurrent in the UV-irradiated MEH-PPV conjugated polymer

We report the quasi-persistent photocurrent in the MEH-PPV conjugated polymer, induced by UV-irradiation in air. It is attributed to the irradiation-induced defects, which also act to accelerate its decay.     

Enhanced Electroluminescent Efficiency Based on Functionalized Europium Complexes in Polymer Light-Emitting Diodes

Efficient red polymer light-emitting diodes are fabricated with the single active layer from the blends of poly（N- vinylcarbazole） （PVK） in the presence of 30 wt. % electron-transporting compound 2-（4-biphenylyl）-5-（ptert- butylphenyl）-1,3,4-oxadiazole （PBD） and europium complexes. The polyphenylene functionalized europium com- plex shows an enhanced electroluminescent efficiency due to the large site-isolation effect. For the polyphenylene functionalized europium complex, the maximum external quantum efficiency of 1.90% and luminous efficiency of 2.01 cd A^-1 are achieved with emission peak at 612nm. The maximum brightness is more than 300cd m^-2.     

Gas phase diagnostics by laser-induced gratings I. theory

Electrostriction and collisional thermalization of absorbed laser energy are the two dominant mechanisms leading to the formation of laser-induced gratings (LIGs) in the gas phase. In this article the results of the theoretical investigations that have been achieved in the past ten years at the Paul Scherrer Institute on this issue are summarized and yield a comprehensive understanding of the underlying physical concepts. Furthermore, a study of the influence of various parameters, such as the alignment and the spatial intensity profile of the beams on the generated electrostrictive and thermal signal is presented for the first time to the authors’ knowledge. The variations of the refractive index responsible for the appearance of laser-induced gratings have been theoretically described by solving the linearized hydrodynamic equations. The contributions from electrostriction, as well as from instantaneous and slow relaxation of the absorbed radiation energy into heat is obtained. These expressions are employed for analysis of experimental data presented in the companion paper [1] which is devoted to the application of the technique for diagnostic purposes in the gas phase. Much effort has been undertaken in order to allow a straightforward physical interpretation of the experimental findings of the expressions presented here.     

Sum-frequency generation and temperature-programed desorption studies of formic acid on MgO(001) surfaces

Adsorption and decomposition of formic acid on MgO(001) surfaces were studied by means of Sum-Frequency Generation (SFG), Temperature-Programed Desorption (TPD) and Atomic Force Microscopy (AFM). Formic acid was dissociatively adsorbed on MgO(001) surfaces at room temperature. From SFG and TPD measurements, the existence of several different types of formate species was deduced and all species mainly decomposed into CO and H₂O on raising the temperature of the substrate.Paper presented at the 129th WE-Heraeus-Seminar on Surface Studies by Nonlinear Laser Spectroscopies, Kassel, Germany, May 30 to June 1, 1994