Regina Stockmann

Amplified spontaneous emission in neat films of arylene-vinylene polymers

Amplified spontaneous emission (travelling-wave lasing) was achieved for a set of 18 poly(2-methoxy-5-(2′-ethyl-hexyloxy)-1,4-phenylene-vinylene) (MEH-PPV) related arylene-vinylene copolymers, a part of which substituted with additional phenyl groups at the vinyl double bond. Wave-guiding neat thin films on glass substrates were used. The samples were transversally pumped with single second harmonic pulses of a mode-locked ruby laser (wavelength 347.15 nm). Travelling-wave emission occurred in the wavelength region between 485 and 650 nm. Repeat unit based absorption cross-section spectra, normalized fluorescence quantum distributions, and amplified spontaneous emission spectra are presented. Ground-state absorption cross-sections at the wavelengths of peak amplified emission are extracted from effective gain length measurements. Effective stimulated emission cross-sections are derived from pump pulse energy densities necessary for optical narrowing.

MEH-PPV and dialkoxy phenylene vinylene copolymers. Synthesis and lasing characterization

We report the use of the HORNER-type polycondensation for the synthesis of MEH-PPV and some strictly alternating copolymers (MEH-DOO-PPV) and regular terpolymers (TPD-M3EH-PPV) all of which having MEH-PV subunits. High molecular weight, solution processable waveguiding materials of increased Tg (80 - 160 °C) were achieved. For comparison also statistical dialkoxy-PPV copolymers and blends of MEH-PPV and DPOP-PPV have been studied. All these luminescent polymers exhibit travelling-wave lasing (amplified spontaneous emission-ASE) in neat films. Emission occurs in the red spectral region. ASE is achieved in waveguiding neat films by picosecond pulse excitation. The lasing is identified by spectral narrowing, temporal shortening, and threshold behaviour of light emission.

Energy density dependent fluorescence quenching of diphenyl substituted phenylene-vinylene and diphenylene-vinylene polymers by exciton–exciton annihilation

Abstract The energy density dependent fluorescence efficiency and fluorescence decay of neat films and solutions in 1,4-dioxane of poly[1,4-phenylene-1,2-di(4-phenoxyphenyl)vinylene] (DPOP-PPV), poly[1,4-phenylene-1,2-di(4-phenylthiophenyl)vinylene] (DPSP-PPV), poly[4,4′-diphenylene-1,2-di(4-phenoxyphenyl)vinylene] (DPOP-PDPV), and poly[4,4′-diphenylene-1,2-di(4-fluorophenyl)vinylene] (DFP-PDPV) are investigated with picosecond laser pulses. In low-concentration solutions, upon exciting more than one segment per polymer chain, the fluorescence efficiency reduces. In non-waveguiding thin neat films, the energy density dependent fluorescence signal reduction begins at much lower pulse energy density than in solution. The fluorescence behaviour is thought to be mainly due to exciton–exciton annihilation with time-dependent annihilation constant followed by polaron pair formation and subsequent polaron pair annihilation.

Photophysical characterization of diphenyl-substituted phenylenevinylene and diphenylenevinylene polymers

Abstract Neat films and solutions of poly[1,4-phenylene-1,2-di(4-phenoxyphenyl)vinylene], poly[1,4-phenylene-1,2-di(phenthiophenyl)vinylene], poly[4,4′-diphenylene-1,2-di(4-phenoxyphenyl)vinylene] and poly[4,4′-diphenylene-1,2-di(4-fluorophenyl)vinylene] in 1,4-dioxane are investigated. The optical constants of the neat films are determined, absorption and fluorescence data are analysed, and picosecond laser saturable absorption studies are carried out. The fluorescence lifetimes and fluorescence quantum yields are analysed to determine the emitting chromophore size (electronic delocalization). The saturable absorption measurements are used to determine the average absorption segment size.

Excited state inhibition of luminescence in DPOP-PPV

The optical properties of a luminescent polymer, poly[1,4-phenylene-1,2-di(phenoxyphenyl) vinylene (DPOP-PPV), have been investigated. Measurement of photoluminescence (PL) quantum efficiency shows an unusual increase in efficiency in the solid state over solution, 52% to 6% respectively. Investigations into the stimulated emission properties of this material were carried out but no amplified spontaneous emission was observed. To investigate the presence of excited state absorption features photoinduced absorption (PA) measurements were performed. An extended polaron absorption band, from 1.5 eV to 2.25 eV was observed. This overlaps the emission spectra and therefore inhibits stimulated emission. This highlights the need to consider the effects of excited state absorption on the emission when synthesising new materials.

Photophysical and lasing characterization of some phenylenevinylene-based polymers

Traveling-wave lasing is reported on two phenylenevinylene polymers (MEH-PPV, M3EH-PPV), two triphenylamine-vinylene- phenylenevinylene copolymers (TPA-MOP-PPV, TPA-MEH-PPV), and a ternaric condensation copolymer containing TPA-MEH-PV and M3EH-PV sub-units (TPA-MEH-M3EH-PPV). Waveguiding thin films on glass substrates are transversally pumped with picosecond lasers pulses (wavelength 347.15 nm, duration 35 ps). The laser emission occurs between 515 nm (TPA-MOP-PPV) and 646 nm (M3EH-PPV) with a spectral width between 7 nm and 13 nm. The threshold pump pulse energy densities are between 10 (mu) J cm-2 and 45 (mu) J cm-2. At high pump pulse energies a saturation of the generated amplified spontaneous emission signal is observed which is thought to be due to exciton-exciton annihilation processes. Additionally a diphenyl substituted phenylenevinylene polymer (DPOP-PPV) is investigated which shows no laser action despite high neat-film fluorescence quantum efficiency ((phi) F equals 0.72). Photo-induced absorption dominates over stimulated emission and hinders traveling- wave lasing. An absorption and emission spectroscopic characterization of the investigated polymers is carried out. An energy density dependent fluorescence quenching is observed and this behavior is discussed in terms of exciton- exciton annihilation processes.

Third-order optical nonlinearities of poly(arylamino-phenylenevinylene) studied with femtosecond pulses

Time-resolved degenerate four-wave mixing (DFWM) experiments performed on films of triphenylamino-phenylene vinylene (TPA-PPV) copolymer show the modulus of the nonlinear refractive index to be (2.1#0.4) x10-13 cm2/W at 800 nm. The polymer was synthesized in the Hoerner-type polycondensation reaction. The films were characterized by optical absorption spectra, molecular weight and glass transition temperature measurements. The linear refractive index measurements performed with a prism coupler indicate that the annealed polymer films are isotropic. The films showed waveguiding of light. The DFWM experiments were performed in the forward BOXCARS geometry with simultaneous monitoring of the phase-matched and the non-phase-matched signals. This allowed measuring the nonlinearity of sub-micrometer thick films even in the presence of signals from a thick glass substrate. A cubic power dependence of the diffracted signal vs. pump intensity was observed as expected for the Kerr-type electronic nonlinearity. The signals showed a strong instantaneous response followed by a slow decay with the time constant 9#2 ps. Z-scan measurements showed the presence of two-photon absorption in the polymer.

Design and fabrication of a waveguiding organic double hetero-structure light-emitting device: a milestone in the development of an electrically pumped polymer laser

We present measurements of the optical constants of a series of spin cast conjugated polymer and thermally evaporated small molecule films on silicon substrates. The optical constants are measured, using spectroscopic ellipsometry, from 0.75eV up to just below the band-gap of each material. The optical constants are then obtained by fitting a homogeneous Cauchy model to the ellipsometry data. Using this data the waveguiding behaviour of a double hetero-structure device with edge emission is modeled using the well-established Beam Propagation Method (BPM). The waveguide loss of double hetero-structure devices is modeled for different device geometries and across the emission band of an experimentally prepared light emitting, waveguiding double hetero-structure organic device. The emission characteristics of the experimental device compare favourably with results of the simulation based on the measured optical constants. These results have important implications for the design of future electrically pumped organic laser devices.

Optical properties and waveguiding in films of a nonlinear polymer: Difluorophenyl-polydiphenylenevinylene (DFP-PDPV)

Third-order optically nonlinear polymeric material built of rigid-rod molecules are often insoluble and transmit visible light poorly. We report on optical and waveguiding properties of a new soluble derivative of PPV: a π- conjugated polymer DFP-PDPV. We found that solvents used for processing of the DFP-PDPV polymer may influence the film optical properties. Changes in absorption spectra, birefringence, optical attenuation and the nonlinear refractive index were observed in the films made by spin coating and solution cast using different solvents. The films are birefringent and dispersive. Refractive indices vary from 1.76 to 1.63 for the TE polarisation of the incident light and from 1.70 to 1.60 for the TM polarisation for wavelengths from 476.5 nm to 1.55μ, respectively. The birefringence is an order ofmagnitude lower than that in films ofunsubstituted PPV. DFP-PDPV is suitable for fabrication of optical waveguides. Low loss waveguiding layers could be prepared. Propagation losses were measured at 632.8nm, 8lOnm and 1064nm. The losses decrease at longer wavelengths reaching the level of 1 dB/cm at 1.06μ. The waveguiding properties of DFP-PDPV films depend on the solvents used in processing of the polymer. We investigated thin films of DFP-PDPV for their potential for nonlinear waveguide applications. The modulus of nonlinear refractive index |n2| in the range (0.9-1.5) x10-14 cm2/W was measured in DFP-PDPV films at 8OOnm using a femtosecond degenerate four wave mixing (DFWM) technique. These studies supplement the results of nonlinear optical studies of this polymer in solution using the Z-scan technique described in Proceedings of SPIE 3473, 79-90 (1998).