Rachel Jakubiak

Dendritic sidegroups as three-dimensional barriers to aggregation quenching of conjugated polymer fluorescence

Abstract We study the photophysics of poly-phenylenevinylene (PPV) polymers with large dendritic sidegroups under both isolated chain and aggregated chain conditions. Aggregation causes a substantial reduction in photoluminescence (PL) quantum yield but that decrease is ameliorated by the attachment of an additional generation of dendron to the PPV backbone. Three-dimensional separation of chromophores can be achieved with these sidegroups and the formation of nearly non-emissive interchain excitations leads to a factor of four enhancements in solid-state PL.

Random laser action in organic–inorganic nanocomposites

Random laser action is demonstrated in organic–inorganic, disordered hybrid materials consisting of ZnO semiconductor nanoparticles dispersed in an optically inert polymer matrix. The ZnO particles provide both the gain and the strong scattering power that leads to light trapping due to multiple elastic scattering, whereas the polymer matrix offers ease of material fabrication and processability in view of potential applications. Excitation of the nanohybrids by a laser pulse with duration shorter than the ZnO photoluminescence lifetime leads to a dramatic increase in the emitted light intensity accompanied by a significant spectral and temporal narrowing above a certain threshold of the excitation energy density. Critical laser and material parameters that influence the observed laser-like emission behavior are investigated in a series of nanocomposites.

Tunable two-photon pumped lasing using a holographic polymer-dispersed liquid-crystal grating as a distributed feedback element

A holographic polymer-dispersed liquid-crystal (H-PDLC) grating film was employed as an angle-dependent and narrow spectral-band feedback control element for two-photon pumped lasing in a dye solution, 4-[N-(2-hydroxyethyl)-N-(methyl)amino phenyl]-4′-(6-hydroxyhexyl sulfonyl) stilbene (APSS) in dimethyl sulphoxide. The grating film contained about 80 layers of liquid-crystal domains periodically dispersed in an ∼15 μm thick polymer film, featuring a maximum reflectance of 75% at 561 nm position with an ∼9 nm spectral bandwidth. The output lasing wavelength could be tuned from 561.5 to 548.5 nm and the lasing bandwidth changed from 5 to 3 nm when the incidence angle on the grating film varied from 0° to 22°. The overall lasing efficiency was measured to be 10%.

High contrast switching of distributed-feedback lasing in dye-doped H-PDLC transmission grating structures

Electrically switched distributed-feedback (DFB) lasing action is presented in a Pyrromethene 580 lasing dye-doped holographic polymer dispersed liquid crystal (H-PDLC) transmission grating structure. This design, when compared with the previously utilized H-PDLC reflection grating structure, has the advantage of a greatly enlarged gain length (10 mm) and a low concentration of liquid crystal (20%) while maintaining sufficient refractive index modulation. The experimental results demonstrate that the emitted laser bandwidth (~5 nm) can be obtained with a pump energy threshold of ~0.3 mJ at three different wavelengths, 561 nm, 569 nm and 592 nm, corresponding to three different grating spacings. The near- and far-field measurements have shown a high directionality of the lasing output. The lasing can be electrically switched off by an applied field of 30V/mum. The temporal, spectral, and output/input properties of the laser output are also presented.

Electrically switchable lasing from pyrromethene 597 embedded holographic-polymer dispersed liquid crystals

One-dimensional photonic band gap (PBG) materials created from holographic polymer dispersed liquid crystals (H-PDLCs) provide enhanced light localization in an organic electro-optic device. Distributed feedback within the reflection notch of a H-PDLC grating narrowed the bandwidth of pyrromethene 597 fluorescence from 56 to 8.4nm at a lasing threshold of 0.12mJcm−2, compared to 2.6mJcm−2 required to observe amplified spontaneous emission in a nonstructured, but comparable floodlit (PDLC) sample. Application of an electric field (10–40V∕μm) continuously decreased the diffraction efficiency of the grating and the commensurate dynamic lasing intensity thus demonstrating electrically modulated gain from an optically pumped, all-organic PBG.

Reduction of photoluminescence quantum yield by interchain interactions in conjugated polymer films

Abstract We use new photoluminescence data to conclusively establish that photoexcitation of substituted phenylenevinylene polymer films results in a large quantum yield of nearly nonluminescent interchain excitations at ambient temperatures. These data are acquired in the low excitation density regime where excited state interactions are negligible. This result has significant implications for material design and processing to make polymer light-emitting diodes.

Large concentration-dependent nonlinear optical responses of starburst diphenylaminofluorenocarbonyl methano[60]fullerene pentads

We demonstrate an approach toward the design of starburst C60-keto-DPAF assembly by applying a starburst macromolecular configuration with C60 as the core center, which is encapsulated by multiple bulky groups leading to the increase of intermolecular separation and aggregation barrier. Molecular compositions of the resulting C60(>DPAF-C9)2 triad and C60(>DPAF-C9)4 pentads were clearly confirmed by MALDI-MS (positive ion) detection of protonated molecular mass ions. Both C60(>DPAF-C9)22 and C60(>DPAF-C9)4 (structural isomers, 3a and 3b) exhibited nonlinear optical transmittance reduction responses in the femtosecond (fs) region with a lower transmittance value for the latter at the high laser power above 80 GW cm−2. This was attributed to the larger fs 2PA cross-section values of 3a and 3b than that of 2 at the same concentration and, apparently, correlated to a higher number of DPAF-C9 subunits in the structure of 3. As the concentration was decreased to 10−4 M, a clear monotonous increase of the σ2 value change (Δσ2) from 13.9, 33.2, to 48.1 and 68.2 × 10−48 cm4 s photon−1 molecule−1 (or 6820 GM for the latter) for the structural variation from the monoadduct 1, bisadduct 2, to tetraadducts 3b and 3a, respectively, was observed. We interpreted the concentration-dependent phenomenon as being due to the high tendency of fullerene-DPAF chromophores to form nanoscale aggregates at concentrations above 10−3 M. We also proposed that starburst structures, as exemplified by C60(>DPAF-C9)4, in a multipolar arrangement resembling encapsulation of C60 by DPAF-C9 pendants, provide a useful means to increase the degree of molecular dispersion and maintain high nonlinear optical efficiency.

Facile plasma-enhanced deposition of ultrathin crosslinked amino acid films for conformal biometallization.

A novel method for the facile fabrication of conformal, ultrathin, and uniform synthetic amino acid coatings on a variety of practical surfaces by plasma-enhanced chemical vapor deposition is introduced. Tyrosine, which is utilized as an agent to reduce gold nanoparticles from solution, is sublimed into the plasma field and directly deposited on a variety of substrates to form a homogeneous, conformal, and robust polyamino acid coating in a one-step, solvent-free process. This approach is applicable to many practical surfaces and allows surface-induced biometallization while avoiding multiple wet-chemistry treatments that can damage many soft materials. Moreover, by placing a mask over the substrate during deposition, the tyrosine coating can be micropatterned. Upon its exposure to a solution of gold chloride, a network of gold nanoparticles forms on the surface, replicating the initial micropattern. This method of templated biometallization is adaptable to a variety of practical inorganic and organic substrates, such as silicon, glass, nitrocellulose, polystyrene, polydimethylsiloxane, polytetrafluoroethylene, polyethylene, and woven silk fibers. No special pretreatment is necessary, and the technique results in a rapid, conformal amino acid coating that can be utilized for further biometallization.

Plasma amino acid coatings for a conformal growth of titania nanoparticles.

We report on the conformal synthesis of ultrathin films from the amino acid histidine on flat silicon substrates and 3D periodic polymer structures via plasma enhanced chemical vapor deposition. We demonstrate the efficient utilization of this functional amino acid nanocoating for the formation of individual titania nanoparticles with dimensions from 2 to 15 nm depending upon reduction conditions. The titania nanoparticles were grown directly on histidine-functionalized planar and 3D polymer substrates by a wet-chemistry method that showed uniform surface coverage that reached approximately 75%. This approach demonstrates the potential for modifying the optical properties of periodic porous polymeric structures via direct conformal growth of titania nanoparticles.

Plasma-Enhanced Copolymerization of Amino Acid and Synthetic Monomers

In this paper we report the use of plasma-enhanced chemical vapor deposition (PECVD) for the simultaneous deposition and copolymerization of an amino acid with other organic and inorganic monomers. We investigate the fundamental effects of plasma-enhanced copolymerization on different material chemistries in stable ultrathin coatings of mixed composition with an amino acid component. This study serves to determine the feasibility of a direct, facile method for integrating biocompatible/active materials into robust polymerized coatings with the ability to plasma copolymerize a biological molecule (L-tyrosine) with different synthetic materials in a dry, one-step process to form ultrathin coatings of mixed composition. This process may lead to a method of interfacing biologic systems with synthetic materials as a way to enhance the biomaterial-tissue interface and preserve biological activity within composite films.