Jieyun Wu

PCBM-doped electro-optic materials: investigation of dielectric, optical and electro-optic properties for highly efficient poling

Fullerenes are ubiquitously popular in organic electronic materials and devices. The high electron affinity, electron mobility and percolated networks for electron transport of fullerene derivatives, such as PCBM, have established them as excellent electron acceptors and transport materials in organic solar cells and electronic devices. It is intriguing to utilize these electronic properties and molecular three-dimensional networks to explore their potential applications in new electronic or optical devices. In this work, PCBM was doped into organic electro-optic (EO) materials and their surface morphology, photophysical properties, dielectric properties as well as optical properties (refractive index) were systematically investigated. It was found that the dielectric constant and refractive index of the doped materials were significantly enhanced. Based on temperature-dependent dielectric constant measurements, the relation between relative microscopic dipole moment and dielectric properties was established. It revealed that, at the poling temperature, the dipole moment of chromophores in the PCBM-doped film P1/PCBM was higher than that of the conventional EO film P1. This enhanced microscopic property of chromophores in P1/PCBM well accounted for the improved poling results in electric field induced poling. A larger EO coefficient (197 pm V−1versus 133 pm V−1) and figure-of-merit n3r33 (1002.9 versus 632.2), as well as a higher order parameter (15.7% versus 10.6%) and birefringence were achieved for the PCBM-doped film P1/PCBM, demonstrating the significant potential of PCBM to be used in organic EO materials and devices.

Facile bromine-termination of nonlinear optical chromophore: remarkable optimization in photophysical properties, surface morphology and electro-optic activity

This paper describes how a small molecular structure modification can enhance the microscopic and macroscopic properties of chromophore. A new chromophore WJ10 is synthesized by applying bromine-termination to the remote donor of WJ6, an existing donor–π–acceptor chromophore. This small molecular change can significantly enhance the photophysical properties of chromophore and generate intriguing inverted solvatochromism in solutions. The absorption intensity of chromophore WJ10 in a guest-host electro-optic polymer film is 40–50% higher than that of the chromophore WJ6, which results in the increase of microscopic first-order hyperpolarizability of WJ10 in guest–host electro-optic (EO) polymer film. DFT calculations was carried out to explain this intriguing photophysical property in both solutions and in films. Bromine-termination also has the influence on macroscopic surface morphology of WJ10 in EO films, making WJ10 more homogeneously dispersed than WJ6. In EO activities, EO coefficient obtained with the WJ10 film is more than two times larger (211 pm V−1) than benchmark value 104 pm V−1 obtained from WJ6. The enhanced electro-optic activity with WJ10 is due to the enhancement of the microscopic hyperpolarizability and the better chromophore alignment in the poling process. This study demonstrates the structure–property relationship in bromine-termination of nonlinear optical chromophore, which can be further explored for the synthesis of new organic EO materials.

Low power variable optical attenuator based on a hybrid SiON-Polymer S-bend waveguide

We propose a low-power variable optical attenuator based on the configuration of a hybrid silicon oxynitride (SiON)-polymer S-bend waveguide and two grooves in both sides of the S-bend waveguide core. With such a configuration, the opposite thermo-optic characteristics of SiON and polymer materials can be fully utilized. As a consequence, the heat utilization efficiency is increased. Theoretical simulation shows that an optical attenuation of ∼50  dB can be achieved with an applied electrical power of 3.6 mW. A typical fabricated device, which has a total length of 8 mm, shows a maximum optical attenuation of 46 dB with an applied electrical power of 16.2 mW and an insertion loss of 5.4 dB.

Tuning the strength of intramolecular charge-transfer of triene-based nonlinear optical dyes for electro-optics and optofluidic lasers

Organic conjugated dyes have received extensive attention due to their broad applications in nonlinear optics and light-emitting and fluorescent sensors. Herein, we report the two-step synthesis of donor–acceptor dyes with triene-conjugated bridges. Via Vilsmeier formylation and Knoevenagel condensation, the dyes 1–4 with respectively varied electron-withdrawing moieties were successfully synthesized with high yields in short duration times. Photophysical properties showed gradually enhanced intramolecular charge-transfer (ICT) strength from dye 1 to dye 4 and diverse absorption and fluorescence properties. This structure–property tuning ensured their different applications in optofluidic lasers and nonlinear optics. Moreover, an optofluidic laser obtained using dye 1 in toluene as the gain material was demonstrated. Narrow linewidth (1.6 nm) of the optofluidic laser (peak wavelength: 610 nm) with a threshold of 94.2 μJ mm−2 was obtained. In electro-optic (EO) activities, four dyes in guest–host EO films showed the step-up EO coefficients from dye 1 (3 pm V−1) to dye 4 (76 pm V−1). This trend was in accordance with the results of ICT energy gap and DFT calculations, and it showed the effective tuning of structure–property relationship for electro-optics. The facile synthesis of NLO dyes, fine-tuning of their intramolecular charge-transfer, as well as the experimental demonstration in nonlinear optics and optofluidic lasers indicated their significant applications in organic photonic devices.

Ultra-efficient and stable electro-optic dendrimers containing supramolecular homodimers of semifluorinated dipolar aromatics

In organic electro-optic (EO) materials, strong dipole–dipole interactions hinder the highly efficient poling of nonlinear optical chromophores. Supramolecular self-assembly through π–π stacking of fluoroaromatics was proved to be one of the most effective strategies to simultaneously achieve high chromophore loading density and highly efficient poling. Herein, we demonstrated a new strategy of supramolecular homodimerization to self-assemble EO dendritic films, in which two dendritic units with semifluorinated dipolar 1,2,3-trifluorobenzene (TFB) moieties were attached to the donor end and the π-bridge centre of push–pull tetraene chromophores. In these new dendrimers, the use of monolithic and semifluorinated TFB rings to replace the heterodimers of phenyl and pentafluorophenyl moieties has greatly simplified the synthesis of dendrimers and their intermixing, and can further potentially enable more efficient and rapid intermixing of interacting moieties in the solid states than those in binary and ternary systems. Photophysical property analysis and DFT calculations were carried out to understand the macroscopic supramolecular self-assembly and microscopic polarizability of new TFB-based EO dendrimers. The poled films of these self-assembled dendritic EO films exhibited very large EO coefficients up to 248 pm V−1 at a wavelength of 1310 nm and excellent temporal stability at room temperature with a very minimal change of ∼5% for over 1000 hours. Our study therefore illustrates that homodimer stacking of TFB rings through dipole–dipole coupling provides stabilization energy similar to that of quadrupolar interaction of phenyl and pentafluorophenyl heterodimeric pairs. Due to the highly efficient poling and excellent temporal EO stability, TFB self-assembled EO dendrimers show great potential for application in photonic devices.

Design, synthesis, and properties of nonlinear optical chromophores based on a verbenone bridge with a novel dendritic acceptor

Two novel second order nonlinear optical (NLO) chromophores based on N,N-diethylaniline as a donor, verbenone based tetraene as a bridge, and tricyanofuran (TCF) or tricyanofuran derivatives with a dendritic moiety as an acceptor have been synthesized in good overall yields and systematically characterized. Besides, a facile applicable synthetic approach for a NLO dendritic acceptor was developed. Compared with C7, after introducing dendritic derivative steric hindrance groups into the acceptor, chromophore C8 had good thermal stabilities with high thermal decomposition temperatures which were 33 °C higher than that of chromophore C7. At the same time, cyclic voltammetry (CV) experiments were performed to determine the different redox properties. The conjugated verbenone tetraene segments in two chromophores could significantly improve the glass-forming ability and molecular polarization of chromophores as revealed by UV-vis-NIR absorption measurements. The bulky dendritic moiety linked by a short C–C bond is closer to the TCF acceptor, which is the most polar part in the chromophore, compared to conventional isolation groups. The results obtained from electro-optic (EO) coefficients indicate that this TCF acceptor with a unique dendritic structure can prevent antiparallel packing between chromophores, improving the poling efficiency and enhancing the EO performance. These properties, together with the good solubility, suggest the potential use of these new chromophores as materials for advanced photonic devices.

Ultra-broadband mode converters based on length-apodized long-period waveguide gratings

We propose an ultra-broadband mode converter based on the structure of a length-apodized long-period grating, where π-phase shifts are introduced at strategic locations of the grating profile. Using a 3-section length-apodized grating structure, we design and fabricate an LP01-LP11a and an LP01-LP11b mode converter with a sidewall grating and a surface grating formed along a polymer channel waveguide, respectively. The fabricated LP01-LP11a and LP01-LP11b mode converters provide a conversion efficiency higher than 99% over a bandwidth of ~120 nm and ~150 nm, respectively, or a conversion efficiency higher than 90% over a bandwidth of ~180 nm and ~300 nm, respectively. The transmission characteristics of these devices are weakly sensitive to polarization and temperature variations. These mode converters can find applications in ultra-broadband mode-division-multiplexing transmission systems based on few-mode fibers and the design principle can be applied to general grating-based mode-coupling devices for a wide range of applications.

Ultra-broadband mode conversion with length-apodized long-period grating on polymer waveguide

We design and fabricate a length-apodized long-period grating on a polymer waveguide for ultra-broadbandLP01-LP11b mode conversion. The fabricated device provides a mode-conversion efficiency higher than 99% over a bandwidth of ~150 nm.