Standard models for evaluating the electro-optic (EO) response of organic materials typically assume that the refractive index of the material in the absence of a RF modulation field is isotropic and homogeneous. Such assumptions work very well for low-concentration guest-host materials in bulk devices. However, current generation organic EO materials at high densities and under nanoscale confinement can show sufficient birefringence to affect device performance. We use computer simulations and spectroscopic experiments to characterize and predict changes in the index of refraction under poling. We also demonstrate that poling-induced birefringence can lead to a non-linear relationship between the apparent EO coefficient and poling field strength.
Multi-scale (correlated quantum and statistical mechanics) modeling methods have been advanced and employed to guide the improvement of organic electro-optic (OEO) materials, including by analyzing electric field poling induced electro-optic activity in nanoscopic plasmonic-organic hybrid (POH) waveguide devices. The analysis of in-device electro-optic activity emphasizes the importance of considering both the details of intermolecular interactions within organic electro-optic materials and interactions at interfaces between OEO materials and device architectures. Dramatic improvement in electro-optic device performance--including voltage-length performance, bandwidth, energy efficiency, and lower optical losses have been realized. These improvements are critical to applications in telecommunications, computing, sensor technology, and metrology. Multi-scale modeling methods illustrate the complexity of improving the electro-optic activity of organic materials, including the necessity of considering the trade-off between improving poling-induced acentric order through chromophore modification and the reduction of chromophore number density associated with such modification. Computational simulations also emphasize the importance of developing chromophore modifications that serve multiple purposes including matrix hardening for enhanced thermal and photochemical stability, control of matrix dimensionality, influence on material viscoelasticity, improvement of chromophore molecular hyperpolarizability, control of material dielectric permittivity and index of refraction properties, and control of material conductance. Consideration of new device architectures is critical to the implementation of chipscale integration of electronics and photonics and achieving the high bandwidths for applications such as next generation (e.g., 5G) telecommunications.
Taken together, theory-guided nano-engineering of organic electro-optic materials and hybrid device architectures have permitted dramatic improvement of the performance of electro-optic devices. For example, the voltage-length product has been improved by nearly a factor of 10<sup>4</sup> , bandwidths have been extended to nearly 200 GHz, device footprints reduced to less than 200 μm<sup>2</sup> , and femtojoule energy efficiency achieved. This presentation discusses the utilization of new coarse-grained theoretical methods and advanced quantum mechanical methods to quantitatively simulate the physical properties of new classes of organic electro-optic materials and to evaluate their performance in nanoscopic device architectures, accounting for the effect on chromophore ordering at interfaces in nanoscopic waveguides.
Biopolymer-based thin films, such as those composed of CTMA-DNA, can be used as a host material for NLOactive dyes for applications such as electro-optic (EO) switching and second harmonic generation. Previous work by Heckman et al. (Proc. SPIE 6401, 640108-2) has demonstrated functioning DNA-based EO modulators. Improved performance requires optimization of both the first hyperpolarizabilities (β) and degree of acentric ordering exhibited by the chromophores. The cationic dye DANPY-1 (Proc. SPIE 8464, 846409-D) has a high affinity for DNA and a substantial hyperpolarizability; however, its macroscopic ordering has not been previously characterized. We have characterized the acentric ordering of the dye using sum-frequency generation (SFG) vibrational spectroscopy in surface-immobilized DNA and on planar metal and dielectric surfaces.
This study highlights some of the effects of UV crosslinking of DNA-CTMA on its electrical and optical characteristics. The crosslinking of DNA-CTMA occurs via the photodimerization of attached coumarin moieties under UV irradiation. An exposure time of 30 min to UV light with an output power of 166 mW/cm<sup>2</sup> is needed to complete the crosslinking process. The UV-crosslinked films show a significant increase in the electrical resistivity (decrease in leakage current) and a markedly lower dielectric constant. <br/>
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Understanding the kinetics of dye adsorption on semiconductors is crucial for designing dye-sensitized solar cells (DSSCs) with enhanced efficiency. Harms et al. recently applied the Quartz-Crystal Microbalance with Dissipation Monitoring (QCM-D) to study in situ dye adsorption on flat TiO2 surfaces. QCM-D measures adsorption in real time and therefore allows one to determine the kinetics of the process. In this work, we characterize the adsorption of N3, a commercial RuBipy dye, using the native oxide layer of a titanium sensor to simulate the TiO2 substrate of a DSSC. We report equilibrium constants that are in agreement with previous absorbance studies of N3 adsorption, and therefore demonstrate the native oxide layer of a titanium sensor as a valid and readily available planar TiO2 morphology to study dye adsorption.
Alignment of dipolar chromophores lies at the heart of organic electro-optic materials research. Among all the factors (e.g., external electric field, temperature, conductivity, etc.) affecting alignment efficiency or order parameter, interchromophore electrostatic interaction has been the focus of attention in the last decade. The strength of dipole interaction is highly dependent not only on dipole moment but also on chromophore shape and chromophore number density. Antiparallel interaction is dominant in the solid state of conventional EO chromophores (long and flat) and prevents electro-optic coefficient (r33) from scaling with chromophore concentration. Despite the great amount of research along various approaches to enhancing alignment, order parameters of organic EO materials are still low (0.13- 0.2 v.s. 1 for a perfect alignment). Antiparallel interaction can be selectively attenuated by attaching bulky groups to the middle part of chromophore. However, it is synthetically challenging to provide sufficient steric protection without causing severe reduction of chromophore concentration. In this paper, we will present the first realization of atomeconomic steric protection of chromophore against H-aggregation in all directions and show evidences for the dominance of head-tail interaction over antiparallel interaction of a highly dipolar chromophore. With the novel shape, the EO coefficients of guest-host films of the chromophore do not show attenuation with increasing concentration up to 100 wt%. The dominance of head-tail interaction also enabled fabrication of optical quality thick films from the neat chromophore and allows poling induced alignment to retain at temperatures above the poling temperature – a phenomenon never observed for other chromophores.
Biopolymers such as DNA can be used as a host material for nonlinear optical dyes for photonic applications. In
previous work by Heckman et al. (Proc. SPIE 6401, 640108-2), the chromophore Disperse Red 1 (DR1) was combined
with CTMA-DNA (a water-insoluble DNA/surfactant complex) to produce an electro-optic waveguide modulator.
However, DR1 does not bind strongly to DNA and has a low first hyperpolarizability (β). We have used theory-aided design to develop and synthesize a novel chromophore with strong affinity for DNA and higher β than DR1. We have also developed a surfactant containing a photocrosslinkable moiety that can be used to harden thin films of the DNA/surfactant/dye composite under ultraviolet light. The optical and thermal properties of these materials and outlook for device applications will be discussed.
Theoretical calculations have demonstrated that the ratio of second and third degree order parameters can define lattice
dimensionality and furthermore, that an increased ratio of second to third degree order parameters represents reduced
lattice dimensionality. As a result, the third degree order parameter (i.e. acentric order parameter) is increased, causing
an increase in electro-optic activity with reduced lattice dimensionality. Experimentally, specific spatially-anisotropic
interactions associated with coumarin moieties and Frechet-type (arene/perfluoroarene) dendrons have been incorporated
into chromophore systems and have been shown to lead to lattices of reduced dimensionality, resulting in increased
values of the acentric order parameter and therefore, electro-optic activity. Reductions in lattice dimensionality can also
arise from guest chromophore-host chromophore interactions in binary chromophore organic glasses and from laserinduced
ordering of host lattice chromophores observed in the laser-assisted electric field poling of azo-dye-containing
host lattices. These interactions in various chromophore systems including investigation of EO and order properties are