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/>
This paper, originally published on 1 October 2013, was replaced with a corrected/revised version on 25 October 2013. If you downloaded the original PDF but are unable to access the revision, please contact SPIE Digital Library Customer Service for assistance.
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.
Specific spatially-anisotropic interactions are identified that enhance noncentrosymmetric order required for electro-optic
activity. Enhancement of electric-field-poling-induced noncentrosymmetric order by these specific interactions is
shown to result from a reduction of lattice dimensionality from three to two dimensions. New analytical techniques for
measurement of centrosymmetric and noncentrosymmetric order and lattice dimensionality are introduced.
Measurement of order parameters is correlated with viscoelastic data to gain further insight into the influence of specific
interactions on poling efficiency and thus material electro-optic activity. The integration of organic electro-optic
materials into silicon photonic, plasmonic, and metamaterial devices is also discussed. These device structures can
affect the "effective" optical nonlinearity of organic materials but care must be exercised to control optical loss.
Passive and tunable optical filters as well as optical modulators, directly fabricated on the end-faces of optical fibers can
provide a fast and low cost production. A hybrid layer system can be built up to a passive Fabry-Pérot microcavity,
where alternating dielectric high and low refractive materials are used as mirrors and a highly transparent polymer as the
spacer material. The mirror design and the spacer thickness define the center operation wavelength and the filter
bandwidth. Bandwidths of less than 1 nm (FWHM) at a wavelength of 1560 nm could be achieved for such microcavities
on the end-faces of optical fibers.
Enhancing the hybrid layer system by transparent conductive electrodes and by adding electro-optically active
chromophores to the polymeric spacer material, the filters become tunable. The material used for the electrodes is indium
tin oxide (ITO). The oxidic electrodes have to be merged with the dielectric mirrors and the polymeric spacer. Applying
a voltage to the electro-optically active polymeric spacer utilizing such electrodes, the refractive index of the spacer can
be changed and therefore the resonance criteria of the microcavity.
This communication focuses on the integration of organic nonlinear optical and gain materials into plasmonic and
metamaterial device architectures and most specifically focuses on the integration of organic electro-optic (OEO)
materials into such structures. The central focus is on structures that lead to sub-optical wavelength concentration of
light (mode confinement) and the interaction of photonic and plasmonic modes. Optical loss and bandwidth limitations
are serious issues with such structures and optical loss is evaluated for prototype device architectures associated with the
use of silver and gold nanoparticles and membranes supporting plasmonic resonances. Electro-optic activity in organic
materials requires that chromophores exhibit finite noncentrosymmetric organization. Because of material conductivity
and integration issues, plasmonic and metamaterial device architectures are more challenging than conventional triple
stack all-organic device architectures and electro-optic of a given OEO material may be an order of magnitude less in
such structures. Because of this, we have turned to a variety of materials processing options for such integration
including crystal growth, sequential synthesis/self assembly, and electric field poling of materials deposited from
solution or by vapor deposition. Recent demonstration of integration of silicon photonic modulator and lithium niobate
modulator structures with metallic plasmonic structures represent a severe challenge for organic electro-optic material
plasmonic devices as these devices afford high bandwidth operation and attractive V<sub>μ</sub>L performance. Optical loss
remains a challenge for all structures.
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