Although the promise of organic materials for a variety of photonic applications has been recognized for some time, the
wide-spread use of these materials is limited by photochemical processes that result in irreversible material
decomposition. In this talk, the use of single-molecule microscopy to investigate the photochemistry of organic
molecules in guest-host systems is described. A new method for analyzing the photoluminescence intermittency (PI) or
“blinking” exhibited by single emitters is presented. This method allows for a statistically robust method for analyzing
PI data, and for determining if an external perturbation results in a significant modification of the blinking statistcs. This
method is applied to the blinking exhibited by CdSe/CdS quantum dots (QDs) in poly(methyl methacrylate) and the
rhodamine derivative violamine R (VR) isolated in single crystals of potassium acid phthalate (KAP). For the QDs our
analysis demonstrates that the blinking statistics are not power-law distributed. For VR in KAP we find that deuterium
substitution significantly alters the PI exhibited by VR consistent with proton-transfer contributing to PI.
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
discussed.
Chromophore-polymer composite materials for electro-optical applications are rendered active at the χ(2) level
of susceptibility by inducing chromophore alignment through the interaction of the chromophore dipole moment with an
external electric field, a process referred to as "poling". To provide insight into the molecular details of the poling
process, single molecule microscopy studies of DCM (4-(dicyanomethylene)-2-methyl-6-(4-dimethylaminostyryl)-4Hpyran)
and RhB (Rhodamine B) in poly(methyl acrylate) (PMA) above Tg of the polymer host are performed. Electric
fields of 50 V/μm are employed consistent with typical experimental conditions. The effect of environment is studied
through comparative studies or RhB reorientation in oxidative and inert atmospheres. Single-molecule rotational
dynamics are monitored through the time-evolution of the fluorescence anisotropy. Anisotropy correlation functions
demonstrate non-exponential decay consistent with previous studies of molecular rotation dynamics in polymer melts.
The rotational dynamics of DCM are found to be weakly perturbed in the presence of a 50 V/μm electric field consistent
with the modest alignment potential created by the electric field relative to the amount of available thermal energy. The
relevance of these findings to current models of the poling process is discussed.
This communication primarily deals with utilizing organic electro-optic (OEO) materials for the fabrication of active wavelength division multiplexing (WDM) transmitter/receiver systems and reconfigurable optical add/drop multiplexers (ROADMs), including the fabrication of hybrid OEO/silicon photonic devices. Fabrication is carried out by a variety of techniques including soft and nanoimprint lithography. The production of conformal and flexible ring microresonator devices is also discussed. The fabrication of passive devices is also briefly reviewed. Critical to the realization of improved performance for devices fabricated from OEO materials has been the improvement of electro-optic activity to values of 300 pm/V (or greater) at telecommunication wavelengths. This improvement in materials has been realized exploiting a theoretically-inspired (quantum and statistical mechanics) paradigm for the design of chromophores with dramatically improved molecular first hyperpolarizability and that exhibit intermolecular electrostatic interactions that promote self-assembly, under the influence of an electric poling field, into noncentrosymmetric macroscopic lattices. New design paradigms have also been developed for improving the glass transition of these materials, which is critical for thermal and photochemical stability and for optimizing processing protocols such as nanoimprint lithography. Ring microresonator devices discussed in this communication were initially fabricated using chromophore guest/polymer host materials characterized by electro-optic coefficients on the order of 50 pm/V (at telecommunication wavelengths). Voltage-controlled optical tuning of the pass band of these ring microresonators was experimental determined to lie in the range 1-10 GHz/V or all-organic and for OEO/silicon photonic devices. With new materials, values approaching 50 GHz/V should be possible. Values as high as 300 GHz/V may ultimately be achievable.
Hyper-Rayleigh scattering (HRS) is used to measure the first-hyperpolarizability (β) of electro-optic (EO) chromophores. One of the inherent concerns in any HRS measurement is the extent to which resonant enhancement contributes to the observed intensity thereby leading to inaccuracies when evaluating chromophore potential for application in electro-optical devices. One way to address this concern is to employ increasingly longer excitation wavelengths far from resonance. However, in charge-transfer-based non-linear optical chromophores, enhanced β generally correlates with a red-shift of the charge transfer absorption band so that even at the longest excitation wavelengths generally employed in HRS studies, resonant enhancement remains an issue. We have adopted an alternative approach in which the wavelength dispersion of the HRS intensity is determined by performing measurements at a variety of excitation wavelengths. This approach allows one to ascertain the role of resonance enhancement thereby allowing for more accurate correlation of improved β with molecular architecture. We report the results of our HRS studies for nine chromophores employing excitation wavelengths ranging from 780 to 1907 nm. Our HRS results demonstrate good agreement with the predictions of density functional theory. This synthesis of experimental and theoretical techniques has resulted in more effective designs for the next generations of electro-optical chromophores.
The kinetics of the photochemical ring openings of 1,3-cyclohexadiene (CHD), 1,3,5- cyclooctatriene (COT) and ((alpha) -PHE) were determined by picosecond, time-resolved UV resonance Raman spectroscopy. The time evolution of the photoproduct ethylenic intensity demonstrates that the photolysis of CHD produces ground state cis-hexatriene in 8 +/- 1 ps. Similarly, the photoproducts of COT and (alpha) -PHE appear in 12 +/- 2 ps and 11 +/- 2 ps, respectively. The similar ground state photoproduct formation times of these reactions indicates that the $OM10 ps timescale is a general feature of photochemical electrocyclic ring-opening reactions.
Picosecond resonance Raman spectroscopy has been used to obtain structural information on the primary photointermediates of bacteriorhodopsin. A synchronously pumped dye laser was amplified at 50 Hz to produce a probe pulse at 589 nm. A second, spectrally distinct, pump pulse at 550 nm was generated by amplification of a 10 nm portion of a continuum produced from the probe pulse. This apparatus was used to record spectra of the J, K, and KL intermediates. The J spectrum exhibits strong hydrogen out-of-plane (HOOP) intensity and the fingerprint region consists of a broad series of lines centered at 1180 cm-1. By 3 ps, K has formed and the relative HOOP intensity decreases while the fingerprint collapses to a single mode at 1190 cm-1, characteristic of a 13-cis chromophore. These results argue that J contains a highly twisted chromophore which relaxes upon conversion to K and that isomerization is complete within 3 ps. Between 3 ps and 3.7 ns there is a resurgence in HOOP intensity and the ethylenic frequency rises from 1518 to 1521 cm-1 indicating the conversion of K to KL.
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