In organic molecules, the strength of the linear and nonlinear optical response scales depends on the size of the structure. Power-laws that correlate the length of a structure and its nonlinear structure have been proposed by different researchers. These power-laws are described as function of the number of repeating units, and are derived from the experimental characterization of one set of homologue compounds. Typically, every set of homologues has been reported to obey a different power-law. We show how the sum rules allow to derive universal scaling power-laws that apply to all structures and are in agreement with the experimental data. Using the concept of universal scaling, we propose a classification of the scaling behavior that can be used to determine what are the best molecular paradigms for future nonlinear optical applications.
Understanding the fundamental mechanisms behind the radiation resistance of polymers and molecules would allow us to tailor new materials with enhanced performance in space and adverse environments. Previous studies of the radiation effects on polymer-based photonic materials indicate that they are very dependent on the choice of polymer-host and guest-chromophores. The best results have been reported from the combination of CLD1 as a guest-chromophore doped in APC as host polymer, where improvement of the performance was observed upon gamma-irradiation at moderate doses. In this paper, we report on the different complementary tools that have been tried to characterize the origin of such enhancement: characterization of the linear and nonlinear response, characterization of chemical properties, and application of an all-optical protocol. We derive some general conclusions by contrasting the results of each characterization, and propose complementary experiments based on microscopy techniques.
In organic molecules, the optical response originates from the motion of the pi-electrons, which are constrained to move along the molecule’s conjugated path. As an electron moves through the conjugated path, it interacts with the rest of the charges such that its motion is very dependent on the shape of the molecule. In this paper we introduce a simple model for that allows us to determine how the shape of the conjugated path affects the nonlinear optical response of the molecule. Our results apply to typical second-order dipolar structures: we have determined how the symmetry of the conjugated path affects the optical response, and we have found potential new strategies for making better molecules.
Previous studies on the radiation effects upon polymer and polymer-based photonic materials suggest that the radiation resistance of the material is heavily dependent on the choice of polymer-host and guest-chromophore. The best results to date have been achieved with electro optic polymeric materials based on CLD1 doped in APC, which has resulted in improved performance at the device level upon gamma-ray irradiation at moderate doses. Still, our understanding of the physical mechanisms behind the enhancement of the performance is unclear. In this paper, we discuss how polarized light microscopy could be used as a means to quantify the effect of the different physical parameters that influence the optical response of electro-optic polymeric thin film samples.
The performance of polymer-based electro-optic modulator is very dependent on the wavelength of operation of the device. Surprisingly, most of our intuitive understanding of the molecular performance in such devised are based on studies performed in the off-resonance regime, where the nonlinear optical response of the molecule is by assumption not dependent on the wavelength of operation; or/and two-level model extrapolation, where the response is assumed to be dominated by the contribution of only one excited state. In either case, the effects of quantum interference (cancellation and enhancement of the response due to interactions between multiple excited states) are ignored. In this paper we show how in complex molecules with more than one excited state, quantum interference effects plays an important role in determining the on-resonant response, and hence should not be ignored when studying the response of organic-based electro-optic materials. We use this principle to interpret previous experimental results on the performance of electro-optic modulations under enhanced radiation environments.
Previous studies on the radiation effects upon polymer and polymer-based photonic materials suggest that the radiation resistance of the material is heavily dependent on the choice of polymer-host and guest-chromophore. To date, the best results have been achieved with electro optic polymeric materials based on CLD1 doped in APC, which has resulted in improved performance at the device level upon gamma-ray irradiation at moderate doses. However, the physical mechanisms are yet not fully understood. In this paper, we introduce an all-optical (linear and nonlinear) characterization protocol that is aimed to elucidate the mechanisms of the radiation damage/enhancement of electro-optic polymeric materials. This protocol is used to quantify the damage/enhancement effects upon irradiation in terms of the relevant physical parameters on a collection of electro-optic polymeric thin film samples.
Optical confinement can induce enhancement of the resonance energy transfer between fluorescent molecules by influencing the interaction between the different available energy levels. We study the energy transfer between a pair of molecules, tris(2-phenylpyridine) iridium and bis(2-methyl-8-quinolinato)-4-phenylphenolate aluminum, which are extensively used in organic light-emitting diode technologies. These molecules have previously shown Förster energy transfer. We present the result of the dipolar coupling of these two molecules embedded in a poly(N-vinylcarbazole) film and inserted in a colloidal photonic crystal. Due to the presence of the photonic band gap, the efficiency of the energy transfer has been improved. A thermal study of the emission under the effect of the photonic band gap has been performed.
The continued interest in molecules that possess large quadratic nonlinear optical (NLO) properties has
motivated considerable interplay between molecular synthesis and theory. The screening of viable candidates
for NLO applications has been a tedious work, much helped by the advent of the hyper-Rayleigh scattering
(HRS) technique. The downside of this technique is the low efficiency, which usually means that measurements
have to be performed at wavelengths that are close to the molecular resonances, in the visible area. This
means generally that one has to extrapolate the results from HRS characterization to the longer wavelengths
that are useful for applications. Such extrapolation is far from trivial and the classic 2-level model can
only be used for the most straightforward single charge-transfer chromophores. An alternative is the TKSSOS
technique, which uses a few input-hyperpolarizabilities and UV-Vis absorption data to calculate the
entire hyperpolarizability spectrum. We have applied this TKS-SOS technique on a set of porphyrines to
calculate the hyperpolarizability dispersion. We have also built a tunable HRS set up, capable of determining
hyperpolarizabilities in the near infrared (up to 1600 nm). This has allowed us to directly confirm the results
predicted in the application region. Due to the very sharp transitions in the hyperpolarizability dispersion, the
calculation is subjected to a very precise calibration with respect to the input-hyperpolarizabilities, resulting
in very accurate predictions for long wavelength hyperpolarizabilities. Our results not only underscribe the
aforementioned technique, but also confirm the use of porphyrines as powerful moieties in NLO applications.
Although much of the early developments of organic and polymer-based materials were fueled by the research on space
materials, most of the optoelectronic applications for usage in space or terrestrial adverse environments are still
dominated by semiconductors. In this paper, we review past and present efforts to incorporate organic-based materials
into photonic devices that are suitable for applications in space environments, and discuss what are the main challenges
that materials based on organics must meet in order to become fully integrated into photonic devices that can operate in
space and space-related environments.
Traditionally, the nonlinear optical response at the molecular level has been modeled using the two-level approximation,
under the assumption that the behavior of the exact sum-over-states (SOS) expressions for the molecular polarizabilities
is well represented by the contribution of only two levels. We show how, a rigorous application of the Thomas-Kuhn
sum-rules over the SOS expression for the diagonal component of the first-hyperpolarziability proves that the two-level
approximation is unphysical. In addition, we indicate how the contributions of potentially infinite number of states to
the SOS expressions for the first-hyperpolarizability are well represented by the contributions of a generic three-level
system. This explains why the analysis of the three-level model in conjugation with the sum rules has lead to successful
paradigms for the optimization of organic chromophores.
The use of piezoelectric polymers has been proposed and investigated in different Space-related environments, for
example, as ultra-light mirrors in space telescopes or as piezoelectric actuators. Even though some piezoelectric
polymers have been shown to be as efficient as the more traditional piezoelectric crystals, no systematic exploration of
the different molecular motives available for piezoelectricity has been performed, partly due to experimentally
challenging conditions: new structures must be generated in enough quantity to be able to produce thin films, and with
measurable piezoelectric response. Consequently, few structure-property relationships have been derived for the
piezoelectric performance of polymer based materials. We show how, under certain conditions, the characterization of
the second-order nonlinear molecular response through the Hyper-Rayleigh scattering technique, can be used as a
screening technique for the optimization of the piezoelectric response of poled-doped materials. In contrast to the
piezoelectric characterization, a Hyper-Rayleigh experiment can be performed with minimal amounts of chromophores
(~mg) in solution, and is relatively quick. Therefore, we propose to use the Hyper-Rayleigh scattering technique as a
screening tool for the search of optimized piezoelectric polymers.
We introduce a self-consistent theory for the description of the optical linear and nonlinear response of molecules that is
based strictly on the results of the experimental characterization. We show how the Thomas-Kuhn sum-rules can be
used to eliminate the dependence of the nonlinear response on parameters that are not directly measurable. Our approach
leads to the successful modeling of the dispersion of the nonlinear response of complex molecular structures with
different geometries (dipolar and octupolar), and can be used as a guide towards the modeling in terms of fundamental
physical parameters.
We present a procedure for the modeling of the dispersion of the nonlinear optical response of complex molecular
structures that is based strictly on the results from experimental characterization. We show how under some general
conditions, the use of the Thomas-Kuhn sum-rules leads to a successful modeling of the nonlinear response of complex
molecular structures.
We investigate the role of the conjugated spacer in the optimization of the first hyperpolarizability of organic
chromophores. We propose a novel strategy for the optimization of the first hyperpolarizability that is based on the
variation of the degree of conjugation for the bridge that separates the donor and acceptors at the end of push-pull type
chromophores. The correlation between the type of conjugated spacer and the experimental nonlinear performance of the
chromophores is investigated and interpreted in the context of the quantum limits.
Much progress has been made in improving the molecular hyperpolarizability by constructing larger structures with lots of pi-delocalized electrons. This bigger-is-better approach does not answer a more fundamental question of what intrinsic molecular properties yield the largest response. We introduce a novel analysis (the quantum limits analysis) which is simple to apply and combines first principles with experimental results. The quantum limits analysis allows us to determine the intrinsic nonlinear efficiency of a structure and highlights the underlying physical principles behind the nonlinear response of molecules.
The results of three independently strategies for the optimizations of electro-optic organic chromophores is
presented. The first strategy to enhance the nonlinear optical response, applied at the molecular level, is the
extension of the conjugation path in a ionic chromophore. The second strategy, applied at the supramolecular
level, is the bottom-up nano-engineering of an inclusion complex of the ionic chromophore in an amylose helix.
The third strategy, also applied a the molecular level, is to use a modulated conjugation path between donor and
acceptor in order to localize eigenfunctions on different parts of the molecule. The first hyperpolarizability of
the different series of compounds has been experimentally determined by frequency-resolved femtosecond hyper-Rayleigh scattering. The effects of the three different enhancement strategies are analyzed and interepreted in
terms of the quantum limits.
The effects of a complex hybrid conjugation path in linear molecules as an strategy to optimize the intrinsic first
hyperpolarizability is investigated. A series of 7 novel chromophores with different hybrid conjugation paths were
synthesized and characterized. Hyper-Rayleigh scattering experiments confirm that complex hybrid conjugation
paths, including benzene, thiophene and/or thiazole rings in combination with azo- and/or ethenyl-linkages,
between a dihydroxyethylamino donor group and different acceptor groups, results in an enhanced intrinsic
hyperpolarizability that exceed the apparent limit for two of the chromophores.
We present the results of the combination of two independently valid optimization strategies for the first hyperpolarizability of ionic organic chromophores. The first strategy to enhance the nonlinear optical response, at the molecular level, is the extension of the conjugation path in the chromophore itself. The second strategy, at the supramolecular level, is the bottom-up nano-engineering of an inclusion complex of the chromophore in an amylose helix by self-assembly. We have studied a series of five (dimethylamino)stilbazolium-type chromophores with increasing conjugation length between the (dimethylamino)phenyl donor ring and the pyridinium acceptor ring in combination with four amylose helices of different molecular weights. The first hyperpolarizability of the self-assembled inclusion complexes has been experimentally determined by frequency-resolved femtosecond hyper-Rayleigh scattering at 800 and 1300 nm. These values are compared with experimental values for the free chromophores in solution and with theoretical values. Where experimental values for the hyperpolarizability in solution were lower than theoretically predicted, an enhancement upon inclusion was observed - with the longest chromophore in the best amylose helix showing an enhancement by one order of magnitude. Molecular modelling of the inclusion of the chromophore suggests that the coplanarity of the two rings is more important than all-trans configuration in the conjugation path. The degree of enhancement, however, is not enough to breach the apparent limit of the first hyperpolarizability which is about an order of magnitude below the fundamental limit calculated by Kuzyk. This analysis confirms the determining role of the arrangement of the excited-state energy levels on the nonlinear response.
Sum rules have been shown to impose a fundamental limit on the of nonlinear-optical susceptibility. All of the measured values of the hyperpolarizability and second hyperpolarizability over the last 25+ years, be it on- or off-resonance, fall a factor of 103/2 below these limits. Not only is this result scientifically puzzling on a fundamental level; but, has implications on the kinds of practical devices that can be made. In this work, we discuss molecular engineering techniques that aim to break this bottleneck.
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