tuning optical properties of opal photonic crystals by structural defects engineering

We report on the preparation and optical characterization of three dimensional colloidal photonic crystal (PhC) containing an engineered planar defect embedding photoactive push-pull dyes. Free standing polystyrene films having thickness between 0.6 and 3 microns doped with different dipolar chromophores were prepared. These films were sandwiched between two artificial opals creating a PhC structure with planar defect. The system was characterized by reflectance at normal incidence angle (R), variable angle transmittance (T), and photoluminescence spectroscopy (PL). Clear evidence of defect states were observed in T and R spectra, which allow the light to propagate for selected frequencies within the pseudogap (stop band)

Dimers of Quadrupolar Chromophores in Solution: Electrostatic Interactions and Optical Spectra

Two dimers of a heteroaromatic quadrupolar (acceptor−donor−acceptor) chromophore have been synthesized with different interchromophoric distances. Optical spectra of dimers in solution show a red shift of the linear absorption band upon decreasing the interchromophore distance, while fluorescence and two-photon absorption spectra are only marginally affected by the interactions. A bottom up approach is adopted to describe the spectra: via a detailed spectroscopic analysis of the monomeric species in solution, we define an essential-state model for the isolated chromophore and use this information to set up a model for the dimers also accounting for interchromophore electrostatic interactions. To discriminate between static screening governed by the static dielectric constant and dynamical screening at optical frequencies, we first solve the problem in the mean-field approximation and then define the excitonic Hamiltonian on the resulting best excitonic basis. Along this line, the evolution of spectral properties with the interchromophore distance is properly rationalized

EMISSION PROPERTIES OF ARTIFICIAL OPALS INFILTRATED WITH A HETEROAROMATIC QUADRUPOLAR DYE

Artificial opals are a simple and cheap playground to manipulate the propagation of light. The interest in these kind of photonic crystals is further increased by the possibility to be infiltrated with highly polarisable media like organic semiconductors, i.e. conjugated polymers, push-pull molecules and multipolar chromophores. In this work, we report on the optical properties of polystyrene opals infiltrated with a heteroaromatic quadrupolar derivative endowed with strong nonlinear optical properties (two-photon absorption) in solution. The insertion of tris(ethylene glycol)monomethyl ether chains on the conjugated skeleton allows the molecule to be soluble in water, a non-solvent for polystyrene. This condition is fundamental in order to attempt opal infiltration. Variable angle transmittance and photoluminescence spectroscopy are used to characterize the system. The bathochromic shift of the opal stop band upon immersion in the chromophore solution confirms that the infiltration process easily takes place preserving a dielectric contrast suitable for further investigations. Photoluminescence spectra recorded at different emission angle with respect to the normal of the sample for both the chromophore solution and opals infiltrated with such solution show interesting characteristics. The presence of opal modifies the chromophore emission spectrum by filtering the light for wavelengths corresponding to those of the stop band and according to its dispersion

Dimers of Quadrupolar Chromophores in Solution: Electrostatic Interactions and Optical Spectra

Two dimers of a heteroaromatic quadrupolar (acceptor-donor-acceptor) chromophore have been synthesized with different interchromophoric distances. Optical spectra of dimers in solution show a red shift of the linear absorption band upon decreasing the interchromophore distance, while fluorescence and two-photon absorption spectra are only marginally affected by the interactions. A bottom up approach is adopted to describe the spectra: via a detailed spectroscopic analysis of the monomeric species in solution, we define an essential-state model for the isolated chromophore and use this information to set up a model for the dimers also accounting for interchromophore electrostatic interactions. To discriminate between static screening governed by the static dielectric constant and dynamical screening at optical frequencies, we first solve the problem in the mean-field approximation and then define the excitonic Hamiltonian on the resulting best excitonic basis. Along this line, the evolution of spectral properties with the interchromophore distance is properly rationalized

Emission properties of artificial opals infiltrated with a heteroaromatic quadrupolar dye

Artificial opals are a simple and cheap playground to manipulate the propagation of light. The interest in these kind of photonic crystals is further increased by the possibility to be infiltrated with highly polarisable media like organic semiconductors, i.e. conjugated polymers, push-pull molecules and multipolar chromophores. In this work, we report on the optical properties of polystyrene opals infiltrated with a heteroaromatic quadrupolar derivative endowed with strong nonlinear optical properties (two-photon absorption) in solution. The insertion of tris(ethylene glycol)monomethyl ether chains on the conjugated skeleton allows the molecule to be soluble in water, a non-solvent for polystyrene. This condition is fundamental in order to attempt opal infiltration. Variable angle transmittance and photoluminescence spectroscopy are used to characterize the system. The bathochromic shift of the opal stop band upon immersion in the chromophore solution confirms that the infiltration process easily takes place preserving a dielectric contrast suitable for further investigations. Photoluminescence spectra recorded at different emission angle with respect to the normal of the sample for both the chromophore solution and opals infiltrated with such solution show interesting characteristics. The presence of opal modifies the chromophore emission spectrum by filtering the light for wavelengths corresponding to those of the stop band and according to its dispersion

One- and two-photon absorption and emission properties of heteroaromatic bichromophores

We present the design, synthesis, and characterization of a class of heteroaromatic bichromophores in order to investigate intermolecular interactions and their effect on optical and nonlinear optical properties. As a design strategy we have linked two dipolar or quadrupolar units through a non-conjugated alkyl chain. The two units are connected either through their donor or their acceptor end-groups. This study represents a first step towards the design of bi- and multichromophoric systems with optimized NLO responses in order to exploit collective and cooperative effects from interchromophore interactions