Electroabsorption Data Research Articles

Distance Dependent Electron Transfer at TiO2 Interfaces Sensitized with Phenylene Ethynylene Bridged RuII–Isothiocyanate Compounds

Excess electrons present in semiconductor nanocrystallites generate a significant electric field, yet the role this field plays in molecular charge transfer processes remains poorly understood. Three ruthenium bipyridyl cis-Ru(bpy)(LL)(NCS)2 compounds, where LL is a 4-substituted bpy, with zero, one, or two phenylene ethynylene bridge units, were anchored to mesoporous nanocrystalline TiO2 thin films to specifically quantify interfacial charge transfer with chromophores designed to be set at variable distances from the surface. Injection of electrons into TiO2 resulted in a blue shift of the metal-to-ligand charge transfer absorption consistent with an underlying Stark effect. The electroabsorption data were used to quantify the electric field experienced by the compounds that decreased from 0.85 to 0.22 MV/cm as the number of OPE spacers increased from 0 to 2. Charge recombination on the 10(-8)-10(-5) s time scale correlated with the magnitude of the electric field with an apparent attenuation factor β = 0.12 Å(-1). Slow components to charge recombination observed on the 10(-4)-10(-1) s time scale that were unaffected by temperature, irradiance, or the bridge units present on the molecular sensitizer were attributed to electron tunneling between TiO2 acceptor states. The photocurrent efficiencies of solar cells based on these compounds decreased markedly when the bridge units were present on the sensitizer. Iodine was found to form adducts with all three compounds, K = 1.8 ± 0.2 × 10(4) M(-1), but only significantly lowered the excited state injection yield for those that possessed the bridge units.

Electronic Excited States of Polynucleotides: A Study by Electroabsorption Spectroscopy

Electroabsorption spectra were obtained for single-stranded polynucleotides poly(U), poly(C), poly(A), and poly(G) in glycerol/water glass at low temperature, and the differences in permanent dipole moment (Deltamu) and polarizability (Deltaalpha) were estimated for several spectral ranges covering the lowest energy absorption band around 260 nm. In each spectral range, the electrooptical parameters associated with apparent features in the absorption spectrum exhibit distinct values representing either a dominant single transition or the resultant value for a group of a relatively narrow cluster of overlapping transitions. The estimated spacing in energy between electronic origins of these transitions is larger than the electronic coupling within the Coulombic interaction model which is usually adopted in computational studies. The electroabsorption data allow us to distinguish a weak electronic transition associated with a wing in polynucleotide absorption spectra, at an energy below the electronic origin in absorption spectra of monomeric nucleobases. In poly(C) and poly(G), these low-energy transitions are related to increased values of Deltamu and Deltaalpha, possibly indicating a weak involvement of charge resonance in the respective excited states. A model capable of explaining the origin of low-energy excited states, based on the interaction of pipi* and npi* transitions in neighboring bases, is introduced and briefly discussed on the grounds of point dipole interaction.

Electronic polarization in pentacene crystals and thin films

Electronic polarization is evaluated in pentacene crystals and in thin films on a metallic substrate using a self-consistent method for computing charge redistribution in non-overlapping molecules. The optical dielectric constant and its principal axes are reported for a neutral crystal. The polarization energies P+ and P- of a cation and anion at infinite separation are found for both molecules in the crystal's unit cell in the bulk, at the surface, and at the organic-metal interface of a film of N molecular layers. We find that a single pentacene layer with herring-bone packing provides a screening environment approaching the bulk. The polarization contribution to the transport gap P=(P+)+(P-), which is 2.01 eV in the bulk, decreases and increases by only ~ 10% at surfaces and interfaces, respectively. We also compute the polarization energy of charge-transfer (CT) states with fixed separation between anion and cation, and compare to electroabsorption data and to submolecular calculations. Electronic polarization of ~ 1 eV per charge has a major role for transport in organic molecular systems with limited overlap.

Quantitative Analysis of Absorption and Field-Induced Absorption Changes in InGaN/GaN Quantum Wells

Results of absorption and electroabsorption measurements are reported for wurtzite InGaN/GaN quantum wells. Despite the relatively high exciton binding energy of 25 meV in GaN no pronounced excitonic features could be observed and the absorption spectra are very similar to those obtained for bulk material. The measured field-induced absorption changes are as large as 7000 cm -1 below and 20000 cm -1 above the band edge. A quantitative analysis of electroabsorption data allows the precise determination of the strong internal fields in InGaN/GaN heterostructures originating from strain-induced polarization and differences in the spontaneous polarization. The fields measured on functioning diodes vary between 1.1 MV cm -1 and 1.4 MV cm -1 for indium concentrations in the InGaN quantum wells ranging from about 7 to 9%.

A novel method for synchronizing the pulse carver and electroabsorption data modulator for ultralong-haul DWDM transmission

The authors demonstrate a novel solution to detect and correct the temporal misalignment between the pulse carver and electroabsorption modulator for ultralong-haul dense wavelength-division-multiplexing transmission. The correct timing is achieved by minimizing the chirp of the optical data pulses. The technique is independent of the bit rate and channel spacing and can be implemented at low cost. The device can also be used as a wavelength locker. The use of electroabsorption modulators for ultralong-haul transmission is validated with a 5000-km 10-Gb/s soliton system experiment.

Electrostatic and conformational effects on the electronic structures of distortional isomers of a mixed-valence binuclear Cu complex.

The electronic structure of the binuclear copper complex [Cu(2)(L)](3+) [L = N(CH(2)CH(2)N(H)CH(2)CH(2)N(H)CH(2)CH(2))(3)N] has been investigated by resonance Raman and electroabsorption spectroscopy. Crystallographic Cu(2) distances of 2.364(1) and 2.415(1) A determined for the nitrate and acetate salts, respectively, are consistent with a substantial metal-metal interaction. The Cu-Cu bonding interaction in the binuclear complex is modulated both in the solid state and in solution by the ligand environment through coupling to ligand torsional modes that are, in turn, stabilized by hydrogen bonding. Electroabsorption data on the three major visible and near-infrared electronic transitions of Cu(2)L, lambda(max) (epsilon(max)) = 1000 nm ( approximately 1200 M(-1) cm(-1)), 748 nm (5600 M(-1) cm(-1)), and 622 nm (3350 M(-1) cm(-1)), reveal a difference dipole moment between the ground and excited states (Deltamu(A)) because of symmetry breaking. The difference polarizability for all three of the transitions is negative, indicating that the ground state is more polarizable than the excited state. A general model to explain this behavior in terms of the proximity of accessible transitions involving copper d electrons is proposed to explain the larger polarizability of the ground state. Raman excitation profiles (REPs) provide evidence for multiple conformational states of [Cu(2)(L)](3+). Separate REPs were obtained for each of the components of the two major Raman bands for nu(1) (a Cu-Cu stretching mode) and nu(2) (a Cu-Cu-N(eq) bending mode). The Raman data along with quantum chemical ZINDO/S CI calculations provide evidence for isomeric forms of Cu(2)L with strong coupling between the conformation of L and the Cu-Cu bond length.

Comparison of titanium and platinum Schottky barrier heights to Ga 0.47In 0.53As obtained from Franz Keldysh oscillations and Schottky diode characteristics

The Schottky barrier height of Ti and Pt contacts to Ga 0.47In 0.53As was measured using Franz Keldysh oscillations detected by electro absorption modulation, and compared to values obtained from the Schottky diode current voltage characteristics. Both methods reveal that the Schottky barrier height for holes in Pt contacts is 50–70 meV lower than in Ti contacts. The obtained barrier heights were used to calculate the specific contact resistance of Pt and Ti to p-type Ga 0.47In 0.53As. The results agree well with experimental data. A de-correlation method for improving the resolution of electro absorption data analysis is presented. An experimentally obtained correction factor for the measured electric field is introduced in order to account for a discrepancy between electro absorption theory and experiment.

Electroabsorption spectroscopy of molecular inorganic compounds

Electroabsorption spectroscopy is known to report directly on the changes in dipole moment and molecular polarizability accompanying electronic excited state formation. Because ground-state excited-state dipole moment changes can be equated with effective one-electron transfer distances, experimental electroabsorption spectroscopy holds exceptional promise as a methodology for investigating light-induced charge transfer processes within inorganic systems. A survey of the available studies, including metal-to-ligand charge transfer, ligandto-metal charge transfer and localized and delocalized 'intervalence' charge transfer studies, is presented. Also surveyed are electroabsorption studies aimed at illuminating selected molecular nonlinear optical responses. A general observation from electroabsorption studies has been that experimentally determined oneelectron transfer distances are less than simple geometric descriptions would predict. Mononuclear transition-metal systems have proven to be good models for unravelling the complicating effects of ground-state localizationand many-electron polarization. The capability of electroabsorption spectroscopy to resolve fundamental questions relating to electronic localization and delocalization has been highlightedin a series of studies in bridged dinuclear and tris(diimine) systems. The available electroabsorption data have also been used to reassess a number of molecular charge-transfer related parameters such as the electronic coupling energies and solvent reorganization energies. Very recent studies of putative octupolar complexes and donor-acceptor porphyrinic structures have highlighted the utility of electroabsorption spectroscopy for evaluating second-order nonlinear optical response mechanisms and for providing detailed information about statespecific contributions to overall molecular hyperpolarizabilities.

Optical non-linearity in β-carotene: new insight from electroabsorption spectroscopy

We report experimental electroabsorption data for β-carotene in the energy range from 1.3 to 5 eV which disclose a rich spectrum of excited states including the 2A g state. These data allow a reassessment of the current understanding of the optical non-linearity of this model conjugated system.

Quantum size effects of charge-transfer excitonsin nonpolar molecular organic thin films

We use a quantum-mechanical model to analyze charge-transfer (CT) excitons in closely packed, nonpolar organic molecular crystal thin films grown by the ultrahigh-vacuum process of organic molecular beam deposition. The exciton Hamiltonian includes both polarization effects and the periodic pseudopotential of the crystal. This model takes into account the very large anisotropy characteristic of many organic materials such as the archetype molecular crystal, 3,4,9,10-perylenetetracarboxylic dianhydride (PTCDA) and PTCDA-based multilayers. Using a single-exciton Hamiltonian, we quantitatively model experimental electroabsorption data and the absorption spectral shifts observed in ultrathin organic multilayers or ``multiple quantum wells.'' The data analyzed from several such experiments give independent and consistent estimations of the anisotropic effective mass tensor and exciton radii for PTCDA along different crystal axes. This treatment is general, and is found to extend to other CT and Wannier exciton systems found in many interesting, nonpolar organic molecular and inorganic semiconductor crystals such GaAs, suggesting similar physical origins for Wannier and CT excitons in a wide range of materials.

Dispersion of the electro-optical response in poled polymer films determined by Stark spectroscopy

We demonstrate the application of electromodulated absorption experiments on poled polymer films. Information on the polar order in guest host materials could be deduced precisely by knowing only the mesomeric moment of the 4′-dimethylamino-4-nitrostilbene dye molecule. The dispersion of the real and imaginary parts of the non-linear optical susceptibility χ(2) could be obtained by applying the Kramers-Kronig formalism to the electroabsorption data. The result was in excellent agreement with theoretical two-level calculations in combination with the experimental information on the polar ordering. Furthermore, the contribution of the piezoelectric effect to the electroabsorption signal is proven to be negligible. The results show that electromodulated absorption is a powerful method for obtaining the dispersion of the pure electro-optical response described by ϵ(2)xxz( -ω;ω,0) over a large frequency range provided that the second order non-linearity is dominated by electronic transitions accessible by optical absorption experiments.

Electromodulation of fluorescence in a crystalline organic photoconductor (thionaphthenindole)

The influence of electric fields on the fluorescence of polycrystalline layers of thionaphthenindole has been studied by an electromodulation technique that allows internal electric fields to be accounted for the effect. The in-phase first harmonic (1ω) and out-of-phase second harmonic (2ω) of the fluorescence response to a modulating (sinusoidal) electric field of frequency ω reveal different electric-field behavior, the 2ω response providing nearly internal field-free signal suitable for verification of theoretical models. Fluorescence quenching with this signal has been observed and attributed to electric-field modulation of the probability of charge separation within an excited state as a precursor. It was found that fluorescence quenching data could not be well explained using theoretical models formulated by Onsager, Poole–Frenkel, and Noolandi–Hong–Popovic for the charge separation via delocalized charge-transfer excitons. The macrotrap model which reconciles both the experimental data and their physical interpretation is proposed. The model attributes the fluorescence quenching to carrier photogeneration by the field-assisted thermal dissociation of a trapped charge-transfer exciton which has a higher located molecular singlet S1 state as a precursor. An analysis of 1ω fluorescence signals provides information on the internal fields evaluated on ≂105 V/cm and attributed to spontaneous polarization effects in ordered crystalline layers of this polar compound, in accordance with conclusions drawn previously from electroabsorption data.

Nonlinear optical study of quasi-one-dimensional platinum complexes: Two-photon excitonic resonance effect

Third-harmonic generation measurements have been made for the first time on oriented films of halogen-bridged mixed-valence platinum complexes for the pump wavelength region from 0.6 to 1.0 μm. The ‖χ(3)(−3ω;ω,ω,ω)‖ spectra obtained by the Maker’s fringe method show a distinct peak at about 1.8 eV with the peak value of about 4×10−11 esu. From comparison with the electroabsorption data, this peak has been identified with the two-photon resonance peak with the forbidden exciton in the Pt-halogen chains. One-dimensional exciton states in this compound take an important role in the third-order optical nonlinearity, similar to the case in conjugated polymers.

Absorption and electroabsorption spectra of an In1−xGaxP1−yAsy/InP double heterostructure

Electroabsorption spectra of an InGaAsP/InP double heterostructure are compared with absorption and luminescence spectra. The alloy tuned to emission at 1.33 μm at room temperature shows a surprisingly narrow spectrum at the fundamental gap with many Franz–Keldysh oscillations. The analysis of the electroabsorption data yields the energy gap, its dependence on temperature, and the spin-orbit splitting. Line shape analysis provides information on lifetime broadening, on internal fields, and space charges, and reveals the presence of excitonic effects. Although discrete exciton levels were not observed, an exciton binding energy of 2.3 meV is obtained from the continuum states which is consistent with a reduced effective mass of 0.03m0. While absorption and electroabsorption data yield the same value of the energy gap, emission of light at low temperature occurs at significantly lower energies pointing to the presence of tail states below the gap.

Quadratic electro-optic effect due to the quantum-confined Stark effect in quantum wells

We have performed a semi-empirical calculation of the refractive index changes induced by a perpendicular electric field in quantum wells. This calculation is based on experimental electroabsorption data, together with absorption sum rules that have recently been developed. We find good agreement with published experimental results. Our results are important to the development of electro-optic and high-speed electroabsorption quantum well devices.

Electroabsorption studies on amorphous silicon

The electroabsorption spectra of plasma deposited and chemical vapor deposited amorphous silicon films have been studied in comparison with their absorption. In all cases a peak with 350–400 meV halfwidth is observed similar to the results on chalcogenide glasses. The peak occurs at moderate values of the absorption constant near 10 4 cm −1 and follows the shift of the absorption edge with variation of the deposition temperature. No response has been obtained in the low absorption region below 1.5 eV. The field modulated reflectance which has been calculated from the electroabsorption data is about two orders of magnitude smaller than the electroabsorption signal. The electroabsorption spectrum is interpreted as resulting from the influence of the external field on the localized states only.

Electroabsorption in disordered solids. II. Anthracene crystals and thin films

For pt.I see ibid., vol.7, p.1595 (1974). Electroabsorption data are presented for anthracene in its crystalline and disordered forms. Particular emphasis is placed on the role of the substrate temperature Ts in determining the structure of vacuum-deposited films. For substrate temperatures below the optimum temperature for surface quality (T0=-60 degrees C) two electroabsorption peaks are observed; the one at 396 nm is thought to be associated with anthracene molecules in a crystalline arrangement. However, the other at 405 nm is attributed either to a disordered phase or more probably to diffraction effects produced by orientational randomness of the anthracene crystallites. The magnitude of both peaks vary with the square of the electric field (quadratic Stark effect). The magnitude of the higher-wavelength peak reduces rapidly and monotonically as Ts reaches the region of T0.

Electro-absorption studies on the urbach edge in chalcogenide glasses

New electro-absorption data are reported on the urbach edge in bulk and evaporated films of se and as-se alloys. A comparison with current microfield models suggests the presence of an additional absorption mechanism. A model is proposed, based on local optical anisotropy.

Electroabsorption in disordered solids: selenium

Electroabsorption data are presented for a number of amorphous chalcogenide semiconductors. Particular emphasis is placed on those obtained for amorphous selenium for which evidence is presented which suggests that the electroabsorption is associated with a quadratic Stark effect of localized excitations in the Se8 rings. For this element the electroabsorption curves reveal subtle differences in the properties of evaporated films prepared under different conditions. The possible application of the electroabsorption technique to determine whether or not the boundary between extended and localized states in amorphous materials is sharply defined, is also discussed.