Electric Field Induced Second Harmonic Research Articles

Molecular Origins of the High-Performance Nonlinear Optical Susceptibility in a Phenolic Polyene Chromophore: Electron Density Distributions, Hydrogen Bonding, and ab Initio Calculations

The molecular and supramolecular origins of the superior nonlinear optical (NLO) properties observed in the organic phenolic triene material, OH1 (2-(3-(4-hydroxystyryl)-5,5-dimethylcyclohex-2-enylidene)malononitrile), are presented. The molecular charge-transfer distribution is topographically mapped, demonstrating that a uniformly delocalized passive electronic medium facilitates the charge-transfer between the phenolic electron donor and the cyano electron acceptors which lie at opposite ends of the molecule. Its ability to act as a “push–pull” π-conjugated molecule is quantified, relative to similar materials, by supporting empirical calculations; these include bond-length alternation and harmonic-oscillator stabilization energy (HOSE) tests. Such tests, together with frontier molecular orbital considerations, reveal that OH1 can exist readily in its aromatic (neutral) or quinoidal (charge-separated) state, thereby overcoming the “nonlinearity-thermal stability trade-off”. The HOSE calculation also reveals a correlation between the quinoidal resonance contribution to the overall structure of OH1 and the UV–vis absorption peak wavelength in the wider family of configurationally locked polyene framework materials. Solid-state tensorial coefficients of the molecular dipole, polarizability, and the first hyperpolarizability for OH1 are derived from the first-, second-, and third-order electronic moments of the experimental charge-density distribution. The overall solid-state molecular dipole moment is compared with those from gas-phase calculations, revealing that crystal field effects are very significant in OH1. The solid-state hyperpolarizability derived from this charge-density study affords good agreement with gas-phase calculations as well as optical measurements based on hyper-Rayleigh scattering (HRS) and electric-field-induced second harmonic (EFISH) generation. This lends support to the further use of charge-density studies to calculate solid-state hyperpolarizability coefficients in other organic NLO materials. Finally, this charge-density study is also employed to provide an advanced classification of hydrogen bonds in OH1, which requires more stringent criteria than those from conventional structure analysis. As a result, only the strongest OH···NC interaction is so classified as a true hydrogen bond. Indeed, it is this electrostatic interaction that influences the molecular charge transfer: the other four, weaker, nonbonded contacts nonetheless affect the crystal packing. Overall, the establishment of these structure–property relationships lays a blueprint for designing further, more NLO efficient, materials in this industrially leading organic family of compounds.

Probing Confinement Effects on Multilayered Ferroelectric Polymer Films Using Second Harmonic Generation

ABSTRACTElectrical energy storage plays a key role in mobile electronic devices, stationary power systems, and hybrid electrical vehicles. High energy density capacitors based on dielectric polymers are a focus of increasing research effort motivated by the possibility to realize compact and flexible energy storage devices, taking advantage of light weight and facile processability of organic materials. In addition, dielectric polymers enjoy inherent advantages of self-healing mechanism and high breakdown strength, leading to capacitors with great reliability and high energy density. It is the focus of this group to develop a multilayered ferroelectric PVDF system for improved energy storage efficiency. These systems are fabricated using enabling technology in co-extrusion which allows more cost effective and large area device production as opposed to more conventional layer-by-layer techniques. Many efforts have been made by the team to fabricate these micro- and nano-layered systems resulting in much improved device performance. A three-time improvement of capacitive electrical energy density has been demonstrated. The focus of this research is to understand the physics of why these multilayered systems perform better than a single layer by developing a characterization technique using both confocal second harmonic generation (SHG) and electric field induced second harmonic (EFISH) laser spectroscopy. Our results have shown that SHG is a very sensitive, non-destructive and versatile technique that can be used to study the ferroelectric and structural properties of layered systems. When combined with EFISH this technique allows the interrogation of structural and dielectric properties within the individual layers and at the interfaces between the layers. Further, the proposed techniques can be readily employed in-situ which can provide information in real time during sample processing with static and time-resolved spectroscopic measurements.

Electronic and Vibrational Nonlinear Optical Properties of Five Representative Electrides.

The electrides have a very special electronic structure with diffuse excess electrons not localized on any specific atom. Such systems are known to have huge electronic nonlinear optical (NLO) properties. Here, we determine and analyze the vibrational, as compared to the electronic, NLO properties for a representative set of electrides: Li@Calix, Na@Calix, Li@B10H14, Li2(•+)TCNQ(•-), and Na2(•+)TCNQ(•-). The static and dynamic vibrational (hyper)polarizabilities are computed by the nuclear relaxation method (with field-induced coordinates and the infinite optical frequency approximation) at the UB3LYP level using a hybrid Pople basis set. In general, the static vibrational βvec and γ∥ exceed the corresponding static electronic property values by up to an order of magnitude. The same comparison for dynamic vibrational hyperpolarizabilities shows a smaller ratio. For the intensity-dependent refractive index (IDRI) and dc-Kerr processes, the ratio is on the order of unity or somewhat larger; it is less for the dc-Pockels and the electric field induced second harmonic (EFISH) effects (as well as the static α̅) but still important. The role of anharmonicity, motion of the alkali atoms, and substitution of Na for Li is discussed along with specific aspects of the charge distribution associated with the excess electron.

First comparison of electric field induced second harmonic of near-infrared femtosecond laser pulses in reflection and transmission generated from Si/SiO2 interfaces of a silicon membrane

For the first time electric field induced second harmonic (EFISH) generation of femtosecond (fs) laser pulses (λ=800 nm, τ=75±5 fs, rep. rate=80 MHz, E pulse≤10 nJ) is observed in transmission through a thin free-standing silicon (Si) membrane of 10-μm thickness and compared to the well-known EFISH results in reflection by use of the z-scan technique. EFISH in reflection and transmission unequivocally originate from the front and rear Si/SiO2 interfaces, respectively, with SiO2 being the natural oxide on the Si surfaces. Frequency conversion is enhanced by photoinduced electric fields across the Si/SiO2 interfaces caused by charge-carrier injection from Si into the oxide. The z-scan results and time-dependent measurements allow comparison of the EFISH signal amplitudes and time constants detected in transmission and reflection, demonstrating the need for further investigation.

A Joint Experimental and Theoretical Investigation on Nonlinear Optical (NLO) Properties of a New Class of Push–Pull Spirobifluorene Compounds

AbstractA new class of push–pull spirobifluorene compounds 1–5 has been synthesized. The nonlinear optical (NLO) properties (first and second hyperpolarizability β and γ, respectively)of spirobifluorene derivatives have been investigated forthe first time using electric field induced second harmonic(EFISH) and third harmonic generation (THG) methods. Moreover, a comparison with the corresponding push–pullfluorene monomers 1a–5a was carried out. Besides the experimental data, ab initio theoretical calculations on structural, electronic, and optical properties were carried out for all compounds. The analyses indicate that an increase in the β and γ values occur on passing from monomers to dimers without compromising the optical transparency.

ChemInform Abstract: Amphiphilic Polyenic Push-Pull Chromophores for Nonlinear Optical Applications.

Amphiphilic cationic polyenic push–pull chromophores which offer interesting supramolecular possibilities for second harmonic generation have been synthesised and their optical non-linearities studied for the first time by electric field induced second harmonic (EFISH) generation in solution.

Synthesis and nonlinear optical properties of chromophores for photorefractive polymer materials

The synthesis and nonlinear optical (NLO) properties of a series of nitrostilbene-, dinitrostilbene-, and dicyanomethylenedihydrofuran-based chromophores potentially useful in photorefractive multifunctional polymers are reported. Electronic absorption spectra have been measured in different solvents. Electric field induced second harmonic (EFISH) generation results show that the 2-dicyanomethylen-3-cyano-5,5-dimethyl-2,5-dihydrofuran moieties lead to the best NLO performances. Dinitrophenyl-based chromophores have shown a reduced nonlinear response as compared with the mononitro analogues. Nonlinear optical properties of a selected nitro–dinitro couple have been also determined theoretically.

Synthesis and non-linear optical characterization of novel borinate derivatives of cinnamaldehyde

Herein is reported the synthesis of three new borinates (2a–2c) derived from bidentate imine ligands (1a–1c), which were prepared from the corresponding cinnamaldehyde derivatives. All compounds were characterized by MS, IR and NMR spectroscopy. The nonlinear optical (NLO) characterization of these molecules having push–pull properties is included. Second order nonlinearities were evaluated at 1.907 μm by the electric-field-induced second harmonic (EFISH) technique through the β×μ product, showing a trend for a general increase of the NLO response after boron complexation. For borinate 2c, which has strong electron-donating and electron-withdrawing groups at opposite ends of the π-backbone, βμ = 1560 × 10−30 D cm5 esu−1, showing a three-fold increase with respect to the ligand 1c (βμ = 542 × 10−30 D cm5 esu−1). Third-order nonlinearities, in solid films, were evaluated employing the third harmonic generation (THG) Maker Fringe technique showing χ(3) susceptibilities of the order of χ(3)∼10−12 esu at the infrared wavelength of 1.2 μm. Compounds with the highest χ(3) values correspond to those having the largest βμ product: for 1cχ(3) = 1.1 × 10−11 esu and for 2cχ(3) = 8.4 × 10−12 esu.

Twisted π-Electron System Electrooptic Chromophores. Structural and Electronic Consequences of Relaxing Twist-Inducing Nonbonded Repulsions

The synthesis, structural and spectroscopic characterization, and nonlinear optical response properties of a “slightly” twisted zwitterionic 4-quinopyran electrooptic chromophore FMC, 2-{4-[1-(2-propylheptyl)-1H-pyridine-4-ylidene]cyclohexa-2,5-dienylidene}malononitrile, are reported. X-ray diffraction data and density functional theory (DFT) minimized geometries confirm that deletion of the four o-, o′-, o′′-, and o′′′-methyl groups in the parent chromophore TMC-2, 2-{4-[3,5-dimethyl-1-(2-propylheptyl)-1H-pyridin-4-ylidene]-3,5-dimethylcyclohexa-2,5-dienylidene}malononitrile, relaxes the arene−arene twist angle from 89.6 to 9.0°. These geometrical changes result in a significantly increased contribution of the quinoidal structure to the molecular ground state of FMC (versus TMC-2), reduced solvatochromic shifts in the optical spectra, and a diminished electric-field-induced second-harmonic (EFISH) generation derived molecular hyperpolarizability (μβ = −2340 × 10−48 esu of DFMC, the dendrimer derivative of FMC, vs −24000 × 10−48 esu of TMC-2) in CH2Cl2 at 1907 nm. Pulsed field gradient spin–echo (PGSE) NMR spectroscopy and EFISH indicate that the levels of FMC aggregation in solution are comparable to those of TMC-2 (monomers and dimers) in CH2Cl2 solution. B3LYP and INDO/S computation of chromophore molecular structure, aggregation, and hyperpolarizability trends are in good agreement with experiment.

A second harmonic generation study of the oxidation of copper electrodes

The oxidation of copper in basic media has been studied by in situ second harmonic generation (SHG), where the SHG signal was recorded alongside the cyclic voltammogram. The SHG signal changes markedly as the copper surface is oxidised to first Cu 2O and then CuO in a duplex structure. The development of Cu 2O gives rise to a resonant SHG signal because of the band-gap of the material then the upper CuO layer produces an electric-field induced second harmonic (EFISH) response. A correlation of charge with the SHG signal is informative with regard to the mechanism of reduction of CuO and SHG is shown to be a useful method for the examination of oxidation of electrode surfaces.

Calculation of the Microscopic and Macroscopic Linear and Nonlinear Optical Properties of Liquid Acetonitrile. II. Local Fields and Linear and Nonlinear Susceptibilities in Quadrupolar Approximation

A discrete model based on the multipolar expansion including terms up to hexadecapoles was employed to describe the electrostatic interactions in liquid acetonitrile. Liquid structures obtained form molecular dynamics simulations with different classical, nonpolarizable potentials were used to analyze the electrostatic interactions. The computed average local field was employed for the determination of the environmental effects on the linear and nonlinear electrical molecular properties. Dipole-dipole interactions yield the dominant contribution to the local field, whereas higher multipolar contributions are small but not negligible. Using the effective in-phase properties, macroscopic linear and nonlinear susceptibilities of the liquid were computed. Depending on the partial charges describing the Coulomb interactions of the force field employed, either the linear properties (refractive index and dielectric constant) were reproduced in good agreement with experiment or the nonlinear properties [third-harmonic generation (THG) and electric field induced second-harmonic (EFISH) generation] and the bulk density but never both sets of properties together. It is concluded that the partial charges of the force fields investigated are not suitable for reliable dielectric properties. New methods are probably necessary for the determination of partial charges, which should take into account the collective and long-range nature of electrostatic interactions more precisely.

Ultraviolet pulse laser induced modifications of native silicon/silica interfaces analyzed by optical second harmonic generation

Native silicon/silica (Si∕SiO2) interfaces are investigated by electric field induced second harmonic (EFISH) generation employing near infrared femtosecond laser pulses (782.8nm, 80fs, 10nJ, 80MHz repetition rate). Here, the temporal EFISH evolution induced by the femtosecond laser irradiation is recorded with a time resolution of 0.2s. Comparative EFISH studies are performed with Si∕SiO2 interfaces, preexposed to ultraviolet (UV) laser pulses (308nm, 16ns, 0.45–2.8J∕cm2) as well as virgin Si∕SiO2. After UV irradiation the femtosecond laser induced electron injection and trapping in the ultrathin oxide is found to be drastically accelerated in contrast to the essentially unaffected interfacial hole dynamics. This result is explained by an enhanced interfacial electron trap density caused in most cases by the UV laser induced melting and recrystallization of the near-interface silicon. Furthermore, three-dimensional second harmonic imaging reveals a saturation effect of the UV induced sample modification due to the melting of the near-interface silicon, for which a threshold fluence of 0.47J∕cm2 is extracted for xenon chloride (XeCl) laser irradiation. Below this threshold an accumulative behavior of the UV induced modification is observed for repeated exposure to several UV laser pulses. This is pointing to a dose dependent modification process presumably due to thermally driven interface chemistry and/or microscopic structural and electronic changes of the Si∕SiO2 interface.

Problems in the comparison of theoretical and experimental hyperpolarizabilities revisited

The relationship between nonlinear susceptibilities and hyperpolarizabilities defined using different conventions is reexamined. In previous work [Willetts et al., J. Chem. Phys. 97, 7590 (1992)], relations between different conventions for microscopic hyperpolarizabilities have been derived, but the application of the corresponding conversion factors led to several inconsistencies. It is shown that different conventions for macroscopic susceptibilities have to be taken into account, too, in order to arrive at consistently comparable values. The complete set of conversion factors between several conventions are given for second harmonic generation, electric field induced second harmonic (EFISH) generation, and third harmonic generation. As an illustration, experimental EFISH and hyper-Rayleigh scattering results of p-nitroaniline are compared with each other and with recent results of ab initio computations including solvation effects. Several problems in the comparison of computational and experimental values are also discussed.

PLZT Ceramics Based EFISH Cell for Nonlinear Heterodyne Interferometry

Electric field-induced second harmonic (EFISH) generation effect in paraelectric lanthanum doped lead zirconate-titanate ceramics (PLZT) has been utilized in the local oscillator device of nonlinear optical heterodyne interferometer. Application of PLZT X/65/35 plates in low frequency rotating electric field as the reference sources of EFISH for interferometric analysis of anisotropy of thin films or surfaces of crystals is proposed. Experimental details and properties of multiphase PLZT EFISH modulator has been discussed.

Surface Enhanced Second Harmonic Generation from Macrocycle, Catenane, and Rotaxane Thin Films: Experiments and Theory

Surface enhanced second harmonic generation (SE SHG) experiments on molecular structures, macrocycles, catenanes, and rotaxanes, deposited as monolayers and multilayers by vacuum sublimation on silver, are reported. The measurements show that the molecules form ordered thin films, where the highest degree of order is observed in the case of macrocycle monolayers and the lowest in the case of rotaxane multilayers. The second harmonic generation activity is interpreted in terms of electric field induced second harmonic (EFISH) generation where the electric field is created by the substrate silver atoms. The measured second order nonlinear optical susceptibility for a rotaxane thin film is compared with that obtained by considering only EFISH contribution to SHG intensity. The electric field on the surface of a silver layer is calculated by using the Delphi4 program for structures obtained with TINKER molecular mechanics/dynamics simulations. An excellent agreement is observed between the calculated and the measured SHG susceptibilities.

Synthesis, characterization and nonlinear optical properties in a series of new chiral organotin(IV) Schiff base complexes

Four new chiral organotin derivatives are reported with their crystal structure. They were synthesized by reaction of diphenyltin oxide and four different ligands obtained from the Schiff base condensation of 4-(diethylamino)salicylaldehyde and (1 R,2 S)-(+)-norephedrine, ( R)-(−)-phenylglycinol, ( R)-(−)-1-amino-2-propanol and (1 S,2 R)-2-amino-1,2-diphenylethanol. Their nonlinear optical properties were investigated experimentally in solid state and with the electric field induced second harmonic (EFISH) technique. In particular, the compound obtained with ( R)-(−)-phenylglycinol exhibits an efficiency 11 times that of urea in second harmonic generation at 1.907 μm. The properties are discussed in relation with computational studies conducted within the framework of the DFT theory.

Synthesis, characterization and nonlinear optical (NLO) properties of a push–pull bisboronate chromophore with a potential electric field induced NLO switch

A new bisboronate with five six-membered ring heterobicycles was prepared by reaction of [2-(2-hydroxy-4-diethylaminobenzylidene)amino]-5-nitroaminophenol and 1,4-phenyldiboronic acid, and its quadratic nonlinear optical (NLO) response was compared to that of a related monoboronate species. A computational investigation conducted within the framework of the DFT theory, at the B3PW91/6-31G* level, indicates that, while the diboronate derivative exhibits a centrosymmetric conformation in the ground state, and hence a vanishing quadratic hyperpolarizability (β), the application of an external electric field provides a gradual intramolecular rotation of the two “push–pull” sub-units which reaches 128° for a field intensity of 10−3 a.u. and leads to a β value equal to 49.2 × 10−30 cm5 esu−1. This result suggests a possibility for a molecular NLO switch induced by an electric field in such systems. Experimentally, the NLO response measured by the electric field induced second harmonic (EFISH) technique indicates an NLO response (expressed as the μ × β product) 1.95 times larger for the bisboronate than for the monoboronate analogue, thus suggesting that, although the ground state conformation is centrosymmetric, the “push–pull” sub-units are in quasi-free rotation at room temperature.

Diffusion voltage in polymer light emitting diodes measured with electric field induced second harmonic generation

We apply electric field induced second harmonic (EFISH) to polymer light emitting diodes (PLEDs) and demonstrate the ability to determine the diffusion voltage in PLED devices. The EFISH signal is proportional to the square of the effective field, which is the sum of the diffusion voltage and the applied voltage. By minimizing the EFISH-signal as a function of the applied voltage, the diffusion voltage is determined by measuring the applied voltage that cancels out the diffusion voltage. The PLEDs are fabricated with indium tin oxide (ITO) as the hole injecting contact and two different electron injecting contacts, namely aluminum and calcium. The diffusion voltage originates from the rearranged charges caused by the difference in Fermi levels in the materials in the PLEDs. Different contacts will thus cause different diffusion voltages. We demonstrate here that the EFISH signal is proportional to the square of the effective field in both reverse and forward bias, and discuss the dependence on contact materials. (© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Synthesis and quadratic molecular hyperpolarizabilities of two new chiral boronates: Computational and experimental study

A monomeric boronate and an oxobridged chiral dimer were obtained by reaction of the ligand derived from 4-diethylaminosalicylaldehyde with ( R)-(−)-phenylglycinol, and phenyl boronic acid or boric acid. The compounds were fully characterized by spectroscopic techniques ( 1H, 13C, 11B NMR, elemental analyses, IR and masses spectrometry); and their molecular hyperpolarizabilities were investigated by the electric field induced second harmonic (EFISH) technique and semi-empirical calculations. The experimental quadratic hyperpolarizability which is equal to 9.8 × 10 −30 cm 5 esu −1 at 1.064 μm for the monomeric derivative rises to 19.5 × 10 −30 cm 5 esu −1 in the dimeric specie.

Structural Modulation of the Dipolar−Octupolar Contributions to the NLO Response in Subphthalocyanines

Second order nonlinear optical properties of a series of trinitrosubphthalocyanine (SubPc) isomers were studied experimentally by electric field induced second harmonic (EFISH) generation and hyper Rayleigh scattering (HRS). These experimental values were compared to the ones obtained theoretically employing both sum over states (SOS) and finite field (FF) methods. From these studies, it was shown that the dipolar contributions to the beta tensor are very much dependent on the substitution pattern at the periphery of the subphthalocyanine macrocycle, whereas the octupolar contributions remain mostly unchanged. Consequently, it was deduced that SubPc is extremely well suited for the decoupling of octupolar and dipolar contribution to the NLO response.