Electric Field-induced Second Harmonic Generation Research Articles

Study of orientation induced molecular rectification in polymer films

We underline the intrinsic rectifying nature of an oriented polymeric material. Orientation of the initially symmetric structure is performed through dc-field ordering of the polar molecules contained in the polymer. The internal field induced in the polymeric material is evidenced by the induction of a rectifying current-voltage characteristic. Second harmonic generation is used as a probe of the molecular order. Orientation induced rectification is proportional to the molecular order. This preparation technique opens a new route for the improvement of organic-semiconductor devices efficiency. Additionally, electric-field induced second-harmonic generation (EFISHG) is used as an internal field measurement technique. It offers an original characterization tool for charge injection and transport studies in organic semiconductors.

Structure-Activity Relationship of New Organic NLO Materials Based on Push-Pull Azodyes. 1. Synthesis and molecular properties of the dyes

Several new push-pull azobenzene dyes were synthesized by diazonium salt coupling of electron-poor anilines with N,N-dialkylanilines. Their dipole moments and first-order hyperpolarizabilities were evaluated respectively from the concentration dependence of the dielectric constant of solutions in apolar solvents and by the electric-field-induced-second-harmonic-generation (EFISH) technique. Investigation of the influence of both nature and steric hindrance of the electron-withdrawing group allowed to highlight a significant deviation from linearity in the relationship between dipole moments and first-order hyperpolarizabilities. This effect was particularly noticeable when multiple electron withdrawing groups pointing in different directions were present on the same phenyl ring.

Hyper-Rayleigh scattering measurements of nonlinear optical chromophores at 1907 nm

Hyper-Rayleigh scattering measurements are performed on nonlinear optical chromophores using an excitation wavelength of 1907 nm. This wavelength is the longest that has yet been reported for the hyper-Rayleigh scattering experiment and allows measurements of the first hyperpolarizability of chromophores that are free of contributions from two-photon absorption induced fluorescence and reduces one and two photon resonance enhancement to a minimum. Using the 1907 nm setup we demonstrate good agreement between our results and those obtained with the electric field induced second harmonic generation (EFISHG) technique previously performed at this wavelength.

High-resolution electric-field-induced second-harmonic generation with ultrafast Ti:sapphire laser

The technique of electric-field-induced second-harmonic generation (EFISHG) has been extended by use of a femtosecond, quasi-cw laser pump source. The effects of ultrashort pulse widths on the coherent EFISHG process have been examined and the analysis procedure used for nanosecond pulse EFISHG measurements is found to be suitable for femtosecond measurements in off-resonant regions. A convenient method for measurement of the group velocity mismatch between the fundamental and second-harmonic pulses in solutions has also been developed to accurately describe the femtosecond EFISHG process. Phase-sensitive detection of the modulated second-harmonic signals results in improvements in accuracy and sensitivity over existing nanosecond pulse experiments. This has been demonstrated with a measurement of Γ(−2ω; ω, ω, 0) of an organic molecule, 4-nitroanisole.

Hyperpolarisability of (donor)2-acceptor‐type molecules determined by EFISHG

Molecular first-order hyperpolarisabilities β of (donor)2-acceptor type molecules have been determined by electric field induced second-harmonic generation (EFISHG) experiments at 1.06 μm. The hyperpolarisability values are enhanced in these two dimensional charge transfer molecules, compared with a reference linear model molecule. One of these molecules absorbs part of the second-harmonic beam. We propose a determination method for β that takes into account this phenomenon. Crystals of the title molecules exhibit second harmonic generation by the powder test.

Optical second-harmonic probe for silicon millimeter-wave circuits

Electric field-induced second-harmonic generation (EFISHG) is shown to be a very powerful method for contactless sampling of high-speed electrical signals on silicon millimeter-wave devices. We present picosecond resolution EFISHG measurements of 2 GHz sinusoidal signals on a photoconductive silicon-on-sapphire antenna. We further demonstrate the application of the EFISHG technique for spatial mapping of free-running microwave signals in silicon.

Vibrational hyperpolarizabilities and the Kerr effect in CH4, CF4, and SF6

The hyperpolarizabilities γ of CH4, CF4, and SF6 were measured by the dc Kerr effect at wavelengths from 457.9 to 1092 nm. Vibrational hyperpolarizabilities γv were obtained by combining these measurements with electric-field-induced second harmonic generation (ESHG) measurements. The vibrational contribution to the hyperpolarizability ranges from 6% to 35% of the total. At high optical frequency the difference between γv for Kerr and γv for ESHG is approximately constant, and has values 18, 31, and 51×10−63 C4 m4 J−3 for CH4, CF4, and SF6, respectively. The experimental results are in good quantitative agreement with the results of recent ab initio calculations of the frequency dependence of γv for CH4, except for a small but non-negligible discrepancy at high frequency.

Novel Chiral Bis-dipolar 6,6‘-Disubstituted Binaphthol Derivatives for Second-Order Nonlinear Optics: Synthesis and Linear and Nonlinear Optical Properties

A number of thermally and optically stable, bis-dipolar chiral molecules based on two geometries of the binaphthol (BN) system with different acceptors/substituents have been synthesized for the first time, and the synthetic routes are reported: optically pure 6,6‘-disubstituted 2,2‘-diethoxy-1,1‘-binaphthyls {R, R‘= −Br, −CHO, −CHC(CN)(COOEt), −CHC(CN)2, −CHCHCN, −CHCH(p-NO2Ph)} and optically pure 9,14-disubstituted dinaphtho[2,1-d:1‘,2‘-f][1,3]dioxepins {R, R‘ = −Br, −CHO, −CH=C(CN)(COOEt), −CHC(CN)2, −CHCHCN, −CHCHSO2CH3, −CHCHCHO, −CHCHCHC(CN)2}. All molecules possess two equal donor−acceptor systems linked together to give a bis-dipolar system. Two mono-dipolar 6-substituted 2-butoxynaphthalene (R = −CHC(CN)2, −CHC(CN)(COOEt)) donor−acceptor systems were prepared as references. The linear optical properties including solvatochromic shifts of absorption and fluorescence revealed strong charge transfer excitations in the new dipolar systems. The molecules show a high first hyperpolarizability β (up to (344−364) × 10-30 esu) as measured by electric-field-induced second-harmonic generation (EFISHG) and hyper-Rayleigh scattering (HRS). A model is developed to express the first hyperpolarizability of the bis-dipolar molecules in terms of the molecular geometry and the β of the monomeric donor−acceptor units. The tensor components were determined experimentally using this model and data from HRS and EFISHG. These techniques probe different combinations of the components of the molecular hyperpolarizability tensor. The results obtained are found to be in excellent mutual agreement.

Supramolecular Second-Order Nonlinearity of Polymers with Orientationally Correlated Chromophores

Nonlinear optical chromophores can be organized as orientationally correlated side groups of polymers with a rigid backbone. In such an organization, each chromophore contributes coherently to the second-order nonlinear response of the polymer structure. A first hyperpolarizability exceeding 5000 × 10 −30 electrostatic units was measured for a poly(isocyanide) structure containing ∼100 chromophores by means of hyper-Rayleigh scattering. Electric field-induced second-harmonic generation measurements confirmed that the product of the permanent dipole moment and the first hyperpolarizability was enhanced for the polymer structure. These results provide guidelines for future efforts to optimize supramolecular structures for applications in second-order nonlinear optics.

The determination of first hyperpolarizabilities β using hyper-Rayleigh scattering: a caveat

A comparison of first hyperpolarizabilities β determined by hyper-Rayleigh scattering (HRS; β HRS) and electric field-induced second-harmonic generation (EFISH; β EFISH) for a series of donor-acceptor substitutetd π-conjugated organic compounds shows a marked discrepancy between β HRS and β EFISH values for NLO-chromophores which fluoresce in the frequency-doubled wavelength region (532 nm). For MONS and DANS it is unequivocally established that two-photon absorption (TPA) at the fundamental wavelength (1064 nm) occurs followed by Stokes and anti-Stokes fluorescence around the second-harmonic wavelength. Hence, hyper-Rayleigh scattering at 1064 nm is not suited to determining the β of these fluorescent NLO-chromophores.

Theoretical investigation of the Kerr effect for CH4

The vibrational contributions to the Kerr effect and to electric-field-induced second-harmonic generation (ESHG) are calculated for methane for a number of optical frequencies. The latter results, together with the experimental ESHG values of the total mean second hyperpolarizability, allow for the determination of the ω2L-dispersion curve for the mean electronic hyperpolarizability. Since this curve is identical, to fourth order, for both processes, we are able to combine it with the calculated Kerr vibrational hyperpolarizabilities and predict the total Kerr hyperpolarizabilities for CH4 for several laser frequencies.

Electric field induced second harmonic generation/third harmonic generation measurements on molecules with extended charge transfer: Absorption domain and strong resonance effects

An enlarged method of performing electric field induced second harmonic generation (EFISHG) or third harmonic generation (THG) measurements of molecular hyperpolarizabilities in the strongly resonant regime is proposed. The influence of absorption coefficients and anomalous dispersion are discussed in detail and corrections proposed which require no change in the experimental setup. An alternative method of data interpretation is presented and its validity discussed for two examples of molecular structures.

Electron density study by X-ray and neutron diffraction of an NLO compound: N-(4-nitrophenyl)-L-prolinol. Description of quadratic hyperpolarizability

The study of the electron density and the determination of the dipolar moment of the molecule in the crystalline compound N-(4-nitrophenyl)-L-prolinol have been completed using X-ray and neutron diffraction. The deformation maps and the calculations of the atomic net charges demonstrate the character of the donor-acceptor couple (DA) linked to the phenyl transmitter. The dipolar molecular moments obtained by several methods have similar values, except the method of kappa refinement. The kappa refinement appears inadequate to solve the phase problem. Accurate knowledge of the first molecular moment reduces the ambiguity on the quadratic polarizability coefficient β, measured from electric field-induced second-harmonic generation (EFISHG).The coefficients of the cubic moment (third-order semi-invariant) of the electronic distribution in the crystal state are compared with those of the quadratic hyperpolarizability obtained by theoretical calculations. An estimate of relevant coefficients in octupolar-type material would provide guidance to the choice of systems for non-linear optics. Crystal data: C 11 H 14 N 2 O 3 , M r , = 222.2, monoclinic, P2 1 , Z = 2, F(000) = 236, T = 122 K, a = 5.152 (4), b = 14.790 (3), c = 7.134 (2) A, β = 106.14 (4)°, V = 522.18 A 3 , D x = 1.41 Mg mm -3 at 122 K, μ = 0.084 mm -1 for λ = 0.7107 A

Synthesis and second-order non-linear optical properties of substituted aminobenzoquinones

The preparation and characterization of a new type of donor-acceptor substituted benzoquinones has been performed. Their second-order molecular hyperpolarizabilities (β) were determined by electric field-induced second-harmonic generation (EFISH) experiments.

Electric-field-induced second-harmonic generation mediated by one-dimensional excitons in polysilanes.

The third-order nonlinear-susceptibility ${\mathrm{\ensuremath{\chi}}}^{(3)}$(-2\ensuremath{\omega};\ensuremath{\omega},\ensuremath{\omega},0) spectrum has been investigated for thin films of linear-Si-chain compounds, poly(dihexylsilane), by measurements of electric-field-induced second-harmonic generation (EFISHG). In the ${\mathrm{\ensuremath{\chi}}}^{(3)}$ spectrum for EFISHG, two-photon resonance structures to the first and second levels of one-dimensional excitons were observed. The observed ${\mathrm{\ensuremath{\chi}}}^{(3)}$ spectrum can be accounted for by considering multiple optical processes in the one-dimensional excitonic series characteristic of the Si-chain system.

Nonlinear optical properties of novel thiophene derivatives: Experimental and ab initio time-dependent coupled perturbed Hartree–Fock studies

We report in this paper experimental and theoretical studies of the nonlinear optical properties of a new class of compounds which differ from traditional second-order molecules in the sense that a commonly used electron donor chromophore is replaced by a thiophene ring. The molecular second-order nonlinear optical coefficients, β, as determined by the electric field-induced second-harmonic generation technique, for 2-nitro-1-(2-thienyl)ethene and 4-nitro-1-(2-thienyl)-1,3-butadiene are, respectively, 1 and 3 times that of para-nitroaniline. Ab initio time-dependent coupled perturbed Hartree–Fock results are in agreement with the experimental findings. A Mulliken population analysis indicates that the sulfur atom in the thiophene ring acts as an electron donor. Both the experimental and the theoretical results confirm that a thiophene ring acts as an efficient donor, giving rise to highly efficient second-order nonlinear optical properties. We also show that with this type of chromophore one can use a thiophene oligomer, in place of a thiophene monomer, to amplify the nonlinearity.

Nonlinear optical properties of polymers and thin polymer films

AbstractAn introduction to nonlinear optics (NLO) and the major challenges in the field of NLO‐properties of polymeric materials, is given in this paper. Methods for the investigation of nonlinear optical properties of [4‐(2‐methacryloxyethyl)methylamino]‐4′‐cyanoazobenzene copolymers with methyl methacrylate (MMA) are demonstrated: Electric Field Induced Second Harmonic Generation (EFISHG), Hyper Rayleigh Scattering (HRS), Second Harmonic Generation (SHG) in floating monolayers and SHG in poled polymer films. An example of phase‐transition analysis by SHG is given.

Hyper-Rayleigh Scattering (Hrs) In Isotropic Media

ABSTRACTIn this paper some of the more important Hyper Rayleigh Scattering (HRS) Measurements performed in our laboratory are reviewed. The results of experiments conducted on polymers, the protein Bacteriorhodopsin, retinal derivatives and octopoles are presented. Basic HRS theory, along with a brief description of the HRS set-up, is also included.We demonstrate that the experimental simplicity of HRS allows for an accurate determination of the first hyperpolarizability of molecules that could not have been measured by the more traditional Electric Field Induced Second Harmonic Generation (EFISHG) technique.

Second-order nonlinear optical properties ofp-dimethylaminobenzylidene-1,3-indandione (DABI) measured by EFISH in dioxane

The second-order nonlinear optical properties of p-dimethylaminobenzylidene-1,3-indandione (DABI) have been measured using electric field-induced second-harmonic generation (EFISH) at wavelengths of 1064 nm and 1907 nm in dioxane. The absolute values of the microscopic hyperpolarizabilities are β1064 nm=1064×10−40 m4 V−1 and β1907 nm=150×10−40 m4 V−1. The condensation of 1,3-indandione with substituted aromatic aldehydes offers a pathway to a class of molecules with extended π-electron systems. In the case of DABI, semiempirical calculations and a simple, two-level model indicate that the relatively high values of the microscopic hyperpolarizability are due to several factors which, in combination, positively enhance the electron donor-acceptor group-induced polarizability of DABI. The nonlinear optical properties of DABI are compared with experimental data for 2-methyl-4-nitroaniline (MNA).

Molecular hyperpolarizabilities of new bimetallic ferrocenyl derivatives

We have investigated the influence of complexation of ferrocenyl derivatives on the second-order hyperpolarizabilities β. This was performed using dc electric field induced second harmonic generation (EFISHG) technique at 1.34 and 1.9 μm. For these new bimetallic ferrocenyl derivatives, significantly increased β values were observed. Our best β value (123.5 × 10−30 esu at 1.34 μm) is comparable to the highest reported values for organometallic complexes. The nature of the second metal ion has a weak influence on the β values, in consequence to the change of geometry of the associated complex. The validity of the two-level model has been examined in detail: we found that it applies fairly well for the monometallic complexes if one takes into account only the low energy MLCT transition but the contribution of upper levels cannot be ruled out for bimetallic complexes.