André Persoons

Second-order nonlinear optical materials: recent advances in chromophore design

This paper deals with recent and important developments in the field of organic materials for second-order nonlinear optics. Attention is drawn to current trends in chromophore design with a discussion of current progress and problems in this field. A number of important classes of chromophores, such as one-dimensional charge-transfer molecules, octopolar compounds, ionic materials, multichromophore systems and organometallics, are discussed.

Hyper‐Rayleigh scattering in solution

The experimental setup for the implementation of a new technique to determine the hyperpolarizability of nonlinear optical molecules in solution is presented. The new technique, hyper‐Rayleigh scattering in solution [K. Clays and A. Persoons, Phys. Rev. Lett. 66, 2980 (1991)], has the advantage over the electric‐field‐induced second‐harmonic generation technique that the dipole moment μ and the second hyperpolarizability γ do not have to be independently determined to obtain the first hyperpolarizability β. No electric field is needed to lower the intrinsic symmetry of the isotropic solution, leading to a simpler cell design and a simpler local field factor. The internal reference method, when applicable, completely eliminates the local field factor. With the technique presented, values of the first hyperpolarizability β of 23×10−30 esu for para‐nitroaniline (PNA), 105×10−30 esu for 4‐methoxy‐4’‐nitrostilbene (MONS), and 95×10−30 esu for 4‐hydroxy‐4’‐nitrostilbene (HONS) dissolved in chloroform have been ...

The molecular and supramolecular engineering of polymeric electro-optic materials

Abstract A new class of electro-optic chromophores, of which 2-dicyanomethylen-3-cyano-4-{2-[ E -(4- N , N -di(2-acetoxyethyl)-amino)-phenylene-(3,4-dibutyl)thien-5]- E -vinyl}-5,5-dimethyl-2,5-dihydrofuran (denoted FTC) is the prototype, has been prepared, characterized, and used to fabricate electro-optic devices. The molecular hyperpolarizability and thermal stability of these chromophore molecules are exceptional. Strong intermolecular electrostatic interactions inhibit the efficient poling of these molecules. A statistical mechanical theoretical treatment is used to quantitatively predict the competition of poling, intermolecular electrostatic interactions, and thermal effects in defining achievable acentric order and hence macroscopic optical nonlinearity. Theory is used to predict the optimum chromophore structure and material composition (chromophore loading in a polymer matrix) for maximum electro-optic activity and minimum optical loss. Problems associated with lattice hardening to lock-in poling-induced order are discussed briefly.

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.

Nonlinear Optical Properties of Proteins Measured by Hyper-Rayleigh Scattering in Solution

Hyper-Rayleigh scattering has been used to determine the nonlinear optical properties of a chromophore-containing protein in solution. Because the technique of hyper-Rayleigh scattering allows the measurement of hyperpolarizabilities in an isotropic solution without the application of an electric field, this method is ideally suited for the study of proteins that carry a net charge. The observed orientational correlation between the nonlinear optical chromophores in incompletely solubilized protein molecules suggests that guidelines from protein structures can be used for the engineering of supramolecular structures with high optical nonlinearity.

Switching of molecular second-order polarisability in solution

Second-order nonlinear optical (NLO) polarisabilities or first hyperpolarisabilities, β, of molecules may be manipulated by reversibly modifying the properties of specific parts of active molecules, usually by on/off switching. This may involve reducing the donor capacity of the electron-rich fragment of a typical donor–acceptor species, D–A, by oxidation or protonation. Conversely, the acceptor behaviour of A may be altered by reduction, or by deprotonation. Alteration of the first hyperpolarisability may also involve structural or chemical modification of the bridging group, thereby interfering with the communication between D and A. Much of the article focuses on redox behaviour of NLO-active metal-containing compounds, mainly describing dipolar species in which the donor fragment containing a metal is oxidised, but giving examples of switching behaviour in octopolar species and of protonation/deprotonation behaviour.

Second‐harmonic generation from chiral surfaces

We present a theory of second‐harmonic generation from chiral surfaces including contributions of electric and magnetic dipole transitions to the surface nonlinearity. The nonlinear polarization and magnetization of the surface as well as the second‐harmonic fields that are radiated in the reflected and transmitted directions are expressed in terms of the six possible bilinear combinations of the components of the electric field of the fundamental beam. For the case in which the polarization of the fundamental beam is controlled by means of a quarter‐wave plate between p‐polarized linear and left‐ and right‐hand circular, the second‐harmonic fields can be expanded in terms of only three different functions of the rotation angle of the wave plate. The process exhibits nonlinear optical activity, i.e., it responds differently to the two circular polarizations of the fundamental beam if the phases of certain expansion coefficients are different. The theory is used to explain the results of a recent experimen...

Solvent dependence of the first hyperpolarizability of p-nitroanilines: Differences between nonspecific dipole–dipole interactions and solute–solvent H-bonds

In this paper, the influence of solvents on the first hyperpolarizability β of nonlinear optical (NLO) molecules is studied at 25 °C for solutions of p-nitroaniline and N,N-dimethyl paranitroaniline in some 50 solvents belonging to different classes. The hyperpolarizabilities deduced from measured hyper-Rayleigh scattering are the relative ones with as reference that of paranitroaniline in 1,4-dioxane. Taking for the latter the literature value of 16.9 10−30 esu, the β values vary from 14.4 to 29.9 10−30 esu for paranitroaniline and from 28.7 to 46.2 10−30 esu for N,N-dimethyl paranitroaniline. A selection of the solvents is made on the basis of the fraction of the time γ0 during which the NLO molecule is not involved in H-bonding with the solvent molecules. γ0 can be determined from the solubility in the given solvent. The formation of specific solute–solvent interactions as H-bonds always increases the hyperpolarizability β. In the absence of such interactions, the hyperpolarizability increases with the...

Second-order non-linear optical polymers

In this article we discuss the state of the art in the field of second-order non-linear optical polymers. More specifically, we highlight those results that we think made an important contribution to the field, combined with some of our own results. We start with a general overview of all the aspects involved in characterizing second-order non-linear optical polymers, from thin film formation and poling to second-harmonic generation and electro-optic measurements on such systems. Next, we review the second-order non-linear optical properties of selected polymer systems such as poly(vinyl ether)s, polystyrenes, polymethacrylates, main-chain polymers and high Tg polymers like polyimides and polymaleimides. Finally, we discuss some new polymer systems that might become important in the field of non-linear optics in the near future.

In situ reversible electrochemical switching of the molecular first hyperpolarizability

In situ reversible electrochemical switching of the molecular second-order nonlinear optical (NLO) polarizability, or first hyperpolarizability, has been implemented in a specially designed cell.The redox-switchable NLO chromophore is based on the octamethylferrocene/octamethylferrocenium redox system as electron-donor (D) group, in conjunction with nitrothiophene as the electron-acceptor (A) group and ethenyl as the p-conjugation bridge.This D- p-A chro- mophore has been shown to exhibit reversible redox switching of its linear and nonlinear optical properties.The im- portance and potential of this electrochemical switching of the first hyperpolarizability is discussed in the context of current and future applications of second-order NLO effects. 2002 Elsevier Science B.V. All rights reserved.