D. M. Burland

Problems in the comparison of theoretical and experimental hyperpolarizabilities

Frequently it is useful to compare experimental values of the hyperpolarizabilities β and γ with calculated values. It is also often helpful to compare experimental values of β obtained from dc‐electric field induced second harmonic generation (dc‐SHG) experiments, e.g., with values obtained using the solvatochromism method. In order to do this the hyperpolarizabilities must be defined using consistent conventions. In this paper, four commonly used conventions are discussed and simple factors for converting between them presented. In addition, the sum‐over‐states expression for the calculation of β and γ is described and its correct use in comparing with hyperpolarizabilities obtained using other experimental and theoretical techniques discussed. As an illustration of the consistent use of conventions, ab initio and semiempirical calculations on para‐nitroaniline are compared with experimental dc‐SHG values. This comparison highlights the difference between theoretical values of the hyperpolarizability wi...

Organic glasses : A new class of photorefractive materials

The performance of amorphous organic photorefractive (PR) materials in applications such as optical data storage is generally limited by the concentration of active molecules (chromophores) that can be incorporated into the host without forming a crystalline material with poor optical quality. In polymeric PR systems described previously, performance has been limited by the necessity of devoting a large fraction of the material to inert polymer and plasticizing components in order to ensure compositional stability. A new class of organic PR materials composed of multifunctional glass-forming organic chromophores is described that have long-term stability and greatly improved PR properties.

Solvent dependence of the second order hyperpolarizability in p-nitroaniline

Abstract Values of the second order hyperpolarizability β have been determined for p-nitroaniline (PNA) in solvents of varying polarity using the electric field induced second harmonic generation experimental technique. An observed solvent dependence of β0, the second order hyperpolarizability extrapolated to zero-frequency, is correlated with the solvent-induced shift of the PNA charge transfer absorption maximum λmax. The correlation is similar to the correlation between λmax and β0 observed in donor/acceptor p-substituted benzenes with different substituents. Both dependences can be understood within the framework of a simple four level Huckel model.

Orientational decay in poled second‐order nonlinear optical guest‐host polymers: Temperature dependence and effects of poling geometry

The orientational decay of chemically and thermally stable high‐temperature chromophores doped into thin films made from polyimides and a variety of other polymeric hosts has been investigated. The chromophores are aligned using electric field poling and second‐harmonic generation (SHG) is used to probe the decay of the electric field poling induced alignment. The decay rate of the SHG signal from films poled using both a corona discharge and side‐by‐side in‐plane electrodes was measured. When electrodes are chosen so that the effects of charge injection are minimized, little difference has been observed between the orientational decays from films poled using the two methods for either an amorphous preimidized polyimide host or a highly anisotropic film poled during imidization. The films imidized during poling showed significant orientational stability at 250 °C for over 15 h after a fast initial partial decay. In addition, the decay of the SHG signal was measured as a function of temperature below the g...

Net two-beam-coupling gain in a polymeric photorefractive material

Two-beam-coupling gain coefficients exceeding the absorption coefficient are demonstrated for the first time to our knowledge in an organic photorefractive system. The material is based on the photoconducting polymer poly(N-vinylcarbazole), doped with the optically nonlinear chromophore 3-fluoro-4-N,N-diethylamino-β-nitrostyrene and sensitized for charge generation with 2,4,7-trinitro-9-fluorenone. The photorefractive performance is significantly better than that of any previously described organic. Diffraction efficiencies as large as 1% in a 125-μm sample, grating growth times of the order of 100 ms, and beam-coupling gain coefficients >10 cm−1 were observed at 647 nm (writing intensity of 1 W cm−2, applied field of 40 V μm−1).

Re‐evaluation of the thermal stability of optically nonlinear polymeric guest‐host systems

Guest‐host polymer systems with potential use in electro‐optic devices are discussed. The polymer host is a polyimide and the guest chromophores are 2,4,5‐triarylimidazoles (lophines). Poling stabilities have been obtained by extrapolating the second harmonic generation decay using a stretched exponential function and extrapolated lifetimes greater than a year at 80 °C have been obtained. In addition, an apparent relationship between the stability of poled order and the glass transition temperature is discussed.

Design of optimized photorefractive polymers: A novel class of chromophores

It is demonstrated that the microscopic mechanism of the photorefractive (PR) effect in organic composites with low glass transition temperatures involves the formation of refractive index gratings through a space‐charge field‐modulated Kerr effect. A tensorial formulation of the macroscopic aspects of the PR Kerr effect and its microscopic interpretation is presented. The second‐order dipole orientation term containing the anisotropy of the first‐order optical polarizability α(−ω;ω) is shown to yield the dominant contribution to the Kerr susceptibility χ(3)(−ω;ω,0,0). A class of special chromophores having negligible second‐order polarizabilities β(−ω;ω,0) and large dipole moments μ has been identified in order to optimize this term. These chromophores are not subject to the efficiency‐transparency tradeoff typically encountered with second‐order nonlinear optical (NLO) chromophores, providing highly transparent materials with large PR Kerr response. Contrary to previous approaches in this field, the bes...

Process for optical information storage

Information is stored by a process involving the steps of (1) sensitizing a recording medium comprising a two-photon, four-level material by exposing said medium to flood illumination with ultra-violet or visible light, and then (2) exposing the sensitized medium simultaneously to two beams of coplanar laser irradiation intersecting within said recording medium to create a microscopic interference pattern.

A comparison of calculated and experimental hyperpolarizabilities for acetonitrile in gas and liquid phases

The nonlinear susceptibility of acetonitrile has been measured in both the gas and liquid phases by dc electric field‐induced second harmonic generation (dc‐SHG or EFISH). The EFISH signal for this molecule is dominated by the first hyperpolarizability β. It is shown that local field factors do not adequately describe the effect of the solvent environment. Thus it is not possible to extract ‘‘gas phase’’ values of hyperpolarizabilities from measurements made in solution. First and second hyperpolarizabilities, β and γ, have been calculated for acetonitrile using ab initio techniques. These calculations are compared to the gas phase experimental measurements. Excellent agreement is obtained when electron correlation effects are included.

The design, synthesis, and evaluation of chromophores for second‐harmonic generation in a polymer waveguide

In order to develop nonlinear optical chromophores for frequency doubling applications, it is important to understand the magnitudes of the relevant chromophore properties that will be required. In this paper the use of organic polymers with attached optically nonlinear chromophores to form waveguide second‐harmonic‐generation devices will be discussed. In particular, the use of such a frequency doubling device in optical storage applications will be considered. To this end limits on the quantity μβ are obtained where μ is the dipole moment and β the hyperpolarizability. Experimental values for electron donor/acceptor‐substituted benzenes, stilbenes, and tolanes are compared to these requirements. None of the chromophores treated here have both sufficiently high optical nonlinearity and sufficiently low optical absorption at a doubled optical frequency of 400 nm to be practical in the specific application described.