New multilayer polymer waveguides have been introduced with inverted nonlinear layers for efficient modal dispersion phase-matched second-harmonic generation at the telecommunication wavelength near 1.55 μm. The nonlinear optical core of the waveguides consists of two modified Disperse Red 1–based side-chain polymers with different glass-transition temperatures. The signs of the nonlinear optical coefficients are different in the two polymers after suitable poling above and between the respective glass transitions, thereby optimizing the overlap integral. The optical nonlinearity profile is controlled by in situ electro-optical measurements during the two poling steps. The successful preparation of inverted layers is verified by electro-optical, pyroelectrical, and second-harmonic-generation thermal analysis. Waveguide losses are low at 1.55 μm (4 dB/cm) and high at 800 nm (100 dB/cm) because of the residual absorption of the Disperse Red 1-like chromophores. Phase-matched second-harmonic generation has been demonstrated with a large figure of merit, 14%/W cm-2. Extensive room for improvement in second-harmonic generation is possible with optimized chromophores, because the total conversion efficiency is strongly limited by the harmonic losses in the modified Disperse Red 1.

Modal dispersion phase-matched second harmonic generation is demonstrated in new poled polymer waveguide geometries with a nonlinear optical core consisting of two side-chain polymers with different glass transition temperatures. After poling above and between the respective glass transitions, the sign of the nonlinear optical coefficient is reversed in the two polymers, thereby improving the overlap integral. Conversion efficiencies up to 7%/W cm2 were achieved in the first experiments.

Modal dispersion phase-matched second harmonic generation is demonstrated in polymer-based waveguides with a nonlinear optical core consisting of two side-chain polymers with different glass-transition temperatures. For an optimized overlap integral, a step like nonlinearity profile (chi(2)-inverted structure) is required across the core thickness. The chi(2)-inverted structure is achieved by two consecutive thermally assisted poling steps above and between the respective glass-transition temperatures, with an opposite poling field in the second poling step. The achieved chi(2)-inverted structure is monitored by in- situ electro-optic measurements and proved by electro-optic and second harmonic generation thermal analysis. Conversion efficiencies up to 7%/Wcm2 were achieved in first waveguide second-harmonic generation experiments.

Polymers, even with attached, non-centrosymmetric, side groups are amorphous due to the random orientation of the chromophores and therefore are not second-harmonic active. They can be made macroscopically non-centrosymmetric by applying strong electric fields (poling) near the glass temperature of the host polymer to align the side groups which normally have large dipole moments.[1] The side groups can be engineered to have large non-resonant nonlinearities so that the poled polymers can have d (2) s of the order of 50-100 pV/m.[1] This has made poled polymers interesting doubling media, especially in channel waveguides which can be easily fabricated using a number of polymer techniques. We have been exploring poled polymers for second harmonic generation (SHG) for operation with 1550 nm inputs for potential application to cascading, WDM frequency shifting, etc.[2,3] Here we report our progress using the modal dispersion phase-matching technique.

We have measured the temperature profile of aluminum coated fiber tips used for illumination-mode scanning near-field optical microscopy as a function of the optical input power with a micron sized thermocouple. The temperature coefficients vary from 20 K/mW for tips with a large cone angle to 60 K/mW for the narrow long ones. Temperatures of up to ≊470 °C have been measured close to the aperture with an optical input power of several mW before thermal damage of the coating occurred. The temperature profiles are analyzed theoretically taking into account the optical absorption, the thermal conductivity of the tip, as well as the heat loss to the environment.

Improved nonlinear organic chromophores of varying conjugation length with either thiobarbituric acid or 3-dicyanomethylene-2,3-dihydrobenzothiophene-1,1-dioxide (FORON® Blue) acceptors have been synthesized and investigated for their nonlinear optical properties. Very large quadratic hyperpolarizabilities β(−2ω; ω, ω) have been found, up to 25,700×10^(−48) esu at λ=1.91 μm. In a guest–host polymer very high electro-optic (EO) coefficients, of up to 55 pm/V, have been determined at λ=1.31 μm with 20-wt % chromophore loading. We find good agreement between molecular parameters evaluated by electric-field-induced second-harmonic generation (EFISH) and the measurements of guest–host solid–solid solutions. The latter method is well suited to the determination of the product of dipole moment μ and hyperpolarizability β quickly and reliably at the wavelength of interest for EO applications without the complications associated with EFISH measurements.

New modified polyimide polymers with pendant side group nonlinear optical (NLO) azo chromophores and moderate to high glass transition temperatures (140 degrees C < T-g < 190 degrees C) have been prepared. poled films of these polymers possess large nonlinear optical susceptibilities of d(31) = 23 pm / V and electro-optic (EO) coefficients of r(13) = 6.5 pm / V at a wavelength of lambda = 1.3 mu m. The structural properties (glass transition, molecular weight, chromophore density) and optical properties (refractive index, optical nonlinearity) of these polyimides can easily be varied to fulfill the requirements of potential electro-optic devices. Due to the relatively high glass transition temperatures of these polymers, long-term stability of the optical nonlinearity of typically one to hundreds of years at operating temperatures of 80-100 degrees C is predicted from accelerated time-temperature measurements.

New modified polyimide polymers with pendent side group nonlinear optical (NLO) azo chromophores and moderate to high glass transition temperatures (140-degrees-C < T(g) < 190-degrees-C) have been prepared. Corona poled films of these polymers possess large nonlinear optical susceptibilities of d31 = 23 pm/V and electrooptic (EO) coefficients of r13 = 6.5 pm/V at a wavelength of lambda = 1.3 mum. The structural properties (glass transition, molecular weight, chromophore density) and optical properties (refractive index, optical nonlinearity) of these polyimides can easily be varied to fulfill the requirements of potential electrooptic devices. Due to the relatively high glass transition temperatures of these polymers, long-term stability of the optical nonlinearity of typically one to hundreds of years at operating temperatures of 80-100-degrees-C is predicted from accelerated time-temperature measurements. Using a development of a phenomenological theory of the glass transition, a normalized relaxation law is proposed with (T(g) - T)/T as the relevant scaling parameter.

As the thermal stability of nonlinear optical polymer electrets is essential for their application in waveguide devices, three typical polymers were studied by means of isothermal pyroelectricity measurements and thermally stimulated depolarization experiments. The fundamental difference between the three sample materials is the connection between the polymer backbone and the nonlinear optical molecules: The guest-host polymer is just doped with chromophore molecules, in the side-chain material, the chromophore is chemically linked to the main-chain, and in the cross-linked system, chromophore movement is still further restricted. The numerical evaluation reveals how the isothermal relaxation deviates from an exponential decay, and the TSD results yield the product of activation energy and stretching parameter. The behavior of guest-host and side-chain polymers can be described by a single broad relaxation process, whereas cross-linking yields a rigid dipole polymer network with a high activation temperature of the dipole orientation

Novel modified polyimide polymers with nonlinear optical chromophores as pendent side groups show high glass transition temperatures and large nonlinear optical coefficients up to d33 equals 78 pm/V at (lambda) equals 1313 nm (electro-optic coefficient r33 equals 18 pm/V). Due to the high glass transition temperatures of up to 190 $DEGC an excellent long term stability of the nonlinearity results. Extrapolation from measurements at elevated temperatures predict long term stabilities of the nonlinearity in excess of tens of years at 80 $DEGC and hundreds to thousands of years at room temperature. The structural (molecular weight, glass transition temperature) and optical (refractive index, nonlinearity) properties of these polyimides can easily be varied which allows to taylor active layer and buffer layer materials.

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 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 glass transition in a wide variety of different polymer host systems. The temperature dependence of the decay was found to be non-Arrhenius, but could be strongly correlated with the glass transition temperature of the guest-host system using an empirical relationship similar to the Williams–Landel–Ferry or Vogel–Tamann–Fulcher equation.

Organic nonlinear optical materials have developed to the point where applications in practical devices may now be contemplated. For second order nonlinear systems, these applications fall into two classes: frequency doubling and electro-optic switching and modulation. Chromophores already exist with values of the second order molecular hyperpolarizability β large enough for electro-optic applications. The situation for frequency doubling is not nearly as promising. The absorption maximum of the chromophore and the magnitude of β cannot be independently optimized within a given class of chromophores (e.g. benzenes, stilbenes, tolanes). Large values of β seem to be correlated with absorption maxima that are shifted to long wavelengths. This means that those chromophores that are the most efficient for frequency doubling will also be the most likely to absorb photons generated at the second harmonic wavelength. This relationship can be easily understood, at least in principle, by referring to a simple Hückel model of a prototypical nonlinear chromophore.

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 Hückel model.

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.