Richard Herman Woudenberg

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

Abstract 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.

High hyperpolarizabilities of donor-p-acceptor-functionalized calix[4]arene derivatives by pre-organization of chromophores

A systematic investigation of the conceptofpre-organization of nonlinear optical (NLO) active chromophoric groups in calix[4]arene derivatives and the influence on the absolute second-order nonlinear optical coefficients is reported. Several calix[4]arenes were synthesized by modification of the electron-withdrawing groups at the upper rim of the aromatic and extension of the conjugated system of the pre-organized chromophoric groups. Electrical field induced second harmonic generation (EFISH) experiments showed high (0) values up to 1165·10-48 esu. Compared with the corresponding reference compounds, enhancements of the (0) values varying up to 2.5 times per chromophore were observed which proves the benefit of pre-organization of NLO-active units in a multi-chromophoric system. Another important advantage is that the increase in NLO activity observed for these systems is not accompanied with a shift of the absorption band to longer wavelengths exceeding 20 nm. This makes these calix[4]arene derivatives promising building blocks for the development of stable, NLO-active materials that are suitable for frequency doubling.

Low-loss nonlinear optical polymeric waveguide materials and devices

Recently developed photobleachable polymers show a loss of < 0.1 dB/cm at 1300 nm and < 0.15 dB/cm at 1550 nm. Nonchromophore containing polymers show film waveguide losses of < dB/cm at 1300 nm and 1550 nm. Refractive indices in these materials can be tuned within a range of 0.05 by changing the polymer composition. Multilayers of cross-linked (solvent resistant) layers, each 2-10 microns have been deposited by multiple spinning steps. Using these multilayers, fully embedded, fiber-compatible strip waveguide structures have been created by masked bleaching of multilayers with chromophore containing corelayers. The lateral refractive index contrast is thereby tuned by changing the chromophore content of the corepolymer. Poling-induced loss has been investigated by wavelength and polarization dependent measurements of losses in films. The results indicate that this loss is due to increased scattering. Bleached channel waveguides in a poled (at 125 V/micrometers ) nonlinear optical polymer have been made showing losses of < dB/cm at 1300 nm. Rapid photodegradation at 1300 nm has been observed in stilbene containing channel waveguides. In a nitrogen atmosphere no degradation was seen. The same is true for waveguides in air at 1550 nm. This suggests the attack of the stilbene chromophores by singlet oxygen. Therefore a new generation of low-loss, linear, and nonlinear optical polymers based on singlet oxygen resistant molecules has been developed. The linear optical polymers are used for the realization of low-insertion loss (< 2 dB), digital (switch voltage 3-6 V) and efficient (switching power < mW, cross talk - 20 dB) pigtailed and packaged 1 X 2 switches. They utilize the strong thermo-optic effect in polymers. Their switching time is therefore limited to 1 ms whereas their polarization dependence is < 0.3 dB.