Optical nonlinearity thermal

For measurements carried out closer to resonances, thermal contributions to the optical nonlinearity, given by the following formula, can compete with electronic contributions (54,55) ... 

The synthesis of materials for device applications has very different requirements. Here, the most important questions are What does the device do and what factors will affect its performance The magnitude of the desired optical nonlinearity will always be one of many criteria that will ultimately dictate the material of choice. In many instances the magnitude of the nonlinearity will not be the most important parameter. Depending on the device applications, other considerations such as optical transparency, processability, one- and two-photon optical stability, thermal stability, orientational stability, and speed of nonlinear response will all be important. Our current understanding of NLO materials suggests that these variables are frequently interrelated and that there is often no ideal NLO material. The material of preference for a given application will typically be one that is the best compromise for a variety of variables. Tutorials by G. Stegeman and R. Zanoni, and by R. Lytel outline some of the NLO device applications and the related materials issues. 

The third-harmonic generation method has the advantage that it probes purely electronic nonlinearity. Therefore, orientational and thermal effects as well as other dynamic nonlinearities derived from excitations under resonance condition are eliminated (7). The THG method, however, does not provide any information on the time-response of optical nonlinearity. Another disadvantage of the method is that one has to consider resonances at oj, 2w and 3o> as opposed to degenerate four wave mixing discussed below which utilizes the intensity dependence of refractive index and where only resonances at a) and 2a) manifest. 

In this report, vacuum evaporated PDA(12-8) film is used as an optically nonlinear layer with a grating coupler for nonlinear coupling for all optical bistability. Grating coupler on a substrate was prepared at the same periodicity and depth as the SHG devices. Vacuum evaporation of PDA on a substrate with previously rf-sputtered Corning 7059 buffer layer film were carried out at 5 x 10 5 torr with tungsten boat heater. Rapid evaporation can avoid thermal polymerization of the undesirable red phase PDA during the process. UV polymerization of the film for the useful blue phase PDA was carried out by Xe lamp 500 w for 20 min. at a... 

Assessing thermal and photochemical stability is important. Thermal stability can be readily measured by measuring properties such as second harmonic generation as a function of heating at a constant rate (e.g., 4-10 °C/min) [121]. The temperature at which second-order optical nonlinearity is first observed to decrease is taken as defining the thermal stability of the material [2,3,5,63,63]. It is important to understand that the loss of second-order nonlinear optical activity measured in such experiments is not due to chemical decomposition of the electro-optic material but rather is due to relaxation of poling-induced acentric... 

The hb-PAEs of hb-P13 and hb-P15 contain NLO-active azo-functionalities, which are soluble, film-forming, and morphologically stable (Tg > 180 °C). Their poled films exhibited high SHG coefficients ( 33 up to 177pm/V), thanks to the chromophore-separation and site-isolation effects of the hyperbranched structures of the polymers in the three-dimensional space (Table 5) [28]. The optical nonlinearities of the poled films of the polymers are thermally stable with no drop in d33 observable when heated to 152 °C (Fig. 8), due to the facile cross-linking of the multiple acetylenic triple bonds in the hb-PAEs at moderate temperatures (e.g., 88 °C). 

The coplanarity has endowed arylated TEEs with some of the highest known third-order optical nonlinearities and, in the case of acentricity, also very large second-order nonlinear optical effects. Furthermore, the strain-free planarity allows cis- and trans-arylated TEEs to interconvert upon photochemical excitation without competition from undesirable thermal isomerization. 

In equations (5)-(8), i is the molecule s moment of Inertia, v the flow velocity, K is the appropriate elastic constant, e the dielectric anisotropy, 8 is the angle between the optical field and the nematic liquid crystal director axis y the viscosity coefficient, the tensorial order parameter (for isotropic phase), the optical electric field, T the nematic-isotropic phase transition temperature, S the order parameter (for liquid-crystal phase), the thermal conductivity, a the absorption constant, pj the density, C the specific heat, B the bulk modulus, v, the velocity of sound, y the electrostrictive coefficient. Table 1 summarizes these optical nonlinearities, their magnitudes and typical relaxation time constants. Also included in Table 1 is the extraordinary large optical nonlinearity we recently observed in excited dye-molecules doped liquid... 

The observed dynamics of the self-starting phase conjugation process is governed by the optical nonlinearity and scattered noise amplitude [27], as well as the thermal grating build up time. Both the onset time and build up times are shortened as the input pump power is increased. The total time it takes for the signal to build up to the maximum can be as short as 0.5 ms, at a pump power of about 800 mWatt. These shortenings of the build up and onset times have also been observed as the sample temperature is increased towards T [25]. 

The excellent thermal stability of NLO activity realized by this approach is illustra in Figs. 3 and 4 which show evolution of the second harmonic generation signal as a function of time at both ambient and elevated temperatures. Fig. 4 illustrates the dramatic improvement of the stability of the NLO activity associated with the final crosslinking step which locks in place both ends of the chromophore. Even at 125 °C, 95% of the initial optical nonlinearity is retained for an extended period of time. 

Stability of common polymers, and consequently, thermal degradation of mercaptide molecules ean be also carried out with the mercaptide dissolved into a polymeric medium. In this case, a finely dispersed inorganic solid phase, embedded in polymer, is generated. Materials based on clusters confined in polymeric matrices are called nanocomposites [Mayer, 1998 Caseri, 2000]. Both semiconductor-polymer and metal-polymer nanocomposites have unique functional properties that can be exploited for applications in several advanced technological fields (e.g., optics, nonlinear optics, magnetooptics, photonics, optoelectronics) [Caseri, 2000]. 

The NLO copolyimides with varying NLO chromophore loading levels have been synthesized. As expected, as the chromophore loading level decreased, the glass transition temperature increased as well as the thermal performance of the resisting polymers. The co lymerization provides a general approach to the enhancement of the thermal stabifity and the no inear optical stability of the materials within a tolerable trade-off in optical nonlinearity. 

Polycondensation and imidization of w,w -diaminobenzophenone and pyromellitic dianhydride under microwave radiation was also carried out. The product polyimide was obtained in a two-step process. It is claimed that this product of microwave radiation polymerization compares favorably with a product of conventional thermal polymerization, because it exhibits third-order nonlinear optical coefficient of 1.642 x 10 esu and response time of 24 ps. The third-order optical nonlinearity of this polymer is dependent on the chain length and the molecular structure. 

Because of their large optical nonlinearities and good mechanical, chemical, thermal, and optical stability, organic nonlinear materials are among the leading practical materials for device applications [2]. A number of experimental techniques have been proposed to obtain information about the dispersion, the sign, and the contributions of both the real and imaginary parts of the nonlinear optical response. In this chapter, we will explore the methods most widely used to characterize the third-order nonlinear coefficient and introduce some of the more recent results. 

Here a. is the polarizability (B and y are the first and second hyperpolarizabilities, respectively. E operties such as optical absorption at the operational wavelength (e.g., 980, 1300, or 1500 nm) of an electro-optic device, thermal and chemical stability of the materials under the conditions of integrated device fabrication and operation, etc. must also be carefully considered. As we shall see in the next section, molecular size and shape must also be considered as they will affect poling efficiency and realizable macroscopic optical nonlinearity. Moreover, very little trade-off between desired properties is permitted. Let us review, in turn, each of these aspects of chromophore design. 

It should be noted that chromophore-chromophore interactions, particularly those leading to nontransient aggregation, will affect the efficiency of latticehardening reactions to be discussed later and will contribute to optical loss if not controlled. The derivatization of chromophores that has been used to improve poling-induced optical nonlinearity also leads to improved thermal stability of pol-ing-induced order realized by lattice hardening and to improved optical transparency due to reduced scattering losses. 

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