Nonlinear optics bulk measurement

The ability to perform simultaneous measurements of the TH on the interface and in the bulk of solution containing submicroscopic particles provides a very unique way of measuring the size of those particles and their nonlinear optical susceptibility (Shcheslavskiy et al. 2006a). Indeed, when such measurements are performed on the... 

In this paper it has been attempted to provide an introductory overview of some of the various nonlinear optical characterization techniques that chemists are likely to encounter in studies of bulk materials and molecular structure-property relationships. It has also been attempted to provide a relatively more detailed coverage on one topic to provide some insight into the connection between the macroscopic quantities measured and the nonlinear polarization of molecules. It is hoped that chemists will find this tutorial useful in their efforts to conduct fruitful research on nonlinear optical materials. 

As the local electric field in the particles is enhanced at the SPR, the metal nonlinear optical response can be amplified as compared to the bulk solid one. Moreover, the intrinsic nonlinear properties of metals may themselves be modified by effects linked with electronic confinement. These interesting features have led an increasing number of people to devote their research to the study of nonlinear optical properties of nanocomposite media for about two decades. Tire third-order nonlinear response known as optical Kerr effect have been particularly investigated, both theoretically and experimentally. It results in the linear variation of both the refraction index and the absorption coefficient as a function of light intensity. These effects are usually measured by techniques employing pulsed lasers. 

The nonlinear optical properties of polydiacetylenes are subject of an increasing interest in last years (1-4). This is due to the fact that polydiacetylenes are a one-dimensionnal system of higly polarizable conjugated tt elections The polarization depends strongly on electron delocalization length Moreover, the electronic origin of hyperpolarizability implies short response times On the other hand, the wave dispersed measurements of molecular hyperpolarizabilities yield information about forbidden electronic transitions In fact the bulk polarization can be developped in external electric field power series ... 

Four calculations have been reported in related areas of molecule-field interactions. Evans (1982) has simulated the bulk anisotropy for a collection of molecules, which while dipolar were not polarizable nor based on the properties of a real molecule. The field-induced force was not explicitly defined in terms of a dipole-field interaction, rather a full Lennard-Jones potential was used. Coffey has examined the inertial relaxation of dipolar molecules in intense fields (Coffey et al., 1983). Madden has focused on interaction-induced effects in dielectric absorption (Tildesley and Madden, 1983) and transient nonlinear optical measurements in CS2 (Madden, 1987). Samios and Dorfmuller used a similar local field formalism to ours when they examined far IR absorption and utilized equilibrium fluctations to obtain correlation functions for liquid CS2 (Gburski et al., 1987). More details on these calculations... 

Bulk and molecular nonlinear optical properties have been measured by laser optical techniques such as second and third harmonic generation (SHG, THG), electric field-induced second harmonic generation (EFISH), and degenerate four-wave mixing (DFWM). Molecular NLO responses can also be calculated by quantum-mechanical (ab initio and semiempirical) methods, and suitable computing programs are being developed. 

We measured the third harmonic generation (THG) of la-e using polystyrene matrices and variable laser wavelengths [136]. In order to understand the influence of the chain length on nonlinear optical properties, it is better to discuss the second hyperpolarizability y, which is a molecular parameter, than the macroscopic susceptibility which is a bulk quantity. For the determination of j/jesj, the imaginary part of the total third-order susceptibility j/res j of the films (thickness 50 and 212 nm) was taken. The result is depicted in Fig. 11.13 [136] in a double logarithmic plot of jy gs] versus the length L of the OPE chains I is a linear function of the number n of repeat units. (See Section 11.1.)... 

All of the studies discussed above for silver have been done with an incident beam of 1064 nm. These studies have proven that the anisotropy in the nonlinear polarizability from the silver surface is not purely free-electron-like at these wavelengths, that the anisotropy can be correlated with surface symmetry, and that the SH response measured in situ is nearly identical to that measured in UHV. The issue of the sensitivity of the rotational anisotropy to surface electronic properties has been the topic of very recent work which has been conducted by variation of the incident wavelengths to where optical resonances in the bulk or surface electronic structure can be accessed. 

To fulfill the need for understanding what structures will allow enhancement of optical nonlinearity, we have coupled ab-initio theoretical calculations of optical nonlinearity with synthesis of sequentially built and systematically derivatized model compounds, and the measurement of their optical nonlinearities. Now I would like to discuss very briefly our efforts to compare microscopic optical nonlinearities. An expression, similar to the expansion of the bulk polarization as a function of the applied field, can be written for the induced dipole moment. Naturally, the nonlinear term Y, for example, is the third derivative of the induced dipole moment with respect to the applied field. Also, using the Stark energy analysis, one can write the nonlinear terms 3 (and Y) as a sum over all excited states terms involving transition-dipoles and permanent dipoles, similar to what one does for polarizability. Consequently, the two theoretical approaches are (i) the derivative method and (ii) the sum-over-s1j tes method. We have used the derivative method at the ab-initio level. We correlate the predictions of these calculations with measurements on systematically derivatized and sequentially built model compounds. Some conclusions of our theoretical computations are as follows ... 

Several of the third-order nonlinear effects described in section 4.1.3 can be used to characterize bulk materials. Degenerate four-wave mixing (DFWM) is used for measuring third-order properties of films and solutions [43-45], and though this experiment is complex to set up and interpret, it can give valuable information on the magnitude, sign and speed of the nlo process, as well as an indication of the nature of the excitation process. Results from DFWM can be found in section 4.3. Optical Kerr effect (OKE) [46] and electrical Kerr effect (EKE) [47] measurements have also been used to characterize third-order properties of nlo polymers. It is important to note that THG, DFWM, OKE and EKE all measure different parts of the third-order susceptibility, and... 

A wide variety of techniques have been employed for the characterization of thin film samples of nonlinear polymeric materials. Many of these are similar to techniques described in the previous section for bulk material characterization, and are employed with thin film samples both to assess differences in material properties in the two physical forms and because certain measurements such as absorption or electro-optic effects may be more easily made in thin film samples. Other techniques are specific to thin film samples in which light can be guided, for which parameters can be measured having no bulk equivalent, such as waveguide scatter or nonlinear mode coupling. 

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