Third-order nonlinear optical measurement techniques

Measurement Techniques for Third-Order Nonlinear Optical Effects... 

The most widely employed material characterization techniques in third-order nonlinear optics are third-harmonic generation (THG) [21], degenerate four wave-mixing (DFWM) [22], Z-scan [6], and optical limiting by direct two-photon absorption (TPA) and fluorescence spectroscopy induced by TPA [23]. All of them will be discussed in the following. Further measurement techniques such as electric-field induced second-harmonic generation (EFISH) [24], optical Kerr... 

The first common method for molecular first hyperpolarizability determination is the electric field-induced second harmonic generation (EFISH) technique in solution [6-10]. This technique can be applied only to dipolar molecules. Under an applied external electric field, molecules in solution orient approximately in the direction of the field giving rise to second harmonic generation. The measured third-order nonlinear optical susceptibility is given by the following expression ... 

Coherent Anti-Stokes Raman Scattering (CARS) Thermometry is a technique for temperature measurement in high temperature environments using a third-order nonlinear optical process involving a pump and a Stokes frequency laser beam that interacts with the sample and generates a coherent anti-Stokes frequency beam. 

As with phthalocyanines, the third-order nonlinear optical susceptibility, of porphyrins can be manipulated by chemical substitution. The third-order NLO properties of several tetraphenylporphyrin compounds were first reported by Meloney et The was measured by the degenerate four-wave mixing (DFWM) technique from... 

Table 7. Third-Order Nonlinear Optical Susceptibility Data of Metalloporphyrins Compounds Measured by Different Techniques...

Two-photon absorption can be measured by several techniques. Two of them are two-photon excited fluorescence (TPEF) and nonlinear transmission (NLT). Pulsed lasers are most often used because TPA is a third-order nonlinear optical process and therefore is most efficient at very high intensities. In the nonresonant TPA, two photons combine to bridge an energy gap larger than the energies of each photon individually, and the transition occurs without the presence of the intermediate state. This can be viewed as being due to a virtual state created by the interaction of the photons with the molecule. 

This review covers the theoretical background and some of the practical aspects of nonlinear optics, including a description of the origins of third-order nonlinearities, systems of units that are encountered, experimental techniques that have been used or may be used to probe the third-order NLO properties of organometallic complexes, and computational methods that have or could be used to calculate third-order NLO properties. Subsequent sections collect comprehensive data of organometallic complexes in tables categorized by complex type and discussions of the results of third-order NLO measurements and calculations performed on organometallic... 

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. 

Out of the large range of possible nonlinear optical effects, chemists are likely to encounter only a limited number of measurement techniques. These include both second- and third-order NLO characterization methods. A brief listing of the different types of measurements, the nonlinear susceptibility involved and the related molecular nonlinear polarizabilities is given here. 

H.S. Nalwa, Measurement Techniques for Third-Order Optical Nonlinearities, Chapter 10 in [10]. 

The EFISH method (Singer and Garito, 1981) permitted for the first time the establishment of a correlation between molecular structure of organic chromophores and the first hyperpolarizability p. In this method an electric field is applied to a solution of the nonlinear optical materials, resulting in an alignment of the dipoles. A direct determination of P with the EFISH method is not possible the third-order polarization y is measured, the dipole moment p. must be known, and with these values the hyperpolarizability p can be calculated. The EFISH technique is not readily applied to salts as the solutions conduct electricity. 

Before going into the details of various materials and their third-order NLO properties, it would serve well to have an idea of the characterization techniques used for their study. To study the effect of the real part of third-order susceptibility, Z-scan measurements, degenerate four-wave mixing (DFWM), optical heterodyne detection of optical Kerr effect (OHD-OKE), and differential optical Kerr effect (DOKE) detection are employed. For the study of TPA, techniques such as nonlinear transmission (NLT) method, two-photon excited fluorescence (TPEF) method, and Z-scan measurements are used. The observables from the above-mentioned techniques vary depending on the inherent limitations of the technique. The nature of the light source employed like the central wavelength of the laser. 

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