Second-order nonlinear optical measurement techniques

Principally two techniques are used to measure the second-order nonlinear optical susceptibilities. These are ... 

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 ... 

As already mentioned, the only techniques sensitive to the polar order are even order nonlinear optical techniques such as the already-described second harmonic generation and linear electro-optic effect (cf. Chapter 2). The hrst technique offers a high sensitivity to the fast electronic contributions to susceptibility and is widely used. As already mentioned, it also gives the opportunity to study the kinetics of the poling by in situ measurements [152]. 

Figure 3.13 Electro-optic measurements using the surface plasmon resonance technique indicating (a) the experimental configuration and (b) an experimental differential reflectivity plot indicating a high second-order nonlinear effect.

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. 

Sum-frequency generation (SFG) at second-order and the nonlinear Raman spectroscopy BioCARS at fourth-order can also probe chiral molecules. They have no analog in linear optics. We show that both are only symmetry allowed in a fluid, if the fluid is chiral. However, in contrast to optical activity phenomena, these processes arise entirely from induced electric-dipoles (without magnetic or quadrupolar transitions) and they are not circular differential. All laser beams can be linearly polarized and no polarization modulation is required as the detection of a sum-frequency (yiz. five-wave mixing) photon is in itself a measure of the solution s chirality. Since an achiral solvent can not contribute to the signal, these techniques are sensitive, background-free probes of molecular chirality. The SFG... 

T. Watanabe, H.S. Nalwa, S. Miyata, Measurement Techniques for Refractive Index and Second-Order Optical Nonlinearities, Chapter 3 in [10]. 

Clays and Persoons presented a new technique in 1992 for the determination of the hyperpolarizabilities of nonlinear optical compounds in solution using the hyper-Rayleigh scattering (HRS). The values for first hyperpolarizabilities can be obtained from second-order scattered light intensity. This method has several advantages over the EFISH technique it needs no electric field, and thus the experiment is simplified, the hyperpolarizability P can be directly measured, and the P values for salts can be determined as weU. 

Several techniques are commonly used to determine the nonlinear optical properties of a thin film. Determination of the second-order susceptibility is frequently made by second-harmonic generation (SHG). Figure 4.2 shows the experimental layout employed for these measurements. The sample is illuminated with intense optical pulses from a pump laser (usually a Nd -.YAG Laser operating at 1.064/ m... 

The second-order NLO properties of the poled polymers can be measured by a variety of experimental techniques. We will describe only two very commonly used methods, the SHG and EO techniques. These are the most direct techniques for the determination of the optical nonlinearities of poled polymers. 

The thermal optical nonlinearity of TM74A under short laser pulse excitation has also been studied by a dynamic grating technique. For visible laser pulses (the second harmonic of a Nd YAG laser at 0.53 pm) and an absorption constant a 2 cm (determined by the absorbing dye dopant concentrationX the measured nonUnear index coefficient 2 is on the order of about -2 X10 " cm AV. 

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