Nonlinear optics second harmonic generation measurement

Y.R. Shen, The Principles of Nonlinear Optics, Wiley, (1984) Optical Second Harmonic Generation at Interfaces, Ann. Rev. Phys. Chem. 40 (1989) 327. (Theoretical basis of SHG, experimental set-up, examples of applications to show what types of information are obtained from SHG measurements.)... 

Using the alternating deposition of the amphiphiles with a carboxyl substituent and arachidic add, noncentrosymmetric LB films (hetero Y-type) were prepared, and molecular orientation and second-order optical nonlinearity in the LB films were evaluated with the linear dichroism [4] and the second-harmonic generation (SHG) measurements, respectively. The SHG measurement procedure is mentioned in the section 1.3. 

Calculations which ascribe nonlinear optical polarizabilities in LilOa to the individual I-O bonds and relate experimentally measured second harmonic generation coefficients x 311 and X313 to theoretical results for the structure of LilOs, have been performed by Jeggo ii). 

The nonlinear optical properties of thin films of the polymers were investigated through measurement of second-harmonic generation, and exhibit second-harmonic coefficients, d33, in the range 4.1-34 pm/V. 

The dielectric tensor describes the linear response of a material to an electric field. In many experiments, and particularly in optical rheometry, anisotropy in is the object of measurement. This anisotropy is manifested as birefringence and dichroism, two quantities that will be discussed in detail in Chapter 2. The nonlinear terms are responsible for such effects as second harmonic generation, electro-optic activity, and frequency tripling. These phenomena occur when certain criteria are met in the material properties, and at high values of field strength. 

TDDFT methods have also been applied successfully to the description of the linear and nonlinear optical properties of heteroleptic sandwich complexes. The optical spectrum and the hyperpolarizability of Zr(OEP)(OEPz,) for which large first hyperpolarizabilities, /JSHG (SHG=second-harmonic generation) were measured in an electric field induced second-harmonic generation (EFISH) experiment [182], have been investigated by TDDFT methods [134]. The excitation energies and oscillator strengths calculated... 

Fig-2. Correlation of ionic conductivity with poling efficiency (second-order nonlinear optical activity). The upper graph shows conductivity measured as a function of temperature while the lower graph shows second-order nonlinear optical activity (measured by second harmonic generation, SHG) as a function of temperature. Note that second-order NLO activity starts to decrease with the onset of conductivity. Conductivity in this case was shown to arise from ionic impurities... 

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 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... 

On the assumption of total symmetry of the tensor of third-order nonlinear polarizability c(— co coi, cog, cog), its non-zero and independent elements are the same as those of Table 12. Direct theoretical calculations of c = c(0 0,0,0) have been performed for the atoms of inert gases and some simple molecules. Values of the tensor elements = c(— cu cu, 0,0) have been determined for numerous molecules from static Kerr effect studies and values of c = c(— cd ot>,coi — col) from measurements of optical birefringence induced by laser li t. Measurements of second-harmonic generation by gases in the presence of a static electric field yield the tensor elements c " = c( — 2co co, to, 0), which can also be obtained from second-harmonic scattering in centro-symmetric liquids. The elements of the tensor c = c(— 3co co, co, co)... 

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]. 

Before we close this section, let us emphasize the importance of the degeneracy factor D in Eq. (28). This factor takes different values depending on the nature of the nonlinear optical process. For instance, in the case of second harmonic generation its value is unity, while for an electro-optic process it takes the value 2. Therefore, great care must be taken in reconciling experimental values of nonlinear coefficients measured using different experiments. 

The second order optical nonlinearity shown in these figures has been measured by second harmonic generation as previously described. Relevant optical data and measured values of optical nonlinearity (measured for the polymer systems reported in this communication) are summarized in Table 1. 

Nonlinear optical properties. The optical nonlinearity was characterized by two experiments second harmonic generation (SHG) and electro-optic measurements. All of the results are listed in Table n. The second harmonic generation (SHG) for the poled polyimides was conducted to determine the second harmonic generation coefficients (das). PI 1 possessed the highest value of SHG... 

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