SHG experiment

In order to measure molecular hyperpolarizabilities the now standard D-C induced SHG experiment is used (12). Although it would be more suitable to work in the gas phase to minimize molecular interactions, high molecular weights (low vapour pressure) and chemical decomposition processes make it hardly feasible for the molecules of interest. 

Several important aspects of the SHG experiments are not described in a straight forward way by the model. These are the residual SHG prior to field perturbation and asymmetric response to fields of different polarity. These effects may be due to the fact that the dipoles within the stacks as formed are subjected to remnant fields from surrounding stacks. The asymmetry may be associated with structural asymmetry within the stacks or some higher ordering or arrangement which does not allow for a symmetric hysteresis about zero voltage. A distribution of nonidentical stacks is also possible. 

In the case of SHG in waveguide nonlinear crystals, we describe a theoretical model which accounts for the temporal behavior of the interacting pulses and the possible z-dependence of the phasematching condition. The model also describes the observed saturation and subsequent decrease in SHG conversion efficiency in the waveguide samples, as a result of two-photon absorption (TPA) of the second harmonic (SH) wave. The results of this model are later compared with experimental data from SHG experiments using femtosecond pulses in the waveguide nonlinear crystals of periodically-poled potassium titanyl phosphate (ppKTP) and appKTP. This model is presented in section 2.3. 

The sample, with patterned area of 4x4 mm was contacted with KC1 H20 liquid electrodes, and two 2.2 kV pulses (6 ms long) were applied. The sample was then tested in an SHG experiment and the back metal electrode and the photoresist was removed. 

Figure 12. Laser configuration for SHG experiments, incorporating four single-narrow-stripe (SNS) red laser diodes and a prism (P) for wavelength tuning (DM dichroic mirror HWP half-wave plate PC polarization cube HR high reflector 1.5% output coupler).

As discussed in Chapter 8, enhanced reactions of S02 at the interface have also been observed (Jayne et al., 1990). Surface second harmonic generation (SHG) experiments (Donaldson et al., 1995) subsequently identified a unique adsorbed S02 species at the air-water interface that may be involved in this enhanced reaction. Such SHG work on the uptake and reaction of N02 on water would clearly also be of value in understanding the kinetic anomalies. In addition, the use of sum frequency generation (SFG) spectroscopy, which in effect allows one to obtain the infrared spectrum of species present at interfaces, may shed some light on such reactions. 

From the air/liquid interface, the SHG signal is typically observed in reflection, where the coherent harmonic beam propagates along the same direction as the reflected fundamental beam. The possibility of significant refractive index dispersion in the liquid means that for SHG experiments on the liquid/liquid interface, the harmonic beam path may deviate from the reflected fundamental. 

The polarization curves for concentrations higher than 6 pM can be fit by Equations (5) and (6) and Figure 1.9 shows the S- and P-polarized harmonic data for the lO-pM solution. Similar results have been reported for SHG experiments on the rfaodamine dyes at various interfaces. The best fit is obtained by introducing a phase difference, rj, between the parameters A and B consistent with the complex nature of tiie susceptibilities on resonance. A plot of the concentration dependence of the A/B, given in Figure 1.10, shows that a relatively sharp transition takes place in the structure of the interface at an aqueous dye concentration of ca. 6 pM. 

In Second-Harmonic Generation (SHG) experiments, an input beam of frequency (jt) incident in the material generates an output beam of frequency 2w. The response is described by the second-order nonlinear susceptibility ... 

SHG experiments may also be used to determine molecular orientation at interfaces. By determining the polarity of the SH signal with respect to that of the incident light one may determine the independent components of y. In general, y is a third-rank tensor with 27 elements. When the composition of... 

To obtain hyperpolarizabilities of calibrational quality, a number of standards must be met. The wavefunctions used must be of the highest quality and include electronic correlation. The frequency dependence of the property must be taken into account from the start and not be simply treated as an ad hoc add-on quantity. Zero-point vibrational averaging coupled with consideration of the Maxwell-Boltzmann distribution of populations amongst the rotational states must also be included. The effects of the electric fields (static and dynamic) on nuclear motion must likewise be brought into play (the results given in this section include these effects, but exactly how will be left until Section 3.2.). All this is obviously a tall order and can (and has) only been achieved for the simplest of species He, H2, and D2. Comparison with dilute gas-phase dc-SHG experiments on H2 and D2 (with the helium theoretical values as the standard) shows the challenge to have been met. 

Figure 17.1.21 Experimental apparatus for SHG experiments. A small portion of the beam from a high-power pulsed laser is sent to a reference channel, after frequency doubling, to provide a signal to normalize for fluctuations in the laser intensity. The main beam is linearly polarized and filtered before impinging on the sample. The resulting beam at 2

The most important feature of these results is given by the change of sign in ( 2) value going from gas phase to aqueous solution. This result seems to confirm the data obtained in dc SHG experiments, which give a positive value of the product p,z ifiz)- Assuming the molecule in the same orientation with respect to z axis (C=0 bond along +z), the experiments... 

For the first hyperpolarizability, P, it is harder to define a single simple quantity. In the SHG experiment, when all applied fields have parallel polarization and the molecule has a rotational symmetry axis (defined as the z axis), the permanent dipole moment is oriented along that axis and the NLO quantities of interest are ... 

They as well revealed that the LC monolayer adjacent to the rubbed polymer substrate remains aligned also in the isotropic phase [20], consistent with NMR and AFM experiments (see Chaps. 2 and 3). The opposite is also true on several rubbed polymer films that induce a strong bulk alignment, SHG experiments show the first LC layer has an isotropic azimuthal distribution. An example of that will be shown in Sect. 5.3 below. 

Fig. 5.1. Schematic representation of the SHG experiment denoting the orientation of a rod-shape liquid crystal molecule at the substrate surface. k u)) and fc(2oi) denote the wavevector of the incoming fundamental and reflected second harmonic waves. On the right hand side a 5CB molecule and its orientation vector 63 are sketched. Directions x,y,z indicate the laboratory frame.

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