Thin samples, nonlinear absorption

In the second, the intensity is kept constant and the sample is moved along the beam axis to increase the intensity at the focal point (Fig. 3.16 [53]). The intensity has the largest value at the focus and thus one can measure the largest nonlinear absorption effect if the focus of the beam is inside the sample. This technique requires the use of thin samples. The measured nonlinear transmittance is normalized with respect to linear transmittance measured at low intensity. Because the sample moves along the z-axis, this technique has been often called z-scan. 

Fig. 3.19 Schematic illustration of the measurement geometry for Mossbauer spectrometers. In transmission geometry, the absorber (sample) is between the nuclear source of 14.4 keV y-rays (normally Co/Rh) and the detector. The peaks are negative features and the absorber should be thin with respect to absorption of the y-rays to minimize nonlinear effects. In emission (backscatter) Mossbauer spectroscopy, the radiation source and detector are on the same side of the sample. The peaks are positive features, corresponding to recoilless emission of 14.4 keV y-rays and conversion X-rays and electrons. For both measurement geometries Mossbauer spectra are counts per channel as a function of the Doppler velocity (normally in units of mm s relative to the mid-point of the spectrum of a-Fe in the case of Fe Mossbauer spectroscopy). MIMOS II operates in backscattering geometry circle), but the internal reference channel works in transmission mode...

Tlie usual experimental techniques developed to study the optical Kerr effect in materials have already been described in a preceding chapter of this book. We only mention here the methods which have especially been used for nanocomposite materials as colloidal solutions or thin films Degenerate four-wave mixing (DFWM) and optical phase conjugation, which provide the modulus of x only and may be completed by Interferometry techniques to get its phase as well, optical limiting, optical Kerr shutter, and z-scan, which is probably the most common technique used in recent years due to its ability to provide simultaneously the nonlinear refraction and absorption coefficients of the same sample point [118],... 

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. 

The sample is far from the focal plane at the start (A) (and the finish (E)) of the scan, and so the intensity of the beam is low, and lensing and modification of absorption are not observed. However, for higher intensities the sample acts as a thin lens due to the fact that the refractive index in the sample varies as n = no + n2l(r,z), where ri2 is the nonlinear refractive index (related to / ) and l(r,z) is the intensity. For a positive value of 2, as the sample approaches the focal plane... 

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