Phase and amplitude imaging

PHASE AND AMPLITUDE IMAGING OF EVOLVING WAVEPACKETS BY SPECTROSCOPIC MEANS... 

In spite of the apparent sensitivity to the material properties, the direct assignment of the phase contrast to variation in the chemical composition or a specific property of the surface is hardly possible. Considerable difficulties for theoretical examination of the tapping mode result from several factors (i) the abrupt transition from an attractive force regime to strong repulsion which acts for a short moment of the oscillation period, (ii) localisation of the tip-sample interaction in a nanoscopic contact area, (iii) the non-linear variation of both attractive forces and mechanical compliance in the repulsive regime, and (iv) the interdependence of the material properties (viscoelasticity, adhesion, friction) and scanning parameters (amplitude, frequency, cantilever position). The interpretation of the phase and amplitude images becomes especially intricate for viscoelastic polymers. 

The factor A has been measured for a variety of samples, indicating that the approximation can be applied up to quasi-atomic resolution. In the case of biological specimens typical values of are of the order of 5-7%, as detemiined from images with a resolution of better than 10 A [37,38]- For an easy interpretation of image contrast and a retrieval of the object infomiation from the contrast, such a combination of phase and amplitude hifomiation is necessary. 

Figure Bl.18.6. Schematic representation of Zemike s phase contrast method. The object is assumed to be a relief grating in a transparent material of constant index of refraction. Phase and amplitude are varied by the Zemike diaphragm, such that an amplitude image is obtained whose contrast is, m principle, adjustable.

The interferometric imaging method of synthetic aperture image reconstruction, described in detail in Section 4, detects the THz electric field at multiple locations and then uses the correlated phase and amplitude of the electric field from selected pairs of detectors [72],... 

Usually the rf pulses applied in an imaging sequence are not perfect. The pulses may differ in shape, phase, and amplitude from the required values. As a result, unwanted transverse magnetization may arise which leads to erroneous signals. This is so, in particular, for the 180° refocusing pulse of the Hahn echo. These transverse magnetization components can be dephased by application of homogeneity-spoil or crusher gradient pulses. 

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