Self-phase compensation

From Figure 9.3, the 90° MTN cell exhibits a good dark state, similar to a transmissive TN cell because of the self-phase compensation effect of the orthogonal boundary layers. However, its maximum reflectance is only 88%. On the other hand, the 75° MTN cell has nearly 100% reflectance, but its dark state has a slight light leakage. The contrast ratio at 5 Vrms is around 100 1. This is because the boundary layers are not perfectly compensating each other. 

This setup provides a flat dispersion by the mirrors and dispersion compensation by the prisms. The curved mirrors focus the pump beam into the laser crystal X. The glass plate P in the second focus causes self-phase modulation which results in a significantly wider spectrum of the laser emission and thus a shorter pulse [695]. 

The reason for this absorption-free propagation can be understood as follows The first half of the 2 r-pulse is absorbed and creates a population inversion. Therefore the second half of the pulse causes stimulated emission which just compensates the absorption of the first half (self-phase-modulation). The absorption of the first half pulse area and the amplification of the second half restores the 27t-pulse. But this restored pulse is delayed for each cycle by r/2 against a weak pulse starting at the same time. The group velocity of solitons is therefore smaller than for weak pulses. 

Since short pulses are spectrally broad, one might assume, that the pulse width increases due to linear dispersion. However, for solitons this linear dispersion is exactly compensated by the nonlinear self-phase-modulation. 

The trick introduced by Meiboom and Gill (14) is to dephase all n pulses in the Carr Purcell train by an angle of 90° with respect to the initial ti/2 pulse. It is easily shown that, without this phase change, imperfections of the 71 pulses are cumulative, whereas with the 90° phase change, a self-compensation occurs for all echoes of even number. The CPMG (Carr-Purcell-Meiboom-Gill) experiment can be handled in two ways ... 

II reaction under similar conditions at temperatures between 80 and 100°C and with a four-fold excess of 2-methylpentanal (to compensate for the low solubility), the selectivity for the Aldol II product (80%) was 20% higher in [BMIMJEF NaOH than in the water/NaOH system, both at 100% propanal conversion. The increased selectivity was attributed to the higher solubility of the reactant 2-methylpentanal in the ionic liquid phase than in the water phase. The higher solubility of 2-methylpentanal effectively suppressed the self-aldol condensation in the ionic liquid. 

Phase II describes saturated counterion condensation [47]. The polyelectrolyte charge is nearly compensated by the counterions [47]. The uncondensed counterions are dispersed in a self-similar fashion throughout the cylindrical region [47]. In fact, the number of counterions bounded between radius r and, say, 2r is independent of r [47]. 

The thermod3mamic and kinetic aspects of hole conduction self-compensation were analyzed in detail.It was concluded that the main reason for the monopolar conduction in the compounds under consideration is that the chalcogen is present in the vapor phase in molecular form. It was also shown that the compensation of acceptor centers in II-VI compounds can be suppressed by reducing the s mthesis or doping temperature to below a critical temperature, if the process is run in saturated chalcogen vapor. 

The correlation of the electrons at different JT centers caused by the virtual phonon exchange leads at some temperatures to the ordering of the local distortions and of the self consistently coupled to them electronic states (orbitals). This happens when the loss in the elastic energy and entropy at the ordering is compensated by the gain in the energy of the crystal electronic subsystem. At this case the electronic order parameter (an average of a pseudospin operator) of the phase transition becomes different from zero and because of that the spontaneous lattice (sublattice) strain is also not zero. 

Figure 9.4. The 90" pulse is self-compensating with respect to resonance offset in its ability to generate transverse magnetisation, although there is no compensation for phase errors.

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