Inverted transitions

Phase-preserving transition state (11) Phase-inverting transition state (12)... 

As we shall see below, the solvent plays a crucial role in determining which form is stable at low temperatures. When the helix is stable at low temperatures, the transition to the random coil at high temperatures is called a normal transition. For the case in which the random coil is the more stable form at low temperatures, the transition is called an inverted transition. 

Figure 13, together with the transition dipole moments and transition rates, provides a simple interpretation of the absorption rate shown in Fig. 11. According to Eq. (139), the emissive peak in the absorption rate appears on an almost completely inverted transition ( a,iV) — —, N — 1)), whose dipole moment is significantly reduced by quantum interference. One could expect that the weaker field should not couple to an almost canceled dipole moment. However, we have assumed that the probe field couples only to the dipole moment p12. From Eq. (136), we find that the coupling strength of the probe field to the transition a. (V) —. /V 1) is proportional to j (Sp12 despite the...

Thus, in terms of the quantum dressed states, the gain features predicted by Menon and Agarwal [48] actually appear on completely inverted transitions whose dipole moments are canceled by quantum interference. Therefore, the gain features can be regarded as the amplification on dark transitions [51]. 

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