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Atomic and Condensed Matter
Confused about fluorescence at resonance
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[QUOTE="f95toli, post: 6347441, member: 87679"] You need to be very careful about which model you are using and which physical situation you are trying to understand. One thing to keep in mind is that QM is all about probabilities. If you measure a single system only once you will always find it in [I]either [/I]the excited [I]or [/I]ground state. The equation for Rabi oscillation tells you the [I]probability [/I]to detect the state in either of these two states. This means that an experimental curve is always an [I]average [/I]created by either measuring a single TLS many, many times or (less often) by measuring lots of TLS (an ensemble) at once. The "basic" Rabi model is for a [I]single [/I] two-level system (TLS) and does not take decoherence into account (T1 and T2 are both infinite). In this context the latter means that if you pump your two-level to the excited state it will stay there forever. A real TLS will spontaneously relax (transition back to the ground state) with some characteristic time T1 and de-phase (time T2). This also means that the amplitude of real Rabi oscillations will decay with a time constant T1 (which btw how T1 is measured). Now, if you apply lots of power you can make sure that the system is immediately excited again once it has decayed. If you plot the power dependence you will find that you then reach a point where the dependence becomes nearly flat and this is what is known as saturation. If you work through the math you will find that the amount of power that is needed for saturation depends on T1*T2. But again, this makes no sense in the basic Rabi model since there it is assumed that T1 and T2 are infinite. This is probably the source of your confusion. In lasers and similar systems you are typically dealing with ensembles of TLS but math is similar. However, instead of measuring the same system many times, you are now usually measuring lots of identical systems. Now, you [I]can [/I]observe regular Rabi oscillations in ensembles of TLS as well but the reasons for why this works are often complicated (you need to take interactions between different TLS into account) and goes well beyond the basic model. [/QUOTE]
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Confused about fluorescence at resonance
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