I started reading about laser physics, in various cases (online as well as in books) the atoms are first excited to an "excited" state which is less stable and decays into a more stable "meta-stable" state, hence populating meta-stable state wrt ground state. My question is why atoms are not directly excited to the meta-stable state, why should they be first excited to a higher energy state? Is there some kind of law that prevents direct excitation to the meta-stable state??
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Related: Lasing in a 2-Level system? – Ruslan Apr 27 '21 at 21:32
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To be metastable means, generally, that the transition is not directly (easily) allowed. So, it is unlikely to go from the ground state to the metastable state, but easy to go to the higher state. The transition from the higher to the metastable state does not obey the same transition rules. – Jon Custer Apr 27 '21 at 23:02
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In the three-level configuration, the lifetime of the meta-stable state needs to be very large. If it is not true, then it's impossible to create population inversion: atoms at this level (which we used to call meta-stable) will simply relax to the ground state quickly. Now, large lifetime means small dipole transition element; and thus it is very hard to excite atoms to this meta-stable state directly from the ground state. This is why we need excitation through the third (higher) level: transition from the ground state to this level is probable, and its lifetime is low, resulting in the relaxation to the meta-stable level.
Another way of seeing it is that, if we get rid of the higher level, we essentially go from the three-level to the two-level system. In the latter, it is impossible to create the population inversion due to spontaneous emission (Einstein A coefficient). No population inversion - no laser.
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To your second point, since the lifetime of meta-stable state is high, will not be the rate of spontaneous emission would be much smaller than of absorption. If so, won't there be population inversion? – the_akshit Apr 27 '21 at 18:36
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No because there is also stimulated emission (going 2->1) given by the same Einstein coefficient B as absorption (going 1->2). So you won't be able to create population inversion on a single transition; at most, when the population at 1 and 2 are equal, photon emission (A+B) is more probable than absoprtion (B) so population at the 2nd level will decrease (due to 2->1 transitions) faster than grow (through 1->2 transitions). I neglect the possible levels degeneracy and consider T=0 here. – sleepy Apr 27 '21 at 18:44