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# laser2 - Basic idea of a laser Exploit positive feedback...

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Basic idea of a laser: Exploit positive feedback inherent in stimulated emission: Photon + Excited atom (B) ! Two photons + ground state atom (A) This could lead to “lasing”: an avalanche of photons at the selected frequency: " = # E/ ! Problem: the old and new photons will double the chance of attenuation of these photons by their absorption by the lower state A (since absorption rate proportional to # photons at this frequency). Population inversion Recall the expression for the ratio of populations in states B, A, with # E= E B - E A >0 N B /N A = exp[- # E/(kT) This means that N B < N A . However, this means that the rate of absorption (upward transition, A ! B) exceeds that of spontaneous emission (downward, B ! A) \$ photon system is attenuated and no lasing occurs! This problem can be solved, in principle, if the state population is “inverted”, meaning that there are more atoms in the upper state than in the lower state. Sometimes the situation where N B > N A is called “negative temperature”. I dislike that

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laser2 - Basic idea of a laser Exploit positive feedback...

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