粒子数反转，粒子指的是什么？

是电子吗？为什么不是离子

应当是原子数吧。。。处于不同能级的原子数~~

electrons，掺杂原子的外层电子

是电子，反转是高能级的粒子数大于低能级的粒子数，从而可以跃迁到低能级而发出光子！

当然是电子啊

原子不可能的啊

处于不同能级的电子数

相当于把一楼的的水抽到六楼，使水具有了重力势能啊

应该是电子。电子吸收泵浦能后，跃迁到高能级。
离子中实际上也是电子在起作用。

© Dr. Rüdiger Paschotta
Last update: 2008-06-30
Four-level and Three-level Gain Media previous  |  next  |  feedback
Ask RP Photonics for advice on any aspects of quasi-three-level lasers. Definition: laser gain media without/with reabsorption from the lower laser level

Optical amplification in the gain medium of a laser or laser amplifier arises from stimulated emission, where the input light induces transitions of laser-active ions from some excited state to a lower state.

Three-Level Systems
In a three-level system, the laser transition ends on the ground state. The unpumped gain medium exhibits strong absorption on the laser transition. Only by pumping more than half of the ions (or atoms) into the upper laser level do a population inversion and consequently net laser gain result; the threshold pump power is thus fairly high.
从这段话看不像是电子阿

课本上说是原子。。。。。。

说 电子，离子都可以吧，对搞激光的来说，只要把它理解为可以吸收或发射光子的最小单位就可以了。

非常同意楼上的意见~~~~~