Lasers – population inversion and stimulated emission
PhysicsQuantum & Nuclear PhysicsAges 17–18
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Model a helium–neon laser (four levels, pumped by an electric discharge) and a ruby laser (three levels, pumped by a flash lamp). Raise the pump power until atoms pile up in the metastable level and the population is inverted; once the gain per round trip beats the mirror losses, stimulated emission builds a coherent beam. The single-atom view shows absorption, spontaneous and stimulated emission, and a third view compares the spectrum, phase and intensity of laser light with a lamp.
Lesson: Lasers: stimulated emission, population inversion and threshold
What it shows
A laser amplifies light by stimulated emission: a photon passing an excited atom makes it emit an identical photon, with the same wavelength, phase and direction. To get more emission than absorption, more atoms must be in the upper level than in the lower one (population inversion). Pumping lifts atoms to a high level, from which they drop quickly into a long-lived metastable level. Two mirrors send the photons back and forth; the laser starts when the gain per round trip is larger than the losses. A four-level laser such as He–Ne reaches threshold far more easily than three-level ruby. Gains and output are illustrative.
How to use
In the Laser tab, choose He–Ne or Ruby, raise Pump power and change Output mirror R; watch the level populations and press Record point to plot output against pump. In Single atom, press Pump, then Send a photon to see absorption or stimulated emission. In Laser vs lamp, move Distance d to compare intensities.
Parameters you can change
- View Laser, Single atom, Laser vs lamp
- Laser medium He–Ne (gas), Ruby (crystal)
- Pump power 0–100 %
- Output mirror reflectivity R 80–99.9 %
- Distance to the screen 1–1000 m
Questions to explore
- Why must the upper laser level be metastable?
- Why does the ruby laser need a much stronger pump than the He–Ne laser?
- Why does the output mirror let a little light through instead of reflecting it all?
