The laser — stimulated emission
An extension of the topic of atomic energy levels and the photon. When pumping energy drives the medium into population inversion (more atoms at the higher energy level than at the lower one), a single photon stimulates the emission of further, identical photons — and a coherent, single-colour laser beam is formed (Nobel Prize 1964).
I will give you the upper-level E₂ population — identify the regime of the medium (no slider). Inversion threshold: 50%.
What just happened?
We raised the pumping above the threshold — more atoms ended up at the higher energy level than at the lower one. This reverses the ordinary, "natural" population distribution and triggers an avalanche of photon emission.
Population inversion:
— N₂ is the number of atoms at the higher energy level E₂, N₁ at the lower level E₁. Normally (without pumping) N₁ > N₂ always — this is the Boltzmann distribution. You have to supply energy from outside ("pump") to reverse it.
Stimulated emission — why is the beam coherent?
When there is inversion, one passing photon "knocks" an excited atom down to the lower state, stimulating the emission of another photon — with exactly the same phase, direction and wavelength as the triggering photon. This repeats as an avalanche:
In this way a coherent, single-colour beam is formed — this is exactly what distinguishes a laser (Light Amplification by Stimulated Emission of Radiation) from a light bulb, in which atoms emit photons independently, at random phases and directions.
Why isn't a light bulb enough?
In a light bulb there is no population inversion — atoms glow spontaneously and independently of one another, so the emitted light is incoherent (different phases, directions, wavelengths). Without inversion there is no stimulated emission, and without that there is no laser.
Why you need this for the exam:
Remember the necessary condition for a laser to work: first pumping, then population inversion, and only then stimulated emission and a coherent beam. This discovery (Townes, Basov, Prokhorov) was awarded the Nobel Prize in Physics in 1964.
Remember: without population inversion there is no laser — the photons of stimulated emission are identical: the same phase, direction and wavelength.
Theory & formulas
Population inversion
Normally more atoms are at the lower level. Pumping energy reverses this — a population inversion appears (more atoms at the higher level).
Stimulated emission
A passing photon of energy stimulates the emission of another, identical photon (same phase, direction, wavelength). An avalanche builds up → a coherent, single-colour laser beam (Nobel Prize 1964).
Notation: — photon energy, — difference between energy levels, — Planck constant, — frequency.
Frequently asked questions
›What is population inversion?
A state in which more atoms are at the higher energy level than at the lower one.
›How does stimulated emission differ from spontaneous emission?
Stimulated emission produces identical photons (same phase, direction, wavelength) — hence the coherence of laser light.
›Why is laser light so special?
It is coherent and single-colour — all photons share the same phase and wavelength.
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