If you shine ultraviolet light upon certain minerals, such as calcite, they fluoresce, that is, they glow as long as the energizing ultraviolet
radiation continues. A mineral absorbs high-energy photons of radiation and then spontaneously emits photons in a series of steps. These
minerals only absorb and emit certain wavelengths of light, due to the quantized nature of their energy levels, so they glow a particular color.
Spontaneous emission is an interesting topic but not the controlled process desired in lasers. If all the laser medium did was to absorb light and
then spontaneously re-emit it in random directions, there would be no amplification of the light.
Stimulated emission is the key to lasers. Below, you see an atom that has already been excited to a higher energy state.
A photon of the appropriate frequency passes close to the excited atom. The result: The atom emits its own photon of the same frequency and
returns to its initial energy state. Here is the “gain” produced by lasers: One photon of light causes a net result of two: the process of light
amplification by the stimulated emission of radiation. The presence of one photon causes another to be emitted.
We use words like “passes close” and “presence” because photons do not “collide” with atoms during stimulated emission, but they do cause
the atom to emit a second photon when the energy of the stimulating photon corresponds to a difference in energy levels allowed in the atom.
In sum, with stimulated emission, you start with one photon, and end with two.
To explain further what is occurring in a laser medium, we will use two analogies. The simpler one is mechanistic. Imagine a pool ball that has
been raised off the ground and placed on the surface of a flat kitchen table. This increases its gravitational potential energy and corresponds to
an atom that has been excited. A second pool ball is rolled at the first and both of them fly off the table. The combined kinetic energy of the two
balls as they reach the ground is greater than the original kinetic energy of the ball that was rolled. The collision has “unleashed” the potential
energy of the pool ball that was resting on the table.
There is a limitation here, perhaps an issue that concerned you as you considered this metaphor and applied it to a large number of pool balls,
in order to make it more similar to the many atoms in the laser medium. The process would not produce a coherent stream of pool balls. If you
did this experiment with many pool balls, they would fly off the table in many directions.
It is better to think of the light as a wave and to consider the phenomenon of resonance, as exhibited by mechanical or electromagnetic waves.
Quantum physics states that atoms act like electromagnetic oscillators with particular resonant frequencies. The passing light wave causes the
atoms in the medium to begin vibrating in a resonant, coherent relationship. The oscillations are driven by, and in phase with, the stimulating
light wave. The atoms respond to the incoming light and reradiate like tiny antennae. The reradiated waves reinforce the waves that cause
them.
Stimulated emission
Photon passes near excited atom
Photon triggers emission of 2nd photon
Two photons leave atom36.19 - Population inversion
Population inversion: The number of excited atoms is greater than the number
of lower-energy atoms.
Stimulated emission is required for lasers to function. For there to be stimulated emission, photons must be passing by and interacting with
excited atoms in a medium, from which they can cause coherent photons to be emitted, rather than just passing by atoms whose electrons are
at lower energy levels. Not only do the low-energy atoms fail to participate in stimulated emission, but they may also sabotage the process by
absorbing photons.
Under normal circumstances, more atoms in a medium will be in lower energy states than in higher energy states. A population inversion is
required: The majority of the atoms must be pumped to a higher energy level. When photons strike a medium with an inverted population,
stimulated emission is more likely to occur.
To illustrate the need for an inverted population, we start by showing you the opposite in the diagram above: photons striking a material with a
An “unexcited” population
Lower-energy atoms outnumber higher-
energy atoms
When light shines into medium
·More photons absorbed than emitted
·Light is not amplified