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Population Inversion in Physics: Definition & Example

Instructor: Sadije Redzovic

Sadije has taught high school physics and physical science. She has a bachelor’s in physics and a master’s in biomedical engineering.

In this lesson we will lean about the fundamentals of population inversion. This will include defining the term coherent as it relates to light and discussing an application of population inversion, lasers.

What Makes a Laser Different?

Lasers are used in numerous aspects of modern life: from telecommunications to medicine. Even if you have never worked with a laser, you have probably still played with a laser pointer.

When you have been around a laser, have you ever thought about what makes a laser beam different from light emitted by other typical light sources? Some common answers to this question are color and power. These answers are certainly on the right track, but an even better answer to this question highlights the fact that laser light is coherent.

Argon-ion laser that is used in the treatment of cancer
Picture of laser

Here is a simplified way to think of coherent light: the photons that make up the light beam are in phase, have the same frequency, and are traveling in the same direction. This is not a complete definition of coherence, but it does provide a good basis for delving further into our lesson.

The production of coherent light that makes up a laser beam is achieved through a process known as population inversion. Let's see how this works.

Population Inversion

Before defining population inversion, let's first review the concept of stimulated emission. In stimulated emission, an electron that's in a higher energy state interacts with a photon that stimulates it to return to a lower energy state. When this occurs, a photon is released. The incoming photon and the released photon are coherent.

Stimulated emission - an electron in a higher energy state interacts with a photon that stimulates it to return to a lower energy state, and a photon is released.
illustration of stimulated emission

Population inversion occurs when more electrons, in a particular situation, are in a higher energy state than in a lower energy state. Population inversion can be thought of as an inversion from the standard, since electrons are typically located in lower energy states.

Population inversion involves more electrons in a higher energy state than a lower one.
illustration of population inversion

The true power of population inversion comes from the fact that the input of a single photon can lead to an important end result. That is, all of the higher energy electrons will 'drop' to a lower energy state.

With each electron 'drop', a photon that is coherent with its neighbors is released. The incoming photon does not directly interact with all of the electrons that are in higher energy levels, but it causes the release of additional photons that further interact with more of the excited state electrons. Basically, population inversion leads to an 'amplified' amount of stimulated emission.

Population inversion: release of coherent photons
illustration of population inversion photon release

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