## LS: Speaking of Science Column Janujary 26, 2009

### Lasers start with excited atoms

(Editor’s note: The is the first of a three-column series on lasers.)
Lasers have become commonplace in our lives. The thin red line of light that scans your purchases at a checkout counter is generated by a tiny laser smaller than your fingertip.

Have you ever wondered how that thin red line of light, like the one in a laser level or in an electric saw to aid alignment, is created? Why are these beams so much narrower, even at large distances, than the best flashlight? If you have a CD or DVD player, you own and operate a laser. In this series of three articles I will try to describe the basic physics of lasers.

Understanding how lasers work requires a very basic understanding of how atoms behave.

Atoms can contain only specific, well-defined amounts of energy. I’m going to use a down-to-earth analogy. I call it the “monkey and a tree atomic analog.”

Imagine a monkey by a tree; the tree has a very slippery trunk and well-spaced branches. If the monkey is lazy or tired, he can sit on the ground at the base of the tree. But when he is motivated, such as by an approaching tiger, the monkey likely will get excited and hop up to one of the branches.

If he is really excited and has enough energy, he can hop to one of the higher branches.

When the monkey is sitting at the base of the tree, we could say he is in his “ground state.”

When he sits on a branch, we could say the monkey is in an “excited state.”

Atoms behave in an analogous way. When the atom has the least amount of energy it can have, it is said to be in its ground state. When an atom contains more than the minimum possible energy, we say that it’s in an excited state (the atom sits on a higher “branch”).

Atoms cannot contain any arbitrary amount of energy, and the energy that a particular kind of atom can contain is specific to that kind of atom. It is like the monkey in our fictitious tree that has to sit on a branch, not in between.

Eventually, our monkey in the tree will tire of the excitement (when the tiger goes away), and he will drop down to a lower branch or the ground. Similarly, atoms don’t stay excited.

Eventually, they, too, return to their ground state. How long they take to “decay” to the lower energy state varies widely with the types of atoms and the specific energy states involved.

When an atom changes between energy states, we say that it has undergone a “transition.”

When developing lasers we are often looking for atoms which have a transition which occurs relatively slowly.

Just as it takes the monkey a certain amount of energy to hop to a higher branch, a specific amount of energy must be added to an atom to put it in an excited state. This energy can be added by light. When exciting atoms with light, they are fussy. The amount of energy must be just the right amount to take it from the ground state to one of its permitted excited states.

As it turns out, a dude named Albert Einstein recognized about 100 years ago that light is composed of particles called photons, which have specific well-defined energies. When an atom absorbs energy from a beam of light, it readily gobbles up only those photons that have exactly the energies corresponding to the spacing between its energy states.

An atom cannot take a bite out of a portion of a photon.

In the next article we’ll discuss how we make use of these characteristics of atoms to build a laser.

Jack Kingsley received a doctorate degree in physics from the University of Illinois. He spent his career in research and development with General Electric Co. in Schenectady, N.Y.

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