Conceptual Physics

normal population. Some of the atoms are at higher energy levels, but most are not. This is typical of a material that is in thermal equilibrium

with its surroundings, such as one at room temperature.

When photons strike the material, most of them are absorbed. This raises the energy levels of some of its atoms, which then spontaneously

emit photons in random directions. Occasionally, there is stimulated emission when a photon encounters an atom that already contains an

excited electron, but this is rare. This is no way to run a laser!

In contrast, the medium above has a population inversion. A photon is far more likely to strike an excited atom, and stimulated emission

becomes commonplace. Placing the medium in a reflecting container can further aid this process. Photons will reflect off the container walls

and strike other excited atoms, causing a “chain reaction” of stimulated emissions. Energy can be continually added by pumping the medium,

ensuring a ready supply of excited atoms.

Population inversion

Higher-energy atoms outnumber lower-

energy atoms

When light shines into medium

·More photons emitted than “fired in”

·Light is amplified

36.20 - Physics at work: operating a laser

Today, there is a variety of ways to create a

population inversion that enables light amplification.

To discuss one, let’s consider a type of laser that was

developed early on, in 1960 and 1961, and remains

common, the helium-neon (He-Ne) laser. A glass tube

is filled with helium and neon gases. An electric

current passes through the tube, and its electrons

collide with helium atoms, raising their energy levels

from their ground state E 0 to an elevated state EA.

In this state, the helium atoms are said to be at a

metastable level. They will stay there for a relatively

long period of time, as opposed to quickly and

spontaneously emitting photons and falling back to a

lower energy level. (Relatively long, in this context, means on the order of a thousandth

or a ten-thousandth of a second.)

The helium atoms, including those at the metastable level EA, are continually colliding

with neon atoms in the gas mixture. The energy level of a metastable helium atom is

very close to that of a high-energy neon atom. In a collision with a ground-state neon

atom, a metastable helium atom causes the neon to rise to energy level EB while the

helium atom itself returns to the ground state. This is shown in Concept 1. Neon atoms

atEB are also metastable; they will persist for a relatively long time at this elevated

energy level.

Why not just let the electric discharge excite the neon atoms, and skip the helium?

Helium is required since neon atoms do not respond readily to electron bombardment

by the current that excites the helium atoms.

So far, we remain in the dark, so to speak. Many neon atoms have had their energy

raised to EB. The population has been inverted: There are more neon atoms at EB than

atE 0. The next goal is to stimulate the neon atoms to emit photons, dropping to the

intermediate energy level EC in the process. To accomplish this, light is beamed into the laser medium consisting of photons having an energy

corresponding precisely to the difference between EB and EC. These photons interact with neon atoms whose electrons are at energy level

EB, causing stimulated emissions: The EB neon atoms each emit a photon of the same frequency, and drop to energy level EC. The atoms

undergo what is called a laser transition.

It is important to note that the energy of each stimulating photon must equal the energy difference between EB and EC in order for the laser to

function. Here is where quantum physics comes into play. A precise understanding of the energy levels of helium and neon atoms is required

to design a successful He-Ne laser, as is knowledge of the relationship between the energy and frequency of a photon.

The additional photons that are emitted interact with other energized neon atoms, causing further stimulated emissions. Mirrors on both ends of

the laser reflect these photons back into the laser medium, along the axis of propagation. The distance between the mirrors is crucial since it

ensures the constructive interference of the light. They require extremely precise fabrication, and reflect more than 99.5% of the photons that

hit them.

One last detail: How about the neon atoms that are now at energy level EC? They rapidly and spontaneously decay to their ground state,

He-Ne laser in a laboratory.

Laser process: part 1

Energy of helium atoms raised

Energy transferred to neon atoms

Population inversion results

(^682) Copyright 2007 Kinetic Books Co. Chapter 36