Friday, November 9, 2012

Science Term of the Week: Laser

A guest post by Angela Calchera

Lasers have been used by scientists, surgeons, evil villains, the Jedi, and by non-technical non-evil non-Jedi people, but what are they really, and how do they work?  The first thing you must understand is that the word "laser" is really just an acronym for what a laser does.  It stands for:

Amplification by
Emission of

So a laser amplifies light in much the same way that an amplifier at a rock concert might amplify the sound of the guitars on stage, and it does this by a process called Stimulated Emission of Radiation.

It's kind of a mouthful, which is why they packaged it all up in an acronym.  And actually, a lot of those concepts could use some further explanation.  You need four things to make a laser:
  • Light Source
  • Gain Medium
  • Cavity
  • Output Coupler
Let me walk you through each of these components, using the titanium sapphire (Ti:Sapphire) laser I use for my research as an example.

Light Source
Every laser has a light source.  Lasers don't  make light, they amplify light.  That means you have to have some light to put into it in order to get something out, just like you need some sound in order to make it louder with an amplifier.  The light source is said to pump the laser, promoting electrons in the gain medium from the lowest energy state into higher energy states.  In the first laser, the light source was a flash lamp, which produces bright bursts of white light like a camera flash.  In the Ti:Sapphire laser in my lab, the light source is actually another laser.
Figure 1. Generalized laser diagram
Gain Medium
The gain medium is where the "magic" happens.  At room temperature nearly the entire population of electrons in a given material are in the lowest possible energy level, also known as the ground state.  In order for the laser to operate, it must achieve a population inversion, meaning that more electrons need to be in an upper level than in a lower level.  In some materials, this is impossible.  For example, if a material only has two energy levels, the best we can do is to put half of the electrons in the upper level and half of the electrons in the lower level; the population of the upper level can never exceed that of the lower level as we need it to in a laser gain medium.  It turns out, three levels is the minimum to make population inversion possible.

In a titanium sapphire laser, the gain medium is sapphire with (intentional) titanium impurities.  In this material, there are four levels the electrons can occupy.

Figure 2: Energy Diagram of Ti:Sapphire
Step 1. First, light comes in from the light source, and that promotes some electrons from the lowest energy level into the upper-most energy level.

Step 2.  Electrons don't stay in that level for long at all, and very quickly move down into the second-highest state.  The electrons lose this energy without any light being produced.  Because this step is very fast, and the next step is very slow, many more electrons end up in this level than in the level below it, which is the population inversion we needed.

Step 3.  This step is the one that gives the laser its name.  Photons (packets of light energy) of the right energy cause (stimulate) the electrons in that level to go down to the lower level.  As those electrons "fall," they release (emit) light energy.  The photons coming in did not get absorbed, their only purpose was to stimulate the emission of more photons, so the emitted photons add to the total number of photons coming out of the gain medium.  So, you see this is the amplification step because the total number of photons is amplified here.  Also, this amplification came by the stimulated emission (as opposed to spontaneous emission, which would occur without the photons coming in to get it started) of radiation (light waves/ photons are a form of radiation).

Step 4.  Electrons very quickly return to the lowest energy state without emitting any light.

The name of a laser pretty much always comes from the type of gain medium used.  In a titanium sapphire laser the gain medium is a nice pink crystal.  You may find it odd that the sapphire is pink and not blue.  Sapphire without impurities are actually completely colorless - the colors come from impurities in the crystal structure by changing the energy levels that electrons can occupy, but that's for another post.

Figure 3. Ti:Sapphire gain medium

Cavity and Output Coupler
A mirror is placed on each side of the gain medium so that the light passes through the crystal multiple times, and with every pass that light is amplified.  The two mirrors make up the ends of the cavity.  If both mirrors reflected all the light, the amplified light would be useless because it couldn't escape the cavity, and so one of the mirrors in the cavity, also called the output coupler, is not as reflective and allows some light to be transmitted out of the cavity.

The invention of lasers and their improvement have led to many advances in physics, chemistry, materials science, environmental science, manufacturing, medicine, and media storage, among other areas.  However, it is interesting to note that when they were first invented, they were considered a novelty that had no practical applications.  For this reason, lasers are a great example of why it is important that we invest in fundamental research, even if it doesn't have an apparent practical application.  You never know what lab project will turn out to be yet another versatile tool to enhance your life.

Also, no, I can't make you a light saber.