Feeds:
Posts
Comments

Posts Tagged ‘He-Ne laser’

Laser_squirrel_hd

An image from Zynga’s Mafia Wars, obviously.

One way to say it is…

Let’s say we have a bunch of squirrels congregating underneath a tree.  We can startle the squirrels, and they jump up into the branches of the tree.  But these are ground-loving squirrels, so they very quickly jump back down—sometimes to a lower branch, sometimes all the way down to the ground.

An interesting fact: the squirrels scream as they fall.  This isn’t because of fear, so there’s no need to alert PETA; they’re just emitting squeals of delight as they plummet.  And the frequency of their squeals (and hence the musical note produced) depends upon how far they fall.  A larger drop, a higher frequency.

My goal is to have a whole bunch of squirrels scream with the exact same frequency, all at once.  How can this be done?

Clearly, I need a bunch of squirrels all on the same branch, and I have to hope they all jump off that branch all at the same time.  But this is trickier than it sounds.

It turns out that our tree, a northern elm (Ne for short) has lots of branches, most of them slippery.  In most cases a squirrel will jump off such a branch almost immediately.  However—and this is lucky for us—the 8th branch from the bottom is not-so-slippery.  Squirrels actually like to hang out on this branch for a little while.  Squirrels, being squirrels, do succumb to peer pressure, though, so when one squirrel eventually jumps to the next-lowest branch, the rest of the squirrels follow suit—all screaming in unison—producing a nice, loud, resonant scream of 4.7 x 1014 Hz.

Here’s the problem: when we initially scare the squirrels into the tree, they don’t all jump up to that 8th branch.  Why would they?  They jump up at random, and only a fraction land on the branch we want.  Even if a sizeable number then jump down all at once, producing the desired sound, it is drowned out by all the other screams and squeals of all the other jumping squirrels on all the other branches.  This isn’t what we want.

But maybe we can be super clever.  Let’s get another tree, let’s say a hemlock (He for short), and place it next to the Ne tree.  Why hemlock?  The cool thing about hemlock is that there’s basically only one branch that squirrels can reach (the other branches are just too high).  And here’s the luckiest coincidence of all: this single branch of the He tree is at almost the exact same height as the 8th branch of the Ne tree (the branch that the squirrels kind of like).

So here’s what we do.  We scare a bunch of squirrels beneath the He tree, and they all jump up to that lone branch (they don’t have a choice.)  We then slide the He tree next to the Ne tree.  The naturally curious squirrels climb over to the Ne tree, because the branches are at the same height.  And guess what—the conditions are now just right for our squirrels-screaming-in-unison trick!  We have a large population of squirrels on a not-so-slippery branch, and when the peer pressure clicks in—eeeeeeeekkkk!

Another way to say it is…

Let’s say we have a bunch of electrons in atoms in a gas.  We can excite the electrons with an electric field, and they jump up into higher atomic energy levels.  But being electrons, they very quickly jump back down—sometimes to a lower energy level, sometimes all the way down to the ground state.

An interesting fact: the electrons emit photons as they fall.  And the frequency of these photons depends upon how far they fall.  A larger drop, a higher frequency.

My goal is to have a whole bunch of photons emitted with the exact same frequency, all at once and in phase.  How can this be done?

Clearly, I need a bunch of electrons all on the same energy level, and I have to hope they all jump off that level all at the same time.  But this is trickier than it sounds.

It turns out that our gas, neon (Ne for short) has lots of energy levels, most of them “slippery”.  In most cases an electron will jump off such a level almost immediately.  However—and this is lucky for us—the 8th branch from the bottom (the 5s energy level) is metastable.  Electrons actually like to hang out on this level for a little while.  Electrons, being electrons, do succumb to peer pressure, though, so when one electron eventually jumps to the next-lowest branch, the rest of the electrons follow suit—in a process called stimulated emission—producing a nice, intense, in-phase cascade of photons with f=4.7 x 1014 Hz (about 633 nm, which is ruby red).

Here’s the problem: when we initially excite the electrons in Ne, they don’t all jump up to that 5s level.  Why would they?  They jump up at random, and only a fraction land on the level we want.  Even if a sizeable number then jump down all at once, producing the desired lasing frequency, it is drowned out by all the other photons emitted by all the other jumping electrons on all the other levels.  This isn’t what we want.

But maybe we can be super clever.  Let’s get another gas, let’s say helium (He for short), and place it in the same container as the Ne.  Why helium?  The cool thing about helium is that there’s basically only one energy level that electrons can reach (the 1s2s level).  And here’s the luckiest coincidence of all: this single energy level of He is almost the exact same energy as the 5s state of Ne.

So here’s what we do.  We excite a bunch of electrons in He, and they all jump up to the 1s2s level  (they don’t have a choice.)  We then mix the He with Ne.  Through collisions, many of the electrons in the 1s2s level state of He are transferred to the 5s state of Ne.  And guess what—the conditions are now just right for our cascading electrons trick!  We have a large population of electrons in a metastable state (a population inversion), and when stimulated emission clicks in—we get coherent laser light!

Whether or not those electrons raid your bird feeders for sunflower seeds is another issue entirely.

[Note: my book Why Is There Anything? is now available for download on the Kindle!]

Advertisements

Read Full Post »