Tuesday, February 02, 2010

LaserFest 2010

This year, the laser will turn 50! On May 16, 1960, at the Hughes Research Laboratories in Malibu, California, Theodore Maiman realized for the first time "Light Amplification by Stimulated Emission of Radiation", using a tiny ruby crystal.

Actually, Maiman and his small group of coworkers was back then just one of several teams, all at industrial laboratories, intensely searching for ways to create laser beams. At the end of the year, the ruby laser was replicated and improved, and lasing was realized using other crystals, and helium-neon gas mixtures. So, it's just fair that the American Physical Society, the Optical Society, SPIE, and the IEEE Photonics Society have decided to organize a yearlong celebration of the 50th anniversary of the laser - that's LaserFest.

But in fact, the path to the laser had begun much earlier.

Berlin, 1916

In the summer of 1916, Albert Einstein took a break from general relativity and cosmology and tried to make sense, once more, of the riddle of the quantum. Specifically, he thought about ways to combine the recent ideas of Bohr on discrete energy levels in atoms with the Planck spectrum of blackbody radiation.

Atoms in thermal equilibrium with radiation can absorb radiation, thereby transiting to a state of higher energy, and they can drop from an excited state to a state with lower energy spontaneously, thereby emitting radiation. Could it be, so Einstein's idea, that atoms also will transit from an excited to a lower-energy state when they are hit by radiation with suitable energy?

Indeed, assuming a thermal Boltzmann distribution for the states of the atoms interacting with radiation, and equal rates for absorption on the one hand and spontaneous and stimulated emission – as the newly stipulated process came to be called – on the other hand, as one would expect for a thermal equilibrium between the atoms and radiation, Einstein could reproduce the Planck formula for the spectrum of blackbody radiation. "A splendid light has dawned on me about the absorption and emission of radiation," he wrote in a letter to his friend Michele Besso on August 11, 1916.

Einstein's "splendid light" of stimulated emission of radiation: An atom in a state with energy E2 is hit by a photon with energy = E2E1. This can trigger a transition of the atom to the lower energy level E1, accompanied with the emission of a photon with energy , in phase with the initial photon. After this so-called stimulated emission, there are two photons instead of one, both in the same state – a nice manifestation of the "bunching" Bose character of photons.

It was recognized in the 1920s that theoretically the process of stimulated emission could result in "negative absorption", that is, amplification, of radiation, but nobody had a good idea how to demonstrate this effect in practice.

New York, 1954

To achieve amplification of radiation via stimulated emission, it is necessary to have more atoms in the high-energy state than in the low-energy state. Otherwise, a photon hitting an atom will more likely just be absorbed than trigger stimulated emission, and there is no gain in radiation. This requirement for amplification is called "population inversion".

In 1951, Charles Townes had an idea how to create "population inversion" in an ensemble of ammonia molecules. The ammonia molecule comes with two states which are separated by an energy corresponding to microwave frequencies. A beam of ammonia molecules can be split into two in an inhomogeneous electric field, separating molecules in the higher and the lower energy states, respectively, with an arrangement similar to a Stern-Gerlach apparatus.

In April 1954, Townes and his students Jim Gordon and Herbert Zeiger at Columbia University piped a beam of ammonia molecules in the higher-energy state into a microwave cavity resonating at the frequency of the energy difference between the two states, and obtained "microwave amplification by stimulated emission of radiation" - this was the birth of the maser.

Townes soon started to think about ways how to extend the maser principle to infrared or optical frequencies. With graduate student Gordon Gould, he discussed arrangements of mirrors around the medium in which population inversion is created, replacing the microwave cavity. These mirrors make sure that a beam of light is going back and forth through the medium many times, thus being able to "collect" ever more photons every time it crosses the medium.

Gould realized that such an arrangement, for which he coined the term "laser", could create sharply focussed light beams of extreme intensity, which could be used for communication, as a tool, or as a weapon.

As soon as the concept of the "optical maser", as Townes continued to call it, was explained in detail in a paper written together with Arthur Schawlow, many groups embarked on a race to be the first to actually construct such a device.

Malibu, 1960

Theodore Maiman had received his doctorate in Physics from Stanford University in 1955 to take a job at the Hughes Research Laboratories, which moved to Malibu in 1960. At Hughes, Maiman had constructed masers using ruby crystals, and when he learned of the possibility of the laser, he convinced himself that it should be possible to build a laser using ruby as the "lasing" medium.

Ruby is, chemically speaking, a crystal of aluminum oxide doted with chromium ions. The chromium ions have several energy levels which can be excited by irradiation with light, two of which are metastable and can be used as the upper level of a lasing medium. The energy of the transition to the ground state corresponds to red light with a wavelength of 694 nm.

Maiman's idea was to take a rod of ruby with parallel faces, to coat these faces with silver to realize the mirrors, and to put the rod inside a helical flashlight tube. The flashlight then excites the chromium atoms and creates population inversion, and the spontaneous emission of one photon can trigger an avalanche of photons by stimulated emission.

On the afternoon of May 16, 1960, Maiman and his assistant Irnee D’Haenens saw for the first time directed beams of intense red light emerging from the ruby - they had realized the first laser.

Theodore Maiman holding the first laser. It consists of a small ruby crystal and a helical flashlight which serves to stimulate the chromium ions of the ruby, thus creating the population inversion necessary for laser action. The ends of the ruby rod have been coated with silver to mirror back and forth the light stemming from stimulated emission, thus producing sufficient gain. The whole device is placed in the small white casing. (Source)


Maiman is reported to have said that “A laser is a solution seeking a problem”, Gould's visions notwithstanding. I have no specific idea how fast the laser was used for commercial or industrial purposes, but it immediately grasped public imagination.

When the movie Goldfinger is released in 1964, James Bond has to face a huge laser, looking similar to a scaled-up version of Maiman's first tiny ruby device, and replacing the buzz saw of Ian Flemings original 1959 novel. As Auric Goldfinger explains:

l, too, have a new toy, but considerably more practical. You are looking at an industrial laser, which emits an extraordinary light, unknown in nature. It can project a spot on the moon. Or, at closer range, cut through solid metal. I will show you.







At the LaserFest website, you can find a nice description of the mechanism of the ruby laser, and a video with explanations by Theodore Maiman himself. Moroever, there is a long interview with Charles Townes on the history of the maser and the laser.

If you want to know more about the history of the laser, there are two books I can recommend:
  • The history of the laser, by Mario Bertolotti, actually tells much more than just the story of the laser: It starts back at the beginning of the 20th century with the early atom models and the puzzle of blackbody radiation, and traces the path to the laser via spectroscopy, magnetic resonance, and the maser.

  • Beam: the race to make the laser, by Jeff Hecht, focusses on the developments of the late 1950s and 1960, beginning with just two brief chapters on the early history of stimulated emission and the maser. If you get lost in between all the names, there is a list of dramatis personae at the end of the book which I, unfortunately, discovered only after reading the text.

If you have Feynman's lectures at hand, there is a discussion of Einstein's derivation of the blackbody spectrum using stimulated emission and the Einstein coefficients in Section 42-5 of Volume I, and the whole Chapter 9 of Volume III is devoted to explain the principle of the ammonia maser.


17 comments:

Bee said...

Did you make the first picture yourself? The colors look very Stefan-ish ;-p

stefan said...

Oh, indeed, it's home-made GLE graphics ;-)

Steven Colyer said...

Most excellent, Stefan. Righteous props. One Grandfather's name was also Stefan, but the Ellis Island clerks wrote it as Steven. Good name. Sorry if I sounded like Sean Penn in "Fast Times at Ridgemont High."

Stefan, if it's not too much trouble, could you squeeze in a mention of the carbon dioxide laser? It has tremendous industrial uses, and I happen to know its Bell Labs (he's no longer there) inventor, Dr. Patel.

Steven Colyer said...

From Wiki, to save you the trouble:

The carbon dioxide laser (CO2 laser) was one of the earliest gas lasers to be developed (invented by Kumar Patel of Bell Labs in 1964[1]), and is still one of the most useful. Carbon dioxide lasers are the highest-power continuous wave lasers that are currently available. They are also quite efficient: the ratio of output power to pump power can be as large as 20%.

The CO2 laser produces a beam of infrared light with the principal wavelength bands centering around 9.4 and 10.6 micrometers.

Friggin' awesome. I feel the need to play Star Wars: Battlefront on my (kid's) PSP now.

Kris Krogh said...

Hi Stefan,

Physics World has a piece on the “Cirque du Laisaire” event sponsored by the SPIE. They say:

The highlight of the evening was the laser magic show, in which a magician called Latimer appeared to pick up a laser beam and spin it around his head. The trick didn’t get much applause, but there’s a reason for that; as the man next to me commented, “Right now, 400 physicists in this room are too busy trying to work out how the hell he did that.”

Maybe your readers would enjoy the show:

http://www.youtube.com/watch?v=T-XoN1Ts6Wg

Cheers, Kris

Bee said...

Wasn't there a Nobelprize recently for optical waveguides?

Bee said...

Stefan: Good old gle... Is the wave a sin function? It is ridiculously difficult to make a decent periodic "wavy line" with most drawing applications. They have all kinds of fancy features for polygons (stars, elipses, filled, non-filled etc) but if you want to make a sin function, you're on your own and end up using splines trying to define the coordinates of points periodically. Likewise, if you're drawing by hand, a sin curve is really hard to do. Straight lines, rectangles and with some practice polygons, circles and ellipses are doable, but for whatever reasons periodic functions are difficult. (Unless you use a special tool, like one for Feynman diagrams or so.) Best,

B.

Michael Varney said...

Go CU! (University of Colorado that is)

Maiman's undergraduate Alma mater.

stefan said...

Hi Steven,

so far I hadn't followed the story up to the CO₂ laser ;-)... thanks for bringing it up. Here is the Physical Review paper where it is described, and here is a scheme of the energy levels involved.

BTW, this is one of the most important types of lasers used in industrial/engineering applications, and it was invented in 1964. That's interesting, because I was wondering whether at the time when Goldfinger was made, such powerful lasers as the one in the movie did exist already, or if this was, back then, still science fiction? It seems it wasl science fiction, though very close to reality?...

Cheers, Stefan

stefan said...

Hi Kris,

oh, it seems they also showed the Goldfinger clip at the event - I had not read that post before, seems like I've had a good idea independently ;-)

... in which a magician called Latimer appeared to pick up a laser beam and spin it around his head.

sounds to me like he has invented the laser sword! It also looks like that - the clip is really cool - thanks!

Stefan

stefan said...

Dear Bee,

well, the wavy line has indeed been made with the "photon propagator" from the feyn.gle package.

Wasn't there a Nobelprize recently for optical waveguides?

Absolutely, glass fibres... And the guys from the other half of the price, CCD chips, had both been at Bell Labs at the time when Maiman scooped them - Willard Boyle tried to build a semiconductor laser then (light from electron-hole recombination, it didn't work, but they filed a patent), and in 1962 built the first continuously operating ruby laser - Maiman's laser was a pulsed one.


Cheers, Stefan

kaleberg said...

I remember all the excitement over the laser in the 1960s. I even got an introductory book when I was in high school. It suggested building a ruby laser as a science project, borrowing a ruby crystal, silvering it and building a xenon flash tube rig to set it off. By the late 60s there were some CO2 lasers being used for industrial purposes, but it was the He Ne (pronounced hee-nee) lasers that got used in LaserDisc analog video players with their diffraction grating silvery LP sized video disks. (I still have an old LaserDisk player).

Phil Warnell said...

Hi Stefan,

A very nice piece and just at a time when I was wondering when we might again see something of yours. I noticed the discussion about the Bond film as to whether the laser as depicted was for the time a reality or rather science fiction and would in the context of application suggest it represented neither. That’s because as Einstein had long since written the paper to demonstrate it was theoretically possible, it then was no longer merely fictional fantasy and thus then only required some bright and dedicated people, such as the ones you noted, to have the potential of his splendid light be realized. However the most interesting thing I find about this being it one of those times when the more practical and beneficial aspects of an idea in science to be the first and most dominant one, as that dreaded death ray is still for the most part practically unrealized, although certainly still a potential.

Best,

Phil

Steven Colyer said...

Hi Stefan, I'm pretty sure that laser Magician clip was quite staged. He's a professional illusionist, and it was dark as well. A simple wrist flick could explain the falling hoop, a wire the hanger, and two small laser pointers in his hands (and lots of practice) the other stuff.

Yes, Goldfinger and '63. Hmm. Interesting. Well, I'm sure there was much competition at the time as to who would first develop "ray guns." There is of course the problem, as shown here, that there are often technological issues to be overcome in exploiting the known science of the day.

However, given time, the next generation, shown here(complete with demonstration, as well as a critique by the small-minded), often solves the problem.

Lesson learned? That when it comes to attaching laser beams to the heads of various aquata, Sea Bass are simply no substitute for Sharks, who in turn are no substitute for Bananafish.

Phil Warnell said...

Hi Steven,

Well in terms of mayhem we can’t quite give Doctor evil his wish, yet the hearts of some can still seem find ways to cast Einstein’s wonderful thought in a bad light.

Best,

Phil

Steven Colyer said...

Oh, what have I done? I brought up the subject of Laser pointers a mere 4 days before The Super Bowl. I found that clip disturbing, Phil, but thanks anyway. I always considered Hockey to be a unification of two previously-thought un-unifiable sports: Field Hockey on Skates, and Amateur Boxing. Having said that, it's terrible when the fans intervene, and in such a nasty harmful way.

I do question how effective a laser pointer would be though, as a weapon. There is such a thing as dispersion, and although one can find real ray guns on sale via the internet, they are quite weak. In Iraq the best we have are infraredders that cause headaches when aimed and fired at suspicious persons approaching checkpoints.

Uncle Al said...

Pink laser ruby self-destructs. Internal flaws, dust on a rod face, too much power then internal self-focusing will blow the rod. All is not lost! Cut a gemstone blank, cut and polish facets, then add a megarad of Co-60 gamma sterilization. There results an extraordinary padparadscha color.