Blinded by the Light - Lasers! By William Lama Ph.D.

In grad school I was fascinated by the recently invented laser. Lured by the thrill of working with high power lasers, I hoped to do a thesis on laser-induced fusion at the University of Rochester Laboratory for Laser Energetics. But the lab was not scheduled to open for another year, I was three years into graduate school and our second child had just arrived. Being anxious to graduate, I turned to a laser cousin called “superradiance.” I was happy that the required apparatus included a high power laser. I’d like to tell you about that “electrifying” adventure within the context of laser history.

Laser operation is encapsulated in the name: Light Amplification by Stimulated Emission of Radiation, LASER. Stimulated emission happens when a photon (a light particle) with the right energy (ΔE) hits an atom that is in an excited state (E2) causing it to emit another photon of the same frequency (v) in the same direction. In the figure (h) is Planck’s constant and the photon energy is hv = ΔE = (E2 - E1).

The first laser was built in 1960 by Theodore Maiman at Hughes Research Laboratories in Malibu. That laser utilized a synthetic ruby crystal that produced millisecond pulses of deep red light at a precise wavelength of 6943 angstroms, or 0.6943 microns. The chromium atoms that give the ruby its color are pumped to an excited atomic state by an intense burst of light from a flash tube. Some of the atoms spontaneously emit light that may travel down the ruby rod where it is reflected by mirrors at the ends. This positive feedback stimulates emission from other excited atoms and a coherent beam of light flows from the partially mirrored end. Back in the day, laser power was measured in Gillettes. A 5-Gillette laser would burn through 5 razor blades in one pulse. A beast!

***Essentials of a Ruby Laser Source Wikipedia***

The flash lamp and ruby rod are contained within an elliptical cavity, silver-coated on the inside and designed to focus the flash lamp light on the ruby rod. The lamp is powered by a bank of capacitors that fill a large refrigerator-like box. When the red trigger is pressed the capacitors dump 1000 joules of electrical energy into the lamp in a short millisecond pulse. The peak electrical power is about a million watts. The radiation from the lamp pumps the ruby rod producing a 10 nanosecond pulse of pure laser light with a peak power of about 100 million watts. (Some details are omitted.) You can appreciate the Gilletes involved.

My Thesis

“Superradiance” was predicted in 1954 by Robert Dicke. When a collection of atoms is suitably prepared, the atoms interact and cooperatively emit a burst of light, similar to laser amplification of light without the feedback provided by the laser mirrors. In Dicke’s words, “For want of a better term, a gas which is radiating strongly because of coherence will be called superradiant.” It was 1969 and still nobody had done an experiment to prove or disprove the theory. I should have been cautious. The brilliant Dicke was equally adept at theory and experiment, yet he had not done the crucial experiment. My thesis advisor, the sainted Dr. Lenard Mandel, warned me that the experiment would be difficult (and expensive). Also, that a null result would not disprove the theory. Many things could go wrong with the experiment. I had better get a positive result. But hey, we had just landed on the Moon, I had a NASA doctoral fellowship and, most of all, I was young, brash and …

Blinded by the light / Revved up like a deuce, another runner in the night (Earth Band)

In my experiment an intense short pulse from a ruby laser would excite the atoms in a tiny sample of ruby. If everything worked the sample would then emit a superradiant pulse of light. The first complication was that the ruby sample would need to be cooled to liquid helium temperature so that thermal vibrations would not spoil the effect. My first purchase was a lab cryostat, a dewar with an inner cylinder filled with liquid helium at 4 degrees kelvin, contained within an outer cylinder filled with liquid nitrogen to keep the helium from boiling off too fast. My budget would not go so far as a stainless steel cryostat so I bought a very delicate glass unit. The low temperature also shifted the energy levels of the ruby sample, leading to the second complication. The ruby laser would also need to be cooled so that the wavelength of the laser light would (nearly) match the energy levels of the cooled ruby sample. Fortunately, the ruby laser only needed to be cooled to liquid nitrogen temperature, but where was I going to get a nitrogen cooled ruby laser?

It turned out that the Raytheon research lab in Boston had built such a device and were no longer using it. I took a plane to Boston and returned with the laser in my lap. I should have been suspicious. Why would they give away a working device that took some effort to build? I soon found out why. The polished ends of the cooled ruby rod would ice up. The next laser pulse would destroy the mirror coatings, which had to be recoated in the optical shop. Not every pulse, but often enough to destroy my schedule. Then one day an electrical arc jumped from the end of the flash lamp to the metal bench that held the experiment and the lamp exploded. When Dr. Mandel came to the lab to inspect the damage he first asked if I was OK. Then: “Mr. Lama, do you expect to get out of here alive.”

More Lasers

The ruby laser was the first and it has been a workhorse. One of its coolest applications was when it was used to measure the distance to the Moon in 1969. Laser pulses from Earth were bounced off a retro-reflector put on the surface of the Moon by the Apollo 11 astronauts. By measuring the time it took a laser pulse to travel to the Moon and back to Earth the distance to the Moon could be calculated. Today ruby lasers are used for metal working, high brightness holographic cameras, cosmetic dermatology and tattoo removal.

The first continuous output laser was the helium-neon gas laser containing a mixture of 90% helium and 10% neon producing an output at 6328 angstroms in the red.

***Helium-Neon Laser, invented by Bell Telephone Labs in 1962. Wikipedia***

Neon atoms are excited to the upper state E2 by an electrical discharge and the laser light comes from a transition to a lower state E1. Today He-Ne lasers are used mostly in optical experiments and as a frequency standard. The laser printer in my last Palos Verdes Pulse article uses a He-Ne laser. (See “Digital Publishing Technology,” August 1, 2020)

The most common laser is the diode laser which is a jazzed-up version of a light emitting diode. An LED consists of two semiconductor materials, n-type that freely conducts electrons and p-type that conducts holes (the absence of electrons). The n and p type materials form a sandwich with a very thin “active” layer of material in between. When a voltage is applied across the diode, charge carriers (electrons and holes) are pumped into the active region from the n and p regions respectively. When an electron and hole meet up they “recombine” and emit a photon of light. This is spontaneous emission. Stimulated emission can be produced when photons encounter electron-hole pairs, further generating light of the same frequency and direction. When feedback is provided by reflection off the polished ends of the diode, laser output is produced.

***Diode Laser: PN junction (left) and construction (right). Wikipedia.***

Diode lasers are everywhere. CD players, barcode scanners, fiber-optic phone lines, dental tools, laser hair-removal devices, laser pointers and most laser printers all use diode lasers because they're small, compact, and inexpensive.

https://www.explainthatstuff.com/semiconductorlaserdiodes.html

The lasers used at the Rochester Laser Energetics lab employ synthetic crystals of Yttrium-Aluminum-Garnet doped with Neodymium, aka Nd:YAG. The Omega laser configuration has sixty of these monsters producing a 40 kilojoule/60 terawatt pulse focused onto a target less than 1 mm in diameter. The objective is to heat the target to a temperature comparable to the center of the sun to produce fusion energy. https://www.youtube.com/watch?v=gVX9vh-f_No

Many companies in the US have been developing special high power lasers for “directed-energy” weapon systems. www.lockheedmartin.com/directed_energy

One of the most exotic lasers, and one of the most beneficial, is the Excimer laser. An excimer is an oddball two-atom molecule that can exist only in an excited state (E2 in the first figure). For example, an Argon-Fluorine molecule in the excited state may emit an ultraviolet photon of wavelength 1930 angstroms and end up as two free atoms. If an A-F gas is excited by an electrical current, stimulated emission and feedback may lead to laser action.

In the early 1980s IBM scientists discovered that the excimer laser UV light is absorbed by biological material without burning. Rather, the laser made clean precise cuts, ideal for delicate surgeries, including eye surgery. https://en.wikipedia.org/wiki/Excimer_laser

Today, LASIK, which stands for “laser in-situ keratomileusis,” is the most common method. My wife’s cataract surgery was a piece of cake. It is estimated that lasers could eventually be used in two-thirds of all operations.

Superradiance, postscript

After my near electrocution in the U. Rochester lab, I put the apparatus back together (on a non-conducting wooden lab bench) and fired the laser. Nothing. No superradiant signal. I took the He cryostat apart to inspect the ruby sample …. and accidently knocked off the glass piece that held the sample. I would need a new cryostat! When Dr. Mandel came into the lab he looked at the broken glass and said: “Mr. Lama, you have used up my equipment budget. I can no longer afford your experiment. Would you consider doing a theoretical thesis?”

So I did. “Approaches to the Treatment of Spontaneous Emission” was completed 18 months later and I (safely) graduated.

Dr. William Lama has a PhD in physics from the University of Rochester. Taught physics in college and worked at Xerox as a principle scientist and engineering manager. Upon retiring, joined the PVIC docents; served on the board of the RPV Council of Home Owners Associations; served as a PV Library trustee for eight years; served on the PV school district Measure M oversight committee; was president of the Malaga Cove Homeowner's Association. Writes about science, technology and politics, mostly for his friends.

email: wlama@outlook.com

Featured