The Physics of War (41 page)

Stimulated emission.

It was an interesting phenomenon, but for several years no one took any interest in it. During World War II, however, radar was developed and used extensively, and there was considerable interest in further developing it after the war. One of the areas of interest was the possibility of a microwave amplifier; in other words, a device that would increase, or amplify, microwaves. Joseph Weber of the University of Maryland became particularly interested in the device. After studying the problem in detail he came to the conclusion that it might be possible to build an amplifier using stimulated emission. He pointed out, however, that what is called a “population inversion” would be needed. Such an inversion occurs when high-energy levels of an atom contain more electrons than lower levels. This
is not the normal situation; the electrons in an atom are usually distributed so that most are in lower energy levels, with fewer in upper energy levels.

A typical energy diagram showing the number of electrons in each level.

A population inversion.

But how could a population inversion be created? Obviously an energy source would be required to force the electrons to higher energy levels, and appropriate energy sources were soon found. We now refer to them as pumps.

Weber designed a microwave amplifier that he thought might work, but he didn't build it. This was left to Charles Townes of Columbia University. Townes was also studying microwaves and looking into the possibility of an amplifier. He decided to set up a population inversion with what he called a resonant cavity, which is a box with reflecting walls. He devised a method for pumping
the electrons within this resonant cavity up to excited states, and by doing so he succeeded in creating a population inversion. In addition, he devised a method for allowing the electrons to suddenly fall to the ground state. The radiation they gave off when this occurred was “coherent” microwave radiation; in other words, the wavelengths were all lined up and had the same phase and frequency (see diagram). In the process he produced the first of what is now called a maser (whereas a maser uses microwave radiation, a laser uses visual light).

Charles Townes.

Soon after he created the maser, Townes began to look at the possibility of a similar device that used optical waves or visible light. This did not prove to be easy. Optical photons are quite different from microwave photons, and Townes worked on the device for several years before he managed to build one. The new device, called the laser (short for light amplification by stimulated emission of radiation), has now overshadowed the maser because it has many more uses in modern society.

The basic principle of the laser is similar to that of the maser. A laser creates a beam of light in which all the photons are coherent. In an ordinary light beam the photons are of many different wavelengths (white light is composed of all colors, each of which has a different wavelength), and the waves are not lined
up; as a result, they easily knock one another out of the beam so that the beam cannot be sharply focused. In a laser beam the photons (or waves) are coherent and of the same frequency so that they
can
be sharply focused.

Top beam: coherent light. Bottom beam: incoherent light.

As in the case of microwaves, a resonant cavity is also needed here; in the laser, though, it is usually called an optical cavity. The medium within the optical cavity is called the gain medium; it is a material that has the properties needed for amplification of the light by stimulated emission. For this amplification a pump is needed; it is usually an electrical circuit or a flash lamp. Mirrors are placed at either end of the optical cavity, one of which is partially transparent so that some light can pass through it. Light within the cavity reflects back and forth through the gain medium and is amplified each time it makes a pass. This medium is, in essence, a population of atoms that has been excited by an external source. The medium itself can be liquid, gas, solid, or plasma.

The gain medium is “pumped” to an excited state; in other words, the atoms within it are in the excited state after the pumping occurs. Eventually a population inversion is achieved in which higher energy states are more densely
populated than lower energy states. The reflected beam grows in intensity until finally it is powerful enough to break through the partially reflecting mirror. What emerges is a coherent laser beam.

Townes, along with his student Arthur Schawlow, was the first to design a workable laser, but they did not build it. They did, however, publish a paper on it, and they filed a patent for the idea in July 1958. A research worker at TRG Incorporated by the name of Gordon Gould was also working on a similar device. Gould tried to patent his device in April 1959, but his application was turned down, even though Gould had described the construction of his laser in a notebook prior to Townes and Schawlow filing for their patent. Several court cases followed, and it took years to settle them.
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The two groups are now credited with having invented the laser independently.

The first person to actually build a working laser, however, was Theodore Maiman of Hughes Research Laboratories in California. His device was quite different than that of Townes, Schawlow, and Gould; they had designed a device using gas as the gain medium. Maiman used a ruby rod with a helical flash lamp wound around it that acted as a pump.

The next step was, of course, to use lasers as weapons of war. Laser-like devices such as ray guns had been used in science fiction for years. It turned out, however, to be much more difficult to make laser weapons than expected, and there's little chance that a laser-like weapon will replace small arms in the near future. The main problem is that lasers require a huge power source, and because of this, there are serious engineering problems. Larger weapons, however, are possible, and the navy has recently built one that could disable an enemy ship and knock down enemy drones. The biggest advantage of a laser such as this is that it doesn't require expensive ammunition. However, the laser itself would be relatively expensive.

One form of laser that appears to have considerable potential is the x-ray laser. It produces a coherent beam of x-rays rather than an optical beam; as a result, it has much more energy. It was considered part of the Strategic Defense Initiative that was proposed in 1983 (sometimes referred to as “Star Wars”). Such lasers were to be powered by nuclear explosions. Tests eventually showed, however, that they were not feasible.

TRANSISTORS, MICROCHIPS, AND COMPUTERS

Many scientific breakthroughs have led to important developments in weaponry, but nothing approaches the invention of the transistor. All electronic devices
now use transistors in one form or another, and hardly a form of weaponry exists that doesn't use electronics in some way. The electronic age came about early in the twentieth century with the invention of the triode, or vacuum tube. It gave us radar and many other electronic devices. But it was fragile in many ways, and relatively large. When John Bardeen, Walter Brattain, and William Shockley developed the transistor at Bell Labs in late 1947, however, the world of electronics underwent a revolution. Tiny radios, calculators of all types, and powerful computers soon followed. Today most transistors are actually found in integrated circuits, or microchips, as they are frequently called; nevertheless, it was the invention of the transistor that started the revolution.
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A transistor is a device that can amplify, or switch, incoming electronic signals. It was developed by physicists working in solid-state physics. As the name implies, solid-state physics deals with solids. And, as you no doubt know, solids come in many varieties. Some are good conductors of electricity; others are insulators (nonconductors), and there is a group in between called semiconductors. Semiconductors have proven particularly important because solids of this type have made transistors possible.

To understand things a little better, let's look at the atomic structure of metals and semiconductors. We'll begin with a gas. The atoms of a gas have a nucleus with a number of electrons whirling around it in various energy levels. Assume that we apply pressure to the gas or lower its temperature. What happens to it? The atoms begin to move closer together and eventually the gas turns into a liquid as the atoms get closer and closer. At this point the energy levels of the various atoms are still completely separated, but as you continue applying pressure (or lowering the temperature), the liquid becomes solid, and the energy levels of the individual atoms begin to overlap. They will create what are called energy bands, which are continuous regions of energy.

The exact way these bands form depends on the particular material being compressed or cooled. If you could look at these energy levels closely you would see that some of them contain electrons, and some are empty. There are also gaps between the bands. In most metals and semiconductors there are, in fact, two major bands with a gap between them. They are referred to as the valence band and the conduction band. The size of the gap between the two bands determines whether they are metals, semiconductors, or insulators.