LASERS LASER (Light Amplification by Stimulated Emission of Radiation). Putting it simply lasers take a preexisting light source and amplify it many, many times over. This amplification is called the “gain.” Most lasers amplify the light they receive by passing it through an amplifying medium. This medium can be solid, liquid or gaseous and is usually a mixture of helium and neon, a solution of fluorescent dye and methanol, or a rod of yttrium aluminium garnate. The gain of a particular medium can vary greatly depending on the wavelength of the light being used, the length of the medium and finally the amount the medium has been energized. In order for the amplifying medium to increase the light’s inherent energy, it must be energized (in laser terminology, “pumped.”) The different ways in which a laser may be pumped depends on the amplifying medium used. In the case of a solid amplifying medium, a xenon filled flashtube is ordinarily used. When these flashtubes are energized, they emit an intense flash of very bright light. Some of this light is absorbed by the amplifying medium. When a laser is pumped by flashtubes, it is a pulsing laser instead of a continuous beam. This method of pumping a laser is usually called optical pumping. Sometimes other lasers are used as the source of light. Gaseous amplifying mediums must be contained within a tube or some other type of enclosure. They are usually pumped by passing energy, in the form of electric pulses, through the medium itself. This then raises the energy state of the atoms and allows them to pass it on to the light. Due to the vast differences between each type of gas, there are many different ways of pumping gas lasers and most are usually very complex. Gas lasers can have pulsing or continuous outputs depending on whether the input charge is pulsing or continuous. There are many different ways to pump lasers other than the two described above. However, most rely on the same principles of increasing the energy in the amplifying medium via electric stimulation. The light amplification setups described above, while often used within laser systems, are not regarded as lasers. This is because a laser is a light amplification system that is positioned between two mirrors. One mirror is almost 100 percent reflective the other is between 20 and 98 percent reflective. These mirrors provide what is called “positive feedback.” These mirrors reflect the amplified light back through the amplifying medium, thereby further amplifying the light. This arrangement in which the light is reflected back to be amplified once again is known as an “oscillator.” The space between these two mirrors is known as the “laser cavity.” Each time the beam of light is reflected back through the amplifying medium, its energy is increased. Since one of the mirrors is only partially reflective, once the beam reaches a certain intensity, some of it is transmitted through the mirror. The portion of the beam that is transmitted through the beam is the beam’s “output.” Besides allowing for the positive feedback, the laser cavity has several other important purposes. First, the cavity keeps the divergence of the laser beam extremely small. In order for the light to go through enough amplifications to escape, it must travel almost exactly parallel to the axis of the cavity. If the light does not travel parallel, it is quickly bounced out of the reflective medium by successive reflections. The laser cavity also helps to polarize the light being reflected through it. Only a few wavelengths of light can be amplified by the amplifying medium. The number of wavelengths decreases considerably more because only a few wavelengths can undergo multiple amplifications. However, all of this is based upon the assumption that we can amplify light by passing it through an amplifying medium. In a normal atom, whenever a light photon is absorbed, an electron in the outer orbit is raised one energy state. The atom is then considered in an excited state and remains in the state for around a millionth of a second. The atom then spontaneously releases the energy in another photon of light causing the electron to lose one energy state. In order to be absorbed the light must be of a specific frequency. This is because in order for an electron to increase in energy it must make one full step, there are no in between steps. Therefore, in order to be absorbed the light must be carrying energy equivalent to one of the electronic proportions. If all atoms behaved in this way light amplification would be impossible since the gain when an atom is absorbed by a typical atom is one. You could throw a photon of light through that atom a trillion times and it would still be one photon of light. In order to get light to amplify we must get atoms to release two photons for every one we put in. This can be accomplished because if an atom that is already excited absorbs another photon it will release both photons. Moreover, the photons are said to be in phase in that their respective waves have the same crests and troughs. The process in which two photons are emitted when one is absorbed is called “stimulated emission.” The problem with stimulated emission is that it never occurs in any significance. In order for stimulated emission to become dominate there must be more excited atoms than non-excited atoms, a very rare occurrence. This state is called a “population inversion” and is central to all lasers. The first laser ever developed used synthetic ruby because it has an obvious population inversion. A ruby laser is powered by xenon flashtubes that flash every few seconds. Chromium ions within the ruby absorb light from the flashtubes and are raised to the highest “F” level. They then quickly transit to the metastable “E” levels at which they remain at for an incredible thousandth of a second. This relatively long period at an excited state allows more than half of the Chromium ions to become excited, and a population inversion is created. At least one of these ions will spontaneously release a photon which will then interact with the excited atoms releasing two, then four, then 8 photons. The cascade will continue to grow until all the excited atoms have been used up. This whole time the laser is being recharged and the process started anew so another pulse of energy can be created. In summary, to create a laser four things are required: a light source, an amplifying medium that has an inherent population inversion, a source of energy for the amplifying medium that allows it to “pump” the laser, and an oscillator capable of reflecting the amplified light back through the amplifying medium until it is of sufficient power and intensity. There are very few materials that allow all of the above to occur. Each one has its specific benefits and drawbacks. However, the most important characteristic of a prospective material is its gain. Even the best of today’s lasers only send out 40 percent of what is pumped in. The only way we will ever be able to reach the full potential of lasers, is if we find a material with the necessary gain.