Lasers
A laser is a device that produces a very intense beam of light with properties that make it an essential piece of equipment across the whole spectrum of science.
Normal "white" light is a mixture of different wavelengths, each wavelength associated with a specific frequency and energy. This is why white light separates into a "rainbow" when it is beamed through a prism. Each color in the rainbow is a different wavelength of light, with short wavelength blue light at the energetic end of the spectrum and long wavelength red light at the low-energy end. Lasers are different because they produce light of only certain wavelengths, all of it traveling in the same direction. The light is also coherent , which means all the peaks and troughs of the different waveforms match up peak to peak and trough to trough. They are like soldiers that are not only marching shoulder to shoulder and at the same speed, but are in step too.
Lasers need three things to make them work. The first is a material called the active medium, which can be charged up with energy and then made to give up this energy in a controlled way. The second is a way of charging up the active medium with the necessary energy and the third is an optical cavity to amplify the radiation.
The word laser is an acronym for light amplification by stimulated emission of radiation and suggests how lasers work. In a molecule, each electron can be assigned a specific energy. One way to understand this is to imagine the electrons to be whizzing round the nuclei in an "orbital" that is rather like the orbits of the planets revolving around the Sun. Each orbital and therefore each electron within it has a certain energy associated with it. Unlike the planets, however, an electron is given extra energy so it can jump from one orbital to one with higher energy.
In most materials in their normal state, there will be far more electrons in the unexcited lower energy level orbitals then there are in the more energetic excited ones. If energy that matches an energy gap between these different levels is put in, the electrons are excited from low-energy state orbitals to more energetic excited orbitals. If these excited oribitals are metastable, this means the electrons stay in these orbitals for a short time before decaying back to their original orbitals. Excitation of electrons to metastable orbitals therefore causes a population inversion, because there will be more molecules with electrons populating the higher energy levels than the lower.
The electrons in active mediums give up this extra energy and decay back to their ground states in two ways. The first is called spontaneous emission. This involves an electron spontaneously jumping down from an excited orbital, giving off the energy as a particle of light called a photon. This photon is equal in energy to the gap between the energy levels, but is given off in a completely random direction. The second way is called stimulated, which occurs when a photon with energy that matches the gap in energy between the two levels hits an excited molecule. The photon then effectively "knocks" the electron down from its excited state by causing it to emit a photon. The important thing here is that the emitted photon not only travels in the same direction and with the same energy as the one that caused it to be emitted, but that it is also coherent with it. These identical photons can now travel through the active medium, knocking down more electrons from their excited states. This causes a snowball effect, releasing a whole cascade of coherent photons.
If a mirror is placed at either end of the active medium, an optical cavity is formed, where the light radiation bounces back and forth between the mirrors. As long as the active medium is charged up and there is "room" in the lower energy levels for the excited molecules to decay to, the light will sweep back and forth through the active medium getting amplified with each pass. Any photons produced in the cavity that do not travel in line with the mirrors are not reflected back into the active medium and lose energy and fade away. If the mirror at one end of the optical cavity is a partial mirror, it reflects only part of the radiation back into the cavity, letting the rest out. The radiation emitted through this mirror is called a laser beam.
One factor determining the output of a laser is the length of the optical cavity. It is only possible for certain wavelengths--the so-called longitudinal modes--to exist in the cavity as all others will interfere and cancel each other. A particular active medium will also be capable of supporting only some of the longitudinal modes, and it is the combination of these two factors that shapes the output.
There are several different types of active medium, and the type used gives the laser its name. Gas lasers use gases such as argon, carbon dioxide, neon, and nitrogen. Solid-state lasers can use crystals (ruby for example), glass (Nd3+ in silicate glass, for example) or semiconductors. Another type uses a dye as the active medium and hence is called a dye laser. These are important in chemistry because it is very easy to change the frequency of the laser output and select a desired frequency for a particular experiment.
The nature of the active medium also determines how the energy used to cause the population inversion is supplied. An electrical discharge supplies the energy to a gas laser as electricity. Optical pumping, where a light is shone on the medium, is used in crystal and glass lasers. Chemical lasers work by reacting two species (such as hydrogen and chlorine gases) together to form products with an inverted population. Hybrid electrical discharges work in the same way, but an electric discharge excites the products first, causing them to react.
Lasers operate in two modes. The first is called continuous wave and occurs because the molecules remain in the upper, metastable excited levels longer than in the lower. This means there is always room in the lower levels where the molecules can decay. These lasers can then operate continuously. The second mode of operation is pulsed operation. In this mode the upper level is pumped up before the lower level has had time to empty and, consequently, there is no room in the lower levels for the metastable states to decay to. The radiation from these lasers are therefore produced in short pulses. With both modes of operation, there are advantages and disadvantages that dictate where these different types of lasers are used.
In many applications, pulses of an extremely short duration are required. There are several ways of achieving this. For a lower limit of nanoseconds (a one-thousand-millionth or 10-9 of a second), Q switching is used. This uses an optical switch, which "spoils" the optical cavity by blocking one of the mirrors. This allows the active medium to charge up, but there is nowhere for the energy to go. When a very short electric pulse is applied to the switch, the cavity is momentarily restored and the radiation rushes out in one big pulse.
For even shorter pulses, mode-locked lasers are used. One type of mode-locked laser contains a dye cell in the optical cavity, which absorbs light at the operational frequency of the laser. Normally the dye absorbs all of the energy of the laser and nothing happens. If by chance, however, the active medium releases a big pulse, it forces its way through the dye by saturating it with so much light that it cannot absorb any more and allows the rest of the pulse through. This pulse is then free to bounce back and forth in the cavity, soaking up all the energy from the active medium. When the big pulse is not traveling through it, the dye absorbs all other radiation, and so the only output of the laser is the big pulse. The frequency of the pulses in such a laser is fixed by the time it takes the pulse to complete the passage from one end of the cavity to the other. These pulses can be so short that quantum effects such as the uncertainty principle come into play and mean that instead of just one specific frequency, there are many. This can be solved to an extent by beaming in radiation from another laser that singles out just one of the possible frequencies present. These pulses can be even shorter than femtoseconds (a femtosecond is a thousand-million-millionth or 10-15 of a second!).
Lasers are extremely useful in spectroscopy, because they combine a focusable beam with a precise frequency and high energy. In absorption spectroscopy, a beam is split in two, one part being sent through the sample, the other going straight to a detector. By comparing the intensity of the two beams it is possible to see whether the one that passed through the sample was absorbed by it to any extent. By doing this at a number of different frequencies it is possible to get a very accurate absorption spectrum of even the most dilute samples.
Lasers with these extremely short pulses have given rise to hyperfast spectroscopy, in which it is possible to study processes such as bonds breaking step by step. This has huge implications for the study of the incredibly short-lived transition states that are characteristic of so many chemical reactions.
Lasers have also revolutionized Raman spectroscopy. Raman spectroscopy relies on the distortion--called the change in polarisability--of the electron cloud in a molecule when it is exposed to light radiation. Low-intensity normal light produces Raman spectra, but they are very faint. The intense radiation from lasers produces very clear Raman spectra.
Coherent anti-stokes Raman spectroscopy (CARS) has been developed on the back of developments in lasers using the nonlinear effects on the polarizability of molecules when placed in an even more powerful field than that used for normal Raman spectroscopy. The fact that the probe in Raman spectroscopy is light means that CARS can be used to look at the chemistry that occurs in hostile and otherwise inaccessible environments such as jet engine exhausts.
Lasers have also revolutionized the separation of chemical isotopes, which had otherwise been extremely difficult, if not impossible. Laser radiation of a very specific frequency picks out a molecule with isotope specific features in the spectrum and can then excite just the isotope leaving the rest of the molecules unaffected. The selectively excited isotope can then be captured either by ionizing it with more laser radiation or using the enhanced reactivity of an excited molecule to make it undergo a chemical reaction. This technique has facilitated the separation of isotopes of uranium, which has had a big impact on both nuclear power and nuclear weapons.
The quantum nature of light, in which it can be described as both a wave and a particle, has made it possible to use lasers to trap and cool individual atoms. The momentum of laser beams can be used to repeatedly "punch" atoms to slow them down. The wave nature of light means that beams can interfere with each other, causing regions in space with high and low laser intensity. The combination of these two techniques have made it possible to trap individual atoms. First a laser is used to slow the atoms down, and then they are caught in an "optical cage" formed by the interference pattern that arises when many laser beams are crossed at a point in space.
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