The answer is that lasers produce single-wavelength EM radiation that is also very coherent—that is, the emitted photons are in phase.
Laser output can, thus, be more precisely manipulated than incoherent mixed-wavelength EM radiation from other sources. The reason laser output is so pure and coherent is based on how it is produced, which in turn depends on a metastable state in the lasing material. Suppose a material had the energy levels shown in Figure 7. When energy is put into a large collection of these atoms, electrons are raised to all possible levels.
This includes those electrons originally excited to the metastable state and those that fell into it from above. It is possible to get a majority of the atoms into the metastable state, a condition called a population inversion. Figure 7. If a majority of electrons are in the metastable state, a population inversion has been achieved.
Once a population inversion is achieved, a very interesting thing can happen, as shown in Figure 8. An electron spontaneously falls from the metastable state, emitting a photon. This photon finds another atom in the metastable state and stimulates it to decay, emitting a second photon of the same wavelength and in phase with the first, and so on. Stimulated emission is the emission of electromagnetic radiation in the form of photons of a given frequency, triggered by photons of the same frequency.
For example, an excited atom, with an electron in an energy orbit higher than normal, releases a photon of a specific frequency when the electron drops back to a lower energy orbit. If this photon then strikes another electron in the same high-energy orbit in another atom, another photon of the same frequency is released. The emitted photons and the triggering photons are always in phase, have the same polarization, and travel in the same direction.
The probability of absorption of a photon is the same as the probability of stimulated emission, and so a majority of atoms must be in the metastable state to produce energy. Einstein again Einstein, and back in ! Among other things, Einstein was the first to realize that stimulated emission and absorption are equally probable. The laser acts as a temporary energy storage device that subsequently produces a massive energy output of single-wavelength, in-phase photons. Figure 8.
One atom in the metastable state spontaneously decays to a lower level, producing a photon that goes on to stimulate another atom to de-excite. The second photon has exactly the same energy and wavelength as the first and is in phase with it.
Both go on to stimulate the emission of other photons. A population inversion is necessary for there to be a net production rather than a net absorption of the photons. The name laser is an acronym for light amplification by stimulated emission of radiation, the process just described. The process was proposed and developed following the advances in quantum physics. The Nobel Prize in went to Arthur Schawlow for pioneering laser applications.
The original devices were called masers, because they produced microwaves. It used a pulsed high-powered flash lamp and a ruby rod to produce red light. Today the name laser is used for all such devices developed to produce a variety of wavelengths, including microwave, infrared, visible, and ultraviolet radiation. Figure 9 shows how a laser can be constructed to enhance the stimulated emission of radiation.
Energy input can be from a flash tube, electrical discharge, or other sources, in a process sometimes called optical pumping. A large percentage of the original pumping energy is dissipated in other forms, but a population inversion must be achieved.
Mirrors can be used to enhance stimulated emission by multiple passes of the radiation back and forth through the lasing material. One of the mirrors is semitransparent to allow some of the light to pass through.
Figure 9. Typical laser construction has a method of pumping energy into the lasing material to produce a population inversion. Lasers are constructed from many types of lasing materials, including gases, liquids, solids, and semiconductors.
But all lasers are based on the existence of a metastable state or a phosphorescent material. The helium-neon laser that produces a familiar red light is very common. Figure 10 shows the energy levels of helium and neon, a pair of noble gases that work well together.
An electrical discharge is passed through a helium-neon gas mixture in which the number of atoms of helium is ten times that of neon.
The first excited state of helium is metastable and, thus, stores energy. This energy is easily transferred by collision to neon atoms, because they have an excited state at nearly the same energy as that in helium. That state in neon is also metastable, and this is the one that produces the laser output. The most likely transition is to the nearby state, producing 1. A population inversion can be produced in neon, because there are so many more helium atoms and these put energy into the neon.
Helium-neon lasers often have continuous output, because the population inversion can be maintained even while lasing occurs. Probably the most common lasers in use today, including the common laser pointer, are semiconductor or diode lasers, made of silicon. Here, energy is pumped into the material by passing a current in the device to excite the electrons.
Special coatings on the ends and fine cleavings of the semiconductor material allow light to bounce back and forth and a tiny fraction to emerge as laser light. Diode lasers can usually run continually and produce outputs in the milliwatt range. Figure Energy levels in helium and neon. In the common helium-neon laser, an electrical discharge pumps energy into the metastable states of both atoms.
The gas mixture has about ten times more helium atoms than neon atoms. Excited helium atoms easily de-excite by transferring energy to neon in a collision. A population inversion in neon is achieved, allowing lasing by the neon to occur. There are many medical applications of lasers. Lasers have the advantage that they can be focused to a small spot.
They also have a well-defined wavelength. Eventually, the "excited" electron loses the extra energy by emitting electromagnetic radiation of lower energy and, in doing so, falls back into its original and stable energy level.
The energy of the emitted radiation equals the energy that was originally absorbed by the electron minus other small quantities of energy lost through a number of secondary processes. Electromagnetic radiation energy levels can vary to a significant degree depending upon the energy of source electrons or nuclei. For example, radio waves possess significantly less energy than do microwaves, infrared rays, or visible light, and all of these waves contain far less energy than ultraviolet light, X-rays, and gamma waves.
As a rule, higher electromagnetic radiation energies are associated with shorter wavelengths than similar forms of radiation having lower energy. The relationship between the energy of an electromagnetic wave and its frequency is expressed by the equation :. Based on this equation, the energy of an electromagnetic wave is directly proportional to its frequency and inversely proportional to the wavelength. Based on this equation, the energy of an electromagnetic wave is directly proportional to its frequency and inversely proportional to the wavelength.
Thus, as frequency increases with a corresponding decrease in wavelength , the electromagnetic wave energy increases, and vice versa. Mortimer Abramowitz - Olympus America, Inc.
Matthew J. Parry-Hill , Robert T. Sutter , and Michael W. Microscopy Primer. Light and Color. Microscope Basics. Special Techniques. Digital Imaging. Confocal Microscopy. Live-Cell Imaging.
Microscopy Museum. The energy in a hydrogen atom depends on the energy of the electron. When the electron changes levels, it decreases energy and the atom emits photons. The photon is emitted with the electron moving from a higher energy level to a lower energy level. The energy of the photon is the exact energy that is lost by the electron moving to its lower energy level. We call these lines Balmer's Series.
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