Laser: The Wonder of Light
Masers preceded lasers. In essence, a maser is an amplification process for electromagnetic radiation. The process was first demonstrated by Townes in the microwave region, who later predicted the possibility of realizing this effect in the more common visible region. Initially named "optical masers," they were christened as lasers by Gordon Gould. Maser is an acronym for microwave amplification by stimulated emission of radiation, and laser thus became light amplification by stimulated emission of radiation.
Lasers are present in any modern household. The laser printer that we use with a computer, or the CD and DVD player that we operate to entertain us contain laser sources. The special properties of lasers that this article discusses permit the functioning of many appliances. As it is, the influence of lasers has percolated into most of the areas of human interest. For example, the psychedelic lighting effects in entertainment theaters, concert halls and light shows are embellished with lasers.
In the human genome project, DNA sequencing machines used detection systems with fluorescence markers that depended on lasers for its efficacy. In space programs and astronomy, the highly collimated nature of laser beams produced accurate measurements by directing laser beams from a planet into space and vice versa. Its military application of accurate targeting by missiles is recent history and the fiction-like ambitious Star War project may be revived any time. In medicine, the laser's ability to shower energy in microscopically localized areas is a boon in fixing detached retinas. The spectacular holographic three-dimensional pictures are a contribution of laser technology. The projected clean energy source by nuclear fusion is heavily dependent on the power of lasers to fuse hydrogen isotopes in a billionth of a second at a temperature of millions of degrees.
SpectroscopyThe more significant effect of lasers is in the area of spectroscopy, where a renaissance occurred due to their multi-faceted versatility as a research tool. Here the property of a laser as a pointed coherent high energy source with optical purity is taken advantage of. Lasers could demonstrate the existence of non-linear optical laws that were possible only with the high intensities that they carried. Thus, some special asymmetric materials could emit light with half the wavelength by interacting with lasers through a process called second harmonic generation. A futuristic but attainable application of lasers is to incorporate and transmit all the information carried by telephone lines, television stations, radio stations and all the computers in the world packed in one laser beam through fiber optic cables.1 The computers of the future may use microscopic lasers instead of electric impulses. Many miraculous applications of lasers that now exist in the realm of fiction will materialize if enough money and time are directed toward them.
Any modern ink laboratory has particle size distribution equipment where the light scattering principle materialized through a laser beam provides accurate information of particle size of pigments in dispersions. This is in addition to other sundry uses in several general appliances.
The expansion of the acronym laser (light amplification by stimulated emission of radiation) bears the significance of the process itself. In essence, it is the amplification of light. By amplification, we mean the magnification of the strength of the light signal. And the method involved is the stimulated emission. To appreciate stimulated emission, we should know what is normal or spontaneous emission.
Light is electromagnetic radiation, which literally means it is a propagating wave where an electric field and a magnetic field move in unison perpendicular to each other in the direction of its propagation vibrating at a definite frequency. In quantum theory, light contains packets of energy called quantum whose energy E is related to the frequency of vibration u as E = hu, where h is Planck's constant with a value 6.6 x 10-27 erg.sec. This revolutionary relation that changed the face of classical physics evolved from the emission studies on black body spectral density parameter by Kirchoff and Rayleigh/Jeans, which was modified by Planck in 1900. Planck correlated the energy of quantum with the frequency from his studies based on elementary thermodynamics in which he coupled Boltzmann's statistical approach to derive two similar expressions, which contained E in one equation and hu in the other.2 It is an irony of the history of science that Planck himself was hesitant to publish these novel results due to the apocalyptic nature of these revelations.
Materials absorb light routinely and the absorption process is responsible for the color in many instances. During absorption the energy in the light quantum (called photon) is transferred to the molecule that in turn uses this energy to excite the molecule into a higher energy state. This energy, depending on the energy levels in the molecule, gets distributed into rotational, vibrational and electronic energy levels in accordance with the quantum theory. The rotational transitions need the lowest energy, which is obtained from the microwave region; vibrational transitions need more energy, which corresponds to the infrared region; the electronic transitions require greater energy and occur by absorption of visible and ultraviolet light. At still higher energies, such as X-rays and g-rays, the molecule on absorption responds by knocking out an electron and ionizing.
Let us focus on the absorption in the visible or near UV region that leads to electronic transitions. The excited molecule formed by the absorption of light in this region will be unstable and usually has a lifetime in the range of nanoseconds or more. The molecules lose a part of this energy by a process called spontaneous emission in which the energy is emitted in the form of light with higher wavelength. The familiar processes of fluorescence and phosphorescence happen by spontaneous emission. We ignore here the nonradiative processes wherein a part of absorbed energy is dissipated as heat.
Stimulated EmissionFollowing is a look at stimulated emission. Consider an excited state of the molecule that is ready to emit. If we shine this excited state with the light corresponding to its emission wavelength, the excited state may be made to emit in unison with the striking light. The output light will have special characteristics. Such a process was first postulated theoretically by Einstein3 and is called stimulated emission. In essence, an atom in an upper energy level is stimulated or forced by an incoming photon to give up its energy and fall to lower energy level. The photon must have the proper energy corresponding to the energy difference between the original state and a state of lower energy. The result of this process is that one photon interacts with the atomic system and two photons emerge. Both photons have the same energy and travel in the same direction. The energy is extracted from the atomic system and appears as additional light of the same frequency. The original light is still present and one has amplified the light intensity in the stimulated emission process. Figure 1 schematically depicts the absorption, spontaneous emission and stimulated emission processes.
The principle of stimulated emission is used in laser action. The laser process requires satisfying three important criteria: an active medium that can sustain a reasonable population of excited state, an excitation source to generate excited state with a favorable population, and a structure called resonant cavity to provide feedback by multiple passage of light to initiate stimulated emission.
The active medium should support appropriate energy levels for sustaining laser action, and the number of molecules in the excited state should exceed the required amount or population. For this, the excited state population should overcome the normal ground state population given by Boltzmann distribution law of states at a given temperature. An energy source is required to realize the excited state population. Such a process is referred to as population inversion and is brought about by the pumping action of an external source. If the medium has reached the required population inversion, it is called an active medium. The active medium can be a gas, liquid or solid, and accordingly we have a gas, liquid, or solid laser. The energy levels supporting the laser action may involve two, three or four levels and accordingly we have a two-level, three-level, or four level laser. The pumping source could be a high intensity light as in a flash lamp or an electric discharge (optical and electric pumping).
The stimulated emission thus happens when a photon hits a molecule or atom in an excited state. For this a positive feed back of photons produced within the medium is essential. In the usual laser arrangement, this is attained by placing two mirrors on either end of the laser medium. One mirror will be highly reflective and the other will be partially reflective in order to allow some radiation to be tapped. The region covering the laser medium and the mirrors are termed as the laser cavity. The proper arrangement of laser cavity provides the laser action. The essential components of a laser4 are shown in Figure 2.
Laser light is hailed mainly because of the following special properties it possesses compared to ordinary sources of light. They are (1) Small beam width: laser light can be obtained in small beam width that shows only little divergence or spreading, in the order of milliradians, even after traveling long distances. The highly collimated nature of a laser beam allowed the reflection of a Ruby laser beam sent from earth to the reflectors placed on the moon's surface to be detected back in the United States during the Apollo space program. The laser beam was only about 1,000 yards wide even after travelling the 240,000 miles to moon. This highly collimated directionality of laser beams finds use in various alignment jobs in the construction industry, for tracking targets and for pollution monitoring.
High IntensityLasers are very powerful, bright sources that carry very high energies compared to conventional sources. This is a manifestation of small solid angles over which the power is distributed. An obvious application of this property is in cutting and welding. Carbon dioxide and Nd-YAG lasers with power of hundreds of gigawatts per square meters are used for this purpose. They find application in cutting leather to steel. The area of artificial intelligence, an offshoot of computer technology, applied in the form of robotics coupled with lasers do a great service as cutting and welding tools. Even the meticulous 'spot welding' of detached retina has been attained several decades back. Fictional depiction of lasers as cutting tools had preceded its demonstration as a practical device as early as in the 1964 James Bond movie wherein a laser was shown to perform a radical lobotomy, years before such operation became a reality.
CoherenceIn ordinary light sources, the individual photons are not in phase and they are referred to as chaotic. But the very nature of the stimulated emission produces photons that are in phase or coherent. Coherence may be compared to the orderly marching of a troop of photons as contrasted to the chaotic movement. Coherence is observed with respect to the intensity of the electric field of the light (spatial coherence) and with respect to time (temporal coherence). In the former uniform electric field as a function of distance and in the latter uniform frequency as a function of time will be observed. The most important application of coherence is in holography where three-dimensional image of objects can be constructed.
MonochromaticityWhite light is polychromatic - it contains many wavelengths as observed in the color spectrum. But lasers are characterized by optical purity, which means that a single wavelength can be tapped. This is possible because laser transition, in principle, involves well-defined energy levels. This property has excellent applications in high-resolution spectroscopy to observe specific transitions in a molecule. A practical application is the separation of isotopes in the nuclear industry where the fissionable isotope of Uranium, 235U, is separated from the non-fissionable one 238U by exploiting the minute difference in their energy levels.
Lasers are available as continuous sources and pulsed sources, both with several applications. Mode-locking and Q-switching are two important procedures to get laser pulses with very high repetition rate. Laser pulses in the range of femtoseconds (one quadrillionth of a second, or 10-15) can be produced and they find use in frontiers of chemical research. Available lasers are tiny to gigantic.
It is instructive to become familiar with examples of lasers belonging to different classes based on the state of the active medium. The following are examples of solid state, liquid state and gaseous state lasers.
Solid State LasersThe first working laser demonstrated by Maiman5 in 1960 was a solid state laser where the active medium was a Ruby crystal (0.05 % of Cr2O3 in an Al2O3 lattice), and the chromium ion is the lasing center which provides a three-level system. The Ruby rod with the dimensions of 3-25 mm in diameter and up to 20 cm in length is excited by a flash lamp and the laser emission occurs at 694 nm. Normal operation of this laser yields pulses, but continuous wave may also be obtained.
Another important solid state laser is based on the four-level energy state of Nd3+ ion, which is hosted by matrices like glass or yttrium aluminum garnet (YAG, Y3Al5O12). Nd:YAG lasers are very common where the emission takes place in the IR region at1.06 mm. The YAG rod is of few centimeters in length and contains about 0.5-2.0 % by weight of Nd3+. Both CW and pulsed operations are possible, and a pulse energy of ~1,000 mJ is common. This laser became prominent because by the frequency doubling process of second harmonic generation, it is possible to get laser light in the visible region at 533 nm and in the UV region at 355 and 266 nm with pulse energies of 350, 175, and 75 mJ, respectively. These correspond to the second, third and fourth harmonic of the fundamental at 1060 nm and are produced in frequency doubling asymmetric crystals.
Lasers belonging to the category of diode lasers (semiconductor lasers) and F-center lasers are other examples of solid state lasers.
Liquid State LasersDye lasers are examples of lasers with liquid active medium. Actually, compounds called laser dyes6 function as the active medium. The dye is dissolved in a solvent such as methanol or dioxane. Here the fluorescence property of the dye is made use of. When a dye is irradiated with light whose wavelength corresponds to the energy difference between the states S0 (ground state) and S1 (first excited state), some of the ground state molecules are raised to the level S1. The laser emission takes place from a lower state of S1 to a state of S0, and hence the laser will operate at a different wavelength than the pumping light. The dye should have the desired photophysical properties such as high fluorescence quantum yield and low intersystem crossing efficiencies.7 Usually a laser provides a single wavelength or limited number of lines. But the dye laser can cover a range of wavelength because the fluorescence emission of the dye is spread in a range. They can thus provide laser light in those regions where other lasers do not operate. The whole wavelength range can thus be tuned using different classes of dyes and they operate in the whole visible region and some portions of infrared.
Gaseous State LasersHe-Ne laser is an example for gaseous laser active medium. An electrical discharge excites the helium atoms, which transfer its energy to neon atoms by a collisional deactivation mechanism. Emission by neon is the basis of the action of this laser. The red He-Ne laser emitting at 632.8 nm is so common now that nobody can miss it in a supermarket barcode reader, or in other optical scanning devices, or in the pointing cursor beam of a speaker in a seminar. Other examples are the argon laser, nitrogen laser, carbon dioxide laser, copper-vapor laser and excimer lasers, all of which have applications in industry and basic research. It is worth noting that the first maser was made from ammonia molecules where the energy difference between levels corresponds to the microwave region, although population inversion was achieved by a totally different process.
ConclusionNow laser pulses can be generated with extremely short pulse widths lying in the range of femtoseconds. A very stable and powerful laser system for this purpose employs a Titanium (Ti):Sapphire system pumped by a CW laser that generates laser pulses of approximately 50 femtosescond duration, with a repetition rate of 80 MHz and pulse energies of approximately 20 nJ. The computers of the future are expected to be dramatically influenced by their effects on random access memory (RAM) brought by lasers. A gallium arsenide chip produced by IBM contains the world's smallest lasers in the form of cylinders of semiconductor roughly one-tenth of the thickness of a human hair; a million lasers can fit on a chip the size of a dime.8
Lasers, being essentially light (in the sense of electromagnetic radiation), are revolutionizing the branch of photochemistry where the interaction of light quantum with matter could be investigated with intense interest, unfolding intricate mechanisms and early events in this branch of chemistry. Several popular and esoteric applications of lasers are yet in the horizon. Novel materials and concepts are being applied in developing microscopic and macroscopic lasers of future.
References1 Townes, C.H. How the Laser Happened, New York: Oxford University Press, 1999.
2 Baggott, J. The Meaning of Quantum Theory, New York: Oxford University Press, 1996.
3 A. Einstein, Phys. Z., 18 (1917) 121.
4 Andrews, D.L. Lasers in Chemistry, Berlin: Springer-Verlag, 1986.
5 T. Maiman, Nature, 187 (1960) 493.
6 Kunjappu, J.T. J. Photochemistry and Photobiology A: Chemistry, 56 (1991) 365.
7 K. H. Drexhage, "Dye Lasers," F.P. Schafer (Ed.), New York: Springer-Verlag, 1973.
8 Webster's New World Dictionary of Science, Macmillan, 1998.