THIS YEAR marks the 50th anniversary of the first demonstration of a working laser by Theodore Maiman at California's Hughes Research Laboratory on May 16th, 1960, writes WILLIAM REVILLE
Today, lasers are widely used in everyday life. A laser beam scans barcodes at the supermarket check-out, reads the coded information on your DVD or CD, removes that embarrassing tattoo you once thought looked so cool, remodels your cornea so you can dispense with spectacles, points to your on-screen slide when you are giving a lecture, induces nuclear fusion in experiments designed to develop useful fusion energy, and much more.
As we all know, a laser beam is a powerful narrow beam of light that can travel a long distance with little sideways spread or loss of intensity. The term “laser” is an acronym for “light amplification by stimulated emission of radiation”. As this name suggests, it is not easy to describe how a laser works, relying on words alone, so please take a deep breath while I stumble on through it.
Laser light is emitted from a large collection of atoms under certain conditions. An atom has two parts – a central nucleus where almost all the mass of the atom resides and electrons that orbit the nucleus. It is helpful to visualise the electrons as occupying orbitals arranged at different energy levels. When energy (heat, electrical, etc) is added to an atom, some electrons jump from lower to higher energy orbitals (the atom moves from a ground state to an “excited” state). The excited electrons eventually return to their original lower energy orbitals and, when they do, they radiate away the energy difference between the orbitals as light energy – when the bar in your electric heater glows red, its atoms, excited by heat, are releasing red light.
In a laser, the manner in which the atoms release light is tightly controlled. First of all the atoms are “pumped” into an excited state by exposure to intense flashes of light for example. When the excited electrons relax back to lower energy levels, light of a specific wavelength corresponding to the energy released is emitted. Two atoms with similarly excited electrons will emit light of the same wavelength (colour).
The smallest part of light that can exist is called a photon. When the photon of light is emitted from an excited atom as described above and this photon encounters another atom in a similarly excited state to the atom that emitted itself, it can stimulate the second atom to emit another photon of the same wavelength as itself and moving in the same direction – stimulated emission. Where you originally had one photon, you now have two identical photons, both moving along in step together. As these two photons encounter further atoms, further stimulation occurs, and so on.
A sort of chain reaction is set up which is further amplified by contriving to have the whole process take place in a cylindrical tube capped at either end by mirrors. These mirrors reflect the light back along its path again and again, thereby continually building up the stimulated emission. But, one of the mirrors is only "half silvered", meaning it only reflects some of the light and lets the rest through. And so, the laser beam of light is emitted from this end of the tube as a monochromatic(all the light is of the same wavelength), coherent (all the photons march in step with each other), very directionallight beam (strong and concentrated) that can travel a long distance with little energy loss or sideways spread.
I have described the general principle of the laser, but there are many different types of laser – the laser medium can be solid (eg ruby), liquid (organic dyes in liquid solution) or gas (eg helium, carbon dioxide). Because the eye focuses laser light just like any other light, eye damage is the chief danger in working with lasers. Laser light should not be viewed directly.
The uses to which lasers can be put is restricted only by the imagination. Powerful laser beams are used in industry to heat or vaporise material in a precise manner. Laser beams are used to study tectonic plate movements in geology, to cut and cauterise tissue in medicine, to transmit telephone signals, and lasers are widely used in scientific research. New applications for lasers are mushrooming and laser machines, like computers, continue to shrink in size. And remember, if you have a good new idea you must persevere in order to gain attention. Maiman’s first scientific paper on the discovery of the laser was turned down.
William Reville is associate professor of biochemistry and public awareness of science officer at UCC – understandingscience.ucc.ie