How Does a Laser Work? Light That Marches in Step

An atom holds its electrons at fixed energy levels, and it can absorb a photon of exactly the right energy to lift an electron to a higher level, then release a photon of the same energy when the electron falls back, in a random direction at a random moment. That is ordinary emission, and it is how a neon sign glows. Einstein showed that there is a third possibility. If a photon of the right energy passes an atom that is already excited, it can trigger the electron to fall, and the photon that is emitted is an exact copy of the one that passed: the same energy, the same direction, the same phase, so that the two leave together in step. One photon has become two identical ones, and the two can become four. That is stimulated emission, and it is amplification.

The catch is that a passing photon is just as likely to be absorbed by an atom in the low state as to stimulate one in the high state, and in any ordinary material most atoms are low, so light is absorbed on balance rather than amplified. A laser needs the reverse, more atoms excited than not, which is called a population inversion and does not occur naturally. It is created by pumping: pouring energy into the material with a flash lamp, an electric discharge, another laser or a current through a semiconductor, using a material chosen so that the electrons linger in an upper level long enough to pile up there. Once more atoms are up than down, a photon passing through is more likely to be copied than absorbed, and the light grows as it travels.

Amplification on one pass is small, so the material is placed between two mirrors facing each other, and the light bounces back and forth, growing on every pass, until it is intense. One mirror is made slightly transparent, and the small fraction that leaks through it each time is the beam. The mirrors also select: only light travelling exactly along the axis stays in the cavity long enough to be amplified, and only wavelengths that fit a whole number of times between the mirrors reinforce themselves, which is why the beam is so straight and so pure in colour. The parts of any laser:

• •A gain medium, the material whose atoms are excited: a ruby crystal, a mixture of helium and neon, a gallium arsenide semiconductor, a dye, or a glass fibre doped with erbium
• •A pump that supplies the energy: a flash lamp, an electric current, or another laser
• •A cavity of two mirrors, one fully and one partly reflecting, that feeds the light back through the medium and defines the beam

Three properties follow from the physics. The beam is monochromatic, a single wavelength, which lets it be tuned to a particular atom or molecule in spectroscopy and to a particular pigment in surgery. It is coherent, the waves in step, which lets it form the interference patterns used in holograms and in the instruments that detected gravitational waves in 2015 by measuring a change in length smaller than a proton. And it is collimated, travelling in one direction with almost no spread, which lets it carry energy or information a long way: the beam sent to the Moon from Earth is only a few kilometres wide by the time it arrives, and the light in an optical fibre survives a hundred kilometres before it needs boosting. Focused to a point, a modest laser reaches intensities no other source can, enough to cut, weld, engrave, or vaporise a cataract.

The first laser, built by Theodore Maiman in California in May 1960 from a rod of synthetic ruby wrapped in a flash lamp, was described at the time as a solution looking for a problem. The problems arrived: barcode scanners, CD and DVD players, laser printers, fibre-optic communication, eye surgery, distance measurement, cutting and welding, guidance, and the tiny semiconductor lasers, cheaper than a light bulb, that are now made by the billion.

Films show laser beams as bright lines crossing a room, but a beam travelling through clean air is almost invisible from the side. Light only reaches your eye if it is heading towards you, and a collimated beam sends nearly all its light forwards. You see the line at a concert or in a classroom demonstration because dust, smoke or water droplets scatter a little of the light sideways. In space, with nothing to scatter it, a beam would be invisible until it hit something. Many lasers are invisible anyway, working in the infrared.

That is part of why laser safety matters. The lens of your eye focuses a parallel beam onto a tiny spot on the retina, concentrating its power far more than it would concentrate the light from a lamp, so even a few thousandths of a watt can cause harm if you look into it. Lasers are sold in safety classes according to their power and wavelength, and powerful infrared ones are especially dangerous because the eye cannot see them, so the blink reflex gives no protection. Shining a laser at an aircraft is a criminal offence in many countries.

Lasers are famous for heating and cutting, but one of their most surprising uses is making things colder than almost anything else in the universe. Atoms in a gas are always moving, and temperature is a measure of how fast. Tune a laser to a frequency slightly below the one an atom absorbs, and an atom moving towards the beam sees the light shifted up into resonance by the Doppler effect, so it absorbs photons and receives a small push against its motion each time. It then re-emits light in random directions, so on average it is slowed.

Aim pairs of such beams at a cloud of atoms from all six directions and every atom is slowed whichever way it moves, an arrangement physicists named optical molasses. The method brings atoms to within millionths of a degree of absolute zero, and three of its developers, Steven Chu, Claude Cohen-Tannoudji and William Phillips, shared the Nobel Prize in Physics in 1997. Laser cooling was the essential first step towards the Bose-Einstein condensate, a strange state of matter first made in 1995, and it lies at the heart of the caesium fountain clocks that help define the second and keep international time.

Lasers are sorted into safety classes according to how much harm their beam can do. Low-power lasers, such as those inside CD players and supermarket scanners, are designed so that the beam is either enclosed or too weak to damage the eye. Ordinary laser pointers sit in the lower classes and, in many countries, are legally limited to a few thousandths of a watt.

More powerful lasers sold online, often mislabelled, can damage the retina in an instant, and the damage is usually permanent. Shining any laser at an aircraft is dangerous because the beam spreads into a dazzling patch of light in the cockpit at a critical moment, and in the United Kingdom it has been a specific criminal offence since 2018. Never look into a laser beam or point one at a person.

A laser works by stimulated emission, in which a photon passing an excited atom triggers it to emit an identical photon in the same direction and phase. Pumping energy into a suitable material puts more atoms in the excited state than the ground state, so light is amplified rather than absorbed, and a pair of mirrors bounces it back and forth until an intense, straight, single-coloured, coherent beam leaks out through one of them.