The Nobel prize for physics goes for the invention of better lasers
PHYSICS has a reputation for being intimidating. But while the details can be devilish, the basic ideas are often quite simple. On October 2nd Sweden’s Royal Academy of Science awarded the Nobel prize in physics to a trio of researchers for improving the state of laser physics.
One share went to Arthur Ashkin, honouring his invention of optical tweezers. These are tiny laser beams that can be used to hold minuscule objects, such as biological cells, viruses or even individual atoms. They work because—as James Clerk Maxwell suggested in 1862, and Pyotr Lebedev proved in 1900—the photons that make up light carry momentum. This means they exert a pressure on any surface exposed to them.
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Dr Ashkin’s first invention was essentially the opposite of the tractor beams common in science fiction. Rather than pulling an object towards the laser emitter, he showed that he could use radiation pressure to push it away. Refinements soon followed. Laser beams are more intense in the middle. That generates a force which, counter-intuitively, tends to move the particle back towards the middle of the laser, trapping it within the beam. The addition of a microscopic lens, to focus the laser light even further, generates a pull force to oppose the push. The result is a device that can trap an item and move it about in three dimensions.
It sounds complicated, and working through the maths is not for the faint-hearted. But the Nobel committee demonstrated the basic principle with the aid of a hair dryer and a ping-pong ball. Anyone who can remember physics from school will recall that a hair dryer can levitate a ping-pong ball by trapping it within the current of hot air. Dr Ashkin’s method has since been used in all sorts of science, from probing the structure of tiny molecular machines in cells to assembling chemical compounds one atom at a time.
The other two shares of the prize honoured a different contribution. They were awarded to Donna Strickland (who thus became only the third female physics laureate) and Gérard Mourou for their work on boosting the power that lasers could achieve.
The first laser was built in 1960. Initially, their maximum intensity rose quickly, increasing more than 10,000-fold by 1970. At that point, though, progress stalled (see chart). It only got going again after Dr Strickland (who worked on the problem for her PhD thesis) and Dr Mourou (who was her supervisor) came up with the idea of chirped-pulse amplification.
The problem with generating high-intensity laser beams was that they tended to damage the machines used to make them. Once again, the details of Dr Strickland’s and Dr Mourou’s solution are fiendish. But the essence is simple—take a short-duration laser beam and make it last longer. The same amount of energy spread over a longer time leads to a lower maximum power. That means the resulting beam can then be amplified further without frying any sensitive components. The final stage is to compress the amplified beam back to its initial, short duration. That gives it an extremely high power. Modern lasers can, very briefly, reach a peak power of a petawatt, which is about 1m times more power than is generated by a nuclear power station.
High-power, short-duration lasers have all sorts of uses. The Nobel committee chose to focus on the familiar example of eye surgery, in which a laser beam is used to sculpt the surface of the eye in order to correct short-sightedness. Other uses include everything from industrial machining, via new types of particle accelerator, to the ability to probe the behaviour of matter on ultra-short timescales. Not bad for a PhD thesis.