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Presentation Speech by Professor Anders Irbäck, Member of the Royal Swedish Academy of Sciences; Member of the Nobel Committee for Physics, 10 December 2018

Your Majesties, Your Royal Highnesses, Honoured Laureates, Ladies and Gentlemen,

Sunlight is essential to life on Earth, and we are getting better and better at harnessing its energy. We also use light in many other ways, today often employing a laser as the light source. Two examples of lasers in everyday use are bar code readers and laser pointers. This year’s Nobel Prize in Physics honours two inventions in laser physics that have led to pathbreaking new ways of using light. These methods have given us light-based tools with applications in medicine and other fields.

Arthur Ashkin is being rewarded for the invention of optical tweezers, and their application to biological systems. This invention is based on the ability of light to exert a force on matter, known as radiation pressure. The possibility of using light, via this force, to move physical objects may make one think of Star Trek and tractor beams, and sound like pure science fiction. Of course we can feel that sunbeams carry energy – they make us warm. But we cannot feel any small push – the force of this sunlight is too weak to do so. The starting point that led Ashkin to his optical tweezers was an experiment aimed at showing that the radiation pressure in an intense laser beam is actually strong enough to move microscopic particles. As it turned out, laser light could not only move such particles, but could also be made to grab them by focusing the beam with the help of a lens. This marked the birth of the optical tweezers, an elegant tool that has gained a broad range of applications and that lets us hold on to and move objects such as living cells without touching them. Ashkin’s method has been successfully used to investigate various components of biological cells, among other things providing us with knowledge about the mechanics of tiny molecular motors that perform vital work inside these cells.

Gérard Mourou and Donna Strickland are being rewarded for an invention called chirped pulse amplification, or CPA. This is a method for creating extremely intense, short pulses of laser light. Efforts to create more intense laser pulses had been under way since the first laser was built in 1960. But by the mid-1980s these experiments had reached an impasse, since the intensity of the laser light destroyed the amplifying material itself. Using their CPA technique, Mourou and Strickland were able to get around this limitation. Their strategy was simple and elegant: First stretch out the laser pulse in time, thereby reducing its intensity and allowing it to be amplified. Finally, compress the pulse again to its original length of time, but now with much higher intensity. This method changed the landscape of research about high-intensity lasers, from something that had been carried out in a few large laboratories to something that could be done in many places around the world, leading to a powerful surge in development work. The pursuit of shorter and shorter pulses has enabled researchers to move closer to the attosecond level – which means one billionth of a billionth of a second. This opens the way for studying the movements of electrons in atoms and molecules. Numerous applications of laser pulses are made possible by the CPA technique. One example is eye surgery for correcting near-sightedness, in which laser pulses serve as ultra-precision surgical tools.

Dr Ashkin, Professor Mourou, Professor Strickland.
You have been awarded the 2018 Nobel Prize in Physics for your ground-breaking inventions in the field of laser physics. On behalf of the Royal Swedish Academy of Sciences it is my honour and great pleasure to convey to you our warmest congratulations. I now ask you to step forward to receive your Nobel Prizes from the hands of His Majesty the King.