Arthur Ashkin invented optical tweezers that grab particles, atoms and molecules with their laser beam fingers. Viruses, bacteria and other living cells can be held too, and examined and manipulated without being damaged. Ashkin’s optical tweezers have created entirely new opportunities for observing and controlling the machinery of life.
Gérard Mourou and Donna Strickland paved the way towards the shortest and most intense laser pulses created by mankind. The technique they developed has opened up new areas of research and led to broad industrial and medical applications; for example, millions of eye operations are performed every year with the sharpest of laser beams
Ashkin creates his light trap
Figure 1. Ashkin creates a light trap, which becomes known as optical tweezers.
Figure 2. The optical tweezers map the molecular motor kinesin as it walks along the cell skeleton
Figure 3. The CPA technique revolutionised laser technology.
It enabled the emission of very intense, short pulses of light using an intricate method to avoid the risk of destroying the amplifying material. Instead of amplifying the light pulse directly, it is first stretched in time, reducing its peak power. Then the pulse is amplified and when it is compressed more light is collected in the same place – the light pulse becomes extremely intense.
Strickland and Mourou’s new technique, known as chirped pulse amplification, CPA, was both simple and elegant. Take a short laser pulse, stretch it in time, amplify it and squeeze it together again.
When a pulse is stretched in time, its peak power is much lower so it can be hugely amplified without damaging the amplifier. The pulse is then compressed in time, which means that more light is packed together within a tiny area of space – and the intensity of the pulse then increases dramatically.It took a few years for Strickland and Mourou to combine everything successfully. As usual, a wealth of both practical and conceptual details caused difficulties. For example, the pulse was to be stretched using a newly acquired 2.5 km-long fibre optic cable. But no light came out – the cable had broken somewhere in the middle. After a great deal of trouble, 1.4 km had to be enough. One major challenge was synchronising the various stages in the equipment, getting the beam stretcher to match the compressor. This was also solved and, in 1985, Strickland and Mourou were able to prove for the first time that their elegant vision also worked in practice.
The CPA-technique invented by Strickland and Mourou revolutionised laser physics. It became standard for all later high-intensity lasers and a gateway to entirely new areas and applications in physics, chemistry and medicine. The shortest and most intense laser pulses ever could now be created in the laboratory.
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