2018-10-09

The 2018 Nobel Prize in Physics went to Gérard Mourou and Donna Strickland (the first female physics laureate for 55 years!) for their work on chirped pulse amplification – more on that below – and to Arthur Ashkin for his work on optical tweezers, which use powerful lasers to manipulate tiny objects and microorganisms using a combination of a focused laser beam’s electric field and radiation pressure. Radiation pressure being the tiny forces imparted by the absorption or reflection of photons, which, despite being massless, have momentum. Collectively, the prize was “for groundbreaking inventions in the field of laser physics.”

Chirped pulse amplification takes a little more explanation. It is a technique used to amplify high-intensity laser pulses beyond the usual intensity limits of the amplification media: Ultra-intense light causes optical phenomena such as Kerr self-focusing, where the refractive index of media increases with the intensity of the light, focusing the beam in on itself until expensive laser kit turns to plasma – no bueno! To avoid this, pulses are “chirped”, meaning that they are spread out according to the frequency of the light, so reducing their intensity. To achieve this, different frequencies are separated using diffraction gratings or prisms and then travel different path lengths. The component that does this is called the dispersive stretcher.

The chirped, and therefore stretched, pulse is then sent through the amplification media, which conveniently don’t get turned into plasma in the process. (The usual medium is titanium-doped sapphire crystals or “Ti:Sapphire”.) The amplified pulse can then be dechirped and so recompressed by a dispersive compressor, which works like a stretcher in reverse: the different frequencies are again separated but this time the frequencies that had longer path lengths before take shorter ones and vice versa, recombining the frequencies into an extremely intense pulse of the original length. Et voila! A massively powerful (potentially petawatt)  laser for doing useful science things with.

Categories: Alexander

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