Guest post by Congcong Huang, Associated Editor, Nature Communications and Nicky Dean, Team Manager Physics, Nature Communications.
This week we conclude our series of ‘beautiful experiments with light’ featured in our poll and finally reach the new millennium in which lasers continue to enable powerful and diverse experiments.
Our story begins with the generation of ultrafast laser pulses. Following the invention of lasers in 1960 enormous efforts were made to shorten the pulse duration which led to femtosecond lasers in the late 1980s and finally in 2001 to the first reports of attosecond laser pulses. In one attosecond (10-18 s), light travels slightly more than the length of a water molecule, while molecules are essentially frozen during this time, with molecular vibration at femtosecond (10-15 s) and rotation at picosecond (10-12 s) timescales. This makes it possible to access the timescale of electron dynamics inside molecules.
As light pulses have been made ultrashort –short enough even to capture the motion of electrons – a natural question is whether the speed of light can be controlled to the same extent. It is not surprising that light slows down when it travels through glass or water, but this is only a modest effect. It was thus a stunning observation, made by Lene Hau and her group in Harvard in 1999, that light travels at a cycling speed – 7 orders of magnitude slower than c – in a sodium atom cloud right below its Bose-Einstein condensation temperature. The cold atoms alone cannot do the work; the use of a laser field that efficiently cancels light absorption, known as electromagnetically-induced transparency, makes the trick possible. The demonstration sparked a new chapter for laser controlled optical materials.
Meanwhile, more attempts at controlling the behavior of light were underway. As mentioned above, light slows down when it passes through a medium, an effect characterized by the medium’s refractive index. This index is normally positive, and it tells us how light rays will be bent when they move from one medium into another. You can see this effect by looking at a straw in a glass of water which appears to be sharply bent at the surface. In the late 1960s, Victor Veselago wondered what might happen if the refractive index was negative. He predicted that light entering such a medium should bend in the opposite sense to what we normally expect (as if the straw would bend the ‘wrong’ way). In 2001, David Smith and colleagues realized this prediction by constructing an artificial material, or ‘metamaterial’, made of an array of copper split-rings on circuit boards. Their metamaterial exhibited a negative refractive index at around 10 GHz. Following Smith’s demonstration, many more negative-index metamaterials have been made using all kinds of different structures, across a range of frequencies, including the visible spectrum.
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Diffraction pattern of a virus particle, taken with an X-ray free electron laser.
Our final beautiful experiment is the first operation of a special type of light source in 2010, the X-ray free-electron laser. The term was coined by John Madey in the 1970s who hypothesized the generation of coherent X-ray radiation from a beam of electrons accelerated to approach the speed of light. Given the need for a large facility accelerator, it was not until recently that lasing at Ångstrom-wavelength was first achieved, at the Linac Coherent Light Source in Stanford. This new laser lights up a largely unexplored world of structure and dynamics at the atomic scale given the unprecedented combination of high pulse energy (at keV), peak power (1010 W) and short pulse duration (tens of femtoseconds). It provides a new view for example into the three-dimensional structure of nanocrystals, proteins and viruses.
This brings us back to 2015, our Year of Light. There has been no slowing down in pace of scientific discoveries with light and we can be sure to expect more surprises, from microscopic to astronomical findings, from quantum to classical applications. We hope you enjoyed our overview of beautiful experiments and have voted for your favourites in the poll (link) which will be open for another week. Stay tuned to find out the results on this blog in May.
Experiments covered this week:
2001 Generation of attosecond light pulses
1999 Light slowed down to cycling speed in atomic clouds
2001 Negative refraction with metamaterials
2010 First light from a hard X-ray free electron laser