A team of physicists led by Dr Hrvoje Petek from the University of Pittsburgh’s Department of Physics and Astronomy has detected, for the first time, the exciton – a quasiparticle responsible for the transfer of energy within devices such as solar cells, LEDs, and semiconductor circuits – in a metal.
Scientists present direct evidence for excitons at a Ag(111) surface in the course of a three-photon photoemission process with 15 fs laser pulses.
Mankind has used reflection of light from a metal mirror on a daily basis for millennia, but the quantum mechanical magic behind this familiar phenomenon is only now being uncovered.
When light reflects from a metal mirror, it shakes the metal’s free electrons, and the consequent acceleration of electrons creates a nearly perfect replica of the incident light.
The classical theory of electromagnetism provides a good understanding of inputs and outputs of this process, but a microscopic quantum mechanical description of how the light excites the electrons is lacking.
The optical and electronic properties of metals cause excitons – particles of light-matter interaction – to last no longer than approximately 100 attoseconds. Such short lifetimes make it difficult for scientists to study excitons in metals, but it also enables reflected light to be a nearly perfect replica of the incoming light.
Dr Petek and his colleagues from the University of Pittsburgh and the Institute of Physics in Croatia have managed to observe excitons at the surface of a silver crystal.
They experimentally discovered that the surface electrons of silver crystals can maintain the excitonic state more than 100 times longer than the bulk metal, enabling the excitons in metals to be experimentally captured by a newly developed multidimensional coherent spectroscopic technique.
The results are published in a paper in the journal Nature Physics.
The ability to detect excitons in metals sheds light on how light is converted to electrical and chemical energy in plants and solar cells, and in the future it may enable metals to function as active elements in optical communications.
In other words, it may be possible to control how light is reflected from a metal.
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Xuefeng Cui et al. Transient excitons at metal surfaces. Nature Physics, published online June 01, 2014; doi: 10.1038/nphys2981
