The Science of Existence (44-10-7) Excitons


Quasiparticles exist even when nothing is there. American physicist William Shockley Jr. was working with semiconductors when he had an epiphany that permitted the perfection of the transistor in 1947.

It had been known for a decade that electrons moving through semiconductors left gaps of nothingness. But no one thought of these “holes” as anything more than an electron’s absence.

Shockley proposed to treat holes as particles in their own right: like an electron but with a positive charge. This crucial paradigm shift led to better understanding the flow of energy in semiconductors, and so fashion the junctures and switches that characterize transistors.

Since then physicists have conceived that electrons and holes can combine, yielding a whole new quasiparticle: the exciton. Plants were way ahead of us on this. The light-harvesting proteins responsible for photosynthesis use electrons to absorb photons of sunlight. The resultant energy kick knocks an electron out of position, creating a hole.

The electron and hole link up to form an exciton, which is shuttled about the photosynthetic machinery. When the exciton gets to where it’s needed to do its bit, the electron and hole recombine, releasing energy employed to split water into constituent hydrogen and oxygen; a key step in making sugar from sunlight, air, and water.