September 30, 2022

First ever image of an electron’s orbit within an exciton

First ever image of an electron’s orbit within an exciton Excitons are technically not particles, but quasiparticles (quasi- meaning "almost" in Latin). They are formed by the electrostatic attraction between excited, negatively charged electrons, and positively charged holes. Holes are spaces left behind by the excited electrons and are themselves a type of quasiparticle. Credit: OIST

In a world first, researchers from the Okinawa Institute of Science and Technology Graduate University (OIST) have captured an image showing the internal orbits, or spatial distribution, of particles in an exciton—a goal that had eluded scientists for almost a century.

Excitons are excited states of matter found within semiconductors.

Excitons are formed when semiconductors absorb photons of light, which causes negatively charged electrons to jump from a lower energy level to a higher energy level. This leaves behind positively charged empty spaces, called holes, in the lower energy level. The oppositely charged electrons and holes attract and they start to orbit each other, which creates the excitons. Excitons are crucially important within semiconductors, but so far, scientists have only been able to detect and measure them in limited ways. One issue lies with their fragility: it takes relatively little energy to break the exciton apart into free electrons and holes. Furthermore, they are fleeting in nature so in some materials, excitons are extinguished in about a few thousandths of a billionth of a second after they form, when the excited electrons “fall” back into the holes.

The instrument uses an initial pump pulse of light to excite electrons and generate excitons. This is rapidly followed by a second pulse of light that used extreme ultraviolet photons to kick the electrons within excitons out of the material and into the vacuum of an electron microscope. The electron microscope then measures the energy and angle that the electrons left the material to determine the momentum of the electron around the hole within the exciton. Credit: OIST
The instrument uses an initial pump pulse of light to excite electrons and generate excitons. This is rapidly followed by a second pulse of light that used extreme ultraviolet photons to kick the electrons within excitons out of the material and into the vacuum of an electron microscope. The electron microscope then measures the energy and angle that the electrons left the material to determine the momentum of the electron around the hole within the exciton. Credit: OIST

The researchers first generated excitons by sending a laser pulse of light at a two-dimensional semiconductor—a recently discovered class of materials that are only a few atoms in thickness and harbor more robust excitons.

After the excitons were formed, the team used a laser beam with ultra-high energy photons to break apart the excitons and kick the electrons right out of the material, into the vacuum space within an electron microscope.

n the physics of the very tiny, strange quantum concepts apply. Electrons act as both particles and waves and it is therefore impossible to know both the position and the momentum of an electron at the same time. Instead, an exciton's probability cloud shows where the electron is most likely to be found around the hole. The research team generated an image of the exciton's probability cloud by measuring the wavefunction. Credit: OIST
n the physics of the very tiny, strange quantum concepts apply. Electrons act as both particles and waves and it is therefore impossible to know both the position and the momentum of an electron at the same time. Instead, an exciton’s probability cloud shows where the electron is most likely to be found around the hole. The research team generated an image of the exciton’s probability cloud by measuring the wavefunction. Credit: OIST

Ultimately, the team succeeded in measuring the exciton’s wavefunction, which gives the probability of where the electron is likely to be located around the hole. (Phys.org, OIST)

The work has been published in Science Advances.

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