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 -- a class
of materials that are key to many modern technological devices, such as
solar cells, LEDs, lasers and smartphones.
"Excitons are really unique and interesting particles; they are electrically
neutral which means they behave very differently within materials from other
particles like electrons. Their presence can really change the way a
material responds to light," said Dr. Michael Man, co-first author and staff
scientist in the OIST Femtosecond Spectroscopy Unit. "This work draws us
closer to fully understanding the nature of excitons."
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 -- 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.
"Scientists first discovered excitons around 90 years ago," said Professor
Keshav Dani, senior author and head of the Femtosecond Spectroscopy Unit at
OIST. "But up until very recently, one could generally access only the
optical signatures of excitons -- for example, the light emitted by an
exciton when extinguished. Other aspects of their nature, such as their
momentum, and how the electron and the hole orbit each other, could only be
described theoretically."
However, in December 2020, scientists in the OIST Femtosecond Spectroscopy
Unit published a paper in Science describing a revolutionary technique for
measuring the momentum of the electrons within the excitons.
Now, reporting on 21st April in Science Advances, the team used the
technique to capture the first ever image that shows the distribution of an
electron around the hole inside an exciton.
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.
The electron microscope measured the angle and energy of the electrons as
they flew out of the material. From this information, the scientists were
able to determine the initial momentum of the electron when it was bound to
a hole within the exciton.
"The technique has some similarities to the collider experiments of
high-energy physics, where particles are smashed together with intense
amounts of energy, breaking them open. By measuring the trajectories of the
smaller internal particles produced in the collision, scientists can start
to piece together the internal structure of the original intact particles,"
said Professor Dani. "Here, we are doing something similar -- we are using
extreme ultraviolet light photons to break apart excitons and measuring the
trajectories of the electrons to picture what's inside."
"This was no mean feat," continued Professor Dani. "The measurements had to
be done with extreme care -- at low temperature and low intensity to avoid
heating up the excitons. It took a few days to acquire a single image."
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.
"This work is an important advancement in the field," said Dr. Julien Madeo,
co-first author and staff scientist in the OIST Femtosecond Spectroscopy
Unit. "Being able to visualize the internal orbits of particles as they form
larger composite particles could allow us to understand, measure and
ultimately control the composite particles in unprecedented ways. This could
allow us to create new quantum states of matter and technology based on
these concepts."
Reference:
Michael K. L. Man, Julien Madéo, Chakradhar Sahoo, Kaichen Xie, Marshall
Campbell, Vivek Pareek, Arka Karmakar, E Laine Wong, Abdullah Al-Mahboob,
Nicholas S. Chan, David R. Bacon, Xing Zhu, Mohamed M. M. Abdelrasoul,
Xiaoqin Li, Tony F. Heinz, Felipe H. da Jornada, Ting Cao, Keshav M. Dani.
Experimental measurement of the intrinsic excitonic wave function. Science
Advances, 2021; 7 (17): eabg0192 DOI:
10.1126/sciadv.abg0192