Magnetics Business & Technology - March/April 2025 - 30

VISIONS
Making Light 'Feel' a Magnetic Field Like an Electron Would
Scanning electron microscope image of a " strained " photonic
crystal showcases the curved pull that allowed researchers to see a
property in photon behavior that was previously observed in electrons.
Researchers have now experimentally made light effectively
" feel " a magnetic field within the complicated structure of the
photonic crystal, which is made of silicon and glass. Credit: Rechtsman
laboratory / Penn State.
Unlike electrons, particles of light are uncharged, so they do
not respond to magnetic fields. Despite this, researchers have
now experimentally made light effectively " feel " a magnetic
field within a complicated structure called a photonic crystal,
which is made of silicon and glass. Within the crystal, the light
spins in circles and the researchers observed, for the first time,
that it forms discrete energy bands, paralleling a well-known
phenomenon seen in electrons. This finding could point to new
ways to increase the interaction of light with matter, an advance
that has the potential to improve photonic technologies,
like very small lasers.
Mikael Rechtsman,
Professor of Physics
at Penn State
This work, led by researchers at Penn
State, was based on an earlier theoretical
prediction by team members
Penn State Professor of Physics
Mikael Rechtsman, Penn State graduate
student Jonathan Guglielmon and
Columbia University mathematician
Michael Weinstein. A paper describing
the experiments published in April 2024
in the journal Nature Photonics alongside
another paper by a separate group
of researchers in the Netherlands, led
by Ewold Verhagen, who independently
observed the same phenomenon.
" For charged particles like electrons, there is a lot of interesting
physics that results from their interactions with magnetic
fields, " said Rechtsman, the leader of the research team.
" Because of this, there has a been an interest in emulating
this physics for photons, which are not charged and so do not
respond to magnetic fields. "
When electrons confined to a two-dimensional surface are
exposed to a strong magnetic field they move in circular, or
" cyclotron, " orbits. The motion of these orbits becomes quantized
- the electrons become constrained to certain discrete
energies, which are called Landau levels.
" Landau levels are sort of akin to the energy levels of electron
30 Magnetics Business & Technology * March/April 2025
orbitals around the nucleus of an atom, " Rechtsman said. " In
an atom, the energy levels result from the attraction of negatively
charged electrons to the positively charged nucleus,
whereas Landau levels result from the interaction of the electrons
with a magnetic field. We employed a method of emulating
a magnetic field - called a pseudomagnetic field - for light
by precisely manipulating the structure of a photonic crystal. "
The research team creates these crystals in tiny slabs of
silicon, similar to what is used to make computer chips, at the
Nanofabrication Laboratory within the Materials Research
Institute at Penn State. They create a honeycomb-like lattice of
holes within the silicon slab, which is only 1/1000th the thickness
of a human hair. The researchers shine laser light into
the crystal-containing slab, and the lattice pattern causes some
of the light to bounce around within the crystal. The team can
then measure the spectrum of the light when it exits the crystal.
To mimic the effects of a magnetic field, the researchers add a
" strain " to the pattern of the lattice.
" For the unstrained lattice, we fabricated a honeycomb structure
out of nanoscale triangular holes that repeats throughout
space in a two-dimensional pattern, " Rechtsman explained.
" To add the strain, we made another slab, but deformed the
pattern. The new pattern looks as if we pulled up on the two
sides, while pulling down on the bottom side. "
When the researchers shine the laser into the unstrained lattice,
the light spreads out evenly in the crystal. In the strained
lattice, the light instead moves in circles and the energy spectrum
of the light changes, forming discrete bands just like Landau
levels. Unlike Landau levels in electrons, the energy bands
are not flat. Instead, they are curved, which the researchers
said results from the curved pattern in the strained crystal.
" The curved nature of the bands is known as dispersion, "
Rechtsman said. " To try to mitigate the dispersion, we added
an additional strain to the pattern. This added strain, which
acts as a pseudo-electric potential, counteracts the dispersion,
giving us flat-band Landau levels just like those from electrons. "
The
flat bands represent a concentration of photons at certain
discrete energies, providing an avenue to increase the interaction
of light with matter.
" There's a bunch of applications where increasing the interaction
of light and matter can improve their function, " Rechtsman
said. " When you have flat bands, that means that the light
is sticking around in one place for longer, which means that
whatever you're trying to do with the light, you can do it more
efficiently. Right now, we're looking into whether we can use
this design for more efficient lasers on photonic chips. "
To carry out this research, Rechtsman's group teamed up with
Randall McEntaffer and Fabien Grisé, X-ray astronomers in
Penn State's Department of Astronomy and Astrophysics, who
are experts on highly precise nanofabrication. Graduate students
Maria Barsukova and Zeyu Zhang in Rechtsman's group
took the lead carrying out experiments and numerical simulations,
with the help and guidance of another graduate student,
Sachin Vaidya. The team also collaborated with Li He and Bo
Zhen of the University of Pennsylvania.
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