Let light “feel” magnetic fields like electrons

Unlike electrons, light particles are not electrically charged, so they do not respond to magnetic fields. Nonetheless, researchers have now experimentally made light effectively “feel” magnetic fields in a complex structure called a photonic crystal, which is made of silicon and glass.

Within the crystal, the light swirled in circles, and the researchers observed for the first time that it formed discrete energy bands called Landau levels, similar to a well-known phenomenon common in electrons.

The discovery could point to new ways to increase the interaction of light with matter, an advance that could lead to improvements in photonic technologies such as very small lasers.

The work, led by Penn State researchers, is based on early theoretical predictions by team members Penn State physics professor Mikael Rechtsman, Penn State graduate student Jonathan Guglielmon, and Columbia University mathematician Michael Weinstein.

A paper describing the experiment was published in the journal on April 23 Nature Photonics A separate paper was published alongside this, by a team of Dutch researchers led by Ewold Verhagen, who independently observed the same phenomenon.

“For charged particles like electrons, their interaction with magnetic fields can produce many interesting physical phenomena,” said Reitzman, the leader of the research team. “Because of this, there is interest in simulating the physics of photons, which are uncharged and therefore do not respond to magnetic fields.”

When electrons confined to a two-dimensional surface are exposed to strong magnetic fields, they move along circular or “cyclotron” orbits. The motion of these orbits becomes quantized—the electrons are confined to certain discrete energies called Landau levels.

“The Landau energy level is somewhat similar to the energy level of electron orbits around the nucleus,” Reitzman said. “In atoms, energy levels are generated by the attraction of negatively charged electrons to positively charged nuclei, while Landau levels are generated by the interaction of electrons with magnetic fields. We used a method that simulates magnetic fields — called pseudomagnetic fields — produce light by precisely manipulating the structure of photonic crystals.

The team created the crystals in tiny silicon wafers, similar to those used to make computer chips, in Penn State’s Materials Institute’s Nanofabrication Laboratory. They created a honeycomb-like lattice of cells within a silicon plate that is only 1/1,000th the thickness of a human hair.

The researchers shined a laser onto a flat plate containing crystals, and the lattice pattern caused some of the light to reflect within the crystals. The team can then measure the spectrum of light as it leaves the crystal. To simulate the effects of magnetic fields, the researchers added “strain” to the lattice pattern.

“For the strain-free lattice, we created a honeycomb structure with nanoscale triangular holes that repeat throughout space in a two-dimensional pattern,” Rechtsman explains. “To increase the pressure, we made another plate but deformed the pattern. The new pattern looked as if we were pulling up on the sides and pulling down on the bottom.”

When the researchers shined a laser into the strain-free lattice, the light spread evenly throughout the crystal. In the strained lattice, light moves in a circular pattern, and the energy spectrum of the light changes, forming discrete bands that act like Landau levels. Unlike the Landau levels in electrons, the energy bands are not flat. Instead, they are curved, which the researchers say is caused by bending patterns in the strained crystals.

“The bending property of the energy bands is called dispersion,” Reitzman said. “To try to mitigate the dispersion, we added an additional strain to the pattern. This additional strain acted as a pseudopotential, canceling out the dispersion and giving us flat-band Landau levels, just like the flat-band Landau from the electrons The energy levels are the same.

Flat bands represent concentrations of photons of specific discrete energies, providing a way to increase the interaction of light with matter.

“In many applications, increasing the interaction between light and matter can improve their functionality,” Reitzman said. “When you have flat bands, it means the light stays in one place longer, which means whatever you want to do with light, you can do it more efficiently. Now, we’re looking at whether we can combine this This type is designed for more efficient lasers on photonic chips.