3D light field lets scientists unlock electron states that were once impossible to reach

Some quantum states of electrons have remained out of reach not because they do not exist, but because scientists lacked the right kind of light to reach them.

Now, physicists in Germany have created an unusual three-dimensional light field by making two ultrashort laser pulses meet from different directions—and used it to access those elusive states.

“With our method, we can generate electronic quantum states that have previously only been described theoretically and also make them spatially visible,” Matthias Wollenhaupt, one of the study authors and a professor at the University of Oldenburg, said.

This achievement could give scientists a new way to control how light interacts with matter, study extremely fast quantum processes, and eventually distinguish molecules that are mirror images of each other.

How they turned light into a 3D tool

The researchers started with laser pulses lasting only about 20 femtoseconds. A femtosecond is one millionth of one billionth of a second, so these flashes are over almost before light has time to travel a few micrometers.

They shaped the pulses and separated them into two different colors. One pulse was redder, centered around 929 nanometers, while the other was bluer, centered around 720 nanometers. They then sent the two pulses toward the same point from different directions.

When the pulses met at an angle of 45 degrees, their electric fields combined in a way that made the resulting light field oscillate in all three spatial directions. The researchers could control its shape by changing the polarization of the two pulses.

This matters because ordinary laser fields do not provide the same freedom. Their electric fields are generally confined to a plane, which limits the ways they can push and manipulate electrons.

“We have thus expanded the experimental optics toolkit to include a new class of three-dimensional light fields,” Wollenhaupt added.

Using the new light to watch electrons

The team tested its approach on potassium atoms. They used the 3D light field to push electrons into higher-energy states and then knock some of those electrons out of the atoms.

The researchers could then examine the released electrons to learn about the quantum states they had occupied. Instead of simply detecting whether an electron had moved to a higher energy level, the experiment allowed them to observe how its quantum state was distributed in space.

They also turned the setup into an ultrafast imaging system. By changing the timing between laser pulses, they captured successive snapshots of the evolving electron state.

The resulting sequence was similar to making a very fast movie. Each snapshot showed the electron distribution at a slightly different moment, allowing the researchers to follow quantum motion that occurs on extremely short timescales.

The experiment showed that the 3D field could create electron states that conventional light fields cannot readily produce. That is important because scientists have long relied on the shape and direction of light to control which quantum states can be reached.

Why giving light a third dimension matters

The technique could eventually be useful far beyond individual atoms. One particularly promising application involves chiral molecules—molecules that come in two versions that are mirror images of each other, much like a person’s left and right hands.

Many molecules important to biology and medicine are chiral, including amino acids and some drug molecules. The two mirror-image forms can behave differently, making it important to identify and distinguish them.

Three-dimensional light fields could provide a new way to probe these subtle differences because the fields themselves can have chiral, or handed, properties. The researchers say their experiment lays an important foundation for using such fields to investigate molecular chirality.

The next challenge is to apply these controllable 3D light fields to increasingly complex systems and develop better ways of reconstructing the resulting electron motion. If that succeeds, light could become more than a way to illuminate or excite matter.

Its three-dimensional shape could become a tool for steering and observing quantum behavior itself.

The study is published in the journal Physical Review Research.