The first-ever photograph of light as both particle and wave

© 2015 EPFL

© 2015 EPFL

Since the days of Albert Einstein, scientists had been trying to observe light as a wave and as a particle at the same time. Now, for the very first time, researchers at the Swiss Federal Institute of Technology, Lausanne (EPFL) have succeeded in capturing a snapshot of both the particle- and wave-like behaviors of light at the same time.

A research team led by Prof. Fabrizio Carbone carried out the experiment with a clever twist: using electrons to image light with EPFL’s ultrafast energy-filtered transmission electron microscope – one of two in the world. The breakthrough work is published in Nature Communications.

See EPFL’s original press release with video here.

A new approach on a classic effect

When UV light hits a metal surface, it causes an emission of electrons. Einstein explained this “photoelectric” effect by proposing that light – thought to only be a wave – is also a stream of particles. However, there has never been an experiment able to capture both natures of light at the same time; the closest we have come is seeing either wave or particle, but always at different times.

The experiment by Prof. Carbone’s team was set up like this: A pulse of laser light is fired at a tiny metallic nanowire. The laser adds energy to the charged particles in the nanowire, causing them to vibrate. Light travels along this tiny wire in two possible directions, like cars on a highway. When waves traveling in opposite directions meet each other they form a new wave that looks like it is standing in place. Here, this standing wave becomes the source of light for the experiment, radiating around the nanowire.

This is where the experiment’s trick comes in: The scientists shot a stream of electrons close to the nanowire, using them to image the standing wave of light. As the electrons interacted with the confined light on the nanowire, they either sped up or slowed down. Using the ultrafast microscope to image the position where this change in speed occurred, the standing wave, which acts as a fingerprint of the wave-nature of light, could be visualized.

While this phenomenon shows the wave-like nature of light, it simultaneously demonstrates its particle aspect as well. As the electrons pass close to the standing wave of light, they “hit” the light’s particles, the photons. As mentioned above, this affects their speed, making them move faster or slower. This change in speed appears as an exchange of energy “packets” (quanta) between electrons and photons. The very occurrence of these energy packets shows that the light on the nanowire behaves as a particle.

“This experiment demonstrates that, for the first time ever, we can film quantum mechanics – and its paradoxical nature – directly,” says Prof. Carbone. In addition, the importance of this pioneering work can extend beyond fundamental science and to future technologies. As Prof. Carbone explains: “Being able to image and control quantum phenomena at the nanometer scale like this opens up a new route towards quantum computing.”

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