In 1905, Albert Einstein provided an explanation of the photoelectric effect – that various metals emit electrons when light is shined on them – by suggesting that a beam of light is not simply a wave of electromagnetic radiation, but is also made up of discrete packets of energy called photons. Though a long accepted tenet in physics, no experiment has ever directly observed this wave/particle duality. Now, however, researchers at the École polytechnique fédérale de Lausanne (EPFL) in Switzerland claim to have captured an image of this phenomenon for the first time ever.
To achieve this, a team of researchers led by Assistant Professor Fabrizio Carbone at EPFL has performed an experiment using electrons to image light.
In essence, the team used extremely short (femtosecond) pulses of laser light directed at a miniscule nanowire made of silver and suspended on graphene film that acted as an electrical isolator (or metal-graphene dielectric). The laser light pumped energy into the system that then directly affected the charged particles in the nanowire, causing them to vibrate and effectively making the nanowire behave as what is known as a quasi-1D plasmonic nanoantenna.
In other words, the nanowire acted as a tiny antenna that generated radiation patterns in sympathy with the received laser excitation. This laser light then oscillated back-and-forth between the two ends of the nanoantenna and, in so doing, set up a standing wave of surface plasmon polaritons (electromagnetic waves that travel along the surface of a metal-dielectric or metal-air interface) in the wire.
Put simply, the light traveled along the wire in two opposite directions and, when these waves bounced back to the middle, they intersected with each other to form a new wave that appeared to be standing in place. This standing wave, radiating around the nanowire, then became the source of light used in the experiment.
Next, the researchers aimed a stream of electrons into the field generated around the nanowire, and used them to image the standing wave of light. When the electrons intermingled with the restrained light contained on the nanowire – that is, where they crashed into individual photons – they either sped up (gained energy) or slowed down (lost energy).
The team then used an imaging filter to select out only those electrons that had gained energy, and focused a UTEM (ultrafast transmission electron microscopy) instrument on these to image where each of the changes in energy state occurred, thereby allowing them to visualize the standing wave and make visible the physical makeup of the wave-nature of the light.
Simultaneously, this also demonstrated the particle nature of the imaged light by demonstrating that the change in speed of the interacting electrons and photons shows as an exchange of energy "packets" (quanta) between the electrons and the photons. This demonstrated that the light on the nanowire was also behaving as particles.
"This experiment demonstrates that, for the first time ever, we can film quantum mechanics – and its paradoxical nature – directly," said Professor Carbone.
Professor Carbone also believes that this experiment not only illustrates the physical observation of the wave/particle duality of light, but it is another step toward the realization of light-based quantum devices and future technologies.
"Being able to image and control quantum phenomena at the nanometer scale like this opens up a new route towards quantum computing," he says.
The research was a collaboration between the Laboratory for Ultrafast Microscopy and Electron Scattering of EPFL, the Department of Physics of Trinity College (US) and the Physical and Life Sciences Directorate of the Lawrence Livermore National Laboratory.
The results of this research were recently published in the journal Nature Communications
The short video below shows an illustrated representation of the experiment.