Taking a radically different experimental approach, EPFL scientists were able to take the first ever snapshot of light behaving simultaneously both as a wave AND as a stream of particles. You need light to take a photograph. But how do you take a photograph of light?
Quantum Mechanics tells us that light can behave simultaneously as a particle or a wave. However, no experiment had until now been able to capture both natures of light at the same time. Until eartlier this year, the closest we had ever come to it involves seeing either wave or particle, but always at different times.
In 1905, Albert Einstein explained the “photoelectric effect” by proposing that light – thought to be a wave only – is also a stream of particles. This discovery led to the quantum revolution. In 1914, Robert Millikan’s experiment confirmed Einstein’s law on photoelectric effect. In 1921, Einstein was awarded the Nobel Prize for “his discovery of the law of the photoelectric effect”, and Millikan was awarded the Nobel Prize in 1923 for “his work on the elementary charge of electricity and on the photoelectric effect.
Scientists have been trying to directly observe both of these aspects of light at the same time ever since.
Even though a variety of experiments have successfully observed both the particle- and wave-like behaviours of light, they have never been able to observe both at the same time. Now, scientists at EPFL have succeeded in capturing the first-ever snapshot of the dual behaviour of light.
The First Ever Photography of Light
The breakthrough work was published in March 2015 in Nature Communications in such esoteric terms as ‘Simultaneous observation of the quantization and the interference pattern of a plasmonic near-field’, and the abstract reads like this:
Surface plasmon polaritons can confine electromagnetic fields in subwavelength spaces and are of interest for photonics, optical data storage devices and biosensing applications. In analogy to photons, they exhibit wave–particle duality, whose different aspects have recently been observed in separate tailored experiments. Here we demonstrate the ability of ultrafast transmission electron microscopy to simultaneously image both the spatial interference and the quantization of such confined plasmonic fields. Our experiments are accomplished by spatiotemporally overlapping electron and light pulses on a single nanowire suspended on a graphene film. The resulting energy exchange between single electrons and the quanta of the photoinduced near-field is imaged synchronously with its spatial interference pattern. This methodology enables the control and visualization of plasmonic fields at the nanoscale, providing a promising tool for understanding the fundamental properties of confined electromagnetic fields and the development of advanced photonic circuits.
The team led by Fabrizio Carbone at the École Polytechnique Fédérale de Lausanne (EPFL) carried out an experiment with a clever twist: using electrons to image light. The result? The scientists managed to capture for the first time a single snapshot of light behaving simultaneously like a wave and a quantum of particles.
The experiment was set up as follows:
A pulse of laser light is fired at a tiny metallic isolated Ag nanowire suspended on a TEM grid with a few-layer graphene support layer.
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 travelling in opposite directions meet each other they form a new wave that looks like it is standing in place. This ‘standing wave‘ becomes the source of light for the experiment, radiating around the nanowire
This is where the experiment’s trick comes in !!
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, Carbone’s team could now visualize the standing wave, which acts as a fingerprint of the wave-nature of light.
While this phenomenon shows the wave-like nature of light, it simultaneously demonstrated 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.
For the first time ever, Carbone et al.’s experiment demonstrates that we can do something that had never been previously achieved! We can film Quantum Mechanics in action – along with its paradoxical nature!! We can do so directly!! Simultaneously!! At the same time!!
The importance of this pioneering work has the potential to extend way beyond fundamental science and to future technologies. Being able to image and control quantum phenomena at the nanometre scale like this opens up a new route towards quantum computing…