Seeing the Light Fantastic – Particles, Waves and All

This seminal photograph shows, for the first time, light in all its beauty, simultaneously seen as a wave and as a particle.Light in All its Beauty

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 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.

 

The Photoelectric Effect

 

When UV light hits a metal surface, it causes an emission of electrons.

According to Classical Electromagnetic Theory, the ‘photoelectric effect‘ can be attributed to the transfer of energy from the light to an electron in the metal.  Viewed from this perspective, any alteration in amplitude or wavelength of light ought to induce changes in the rate of emission of electrons from the metal.  According to this theory, a sufficiently dim light would be expected to show a lag time between the initial shining of its light and the subsequent emission of an electron.  However, the experimental results did not correlate with either of the two predictions made by this theory.

A graphic illustrating the photoelectric effect. Under infrared radiation, no electrons emitted. Under visible or ultraviolet light, electrons emitted depending on the surface material. Under X-rays, electrons always emitted. Under gamma-rays, electrons always emitted.
Photoelectric Effect Explained  Source: ESA

Instead, electrons are only dislodged by the photoelectric effect if light reaches or exceeds a threshold frequency, below which no electrons can be emitted from the metal regardless of the amplitude and temporal length of exposure of light.  To make sense of the fact that light can eject electrons even if its intensity is low, Albert Einstein proposed that

A beam of light is not a wave propagating through space, but rather a collection of discrete wave packets (or photons), each with energy hf.

 

This idea shed revolutionary new light on Max Planck’s previous discovery of the Planck relation, that is

E_{photon} = hf

where the photon energy E is proportionally linked to the frequency f.

The factor h is now known as the Planck constant.

 

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.

 

A graphic illustrating the experimental set-up and apparatus used to realize the first ever photograph of light.
A schematic of the experimental set-up.  Light and electron pulses at a variable time delay are spatially overlapped on an isolated Ag nanowire suspended on a TEM grid with a few-layer graphene support layer.  Probing electrons are detected using a CCD camera after passing through an electron imaging filter.  Source: Carbone et al., 2015

Experimental Set-Up

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.

 

Results

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 of light, they “hit” the light’s , 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.

A graph documenting the first ever photograph of light experiment. And maps of the electron energy loss intensity.
Map of the electron energy loss intensity versus the relative time delay Dt between the optical pump and electron probe pulses, taken on a single photoexcited nanowire (5.7 mm length, C67nm radius).  Excitation wavelength and polarization angle are 800nm and j¼45, respectively.  Energy spectra at negative (Dt¼ 1.6 ps, black trace) and zero delay (Dt¼0 ps, orange trace) are superimposed.  The intensity in both the map and the spectra is plotted on a logarithmic scale.  From c to g, snapshots of an isolated nanowire at different time delays obtained using only the electrons that have gained energy, that is, those in the region indicated by the white arrow in map above.  Electron counts are on a linear scale.  The vertical scale bar in snapshot c corresponds to 2 mm and holds for all images. Source: Carbone et al., 2015

 

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…