Physics

Image captures light as both wave and particle for very first time

Image captures light as both w...
Light simultaneously showing both wave pattern and particle energy attributes (Photo: Fabrizio Carbone/EPFL)
Light simultaneously showing both wave pattern and particle energy attributes (Photo: Fabrizio Carbone/EPFL)
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Light simultaneously showing both wave pattern and particle energy attributes (Photo: Fabrizio Carbone/EPFL)
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Light simultaneously showing both wave pattern and particle energy attributes (Photo: Fabrizio Carbone/EPFL)

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.

Source: EPFL

Two-in-one photography: Light as wave and particle! (sous-titres français)

10 comments
quax
The headline is very misleading. It makes the reader think that the standing waves are the 'particles'. But they are as much particles (not) as standing waves made of water.
The only thing 'particle' is the quantum exchange of the light field with the electrons used in creating this image.
(No offense to Colin Jeffrey, but couldn't these kind of articles be run by Gizmag's very own Brian Dodson to ensure the physics is correctly reported on? C'mon you have a physics PhD handy so put him to some good use!)
Maggie
... or maybe the egg carton-like effect observed is an interference effect given that the light is traveling from two Different directions...ie. Both ends of the wire. We see a similar phenomenon in geology called hummocky cross stratification where the bedform looks just like the photo above, however it is formed by waves travelling from 2 directions....causing the interference. Just a thought.
Doc Rock
I think, Maggie, that the interference effect is the whole point. That's how they got the standing wave. Heavy physics indeed. I really had to read it carefully.. Impressive. Couldn't have been done wiithout graphene, either.
Steve White
Surely there is no wave-particle duality, reality is wavy. It's just that to interact with the waviness energy has to be exchanged, which can only be done in multiples of discreet quanta - and that's what can make it look particle-ish. The experiment - amazing though it is - doesn't demonstrate that light is both a wave and a particle, but that energy isn't endlessly divisible.
Kevin Ritchey
I'm convinced. The picture is very pretty. But I still believe photons are small plasma packets acting as waves. I'm a believer in quantum mechanics. What photonic energies are composed of, I have no clue and it may still be some time before we know for sure. But I applaud the researchers for following their instincts and giving it the old college try. And thank you Colin for bringing this to our attention. Nicely done.
ringo the Baptist
Whew boy - gee, I wish somebody could make photons make sense.
Somebody please explain to me how we logically conclude that light is granular (photons) when the means of determining this "fact" (photo-electrons) are of this same granularity!
Surely this not unlike insisting that water only comes in bucket fulls, just because the only measure at our disposal is a bucket?
Can anybody point me to an explanation that LOGICALLY explains the quantum nature of light without resorting to simply parroting maxims and quotations?
I believe that much of confusion in the study of the nature of light is due to the idiosyncrasies of the human vision system.
A shower head analogy may help to give some idea of what I mean:
What we perceive as colour is not the colour of the water, but rather the water pressure inside the shower rose. What we perceive as brightness is not analogous to the water pressure, but rather the density of the holes across the face of the shower head.
In my mind this explains why colour is what delivers the motive force in the photoelectric effect.
There is no such thing as colour! Our vision system codes "luminous EMF" as colour, rather like the colour coding commonly used on geographic temperature maps.
Light “colour” is little more than a fairly common "biological convention" – it is not a physical reality.
The granular nature of the photoelectric effect is obviously a direct consequence of measuring light with something that is granular! (Photo-electrons.)
It makes no sense transferring the attributes of the measuring method onto the quantity that we are attempting to measure - surely? Water certainly comes in smaller quantities than buckets.
What will happen to quantum theory if we should one day contrive a means to measure light in increments thousands of times smaller than that which can liberate a photo electron?
What if the double slit experiment produces interference patterns with individual dots simply because of the coarseness of MATTER, and not because of the alleged mystical properties of “photons”?
Why are many compelled to believe that "photons" can tell when there is an alternative slit available, and then they conspire to mess with human heads by bunching up into fringes?
Anybody else on this bus? Can anybody explain why it is the wrong bus (without simply quoting mantras)?
Ever so ta!
Facebook User
@ringo the Baptist, in this case your scepticism serves you well. You cannot see or image photons. What you see in the image are the light field knots of a standing wave. Now the latter sounds far less exciting than the off-the wall headline.
And no you don't measure buckets of water with the same buckets. Nicely spotted that this doesn't make sense. You measure the light field by its interaction with electrons, the measurement of the latter are then used to reconstruct this image. All this is explained in the research but only hinted at in the pop-sci articles, because the latter are not there to educate, but just another form of clickbait.
As to your speculation that it is the structure of matter that makes light only come in form of quanta, you are in good company. Max Planck held the same believe. On the other hand you can apply quantum statistics to photons, but then again you can do the same to any quantum quasi-particle. At any rate the photon as a model should certainly not to be confused with a real particle that has rest mass. It really is nothing but a shorthand for a field's quantum energy that is only localized when interacting with matter.
EH
Steve White was the clearest in this thread, and Ringo and FB User also are on the right track. See Carver Mead's book "Collective Electrodynamics" for more on the wave-only perspective.
(Mead pioneered microcircuits, discovered that microelectronics work better when made smaller, leading his friend Moore to his famous law, co-taught courses with Richard Feynman and later worked on biomorphic / neural electronics and superconductors).
ringo the Baptist
Thanks EH - I will take a look at Collective Electrodynamics - but I would still like to hear some clear rationale for photon belief.
@Steve White "...energy isn't endlessly divisible."
With all due respect, what evidence is there for this?
Something finer than matter is needed when detecting light – and there IS something finer.
I believe this has already been done - but many interpret the results in a manner that flings Occam’s poor, battered razor out of the window. The simplest and most logical answer is most often the right one.
We KNOW that wave interaction produces interference patterns. We KNOW that matter is grainy. We KNOW that observing light (using matter) gets very grainy at low light levels. Why are we so eager to discard knowledge that has served us so well?
Back to the double-slit, low-light experiment These appear to be the only two explanations that are up for grabs:
FIRST EXPLANATION: Wild Leap ONE: Photo-electrons faithfully reproduce the true nature of light Therefore light from the two slits must arrive in lumps (photons).
Wild Leap TWO: Since these photons (when arriving one at a time) still accumulate to produce a speckled fringe pattern, they obviously possess the ability to “KNOW” that their mates have been arriving from different directions, and then, armed with this knowledge, they bias their courses to concentrate their crash sites in a fringe pattern. (Note that covering one slit at a time doesn’t fool these suckers, photons are “bright” ;-) have long memories, and know when not to do their dance)
SECOND EXPLANATION: The interference fringes only APPEAR grainy because of our crude and coarse means of detecting light, and the reason there are fringes at all is because CONTINOUS, SIMULTANIOUS light radiation approaching from two different directions is present. The fact that photomultipliers and photographic film and digital image sensors cannot detect a continuous arrival is only due to our severely limited light detection capabilities - they all depend on light dislodging photo-electrons.
Surely it makes far more sense to read the interference fringe as solid evidence that continuous waves ARE present?
This fringing IS the sensitive light detection method that I spoke of before – here we see light detecting light – and so light is clearly capable of existing in smaller quantities than is necessary to dislodge an electron. Unfortunately we can only "look" at light (and these fringes) using coarse, grainy matter and clunky photo-electrons.
What evidence is there to convince us that option B is wrong?
Any valid reasons for choosing option A must surely be able to be expressed in terms that any engineering mind could easily grasp.
Einstein is believed to have said that “if you can’t explain it simply, you don’t understand it well enough” – and I agree absolutely. If nobody is able to offer a clear explanation, then nobody really understands it – in which case it is religion, not science.
Can nobody walk us through the COMPELLING LOGIC for choosing A over B?
- or point us to a website that does?
It can’t be too difficult, come on guys!
spike50
Why does something seem out of place in the above "photo". I thought red "waves" @ approx. 640nM were longer than blue "waves" @ approx. 440nM. Just seems like an illustration out of kilter more than a photo.