By Erik Streed, Griffith University
My colleagues and I at Griffith University have been able to photograph the shadow of an atom for the first time – the culmination of five years of work by our team.
The image, and attendant paper, are published today in the journal Nature Communications.
So, in a nutshell, how did we get the image? The following analogy might help.
On a sunny day at the beach, your shadow is a constant companion. Holding your hand up will block the bright sun, but a few rays will still penetrate the thinner parts of your fingers.
If we were to take a closer look using a microscope we would see dark strands of tightly wound DNA in the nucleolus (composed of proteins and nucleic acids found within the nucleus) of the skin cells. Looking closer still, we might wonder: how small can something be and still cast a shadow?
The picture leading this article shows the shadow cast in a laser beam by a single Ytterbium atom suspended in empty space. At Griffith University, we have has pioneered the use of Fresnel lenses (a type of lens for large aperture and short focal length – producing an ultra hi-res miscroscope) to capture high-resolution images of atoms.
Our lens is like a smaller versions of the lenses used in lighthouses – both have many separate segments all working in concert to focus the light.
The figure above shows how a laser beam (orange) passing by a single atom (blue) leaves a dark shadow in its wake, with the actual picture of the single atom shadow shown on the right end.
Since a single atom casts a very small shadow, our advances allowed us to be the first to take a picture of this effect. The size of the shadow is set by the wavelength of light, which is about a thousand times larger than the actual atom.
We hold the Ytterbium atom in empty space by removing one of its electrons and using high voltage electricity to fix its position. Ytterbium was chosen because we could build lasers of the right colour to be strongly absorbed by the atom.
Our work has implications for research ranging from quantum computing to microbiology. In quantum computing, light is the most effective method for communication, while atoms are often better for performing calculations.
In observing the shadow from a single atom we have shown how to improve the input efficiency in a quantum computer. Single atoms have well-understood light absorption properties. We used this knowledge to predict how dark the shadow should be for a given amount of light.
Since Dutch scientist Antonie van Leeuwenhoek's first observations of red blood cells in 1674, absorption microscopy has played a prominent role in biology. X-ray and ultraviolet light are very useful for imaging cells but can also damage them at high dosages.
By knowing how much light is required to achieve a particular image quality, our work will be useful to predict when a little damaging light is enough to take a good image.
We're pleased to be the first to capture a snap of the long shadow from an single atom's dark side.
Erik Streed receives research funding from the Australian Research Council. He is presently a Lecturer in Physics at Griffith University's Gold Coast campus.