Director, Head of Attosecond- and High-Field Physics Division, Max Planck Institute for Quantum Optics Munich, Chair of Experimental Physics, Ludwig-Maximilians-Universität München, Germany.
Breaking the Wall of Microscopic Motion. How Attophysics Captures Fastest Phenomena of the Microcosm.
Good evening ladies and gentlemen. It is a great honour for me to be here and be allowed to tell you about breaking the walls of gaining access to the fastest motions in the microscopic world. You all know the proverb: seeing is believing. It wonderfully expresses the importance of vision. But, have you ever asked yourself how this capability actually comes about and how it can possibly be extended to observe the fastest motions in the microscopic world? So, in my talk I would like to show you how all these capabilities actually relate to a wonderful symbiosis between electrons and light.
Well, for vision, what we need in the first place is light; light emerges in nature from the electrons’ microscopic oscillatory motion. So, if you zoom in now into this light bulb, the green clouds here represent the charged distribution of electrons and atoms oscillating around the core. The oscillation frequency determines the colour of the emitted light. Once this light reaches our eye, it impinges on the rhodopsin molecules in our retina where the light waves bring electrons into oscillation again. This electron oscillation creates an electric signal, which is afterwards transmitted through our nerves again by electrons – through our nerves to our brain where the image is formed. So, the bottom line is: electron motion is responsible for both the emission and detection of light. Put it the other way around – from the perspective of light waves – they originate from the electrons’ microscopic motion and can also induce this motion. So, it is obvious that these two motions, light waves and the electron oscillations, are very closely connected. They are equally rapid, both evolving on an attosecond timescale.
So, how brief are attoseconds actually? The most human unit of time is certainly the second, which is the approximate duration of our heartbeat. One nanosecond is a billion times shorter than one second and defines the characteristic scale for modern electronics. One attosecond is another billion times faster than one nanosecond. Now, how can we possibly capture motions that take place on such a short timescale? I guess you all know the basic concept. To recapitulate it for you, I would like to present a small, very primitive demonstration.
This object, representing a molecule, is moving and in this case is actually rotating so fast that our eye is unable to follow it. If we now illuminate this fast moving object by short flashes of light, we can actually freeze the motion, as you see here, and in the time the light flashes properly, we can even realise a slow motion replay. So, we may draw the conclusion that access to very fast motions requires short flashes of light. Now, particles in the microscopic world move a trillion to a thousand trillion times faster than this dummy molecule; so, we obviously need trillions of thousand trillions times shorter pulses to capture them. These much shorter pulses can be produced by lasers, by simultaneously generating light waves in many different colours and summing them. If we add all the waves of visible light together, we can produce pulses that are as short as shown here, just three femtoseconds. However, even these pulses are far too long to capture the fastest electrons that can occur in nature.
This wave, this white line, actually shows us how the force that the electric field of this light wave exerts on charged particles like electrons actually varies in time. It has been the control of this force that allowed us both to generate and measure attosecond pulses, which constitute the key to capturing electron motion in atoms. For this purpose, we focus these few femtosecond pulses consisting of just one single wave period with a controlled waveform into an assembly of atoms, which is kept in a small tube in a vacuum chamber. If we now zoom onto one atom, we can follow how the charg distribution of the weakest bound electron of this atom responds to the electric field of this light wave. Once this field, once this light force, becomes strong enough near the peak of the pulse, the electron can be liberated from its atomic binding and first pulled away from the atom by the strong force, but a little later the force reverses its direction and pushes the electron back to the vicinity of the core, where the re-encounter of the electron with its parent atom emits a short burst of extreme ultraviolet light.
Now, you remember that I mentioned at the beginning that it is the cooperation between light and electrons that allow access to the microscopic world. Here, you see what I meant by that. The light wave first sets the electron in motion. Afterwards, the electron comes back, emits an even shorter light pulse, and as you will see in a few seconds, we use at the end these very short attosecond pulses for actually following the motion of the electrons in real time. So, that is how the circle is closed at the end. So, this process that I have shown you here, actually takes place simultaneously in millions of atoms, and that is how actually a macroscopic pulse with a macroscopic intensity is set up. Before we can set out to use them for capturing electron motion, we have to measure them. That is what we devised – a new concept, a new device, which we dubbed a light-field-driven streak camera for. Here you see the first result that we obtained with this back in 2004, which yielded 250-attosecond pulses a little later. We could shorten these pulses down to 80 attoseconds, which constitute the shortest isolated events produced and measured to date.
As a first application, we can use these attosecond pulses to actually measure the waveform of light of the light wave, which we used previously for its generation. With these tools, with this field cycle laser wave and the attosecond pulse, we can now set out to capture electron motion with a never-before-achieved resolution. Just as snapshots of a macroscopic object can be taken with a high-speed camera, here with a microsecond exposure time, and from these snapshots the motion can be reconstructed in slow motion and replayed to make it perceivable to our eyes. Attosecond pulses allow, meanwhile, to take very similar snapshots of electrons moving deep inside atoms, as you see here from the series of pictures. Again, from these snapshots we can reconstruct the motion in a slow motion replay. For this purpose, we had to slow down time, or in the language of microscopy, we had to magnify time by a factor of 1015. You see, this is a really big number. Believe it or not, it is even bigger than the total debt of all countries of the European Union in Euros – hard to believe, but it is true.
So, what is this good for? Well, you have seen at the beginning of my talk how electron motion in molecules is responsible for the detection of light and allows us to see the world around us – thanks to this motion – and how this motion actually is able to transport biological information through our nerves – to mention just two functions, two functions of vital importance for our life.
Unfortunately, electron motion can also have fatal consequences, particularly if this motion happens in a chemical bond, which actually glues atoms together to form molecules, which actually form our body. So, if electron motion is initiated in such a chemical bond, the structure of the molecule may change. It may cause a malfunction of this molecule of possibly vital biological function, which, at the end, may result in the emergence of some very serious disease like cancer. Developing efficient broadband therapies for diseases like cancer will rely on a deep insight into the microscopic origin of these diseases – I guess down to the lowest level, down to the level of electrons.
But electrons are also responsible for the revolution that we have witnessed over the past decades in information technology. As you know, in present-day electronics, it is basically microwave voltage that switches electric current on and off on a timescale of nanoseconds – so one billionth of a second, which means that the corresponding electric circuits are able to perform up to billions of operations per second.
Now, attosecond technology will, hopefully, sometime – maybe in 20 years in the future – allow us to replace microwaves with light waves doing the same job, in switching current on and off in some future version of electronics like maybe molecular electronics or spintronics. This would mean that the potential would be open for speeding up information technologies by at least a factor of 100,000. The corresponding boost in computer power would definitely have a number of implications. I guess you could certainly conjecture about many possible implications. I guess the reliable prediction of natural disasters might be among the least spectacular ones.
With these prospects, I would like to conclude my talk and thank the Max Planck Society, the European Research Council, and the German Science Foundation for the general support, my colleagues and co-workers, the team, the valuable contributions of whom made these results possible, Christian Hackenberger, and WoogieWorks Animation Studio for the graphics and animations; and last but not least, thank you very much for your attention.