18 Apr 2006
Physicists are now producing attosecond laser pulses that are short enough to probe atomic and subatomic electron processes. Rob van den Berg spoke to Ferenc Krausz and Reinhard Kienberger, two pioneers in attosecond physics, about their work.
Electrons in motion play a crucial role in science and technology, but they move so fast that ultrashort flashes of light are needed to measure and control their behaviour. Laser pulses lasting just a few femtoseconds (10-15 s) were first generated in the 1980s, which has made it possible to monitor molecular dynamics with unprecedented temporal resolution. But electron processes in atoms occur on much faster timescales, ranging between tens and hundreds of attoseconds (10-18 s).
This time regime, for which the term ultrafast no longer seems adequate, has in the last few years been opened up by the work of Prof. Ferenc Krausz, currently at the University of Munich and the Max Planck Institute for Quantum Optics in Garching, Germany. Indeed, Krausz has just been awarded the Gottfried Wilhelm Leibniz prize by the German Research Foundation for his research in ultrafast optics, which at €1.55 m is the highest value prize in German research. OLE spoke to Krausz and his co-worker, Reinhard Kienberger, about their work and future plans to generate even shorter laser pulses.
What can we observe with an attosecond pulse that we cannot already with a femtosecond pulse?
From photography we know that exposure times must be much shorter than the process under investigation to prevent the picture from becoming blurred. As we look deeper and deeper inside matter, the dimensions not only get smaller, but the timescale on which processes occur also gets shorter. Femtosecond pulses are fast enough in molecular science, as was beautifully shown in the field of femtochemistry - and for which Ahmed Zewail from CalTech received a Nobel prize in 1999. However, our goal is to control the motion of electrons on the length and timescales of atoms, and to observe these motions in space and time with subatomic resolution. This atomic and subatomic regime requires laser pulses in the attosecond range.
What techniques are used to generate attosecond pulses and how do these differ from the way femtosecond pulses are produced?
We generate attosecond pulses using a technique known as high-order harmonic generation (HHG), in which a short ultraviolet laser pulse is fired at a cloud of neon atoms. These visible laser pulses - which have a controlled electric-field waveform of less than two cycles - force an electron to tunnel out of a neon atom. The laser pulse's electric field accelerates the electron and then pulls it back when the field changes direction. At that moment the electron recombines with its parent ion and releases its energy in a burst of radiation lasting a few hundred attoseconds. The resulting photons have a wavelength in the extreme ultraviolet (XUV) or even soft X-ray part of the spectrum.
All of the attosecond techniques developed so far rely on a similar kind of nonlinear frequency conversion of visible laser pulses to shorter wavelengths. And these short wavelengths are needed because Heisenberg's uncertainty principle places a lower limit on the duration of the pulse - that is, a single oscillation of the electric field - since shorter pulses must also have a broader frequency range.
For a purely optical pulse spanning the visible spectrum, a single oscillation period lasts for about 2.5 fs. There is no simple method to go beyond this, such as using a conventional femtosecond laser to generate attosecond pulses, since there is no suitable laser material for very short wavelengths.
Are there other ways of generating attosecond pulses?
Schemes have been proposed for free-electron lasers like the ones in Hamburg or Stanford, which could have much higher photon energies than can be achieved with high-harmonic generation and could therefore be used to study processes occurring even closer to the nucleus of an atom. However, HHG-generated attosecond pulses have the advantage of an intrinsic, jitter-free synchronization to the driving laser pulse, which constitutes an ideal tool for pump/probe investigations.
When and how was the first attosecond pulse produced?
The first single, isolated pulses lasting for less than 1 fs (measured at 650 as) were produced in 2001 in our group, then at the Vienna University of Technology. To achieve this, we first had to develop the capability to generate few-cycle (5 fs) laser pulses, and, more importantly, to devise a method to obtain full control over the waveform (phase) of these pulses.
Fortunately, the invention of chirped multilayer dielectric mirrors by Robert Szipöcs and myself back in 1994 meant that producing few-cycle pulses became routine. These mirrors made it possible to control the subtle interplay between two effects: a spectral broadening of the pulses and a subsequent realignment of the various spectral components.
Obtaining similar control over the phase of the pulses resulted from the Nobel-prize-winning optical frequency-comb technique developed by Ted Hänsch, who is also at the Max Planck Institute here in Garching. These breakthroughs, combined with a great deal of effort to improve the relevant technologies, means that we can now routinely generate single pulses shorter then 300 as in the 100 eV energy range in our Laboratory for Attosecond and High-Field Physics in Munich.
What kind of information can be gained from such attosecond pulses?
Electrons released from an assembly of atoms by an attosecond XUV are confined to a time window that is a small fraction of the wave period of visible light. If these electrons are launched into the light field to be measured, they suffer a well-defined energy shift that depends on their timing with respect to the waves in the light pulse.
Scanning this attosecond electron probe across the light pulse causes the electron energies to be shifted up and down according to the oscillations of the light field. This kind of trace would make it possible to determine both the temporal evolution and the instantaneous strength and direction of the electric field of a visible light wave, allowing a light wave to be imaged directly for the first time.
What kind of atto-stopwatch do you use to measure these pulses?
That is indeed a real challenge. We have developed the technique of volume autocorrelation to characterize attosecond pulses with low photon energy. In this technique, the attosecond pulse is first divided into two replicas by a split-mirror arrangement that focuses the XUV beam into a helium gas jet. When the two parts of the mirror are moved in small steps, their relative motion alters the intensity distribution at the focus. A two-photon ionization process then takes place in the helium gas, and the variations in intensity distribution act to modulate the ion signal.
We also exploit a technique known as the attosecond streak camera, which is basically a cross-correlation technique. It relies on a time reference to extract the duration of the attosecond burst, although in this case the time reference is not given by the envelope of the laser pulse, but by the oscillating field of the laser field itself. This method can be used for XUV pulse durations shorter than a half-period of the oscillating laser field, and it can also be used to measure the chirp of the attosecond pulse.
What are attosecond pulses being used for today?
Our laser systems that deliver X-ray pulses with a duration of less than 300 as are being used to control and observe the motion of electrons inside atoms and molecules in real time. We are investigating processes such as ionization dynamics, outer-shell electron dynamics - including Auger decay and auto-ionization - and dissociation processes in molecules.
Challenging tasks for the future include time-resolving surface phenomena, such as plasmon dynamics and charge transfer in biomolecules. Higher photon energies will also open the way to the so-called water-window, a part of the X-ray spectrum where water is transparent but carbon is not. This will enable biological structures to be investigated in vivo - in other words, without being dehydrated. One day, it might even become feasible to perform 4D microscopy of electrons and complex molecular systems.
When will we see shorter or more intense pulses? And what will happen next in this field?
Our efforts are currently focused on obtaining higher photon energies, since shorter pulses - with wavelengths of a few tenths of a nanometre - are achievable with photons in the keV energy range. Ultimately, there is no limit to how short the pulses can be, and I am convinced that pulses at the same timescale as the atomic unit of time (24 as) will be available within the next five years.
For now, however, we still have a long way to go before we can time-resolve processes that occur in the inner core of an atom, but that is just a matter of increasing the photon energy. To move towards that goal, we are developing a unique laser, the Petawatt Field Synthesizer, although some major hurdles still need to be overcome - for instance, in developing X-ray optics such as multilayer mirrors.
Once it is built, which we expect to take about three years, this laser will enable pioneering experiments to be conducted in a range of research areas. As well as applications in attosecond physics and structural biology, it could also be used to accelerate electrons and protons in a controlled way to the GeV energy range.
More generally, our work could help in the quest for compact, bright X-ray lasers. Today, X-ray beams can only be produced in huge particle accelerators, but smaller and cheaper X-ray sources are needed in hospitals to target medical X-rays at specific parts of the body, which would prevent unnecessary exposure to radiation. Free-electron lasers also offer the prospect of intense pulses in the hard X-ray range. Whatever lies ahead, it promises to be an exciting race.
If you would like to find out more information about the work of Ferenc Krausz and his team, see www.attoworld.de.