Scientists at TU Wien (Vienna) found a way to compress ultrashort laser pulses, increasing its peak power to half a terawatt – which is equivalent to the output of hundreds of nuclear reactors.
It is a very unusual kind of laser: researchers at the photonics institute at TU Wien (Vienna) have built a device which emits ultrashort flashes of infrared light with extremely high energy. “It is very hard to combine these three properties – long infrared wavelength, short duration and high energy”, says Valentina Shumakova. “But this combination is exactly what we need for many interesting strong-field applications.”
Now the team has achieved a major breakthrough: By sending very energetic pulses in the infrared regime through a solid medium, the pulses can be compressed in time and space. The energy stays roughly the same, but it can now be deposited in an even shorter period of time, resulting in an incredible peak power of up to half a terawatt. This power corresponds to the output of hundreds of nuclear reactors. But unlike power plants, which produce the power steadily, the compressed laser pulse only lasts 30 femtoseconds (millionths of a billionth of a second). The new results have now been published in the journal “Nature communications”.
Playing with Invisible Colours
“Under certain conditions, laser pulses can self-compress and become shorter. This is a well-known phenomenon in laser science”, says Audrius Pugzlys. “But until now, people used to believe that self-compression in solid media at extremely high intensities is impossible.”
Unlike the light of a simple laser pointer, an ultrashort laser pulse does not only have one specific colour. It is a mixture of a spectrum of different wavelengths – in this case centred around 3.9 micrometers, in the long infrared region, invisible to the human eye.
In vacuum, light always travels at the same speed, regardless of its wavelength. But this is not the case for light traversing a solid material. “The material causes some components of the laser pulse to move faster than others. If this effect is cleverly used, the laser pulse is compressed, it becomes shorter just by travelling through the material”, says Skirmantas Alisauskas.
This technique, however, is not always applicable. “If a pulsed laser beam of very high intensities is sent through a material, the beams tends to collapse chaotically into many separate filaments”, says Audrius Pugzlys. “It is like a bolt of lightning that spontaneously breaks up into various branches.” Each of the branches only carries a small part of the energy of the original beam, the resulting laser beam cannot be used for advanced strong-field laser experiments any more.
Breaking the filamentation threshold by four orders of magnitude
The Viennese research group, in collaboration with researchers from Moscow state university, has now identified conditions which lead to self-compression and an extremely high peak power without causing the beam to collapse into filaments. “As it turns out, we are dealing with two different length scales”, says Valentina Shumakova. “The length scale of the unwanted filamentation is longer than the length on which self-compression occurs. Therefore, it is possible to find a parameter regime in which the pulse is compressed but filamentation does not yet set in.” The power of the lase pulse is 10,000 times higher than the filamentation threshold – and still it does not collapse.
The team used an Yttrium aluminium garnet (YAG) crystal with a width of only a few millimetres – and the results are remarkable: By sending the laser pulse through the crystal, its duration decreases from 94 femtoseconds to a mere 30 femtoseconds. Its energy stays almost the same, and the power (energy per time) increases by a factor three, to almost half a terawatt. “As the pulse is very short, its extremely high power opens the door to many exciting experiments and maybe even to new technologies in laser science”, says Audrius Pugzlys.
Learn more: With Great Power Comes Great Laser Science
Electrons reveal their quantum properties when they are confined to small spaces. Scientists from TU Wien (Vienna), Aachen and Manchester have created tiny quantum dots in Graphene
In a tiny quantum prison, electrons behave quite differently as compared to their counterparts in free space. They can only occupy discrete energy levels, much like the electrons in an atom – for this reason, such electron prisons are often called “artificial atoms”. Artificial atoms may also feature properties beyond those of conventional ones, with the potential for many applications for example in quantum computing. Such additional properties have now been shown for artificial atoms in the carbon material graphene. The results have been published in the journal “Nano Letters”, the project was a collaboration of scientists from TU Wien (Vienna, Austria), RWTH Aachen (Germany) and the University of Manchester (GB).
Building Artificial Atoms
“Artificial atoms open up new, exciting possibilities, because we can directly tune their properties”, says Professor Joachim Burgdörfer (TU Wien, Vienna). In semiconductor materials such as gallium arsenide, trapping electrons in tiny confinements has already been shown to be possible. These structures are often referred to as “quantum dots”. Just like in an atom, where the electrons can only circle the nucleus on certain orbits, electrons in these quantum dots are forced into discrete quantum states.
Even more interesting possibilities are opened up by using graphene, a material consisting of a single layer of carbon atoms, which has attracted a lot of attention in the last few years. “In most materials, electrons may occupy two different quantum states at a given energy. The high symmetry of the graphene lattice allows for four different quantum states. This opens up new pathways for quantum information processing and storage” explains Florian Libisch from TU Wien. However, creating well-controlled artificial atoms in graphene turned out to be extremely challenging.
Cutting edge is not enough
There are different ways of creating artificial atoms: The simplest one is putting electrons into tiny flakes, cut out of a thin layer of the material. While this works for graphene, the symmetry of the material is broken by the edges of the flake which can never be perfectly smooth. Consequently, the special four-fold multiplicity of states in graphene is reduced to the conventional two-fold one.
Therefore, different ways had to be found: It is not necessary to use small graphene flakes to capture electrons. Using clever combinations of electrical and magnetic fields is a much better option. With the tip of a scanning tunnelling microscope, an electric field can be applied locally. That way, a tiny region is created within the graphene surface, in which low energy electrons can be trapped. At the same time, the electrons are forced into tiny circular orbits by applying a magnetic field. “If we would only use an electric field, quantum effects allow the electrons to quickly leave the trap” explains Libisch.
The artificial atoms were measured at the RWTH Aachen by Nils Freitag and Peter Nemes-Incze in the group of Professor Markus Morgenstern. Simulations and theoretical models were developed at TU Wien (Vienna) by Larisa Chizhova, Florian Libisch and Joachim Burgdörfer. The exceptionally clean graphene sample came from the team around Andre Geim and Kostya Novoselov from Manchester (GB) – these two researchers were awarded the Nobel Prize in 2010 for creating graphene sheets for the first time.
The new artificial atoms now open up new possibilities for many quantum technological experiments: “Four localized electron states with the same energy allow for switching between different quantum states to store information”, says Joachim Burgdörfer. The electrons can preserve arbitrary superpositions for a long time, ideal properties for quantum computers. In addition, the new method has the big advantage of scalability: it should be possible to fit many such artificial atoms on a small chip in order to use them for quantum information applications.
Learn more: “Artificial Atom“ Created in Graphene
When rain falls on a lotus leaf, the leaf doesn’t get wet. Thanks to its special structure, the water drops roll off without wetting the surface. Artificial materials can be made water-repellent, too. It is, however, extremely challenging to produce a surface with switchable wetting. Now, a research team from TU Wien, KU Leuven and University of Zürich has managed to manipulate a surface of a single layer of boron nitride in such a way that it can be switched back and forth between states with high and low wetting and adhesion.
Hexagons making waves
One of the most interesting physical properties of a surface is its stiction or static friction” says Stijn Mertens (Institute of Applied Physics at the Vienna University of Technology, and associated with KU Leuven in Belgium). „This force has to be overcome for an object on the surface to start sliding.” The nanostructure of the surface determines its stiction to a large extent: the details of the contact between the surface and another object (for example, a drop of liquid) depend on the geometry of its atoms and other properties. This in turn is crucial for adhesion, stiction and wetting. The relationship between stiction and wetting, however, is so far only poorly understood.
“Just as the material graphene consists of only one layer of carbon atoms, our boron nitride — which contains as many boron as nitrogen atoms — has a thickness of only one atomic layer”, explains Thomas Greber from the Physics Institute at the University of Zürich. This ultrathin layer can be grown on a rhodium single crystal. The atoms on the rhodium surface and in the boron nitride form a hexagonal pattern, but the distances between the atoms in the two materials are different. Thirteen atoms in boron nitride take the same space as twelve rhodium atoms, so that the two crystals do not fit together perfectly. Because of this mismatch, the boron nitride hexagons must bend, they appear as a frozen wave with a wavelength of 3.2 nanometres and a height of about 0.1 nanometres.
Precisely this two-dimensional nanowave influences the wetting of the surface by water”, says Stijn Mertens. In any case, the boron nitride superstructure can be made flat with a simple trick: by putting the material in acid and applying an electrical voltage, hydrogen atoms creep under the boron nitride layer and change the bond between nitrogen and rhodium. This makes the boron nitride flat. Suddenly the adhesion of a water drop on the surface changes dramatically – even though the drop is 100’000 times bigger than the tiny waves in the boron nitride. If the voltage is decreased, this effect is reversed: „We can switch the surface again and again between these two states”, explains Stijn Mertens.
The Vienna University of Technology (German: Technische Universität Wien) is one of the major universities in Vienna, the capital of Austria.
Founded in 1815 as the “Imperial-Royal Polytechnic Institute” (k. k. Polytechnisches Institut), it currently has about 26,200 students (19% foreign students/30% women), eight faculties and about 4,000 staff members (1,800 academics). The university’s teaching and research is focused on engineering and natural sciences.
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