Strong magnetic fields are important tools, not only in physics and materials research, but increasingly also in other branches of science and medicine. Today, very high field strengths can be achieved in special magnetic field laboratories. However, conventional wire-wrapped solenoids react too slowly to be able to track extremely fast magnetic phenomena. Especially in atomic, molecular and solid-state physics and in numerous applications of new materials, however, an understanding of very fast magnetic processes even at high field strengths is necessary. Although conventional solenoids achieve high field strengths, the speed at which their magnetic fields can be increased is limited. Switching rates in the microsecond range can be achieved. However, this is a far cry from the femtosecond regime in which many interesting electronic processes take place. A team of Canadian scientists has now proposed a new concept for the lightning-fast generation of strong magnetic fields, which is based on laser pulses and enables much faster rates of change in the magnetic field.
The idea behind the new proposal is based on having the electrons orbit in a plasma rather than in a wire. In contrast to previous studies, the researchers' proposal also requires only moderate laser power with pulse energies in the range of a few microjoules. Earlier concepts were based on a vortex laser beam in which each individual photon carries an angular momentum which it transfers to the electrons of the target material. The new concept, on the other hand, envisages an azimuthal laser beam in which the electric field lines form a circle around the beam axis. The field strength is strongest in a ring-shaped area around this axis.
If an electron is knocked out of its atom by such a pulse, it would be accelerated back and forth by the oscillating field. In order to force a circular path instead, a second laser field of double frequency must be superimposed, which is provided with a suitable phase shift. The forced circular motion of the electrons then leads to a magnetic field in the direction of the laser beam. According to the calculations, field strengths of up to 8.4 Tesla should be possible, which can be achieved within an extremely short time span of just fifty femtoseconds. This does not even require excessively sharp focusing conditions, and a laser in the mid-infrared and atomic hydrogen or helium as a target should be sufficient for the setup.
Using purely optical means, it should therefore be possible to generate spatially isolated magnetic fields whose field strength corresponds to the possibilities in typical magnetic field laboratories. With these field strengths and correspondingly high-energy laser pulses, a sample located directly at the beam path could be destroyed. However, as the researchers report, it is also possible to place a sample at a safe distance and still benefit from the fast magnetic fields. The new method for generating magnetic fields is suitable for studying very different problems and for numerous applications.
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