How can light lose energy

The wondrous transformations of light and matter

(abridged version published in "Ostseezeitung", June 1999)


The nature that surrounds us consists largely of atoms: in the center of the nucleus and "above" a kind of "ladder" rises, on whose rungs (energy levels) the electrons are distributed. If one of them finds a free lower rung, it drops down. As in billiards, the force of the impact hits a small particle out of the atom photon - which can be perceived from the outside as a weak flash of light. If a trick succeeds in causing a large number of electrons to jump down (coherently) at the same time, the flashes combine to form a strong light beam, a laserbeam. This can now be used the other way around, e.g. to place electrons from a solid on their conductor up to bump, causing the material to get into "great excitement". The wild hopping that now begins can be used - with a lot of feeling and experience - to let the electrons do various useful things that have long since found their way into our everyday lives: think of lasers in CD players, laser pointers, at the doctor's, at the accurate cutting of metals, etc.

Now what happens if you give an electron such a big push upwards that it shoots over the top rung of the ladder? It can move freely, and what remains is an ion. If you bombard a solid with more and more laser photons, it will lose more and more electrons and at the same time heat up so much that ultimately a gas of electrons and ions remains - a hot plasma, similar to that of which our sun is made and for which it is There are many interesting applications in the research laboratory and in material processing.

Now it is easy to see that the energy of the ejected photons is greater, the deeper the electrons jump down. Interestingly, the color (frequency) of the light changes from red to blue, ultraviolet, even to X-rays. That is why many researchers all over the world are now concerned with specifically producing new materials with the desired energy "rung" spacing.

In addition to the frequency of the light beam, one would also like to be able to vary its energy, but this is limited - namely by the number of photons knocked out of the atoms, i.e. by the number of electrons that can be moved to jump off at the same time, and this is difficult to do increase beyond a few joules1. The way out is very simple and was also realized technically in a spectacular way a few years ago: You "compress" the entire laser beam to a few ┬Ám[1]Length together so that all photons almost simultaneously (within a few femtoseconds[2]) hit like a bomb. The result is fascinating: all electrons are snatched from even the heaviest atoms in one fell swoop, and these can be accelerated to almost the speed of light. Even more - solids are literally squeezed together by the light, and even such exotic predictions of quantum theory as the transformation of photons into matter (electron-positron pairs) are within reach. And all of this is only the beginning of a completely new dramatic development that will be in no way inferior to the current computer revolution.

It is a great challenge for the physicist to understand in detail how matter behaves in such short periods of time. Here, too, femtosecond lasers open up completely new possibilities - as a measuring probe with unsurpassed sensitivity and time resolution. The physics department at the University of Rostock has achieved remarkable success here. On the other hand, explaining experiments or predicting new phenomena is the task of theory. Together with Rostock scientists as well as colleagues at home and abroad, I try to change the behavior many To model electrons, ions, atoms and photons, their diverse interactions with one another and transformations on these extremely short time scales. The starting point are the fundamental equations of many-particle physics, quantum field theory[3]that we have recently been able to solve on modern supercomputers.

Making these models more and more realistic, in order to make the above-mentioned fascinating phenomena practically usable and to discover new ones, is a worthwhile task that requires great people. In order to be successful here, one must grasp complex interrelationships, but first and foremost acquire in-depth physical understanding and solid basic knowledge in school.



2 1fs = 0.000 000 000 000 001 seconds

3 see e.g. Michael Bonitz "Quantum Kinetic Theory", Teubner Stuttgart / Leipzig 1998