Sources and Diagnostics for Attosecond Science

Abstract: Popular Abstract in English What is a photographic camera flash good for? The easy answer is “to illuminate”. There is more to it though: the duration of a camera flash is usually much shorter than the shutter speed of the camera. This allows us to take sharp pictures of fast objects with an inexpensive camera. Ultrafast science is based on a similar principle: no shutter is capable of opening and closing fast enough to “freeze” the motion of molecules breaking up and forming new ones on a chemical reaction; or, much faster, electrons “spinning” around the nucleus of an atom. The trick is to use very short light pulses (our flashes). Events like the ones described take place in times as short as femtoseconds and attoseconds, respectively. If we want to see what happens, for example, during a chemical reaction, and not only the before and after, we need a flash shorter than the time it takes to occur. But how short is a femtosecond? And an attosecond? A femtosecond is 0.000000000000001 seconds (or 10−15 s), and an attosecond is one thousand times smaller. To put it in perspective, suppose you have a clock and that, at each second, your clock would fall behind one femtosecond. How long would it take for it to be one second off? It would take longer than thirty million years. There is a fundamental limitation to how short a light pulse can be. Light is an oscillation, or vibration, of the electric and the magnetic fields, that propagate as waves. Visible light, that our eyes can perceive, has oscillations periods of about two femtoseconds, and a light pulse cannot be shorter than that. To create even shorter pulses, we have to go higher in the frequency spectrum, towards X-rays. Light pulses with durations of some femtoseconds are nowadays generated directly from lasers. These laser pulses can then be used to interact with matter and generate light at higher frequencies, and even shorter pulses can be created, with durations of around one hundred attoseconds. This thesis describes the work undertaken on creating, taming, measuring and using such short light pulses, both in the femtosecond and attosecond regime. Creating such short light pulses poses a considerable technical challenge. Interestingly, it is as difficult to keep them short as it is to create them, and the same goes for characterizing them: since these are the shortest events artificially created, we don’t have an even shorter light pulse to measure them, so we have to use these pulses to measure themselves.