There is a profound, almost symbiotic, relation between electrons and photons. When light, i.e., photons, interacts with matter the electrons in the material will start to move and oscillate. When a charged particle, such as an electron, oscillates it will act as a dipole and emit light. We intend to utilize this symbiotic relation and extend the control of light and matter to the XUV region and attosecond timescales.
The interaction of light and matter is generally referred to as quantum optics. While quantum optics deals with the interaction of matter and light of any frequency, most techniques for controlling light have been developed for light in the visible and IR wavelength regions. We propose to adopt and transfer the concepts developed to control and exploit visible light to the XUV region and ultrafast timescales. The atomic states that can be probed with XUV pulses are generally short-lived, and very short laser pulses are therefore required. This is a new direction in attosecond physics research, and expanding the field to incorporate and utilize quantum optics will allow us to work towards ultrafast quantum information, for instance, quantum memories operating at room temperature with ultrafast readout.
With the development of the laser came the possibility to exert the above-mentioned, symbiotic two-way control both coherently and collectively. The ability to control the properties of laser light in the IR to visible spectral regions has enabled an abundance of new experimental areas, ranging from basic research such as quantum optics, and femtochemistry, to applied applications like environmental monitoring and medical diagnostics. The use of lasers and the ability to control the properties of light has led to a deeper understanding of the world around us, but also important, society-changing technological advances in telecommunications and data storage. We now use lasers and the ability to control the light in our everyday lives. However, to study the mechanisms behind the symbiotic interaction between light and electrons, we need laser pulses that are shorter than those can be produced with visible light. The natural timescale for electron motion is on the order of attoseconds. The atomic unit of time is 24 as, since it takes 2p×24 as for an electron to orbit the proton in a hydrogen atom in the Bohr model of the atom. Since a laser pulse can never be shorter than a single cycle, and a single cycle in the visible region is longer than one femtosecond, it is necessary to use light with shorter wavelengths in the XUV to X-ray regions (see Figure 1). However, it is not sufficient to use shorter wavelengths in order to produce short light pulses; we also need a laser with a very broad and coherent bandwidth. This coherence will also be necessary when the pulses are used to control matter.
The generation of attosecond XUV pulses became a reality with the new millennium. The process used to break the femtosecond barrier and produce pulses in the XUV region with durations around 100 as is the so-called high-order harmonic generation (HHG) process. In an intense laser pulse interacts with a gaseous target. The laser intensity is so high that the electric field strength is comparable to the intra-atomic forces binding the electron to the nucleus. Attosecond pulses are generated through the conversion of ultrafast, high-power laser pulses, which relies on the use of state-of-the-art laser systems. The research environment is stimulating for students, since it includes basic research, quantum mechanics, high-power laser systems, advanced optical designs, and vacuum systems.
When an atom or a molecule is exposed to a light field with a field strength comparable to the intra-atomic forces, an electron can tunnel out and thus ionize the system. Once freed, the electron is accelerated by the laser field and, depending on the structure of the field and the time of ionization, the electron may return to its parent ion with high kinetic energy. A number of different processes may occur when the electron returns:
- the electron may scatter off the ion, potentially resulting in a much higher final energy,
- the electron may knock out another electron, leaving the ion in a doubly ionized state, or
- the electron may recombine to the ground state, emitting its excess energy in the form of a very short burst of light – an attosecond pulse.
This process, schematically depicted in Figure 2, which is the quintessence of strong-field physics, is known as the semi-classical three-step model. With the generation of attosecond pulses, attophysics has become the short-timescale frontier of physics. In this regard it replaces femtochemistry, which uses femtosecond laser pulses to capture the ultrafast motion of atoms during a chemical reaction. While femtochemistry is concerned with the dynamics of atoms and molecules, attosecond pulses are better equipped to capture the even faster motion of electrons within atoms and ions.
We are facing a formidable challenge, but also great opportunities, when it comes to experiments using attosecond pulses since they represent a completely new form of light–matter interaction. These pulses form a coherent light source and not only are they shorter than any pulses previously available; they are also in the XUV, with bandwidths comparable to their central frequencies. This means that methods developed for longer pulses (coherent in the visible or incoherent for shorter wavelengths) cannot be directly adopted, and completely new methods will have to be developed for attosecond experiments. The keyword here is phase; attosecond pulses enable us to do phase measurements and control the phase of the light in a way that has not been possible before.
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