Innsbruck Physics Lecture 2016

Attosecond pulses are generated by electrons that are extracted from a quantum system by tunneling in an intense light pulse and travel through the continuum.  Portions of each electron wave packet are forced to re-collide with its parent ion by the oscillating force of the time dependent electric field.   Upon re-collision, the electron and ion can re-combine, emitting VUV or soft X-ray radiation.  

Since nonlinear optics underlies every ultrafast pulse that we create, every ultrafast measurements that we perform and many other measurements in physics, a new nonlinear process is bound to be important.  If we ensure that the intense driving pulse only permits re-collision within a fraction of a light period, in rare gas atoms re-collision generates the world’s shortest pulses (currently ~ 65 x 10-18 sec).  If we use a mid-infrared driver, it can also generates coherent soft X-ray radiation by combining 1000s of IR photons into one with photon energy > 1 keV.  But the re-collision electron, with wavelength in the 3-0.3 Angstrom range, is also a probe of structure.  In semiconductors we can measure the band structure of the material in which the process occurs.  Band structure information is encoded in the emitted light.  Determining band structure in this way is very much like inverse photoelectron spectroscopy.  Using small molecules, the emitted spectrum contains the information needed to image the structure of the wave function of the orbital (or orbitals) from which the re-collision electron was taken.  As time permits, the talk will cover these issues.