Probing Quantum Systems from the Inside While Producing the World’s Shortest Optical Pulses¹

Abstract—In 1964, Professor L.V. Keldysh, with whom I share winning the Lomonosov Gold Medal, published what was to become a very influential paper. In his paper, he developed the theory of multiphoton ionization for atoms and the creation of electron-hole pairs for solids. Fifty years later, we generate the world’s shortest light pulses using electron wave packets that are extracted from rare gas atoms by an intense infrared pulse much as Professor Keldysh described. The ultrashort bursts of XUV radiation from many atoms add coherently to produce intense pulses as short as 65 attoseconds—the current world record. Similar highly nonlinear processes occur in other atoms, molecules and solids. In addition to its importance as a new source of soft X-ray radiation and ultrashort pulses, the radiation generated from ionizing material encodes information on the quantum system from which it was made. By analyzing the XUV radiation, not only can we image molecular orbitals but also determine the band structure of solids.