It is theoretically shown that coherent control of atomic bound states can achieve macroscopic control over the process of High-order Harmonic Generation. In particular purely temporal Quasi-Phase-Matching is numerically demonstrated.
We propose a paradigm for macroscopic control of high-order harmonic generation by modulating the bound-state population of the medium atoms. A unique result of this scheme is that apart from regular spatial quasi-phase-matching (QPM), also purely temporal QPM of the emitted radiation can be established. Our simulations demonstrate temporal QPM by inducing homogenous Rabi oscillations in the medium and also spatial QPM by creating a grating of population inversion using the process of rapid adiabatic passage. In the simulations a scaled version of high-order harmonic generation is used: a far off-resonance 2.6 μm source generates UV-visible high-order harmonics from alkali-metal-atom vapor, while a resonant near IR source is used to coherently control the medium.
High harmonic generation has been considered for many years to be a coherent source of upconverted laser radiation. We propose a method for the generation of essentially incoherent high harmonic generation with a controlled degree of temporal coherence, obeying random-walk statistics. This is achieved by a unique combination of single-atom and macroscopic effects involving preparation of the medium in an excited state with coordinate-dependent population inversion and using an intense off-resonant and phase-mismatched pumping.
Summary form only given. In the visible region of the spectrum, nonlinear conversion of light from one frequency to another is efficient because of phase matching. The phase velocity of the fundamental and nonlinear signal is matched such that the generated nonlinear signal sums coherently throughout the entire interaction region. At visible frequencies, this is accomplished by appropriate orientation of an anisotropic crystal. Alternatively, quasi-phase matching (QPM) techniques, which restrict nonlinear light generation to specific spatial regions, can be used. Both of these techniques rely on propagation effects. In this work, we present four new results. First, we present two new phase matching techniques that can be applied uniquely to extreme nonlinear optics, to upshift light into the XUV using high harmonic generation (HHG). The first uses a type of phase matching that can occur within single atoms. The second demonstrates a new type of QPM in the XUV based on amplitude modulation. We also show that these new techniques require an electron trajectory to be adjusted by 25 attoseconds. Finally, we demonstrate excellent agreement between theory and experiment for the first time in learning control of high-order nonlinear processes.