Phase-matched high-order difference-frequency mixing in plasmas

Macroscopic effects in noncollinear high-order harmonic generation

We study two-color high-order harmonic generation using an intense driving field and its weak second harmonic, crossed under a small angle in the focus. Employing sum- and difference-frequency generation processes, such a noncollinear scheme can be used to measure and control macroscopic phase matching effects by utilizing a geometrical phase mismatch component, which depends on the noncollinear angle. We further show how spatial phase effects in the generation volume are mapped out into the far field allowing a direct analogy with temporal carrier envelope effects in attosecond pulse generation.

Grating-assisted phase matching in extreme nonlinear optics

We propose a new technique for phase matching high harmonic generation that can be used for generating bright, tabletop, tunable, and coherent x-ray sources at keV photon energies. A weak quasi-cw counterpropagating field induces a sinusoidal modulation in the phase of the emitted harmonics that can be used for correcting the large plasma-induced phase mismatch. We develop an analytical model that describes this grating-assisted x-ray phase matching and predicts that very modest intensities (<10(10) W/cm2) of quasi-cw counterpropagating fields are required for implementation.

Unified microscopic-macroscopic formulation of high-order difference-frequency mixing in plasmas

We investigate high-order difference-frequency mixing in plasmas, taking into account the microscopic rescattering physics and propagation effects for the first time. We show that phase matching can occur over a broad frequency range, up to very high photon energies, and that it is confined to specific temporal and spatial windows. This gated phase matching mechanism is driven by the continuous phase slip between two driving fields and can be employed for manipulating the temporal, spatial, and spectral properties of high harmonic emission.

Cascaded highly nondegenerate four-wave-mixing phenomenon in transparent isotropic condensed media

Broad-bandwidth light pulses with different frequencies, extending from the IR to the UV, are simultaneously generated when two noncollinear, ultrafast laser pulses from a visible dual-frequency laser propagate through bulk isotropic transparent media such as common glass. This phenomenon, which is believed to have been previously unreported, can be explained by a cascade of highly nondegenerate four-wave-mixing processes and corresponds to a coherent scattering effect with geometrically minimized phase mismatch. Frequency-upconverted beams were observed up to the 11th order.