Selection of quantum path in high-order harmonics and isolated sub-100 attosecond generation in few-cycle spatially inhomogeneous laser fields**Project supported by the National Natural Science Foundation of China (Grant Nos. 11174108, 11104108, and 11271158).

Abstract

We theoretically study the selection of the quantum path in high-order harmonics (HHG) and isolated attosecond pulse generation from a one-dimensional (1D) model of a molecule in few-cycle inhomogeneous laser fields. We show that the inhomogeneity of the laser fields play an important role in the HHG process. The cutoff of the harmonics can be extended remarkably, and the harmonic spectrum becomes smooth and has fewer modulations. We investigate the time-frequency profile of the time-dependent dipole, which shows that the short quantum path is enhanced and the long quantum path disappears in spatially inhomogeneous fields. The semi-classical three-step model is also applied to illustrate the physical mechanism of HHG. The influence of driving field carrier-envelop phase (CEP) on HHG is also discussed. By superposing a series of properly selected harmonics, an isolated attosecond pulse (IAP) with duration 53 as can be obtained by a 15-fs, 1600-nm laser pulse with the parameter ε = 0.0013 (ε is the parameter that determines the order of inhomogeneity of the laser field).