Quantitative identification of tunneling ionization (TI) and multiphoton ionization (MPI) with Keldysh parameter γ in intermediate regime is of great importance to better understand various ionization-triggered strong-field phenomena. We theoretically demonstrate that the numerical observable ionization delay time is a more reliable indicator for characterizing the transition from TI to MPI under different laser parameters. Using non-linear iterative curve fitting algorithm (NICFA), the detected time-dependent probability current of ionized electrons can be decoupled into weighted TI and MPI portions. This enables us to confirm that the observed plateau-like structure in ionization delay time picture at the intermediate γ originates from the competition between TI and MPI processes. A hybrid quantum and classical approach (HQCA) is developed to evaluate the weights of TI and MPI electrons in good agreement with NICFA result. Moreover, the well separated TI and MPI electrons using HQCA are further propagated classically for mapping their final momentum, which well reproduces the experimental or ab-initio numerical calculated signatures of ionized electron momentum distribution in a rather broad γ regime.

With the semiclassical ensemble model, we systematically investigate the correlated electron dynamics in strong-field nonsequential double ionization (NSDI) by the counter-rotating circularly polarized two-color (CPTC) laser pulses. Our results show that the angular distributions of the electrons in NSDI sensitively depend on the intensity ratio of the CPTC laser fields. At the small ratio, the electron pairs emit with a relative angle of about 120°, and this angle shifts to 40° as the ratio increases and finally it exhibits a wide range distribution as the intensity ratio further increases. Back analysis of the NSDI trajectories shows that this behavior results from the relative-intensity-dependence of the release time of the electron pairs in the CPTC laser fields. The release times of the electron pairs are directly mapped to the angular distribution. Our results indicate that the emission times of the correlated electrons in NSDI can be controlled with the CPTC laser fields.

We investigate the orientation dependence of high-order harmonic generation (HHG) in nanowires by solving the time-dependent Schrödinger equation. It is found that the harmonic spectrum exhibits a two-plateau structure and the two plateaus have different orientation dependences. The harmonic yield of the first plateau reaches a maximum when the laser field is parallel to the nanowire and gets a strong suppression when the laser field is perpendicular to the nanowire. The harmonic yield of the second plateau increases first and reduces eventually with the change of the nanowire orientation. These phenomena can be explained by the time-dependent population imaging and the analysis of the transition probability between different energy bands. Our analysis shows a strong band-sensitive orientation dependence in low-dimensional material.

A robust optical parametric amplifier (OPA) scheme is proposed in which we have experimentally achieved broadband bandwidth for a collinear OPA design that exhibits good beam quality and spatial distribution using a type-I $\beta$ barium borate crystal. The applied pump pulses are simultaneously spectrally broadened and temporally stretched in a multi-plate system before being used to amplify the temporally stretched white light. In this case, the phase matching can be obtained for a broad range of wavelengths, and we have managed to achieve bandwidths three times broader than a conventional narrowband pumped collinear OPA. With a bandwidth as broad as 400 nm centered at 1400 nm, as well as a broadband angular dispersion-free idler, the signal bandwidth supports transform-limited pulses as short as 7.5 fs which correspond to sub-two optical cycles for this center wavelength. Furthermore, the system is easily tunable over a 400 nm range of bandwidths starting from 1100 to 1500 nm. The proposed scheme provides a versatile near-infrared/middle-infrared source with a broad bandwidth and fine tuning capability which paves the way for ultrafast spectroscopy and strong field applications.

We demonstrate an all-optical method for time-domain characterization of ultrashort optical pulses using nonionizing lasers. A moderately intense laser pulse couples the doubly excited states of helium and induces the ac Stark effect. A synchronized weak signal field perturbs the coupling process and alters the quasienergies of the dressed atom, upon which the carrier oscillation of the signal field is encoded. The nonionizing lasers used in current approach minimize plasma-induced pulse distortion and provide a complementary detection scheme for effective and reliable optical oscilloscope.