An improved method for the investigation of high-order harmonic generation from graphene

High-order harmonic generation (HHG) of bulk crystals in strong laser field is typically investigated with semiconductor Bloch equations (SBEs). However, in the length gauge, it suffers from the divergence for the crystals with a zero band gap, such as graphene, using both Bloch- and Houston-states expansion methods. Here, we present a method of solving the SBEs based on time-dependent Bloch basis, which is equivalent to semiconductor Bloch equations in the velocity gauge. Using this method, we investigate the HHG of a single-layer graphene. It is found that our results for population are in good agreement with the other results. For a initial condition py = 0, we find the electrons just move in single valence band or conduction band, which are in accord with classical results. Our simulations on the HHG dependence of polarization of driving laser pulse confirm that 5th, 7th, and 9th harmonic yields increase to the maximal value when laser ellipticity ε ≈ 0.3. What is more, similar to the case of atoms in the laser field, the total strength of 3rd harmonic decrease monotonically with the increase of ε. In addition, we simulate the dependence of HHG on crystallographic orientation with respect to the polarization direction of linear mid-infrared laser pulse, and the results reveal that for higher harmonics, their radiation along with the change of rotation angle θ reflects exactly the sixfold symmetry of graphene. Our method can be further used to investigate the behaviors of other materials having Dirac points (i.e., surface states of topological insulators) in the strong laser fields.

Experimental investigation of the reaction-build-up for plastic bonded explosive JOB-9003

In order to measure the shock initiation behavior of JOB-9003 explosives,Al-based embedded multiple electromagnetic particle velocity gauge technique has been developed.In addition,a gauge element called the shock tracker has been used to monitor the progress of the shock front as a function of time,thus providing a positionetime trajectory of the wave front as it moves through the explosive sample.The data is used to determine the position and time for shock to detonation transition.All the experimental results show that the rising-up time of Al-based electromagnetic particle velocity gauge is very short(<20 ns);the reaction-build-up velocity profiles and the positionetime for shock to detonation transition of HMX-based plastic bonded explosive(PBX)JOB-9003 with 1e8 mm depth from the origin of the impact plane under different initiation pressures were obtained with high accuracy.