Editorial – Topical Review

Starting from a family of equations of motion for the dynamics of extended systems whose trajectories sample constant pressure and temperature ensemble distributions (Ferrario, M., 1993, in Computer Simulation in Chemical Physics, edited by M. P. Allen and D. J. Tildesley (Dordrecht: Kluwer)), explicit time reversible integration schemes are derived through a straightforward Trotter factorization of the dynamic Liouville propagator, along the lines first described by Tuckerman, M., Martyna, G. J., and Berne, B. J., 1992, J. chem. Phys., 97, 1990. The original Andersen's constant-pressure dynamics are recovered in the limit of zero coupling with the Nose thermostat. Reversible integration schemes are derived as a generalization of the velocity Verlet algorithm, with direct handling of the velocity dependent forces in such a way that both predictions and relative iterative corrections are not required. For the sake of clarity both the equations of motion and the Trotter factorization are kept to the basic level. The proposed structure can accommodate easily, when needed, complications such as multiple timesteps and more effective thermostats (Nose-Hoover-chain). Finally, an application is made to a model molecular system subjected to holonomic constraints by means of the SHAKE algorithm. In the constant pressure case it is no longer possible to avoid using a prediction for the constraint contribution to the volume acceleration; however, recourse to a minimal iteration scheme still achieves excellent overall behaviour for the proposed integration algorithm, with no perceptible difference from the unconstrained case.     

具有缓变折射率的太赫兹宽带增透器件

用高阻硅制作的光学元件是太赫兹系统里常用的器件, 但是其高达3.42的相对折射率所引起的阻抗失配严重影响了太赫兹系统的功率, 因此研究人员尝试了各种各样的方式在高阻硅表面镀上有效的增透膜. 在太赫兹波段, 缺乏合适的材料是增透研究中亟需解决的一个重要问题. 介绍一种结构新颖的硅材料增透器件----- 三维光子倒置光栅. 与普通高阻硅片相比, 当结构周期为15 μm时, 该器件在0.2---7.3 THz范围内对太赫兹波具有明显的增透作用, 且覆盖了大部分太赫兹波段. 此外, 该器件的使用不受太赫兹偏振方向限制, 适用于大入射角情形, 并具有高达116.3%的相对3 dB带宽.