Massively-parallel time-dependent density functional theory calculations for optical near-field excitations in silicon

In current frontiers of optical science, electron dynamics in nano-materials has been explored in time domain using ultrashort light pulse. To describe such phenomena, we have been developing a program package SALMON (Scalable Ab-initio Light-Matter simulator for Optics and Nanoscience) that is based on first-principles time-dependent density functional theory. SALMON solves time-dependent Kohn-Sham equation in real time using three-dimensional grid representation. The code is efficiently parallelized with respect to spatial grids, orbitals, and k-points. We have applied the code to wave vector excitations in three silicon bilayers with a Si(111) surface terminated by hydrogen atoms by optical near fields (ONF). To describe electronic excitations accompanying wave vector changes that are promoted by the ONF, we calculated the system composed of 10,240 atoms. The calculation costs 13 hours using 4096 nodes on the K computer at RIKEN Center for Computational Science. We have found that the excitation by ONF is a few orders of magnitude larger than that by the far field. We also find the lowering of the absorption edge by the ONF excitation that is attributed to direct interband transitions with finite wave vector differences.