Atto-femtosecond Control of Coherent Electrons in Low-temperature and Low-dimensional Nanomaterials (ACCEL-Nano)

超快纳米电子器件工作在飞秒时间尺度上，作为具有前所未有的工作速度的器件的候选者，受到了极大的关注。这些设备依赖于超短激光脉冲驱动的纳米物体中的超快电子动力学。在纳米物体中，如果电子的相干长度超过物体的大小，电子波将相干地扩展到整个物体，从而形成相干电子系统。这样的系统可以使用低温和低维的纳米材料来构建。实现对这种相干电子系统的时空控制将为新型超快纳米电子设备打开大门。由于几个周期的激光脉冲的持续时间远远短于物体中电子的时间相干性，因此可以在相干性时间内对电子的激发进行时间操纵。然而，操纵空间中的相干电子系统是困难的，因为激光的衍射极限通常大于电子相干长度。现在，利用激光感应场发射来实现对超快电子动力学的纳米尺度空间控制是可能的。在金属纳米针尖上施加强大的电场，可以使电子通过隧道进入真空而产生场发射。这种场发射从尖端径向传播，并将尖端上的纳米级几何信息放大到宏观尺度，从而起到场发射显微镜的作用。因此，有限元方法成为研究纳米电子系统的有力工具。由于场发射固有的小源尺寸可以与金属内部的相干长度相媲美，场发射也被广泛用于产生高度相干的电子束。用飞秒激光脉冲照射这种纳米针尖，实现了激光诱导的脉冲场发射，从纳米尺度区域产生超短相干电子脉冲。激光诱导的电子发射在纳米领域表现出许多有趣的阿特飞秒电子动力学，如电子-电子相互作用、再散射过程或亚周期发射。在完成脉冲场发射后不久，我们就研制出了发射位置选择性在几十纳米级的超快脉冲场发射源，这将使我们能够对远远超出激光衍射极限的相干电子系统进行纳米级的控制。在这个项目中，我们希望利用激光感应场发射来实现相干电子波的时空控制，并将这一技术应用于研究和控制低维低温纳米材料中相干电子系统的飞秒动力学。计划中的实验还将在超快相干电子动力学、表面科学、自旋电子学、时间分辨电子全息术和量子信息科学方面创造新的方向。

Ultrafast nanoelectronic devices operating on atto-femtosecond timescales have received much interest as candidates for devices with unprecedented operating speed. These devices rely on ultrafast electron dynamics in nano-objects driven by ultrashort laser pulses. In a nano-object, if the coherence length of electrons exceeds the size of the object, the electron waves will coherently extend all over the object, thus forming a coherent electron system. Such a system can be constructed using low-temperature and low-dimensional nanomaterials. Realizing spatio-temporal control of such a coherent electronic system would open a door to new types of ultrafast nanoelectronic devices. Because the duration of few-cycle laser pulses is far shorter than the temporal coherence of electrons in the object, electron excitations can be manipulated in time within the coherence time. Manipulating coherent electron systems in space, however, is difficult because the diffraction limit of a laser is typically larger than the electron coherence length. Now it may be possible to achieve nanoscale spatial control of ultrafast electron dynamics by using laser-induced field emission. Applying strong electric fields to a metallic nano-tip enables field emission due to electrons tunneling into a vacuum. This field emission radically propagates from the tip apex and magnifies nanoscale geometrical information on the tip apex to a macroscopic scale, which thus serves as field emission microscopy (FEM). As a result, FEM becomes a powerful tool to investigate nanoscale electronic systems. Because its inherently small source size is comparable to the coherence length inside metals, field emission is also widely used for producing highly coherent electron beams. Illuminating such a nano-tip with femtosecond laser pulses has realized laser-induced pulsed field emission, which generates ultrashort coherent electron pulses from nano-scale areas. Laser-induced electron emission has exhibited many intriguing atto-femtosecond electron dynamics in nanometer areas such as electron-electron interactions, rescattering processes or subcycle emissions. Shortly after accomplishing pulsed field emission, we have produced an ultrafast pulsed field-emission source with emission site selectivity on a few tens of nanometers scale, which would enable nanoscale control of the coherent electronic system that is far beyond the diffraction limit of a laser. Using a laser-induced field emission, we would like in this project to realize spatio-temporal control of coherent electron waves, and apply the technique to investigate and control the atto-femtosecond dynamics of the coherent electronic system in low-dimensional and low-temperature nanomaterials. The planned experiments should also create new directions in ultrafast coherent electron dynamics, surface science, spintronics, time-resolved electron holography and quantum information science.