We are interested in
Manipulation of coherent electron waves
Ultrafast electron and phonon dynamics
Atomic resolution microscopy
When we apply a high voltage to a metallic tip of nanometer sharpness (a nano-tip), electrons can be emitted from the tip’s apex to a vacuum via the electron tunneling processes. This is what is called field emission. The emitted electrons propagate radially from the tip apex. As a result, the emitted electrons magnify nanoscopic information on the apex to a macroscopic scale, which enables field emission microscopy (FEM).
This is a typical field emission pattern from a tungsten tip oriented towards the  direction. We can observe four nanoscale emission sites.
Laser pulses induce electron emission from the same emission sites. But the emission patterns become strongly asymmetric and drastically change with varying angles of laser polarization. In effect, we can select the emission sites. For instance, there are two emission sites, which are spaced approximately 30nm apart. Even when emission sites are this close together, we can select either one of them.
Understanding the physics behind is the most enjoyable part for us. We performed plasmonic simulations using OpenMaXwell and investigated how a laser wave propagates through a model tip.
Using the resulting optical field distributions on the nano-tip and the field emission theory, we successfully reproduced the asymmetric emission patterns. For more details.
The above site-selective technique can be used to control the Young’s interference. If we have two emission sites, we can replicate the Young’s interference experiments with field emission. In the nano-tip, a coherent electron wave exists and it will be split into two coherent waves when it is emitted into the vacuum. If the two coherent electron waves overlap each other in the vacuum, the interference can be observed.
Using 7fs laser pulses, we can observe the interference patterns between two consecutive emission sites. They appear as streaky patterns indicated by the red arrows. Let’s have a look at the interference patterns of the electron emissions from sites A and B. If the electron emissions occur from both A and B, an interference can be observed. However, if the electron emissions occur from either A or B, no interference can be observed.
The interference was successfully reproduced by simulating the temporal evolution of an electron wave from a nano-tip to the vacuum based on a time-dependent Schrödinger equation. The simulations revealed the underlying physics that drive the interference patterns in the laser-induced field emission. For more details.
We developed a site-selective ultrafast electron source on scales of nanometers and femtoseconds. Such an electron source can be used for time-resolved electron microscopy and for realizing ultrafast devices. In order to use this source for applications, we need to understand its electron dynamics. Under the illumination of weak field laser, femtosecond dynamics can be expected. For instance, we simulated how the population of excited electrons changes while a laser pulse passes through the tip apex. The upper right panel shows the temporal evolution of the electron distribution function.
These femtosecond electron dynamics can be addressed by the energy spectra of the emitted electrons. The energy spectra can be measured by using an electron energy analyzer. We have a hemispherical analyzer with quite good energy resolution.
The measured energy spectra (green dots) change depending on the laser powers and the tip voltages. Using the above theory, we have successfully reproduced the energy spectra (pink curves), which allows us a direct insight into the electron dynamics involved. For more details.
If the laser field is strong, we have additional electron dynamics in the vacuum. Now some of the emitted electrons can be driven back to the surface due to the oscillating laser fields and elastically (or inelastically) re-scattered from the surface.
In order to understand the physics in the strong laser field regime, we performed 3D electron tracking simulations. A 7fs laser pulse induces electron emission. The emitted electrons feel DC fields, AC fields, and coulomb forces from other emitted electrons and image charges in the nano-tip.
Thus the simulated energy spectra successfully reproduced the observed energy spectra in the strong field regime. For more details.
In even stronger laser fields, thermal effects become prevalent and laser heating modifies the tip surface asymmetrically. For more details.
Dr. Hirofumi Yanagisawa, DFG project leader,
Am Coulombwall 1, 85748 Garching, office: 215
Office Phone: 08928954087