Terahertz-Enabled Ultrafast Electron Field Emission: From Fundamentals to Applications

This PhD work explores ways to use ultrafast electron field emission as an enabling phenomenon for the development of new technologies. The work has been carried out as a research project of both fundamental science and innovation. As a result,the dissertation covers a wide range of scientific observations of which only a few have been selected for further scrutiny based on their potential use in future technologies.

Electron field emission is a phenomenon that occurs in physics when a strong electric field in the order of V/nm is applied to a subject like an atom, a molecule or solid state material. Under such conditions, electrons residing inside the subject can undergo quantum mechanical tunnelling to emit into the surrounding environment.

In a technology context, the required electric field has historically been applied using electronics. With the rapid development of highly intense, ultrafast femtosecond (fs) lasers over the last 2-3 decades however, it has become possible to access the required electric field regime using optical methods.

An important, inherent capability of ultrafast laser pulses is that they lead to on/off switching timesof the electric field that are several orders of magnitude below what can be reached with electronics. The associated electron field emission is therefore also taking place on ultrafast time scales, which ultimately leads to a branch of fundamental science that is still relatively unexplored.

In this work, we use ultrafast laser pulses to generate single-cycle terahertz(THz) electromagnetic transients of∼1 picosecond time duration and peak electricfields∼100 kV/cm. The THz transients enable ultrafast electron field emission from 2-D periodically arranged arrays of subwave length metal antennas called metasurfaces.

By carefully engineering the metasurfaces, we enable field emission with compact table-top laser systems. Such engineering is based on numerical simulations using the software package CST MWS followed by nano-fabrication in the National Danish Cleanroom, located at DTU. We observe via permanently imprinted damage patterns how currents are distributed in our antennas and how the electron emission can be visualized as a projection onto the metasurface substrate.

The linearity of numerical simulations are corroborated with a THz-THz autocorrelation experiment, which experimentally demonstrates the ultrafast fs emission time scale. As a first technological development, we initiate work on how to use the electron emission to initiate chemical reactions. The aim is to resolve the chemical dynamics on a fs time scale using e.g. electron pump optical probe experiments.

Todo this, we cast polymerbased chemical systems directly on top of the metasurfaces and observe both permanent polymer reconfiguration as well as a transient scin-tillating effect. We use Monte Carlo-based numerical methods to explain the latterand gain insight into the electron dynamics inside the polymer.

Next, we utilize the ability of field emitted electrons to ionize a surrounding ar-gon gas, which creates a visible plasma glow discharge that we record with a CCD camera. Such nonlinear mapping from THz into the visible goes via the electricfield seen by the metasurfaces. This enables detection of a wide range of characteristics pertaining directly to the electric field, of which we demonstrate determination of polarization and absolute polarity.

In addition, we demonstrate single-shot2-D imaging of THz transients with peak electric fields∼100 kV/cm.The innovative pinnacle comes from a collaboration with the Japanese company Hamamatsu Photonics. Here, we embed metasurfaces into sensitive light detection devices called photomultiplier tubes (PMTs), in order to extend the PMT workingrange from near infrared light into the THz frequency regime.

We use this to verify that field emission from metasurfaces can also be enabled by laser pulses in the range 2.6→12μm. These results are used to conclude that field emission canbe enabled by all frequencies in the entire THz- and infrared frequency range. We subsequently demonstrate an extended technique where the THz electric field ismixed with the electric field from an electronic source.

This increases the sensitivity of our PMTs drastically. In sequel, DTU and Hamamatsu Photonics will continue the PMT development for years to come, thus proving that fundamental science and innovation can in practice go hand in hand.