Lara Wimmer

Field emission at terahertz frequencies: AC-tunneling and ultrafast carrier dynamics

We demonstrate ultrafast terahertz (THz) field emission from a tungsten nanotip enabled by local field enhancement. Characteristic electron spectra which result from acceleration in the THz near-field are found. Employing a dual frequency pump–probe scheme, we temporally resolve different nonlinear photoemission processes induced by coupling near-infrared (NIR) and THz pulses. In the order of increasing THz field strength, we observe THz streaking, THz-induced barrier reduction (Schottky effect) and THz field emission. At intense NIR-excitation, the THz field emission is used as an ultrashort, local probe of hot electron dynamics in the apex. A first application of this scheme indicates a decreased carrier cooling rate in the confined tip geometry. Summarizing the results at various excitation conditions, we present a comprehensive picture of the distinct regimes in ultrafast photoemission in the near- and far-infrared.

Phase space manipulation of free-electron pulses from metal nanotips using combined terahertz near fields and external biasing

This work studies the manipulation of photoelectron emission from metallic nanostructures by intense single-cycle terahertz transients. Specifically, the authors employ streaking spectroscopy to study the kinetic energy of photoelectrons emitted from metal nanotips exposed to tailored static and terahertz electrical fields. Supported by detailed numerical simulations, the measurements provide quantitative information on the temporal and spatial properties of terahertz near fields at metal nanotips. The study illustrates far-reaching control over the trajectories and phase-space density evolution of photoelectron wavepackets acted upon by strong static and dynamic near fields. The results are relevant for applications of nanoscopic photoelectron sources employed in ultrafast electron diffraction and microscopy.

THz near-field streaking spectroscopy at biased metal nanotips

We demonstrate terahertz (THz) near-field streaking at metal nanotips. In this scheme, photoelectrons are generated at the apex of a nanotip (gold, tungsten) by near-infrared (NIR) femtosecond pulses and accelerated in the THz-induced near-field (cf. Fig. 1a). The high local field strengths and the nanometric spatial confinement of the THz near-field allow electrons to leave the THz electric field within a fraction of an optical cycle, and the kinetic energy of the photoelectrons is determined by the electric field in the moment of photoemission. Such a sub-cycle interaction of the photoelectrons with the streaking field is a characteristic feature of near-field streaking at nanostructures [1]. We record streaking spectrograms (see Fig. 1b) by varying the relative delay between the THz and the NIR pulses to map the temporal evolution of the THz near-field onto the kinetic energy of the photoelectrons [2].

Clocking plasmon nanofocusing by THz near-field streaking

We apply terahertz (THz) near-field streaking in a nanofocusing geometry to investigate plasmon polariton propagation on the shaft of a conical nanotip. By evaluating the delay between a streaking spectrogram for plasmon-induced photoemission with a measurement for direct apex excitation, we obtain an average plasmon group velocity, which is in agreement with numerical simulations. Combining plasmon-induced photoemission with THz near-field streaking facilitates extensive control over localized photoelectron sources for time-resolved imaging and diffraction.

Highly Nonlinear and Ultrafast Optical Phenomena in Metallic Nanostructures

This Chapter presents recent findings on nonlinear ionization and photoemission processes at metallic nanostructures. A particular emphasis is placed on processes which—due to the localized excitation in optical near-fields—exhibit different properties and scalings from their counterparts in the gas phase or at planar surfaces. The Chapter is structured in two parts. The first part discusses various regimes in highly nonlinear photoelectron emission from metallic nanotips, including field-driven photoemission at near- and mid-infrared frequencies, and the control of localized photoemission using intense terahertz transients. In the second part, multiphoton and strong-field ionization of atomic gases in plasmonic antennas and waveguides is presented. It is demonstrated that local ionization is enabled by optical field enhancements in various types of structures. At the same time, fundamental physical limitations preventing efficient high-harmonic generation in nanostructures are discussed.

Controlling ultrafast photoelectron dynamics in nanostructure-enhanced THz fields

We present experimental results on a nanostructure streaking scheme employing carrier-envelope-phase-stable THz pulses to control ultrafast photoemission. The tip-enhanced local THz-field allows for field-driven electron dynamics and effective manipulation of trajectories and kinetic energy spectra.