Katharina E. Echternkamp

Quantum coherent optical phase modulation in an ultrafast transmission electron microscope

Coherent manipulation of quantum systems with light is expected to be a cornerstone of future information and communication technology, including quantum computation and cryptography. The transfer of an optical phase onto a quantum wavefunction is a defining aspect of coherent interactions and forms the basis of quantum state preparation, synchronization and metrology. Light-phase-modulated electron states near atoms and molecules are essential for the techniques of attosecond science, including the generation of extreme-ultraviolet pulses and orbital tomography. In contrast, the quantum-coherent phase-modulation of energetic free-electron beams has not been demonstrated, although it promises direct access to ultrafast imaging and spectroscopy with tailored electron pulses on the attosecond scale. Here we demonstrate the coherent quantum state manipulation of free-electron populations in an electron microscope beam. We employ the interaction of ultrashort electron pulses with optical near-fields to induce Rabi oscillations in the populations of electron momentum states, observed as a function of the optical driving field. Excellent agreement with the scaling of an equal-Rabi multilevel quantum ladder is obtained, representing the observation of a light-driven ‘quantum walk’ coherently reshaping electron density in momentum space. We note that, after the interaction, the optically generated superposition of momentum states evolves into a train of attosecond electron pulses. Our results reveal the potential of quantum control for the precision structuring of electron densities, with possible applications ranging from ultrafast electron spectroscopy and microscopy to accelerator science and free-electron lasers.

Ramsey-type phase control of free-electron beams

Using a technique inspired by Ramsey spectroscopy it is now possible to coherently control free electrons in an electron microscope.

Optical field emission from resonant gold nanorods driven by femtosecond mid-infrared pulses

We demonstrate strong-field photoelectron emission from gold nanorods driven by femtosecond mid-infrared optical pulses. The maximum photoelectron yield is reached at the localized surface plasmon resonance, indicating that the photoemission is governed by the resonantly-enhanced optical near-field. The wavelength- and field-dependent photoemission yield allows for a noninvasive determination of local field enhancements, and we obtain intensity enhancement factors close to 1300, in good agreement with finite-difference time domain computations.

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.

Quantum coherent interaction of electrons with optical near-fields in an ultrafast electron microscope

We study photon-induced scattering of swift electrons with confined light in an ultrafast transmission electron microscope. The fluence-dependent sideband population in the kinetic energy spectra evidences the quantum coherent manipulation of the electron momenta.

Ultrafast Transmission Electron Microscopy with nanoscale Photoemitters

Ultrafast transmission electron microscopy (UTEM) is a rapidly developing technique which aims to elucidate laser-triggered ultrafast processes at the nanometer scale [1]. In UTEM, ultrafast imaging and spectroscopy with sub-picosecond temporal resolution is achieved by utilizing electron pulses in a stroboscopic laser-pump/electron-probe scheme (Fig 1a). The previously employed flat-surface photocathodes put a severe limit on the achievable spatial resolution due to their intrinsically large electron source size, demanding novel pulsed electron source concepts.

Electron Tunneling and Acceleration at Gold Nanostructures Driven by Ultrashort Mid-Infrared Pulses

Strong-field photoemission from resonant and non-resonant gold nanostructures is studied using ultrashort mid-infrared pulses. The photoelectron yield and kinetic energy spectra are governed by both antenna resonances and optical near-field distributions.

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.

Photoemission at metallic nanostructures: multiphoton and strong-field dynamics

Photoemission from nanostructures offers sub-wavelength field localization and enhancement. Excited by ultrashort pulses, electron emission can be confined and controlled in both time and space. Studies with metallic nanotips have examined the transition to strong-field conditions in photoemission. Reaching deeply into this regime with ultrashort mid-infrared pulses, we generate photoelectrons up to hundreds of electron volts and observe dynamics in which electrons are ejected from the field-enhanced region in less than an optical half cycle. Moreover, single cycle terahertz pulses are shown to yield a novel means of control over the photoemission process.