Johannes Hoffrogge

Tip-based source of femtosecond electron pulses at 30 keV

We present a nano-scale photoelectron source, optimized for ultrashort pulse durations and well-suited for time-resolved diffraction and advanced laser acceleration experiments. A tungsten tip of several-ten-nanometers diameter mounted in a suppressor-extractor electrode configuration allows the generation of 30 keV electron pulses with an estimated pulse duration of 9 fs (standard deviation; 21 fs full width at half maximum) at the gun exit. We infer the pulse duration from particle tracking simulations, which are in excellent agreement with experimental measurements of the electron-optical properties of the source in the spatial domain. We also demonstrate femtosecond-laser triggered operation of the apparatus. The temporal broadening of the pulse upon propagation to a diffraction sample can be greatly reduced by collimating the beam. Besides the short electron pulse duration, a tip-based source is expected to feature a large transverse coherence and a nanometric emittance.

Microwave Guiding of Electrons on a Chip

We demonstrate the transverse confinement and guiding of a low energy electron beam of several electron volts in a miniaturized linear quadrupole guide. The guiding potential is generated by applying a microwave voltage to electrodes fabricated on a planar substrate, which allows the potential landscape to be precisely shaped on a microscopic scale. We realize transverse trapping frequencies of 100 MHz and guide electrons along a circular section of 37 mm length. A detailed characterization of the guiding properties in terms of potential depth and dynamic stability is given. This new technique of electron guiding promises various applications in guided matter-wave experiments such as electron interferometry.

Planar microwave structures for electron guiding

We present microwave electrode structures suited for guiding electrons propagating in a purely electric, alternating quadrupole field. In a first experiment using a standing wave on an electrically short structure, we previously demonstrated the general concept of electron confinement in microwave fields. Here, we discuss the extension to electrically long structures supporting travelling microwave excitations. This requires a modal decomposition of the voltage patterns on the multiconductor transmission line formed by the electrodes. We show that the use of a general five-wire structure leads to the distortion of the guiding potential upon propagation due to differing modal propagation constants. This can be avoided by implementing a coupled microstrip configuration with elevated signal electrodes. With structures like these, complex geometries connecting different beam manipulation elements can be realized, enabling new forms of electron experiments with guided matter waves.

From Above-Threshold Photoemission to Attosecond Physics at Nanometric Tungsten Tips

The interaction of few-cycle laser pulses with a nanometric metal tip is described. We find many effects that the strong-field physics community has discovered with atoms in the last 30 years, and describe them here in experiments with solid nanotips. Starting with a clear identification of several photon orders in above-threshold photoemission, via strong-field effects such as peak shifting and peak suppression, to the observation of a pronounced plateau in electron spectra, we show that we have reached the level of control necessary for attosecond physics experiments. In particular, we observe electronic wavepacket dynamics on the attosecond time scale. Namely, by variation of the carrier-envelope phase of the driving laser pulses, we observe a qualitative change in the electron spectra: For cosine pulses we obtain an almost flat plateau part, whereas for minus-cosine pulses the plateau part clearly shows photon orders. We interpret this change by the occurrence of a single or a double slit configuration in time causing electronic matter wave interference in the time-energy domain.

Ultrafast coherent electron emission from ultrasharp metal tips

Sharp metal tips as field emitters are well-known and established electron sources. A DC-electric field leads to a continuous electron tunnel current out of the high-energy tail of the electron thermal distribution. In contrast, here we present a project where electrons are emitted due to the high electric field delivered by ultrashort laser pulses.

Ultrafast nanometric electron sources: current status, first applications, and suitable laser sources

Laser pulses of a femtosecond oscillator focused on a sharp metal tip allow the generation of a pulse train of electrons at the oscillator repetition rate [1–4]. The emission of electrons from the tip takes place on the femtosecond time scale. With the advent of these novel ultrafast electron sources, important experiments have been and more will be performed ([5], see [6] for an overview). We report on the status of our current experiment aiming at a deeper understanding of the emission process by analyzing the energy of the electrons emitted. We will further discuss first experimental steps towards direct laser acceleration of low energy electrons by means of a transparent double-grating structure. This structure imposes boundary conditions that allow a net momentum transfer from the laser beam onto the electrons that travel through the structure [7, 8]. Our simulations show that electrons can be accelerated from zero kinetic energy with laser light only (Fig. 1, 2). The acceleration gradient exceeds that of maximum DC acceleration (∼6 MeV/m) even in the very low energy regime. In order to maintain phase synchronicity, a self-referenced driving laser is mandatory. We will give a progress report.

Microwave near-fields on atom chips

Microwave near-fields are a key ingredient for quantum information processing with atom chips. Our goal is to realize a quantum gate with the following features: the qubit is encoded in the hyperfine states |1rang equiv |F=1, mF=-1rang and |2rang equiv |F=2, mF=+1rang of 87Rb, which are both magnetically trappable and allow for very long coherence lifetimes. Microwave near-fields guided on the atom chip are used to drive single-qubit rotations and provide state-selectivity to the magnetic trapping potential. The quantum phase gate is implemented by state-selective collisions of two qubit atoms in this potential. Besides applications in quantum information processing, microwave near-fieIds on atom chips can also be used for atom interferometry and chip-based atomic clocks.