Methods for studying ultrafast nanoscale dynamics

A central aspect of the thesis at hand is the development of new instrumental capabilities in ultrafast electron microscopy for the study of ultrafast nanoscale dynamics. However, assessing the future prospects of the UTEM methodology requires a comprehensive comparison to other ultrafast high-resolution probing techniques, which is presented in the following and summarized in Fig.7.8.

The most relevant parameters to assess the capabilities of ultrafast probing techniques include (i) real-space (ii) k-space (iii) spectroscopic and (iv) temporal resolution. Further-more, the couplings to the sample and corresponding acquired information depend on the (v) type of particle or radiation and their (vi) respective wavelengths. The (vii) coherence properties determine the usefulness of holography or phase contrast imaging. Finally, it must be distinguished between techniques that use (iix) near-field or far-field probing, being a key aspect for flexibility and required sample geometry and their (ix) sensitivity to surfaces, thin film samples or bulk materials.

Photon based methods

Optical spectroscopy and microscopy using visible light provide for an excellent few-fs temporal and few-meV spectral resolution. Therefore, optical far-field probing methods are ideally suited for the study of ultrafast electronic phenomena in homogeneous condensed matter [317] and gas-phase systems [370] at the femtosecond time scale. Nanoscale local probing of electronic states is provided by scanning near-field optical microscopy [371] and THz-field gated scanning tunneling microscopy [27]. Recently, table-top high-harmonic sources, which can readily reach attosecond pulse durations [23,244], could achieve a real-space resolution on the 10-nm scale by coherent diffractive imaging (CDI) [372,373]

with tailored contrast mechanisms, e.g. to out-of-plane magnetization [374]. Yet, direct far-field optical probing of lattice dynamics requires photon wavelengths matched to the atomic distances and is so far only achieved in large-scale synchrotron or FEL facilities [375,376]. Atomic scale reciprocal space and a 10-100-nm real space resolution were shown by X-ray nanodiffraction [377–379] or lensless imaging approaches [176,380].

Notably, optical methods benefit from almost fully coherent light sources that enable diffraction-limited spatial resolution and bandwidth-limited ultrashort pulses in a routinely manner. Coherent beam techniques like in-line or off-axis holography, interferometry, high-quality phase contrast imaging and advanced beam shaping contribute to the powerful

toolset of optical science. On the downside, lab-scale ultrafast coherent light sources of sub-nm wavelength are not available, and the low scattering cross-sections and high penetration depths render the study of nanoscale volumes increasingly difficult [381].

Electron probing

Ultrashort electron pulses are ideally suited as probes for ultrafast lattice dynamics due to their intrinsically short wavelength, high scattering cross-sections and technological synergy with the well-developed field of electron microscopy. Ultrafast far-field electron diffraction in transmission (UED) [25,114] or reflection (U-RHEED, U-LEED) [78,174]

routinely achieves down to 100-fs temporal [201,214] and sub-Å resolution in k-space [77], typically probing sample areas on the10−100µmscale. Further strategies for improved electron source quality were discussed in Sec. 7.1. One alternative approach is to use localized photoelectron emission from clean surfaces. Time and angle resolved photoe-mission spectroscopy (trARPES) [382,383] achieves laser bandwidth-limited temporal and spectral resolution and time-resolved photoemission electron microscopy (TR-PEEM) [384] gives a below 100-nm spatial resolution, e.g. of local sample magnetization [385].

For gas targets, laser-induced electron diffraction (LIED) [386,387] yields a sub-optical cycle timing-accuracy of inertial molecular dynamics. Another promising approach to probe electronic dynamics and optical near-fields is by lens-less projection to the far-field in ultrafast point-projection microscopy (PPM) [29,182–184] with a few 10-nm spatial and fs temporal resolution.

Coherent ultrafast TEM

Each of the ultrafast electron probing techniques described above demonstrate tailored capabilities for either excellent k-space or real space resolution, combined with high sensitivity to specific sample properties. Usually, a high temporal resolution is achievable, but strongly depending on the experimental geometry. The primary achievement of ultrafast transmission electron microscopy is a combination of either excellent k-space or real space probing, with simultaneous sub-ps temporal and about1 eVenergy resolution.

State-of-the-art TEM optics enable a highly flexible investigation of thin film samples in diffraction, imaging or spectroscopy with sensitivity to electronic, magnetic and lattice degrees-of-freedom.

The instrumental development presented in this thesis further expands the capabilities of UTEM, now spanning orders of magnitude in real-space (1 nmto1 mm) and k-space (sub 1 ˚A⁻¹to1µm⁻¹) for the stroboscopic probing of ultrafast dynamics in condensed matter with a temporal resolution of down to200 fs. Notably, the strongly enhanced coherence properties of electron beams generated from nanoscale photocathodes have not been fully harnessed yet. Access to holography, interferometry, advanced beam shaping and high-resolution phase contrast imaging will drastically expand the addressable scientific questions and sample systems of ultrafast TEM, making it one of the most universal tools for the study of ultrafast nanoscale dynamics (cf. Fig.7.8).

UTEM

cTEM

DTEM UTEM

cTEM (ultrafast)

optics HHG HHG

FEL FEL

LIED trARPES

UED URHEED

ULEED PPM

STMTHz

DTEM

k-space (lattice- and molecular correlations) real-space (inhomogeneous systems)

1/pm 1/nm 1/µm 1/mm mm µm nm pm

as

fs

ps

ns

µs

ms

Figure 7.8:Time and length scales addressed by fast probing techniques. A variety of ultrafast techniques aims for a particular high resolution in real or reciprocal space and tailored sensitivity to specific degrees-of-freedom. Ultrafast TEM features a broad range of applications and explores systems on many orders of magnitude in time, reciprocal space and real space simultaneously.