Max Gulde

Field-driven photoemission from nanostructures quenches the quiver motion

Strong-field physics, an extreme limit of light–matter interaction, is expanding into the realm of surfaces and nanostructures from its origin in atomic and molecular science. The attraction of nanostructures lies in two intimately connected features: local intensity enhancement and sub-wavelength confinement of optical fields. Local intensity enhancement facilitates access to the strong-field regime and has already sparked various applications, whereas spatial localization has the potential to generate strong-field dynamics exclusive to nanostructures. However, the observation of features unattainable in gaseous media is challenged by many-body effects and material damage, which arise under intense illumination of dense systems. Here, we non-destructively access this regime in the solid state by employing single plasmonic nanotips and few-cycle mid-infrared pulses, making use of the wavelength-dependence of the interaction, that is, the ponderomotive energy. We investigate strong-field photoelectron emission and acceleration from single nanostructures over a broad spectral range, and find kinetic energies of hundreds of electronvolts. We observe the transition to a new regime in strong-field dynamics, in which the electrons escape the nanolocalized field within a fraction of an optical half-cycle. The transition into this regime, characterized by a spatial adiabaticity parameter, would require relativistic electrons in the absence of nanostructures. These results establish new degrees of freedom for the manipulation and control of electron dynamics on femtosecond and attosecond timescales, combining optical near-fields and nanoscopic sources.

Ultrafast low-energy electron diffraction in transmission resolves polymer/graphene superstructure dynamics.

Probing interfaces with electrons When molecules move on surfaces, they behave differently from when inside a solid. But surface layers give off limited signals, so to probe these systems, scientists need to act fast. Gulde et al. developed an ultrafast low-energy electron diffraction technique and used it to study how a polymer moved and melted on a graphene substrate (see the Perspective by Nibbering). After hitting the sample with a laser pulse, energy transferred across the graphene-polymer interface, the polymer film became less orderly, and an amorphous phase appeared. Science, this issue p. 200; see also p. 137 Time-resolved low-energy electron diffraction resolves picosecond structural dynamics in a polymer-graphene bilayer. [Also see Perspective by Nibbering] Two-dimensional systems such as surfaces and molecular monolayers exhibit a multitude of intriguing phases and complex transitions. Ultrafast structural probing of such systems offers direct time-domain information on internal interactions and couplings to a substrate or bulk support. We have developed ultrafast low-energy electron diffraction and investigate in transmission the structural relaxation in a polymer/graphene bilayer system excited out of equilibrium. The laser-pump/electron-probe scheme resolves the ultrafast melting of a polymer superstructure consisting of folded-chain crystals registered to a free-standing graphene substrate. We extract the time scales of energy transfer across the bilayer interface, the loss of superstructure order, and the appearance of an amorphous phase with short-range correlations. The high surface sensitivity makes this experimental approach suitable for numerous problems in ultrafast surface science.

Coherent femtosecond low-energy single-electron pulses for time-resolved diffraction and imaging: A numerical study

We numerically investigate the properties of coherent femtosecond single electron wave packets photoemitted from nanotips in view of their application in ultrafast electron diffraction and non-destructive imaging with low-energy electrons. For two different geometries, we analyze the temporal and spatial broadening during propagation from the needle emitter to an anode, identifying the experimental parameters and challenges for realizing femtosecond time resolution. The simple tip-anode geometry is most versatile and allows for electron pulses of several ten of femtosecond duration using a very compact experimental design, however, providing very limited control over the electron beam collimation. A more sophisticated geometry comprising a suppressor-extractor electrostatic unit and a lens, similar to typical field emission electron microscope optics, is also investigated, allowing full control over the beam parameters. Using such a design, we find ∼230 fs pulses feasible in a focused electron beam. The m...

Dynamics and Structure of Monolayer Polymer Crystallites on Graphene

Graphene-based nanostructured systems and van der Waals heterostructures comprise a material class of growing technological and scientific importance. Joining materials with vastly different properties, polymer/graphene heterosystems promise diverse applications in surface and nanotechnology, including photovoltaics or nanotribology. Fundamentally, molecular adsorbates are prototypical systems to study confinement-induced phase transitions exhibiting intricate dynamics, which require a comprehensive understanding of the dynamical and static properties on molecular time and length scales. Here, we investigate the dynamics and the structure of a single polyethylene chain on free-standing graphene by means of molecular dynamics simulations. In equilibrium, the adsorbed polymer is orientationally linked to the graphene as two-dimensional folded-chain crystallite or at elevated temperatures as a floating solid. The associated superstructure can be reversibly melted on a picosecond time scale upon quasi-instantaneous substrate heating, involving ultrafast heterogeneous melting via a transient floating phase. Our findings elucidate time-resolved molecular-scale ordering and disordering phenomena in individual polymers interacting with solids, yielding complementary information to collective friction and viscosity, and linking to recent experimental observables from ultrafast electron diffraction. We anticipate that the approach will help in resolving nonequilibrium phenomena of hybrid polymeric systems over a broad range of time and length scales.

Methods and Concepts

This chapter offers an introduction to the theoretical and experimental methods, which are important in the framework of this thesis. First, the concept of low-energy electron diffraction as a tool for surface structural investigations is outlined.

Ultrafast PMMA Superstructure Dynamics on Free-Standing Graphene

This chapter describes the time-resolved structural analysis of an ultrathin polymer overlayer adsorbed on free-standing graphene by the previously introduced transmission ULEED setup.

Strong-Field Photoemission from Metallic Nanotips

Nonlinear photoemission from single nanostructures is investigated over a broad wavelength range in the near- and mid-infrared. The field enhancement at a nanotip apex and mid-infrared excitation enable sub-cycle dynamics, where electrons are ejected from the near-field within a fraction of the optical half-cycle. The implications of this field-driven acceleration are studied via photoemission spectrocopy and numerical simulations.

Numerical Analysis of a Tip-Based Ultrafast Electron Gun

In this section, a numerical study of an electron source with tip-based geometry is provided. First, a brief introduction to the employed finite element method (FEM) is given (Sect. 4.1). Next, the influence of the individual lens components with respect to focusability and temporal resolution of the electron pulse is simulated (Sect. 4.2).

Aspects of Ultrafast LEED

In this chapter, the potential of ultrafast LEED for investigations with atomic-scale resolution is discussed.

Experimental Analysis of a Tip-Based Ultrafast Electron Gun

In this chapter, the experimental realization of the simulated gun design is presented, starting with the preparation procedure of the needle emitter (Sect. 5.1). Furthermore, the setup of the ULEED experiment is described (Sect. 5.2) and a characterization of key parameters such as pulse duration is given (Sect. 5.3).