Markus Schenk

Attosecond control of electrons emitted from a nanoscale metal tip

Attosecond science is based on steering electrons with the electric field of well controlled femtosecond laser pulses. It has led to the generation of extreme-ultraviolet pulses with a duration of less than 100 attoseconds (ref. 3; 1 as = 10−18 s), to the measurement of intramolecular dynamics (by diffraction of an electron taken from the molecule under scrutiny) and to ultrafast electron holography. All these effects have been observed with atoms or molecules in the gas phase. Electrons liberated from solids by few-cycle laser pulses are also predicted to show a strong light-phase sensitivity, but only very small effects have been observed. Here we report that the spectra of electrons undergoing photoemission from a nanometre-scale tungsten tip show a dependence on the carrier-envelope phase of the laser, with a current modulation of up to 100 per cent. Depending on the carrier-envelope phase, electrons are emitted either from a single sub-500-attosecond interval of the 6-femtosecond laser pulse, or from two such intervals; the latter case leads to spectral interference. We also show that coherent elastic re-scattering of liberated electrons takes place at the metal surface. Owing to field enhancement at the tip, a simple laser oscillator reaches the peak electric field strengths required for attosecond experiments at 100-megahertz repetition rates, rendering complex amplified laser systems dispensable. Practically, this work represents a simple, extremely sensitive carrier-envelope phase sensor, which could be shrunk in volume to about one cubic centimetre. Our results indicate that the attosecond techniques developed with (and for) atoms and molecules can also be used with solids. In particular, we foresee subfemtosecond, subnanometre probing of collective electron dynamics (such as plasmon polaritons) in solid-state systems ranging in scale from mesoscopic solids to clusters and to single protruding atoms.

Strong-field above-threshold photoemission from sharp metal tips

Laser-triggered nanoscale electron emitters should be ideal electron sources for applications such as ultrafast electron diffraction or electron accelerators. Furthermore, they give insight into fundamental questions of light-matter interaction and ultrafast electron dynamics in nanostructures. Recently, a variety of experiments has been done in this field [1]. Here we present recent measurements that enable us to, for the first time, uniquely identify strong-field effects that are clearly resolved in the spectra of emitted electrons [2]. The observation of these effects shows that such a system may be ideal to extend strong-field studies from atomic gases to solid state systems.

Attosecond physics in photoemission from a metal nanotip

Nanometre-scale metal tips irradiated by femtosecond laser pulses represent ultrafast electron sources. The combination of the laser pulse and the tip offers the possibility of extending attosecond science from atomic or molecular gases to surfaces of solid nanoemitters. We first review this emerging research field focusing on electron rescattering at sharp metal tips. In particular, we investigate the carrier?envelope phase effects that reveal attosecond emission dynamics. Furthermore, we present detailed theory models that support this interpretation.

Electrostatic Extraction of Cold Molecules from a Cryogenic Reservoir

We present a method which delivers a continuous, high-density beam of slow and internally cold polar molecules. In our source, warm molecules are first cooled by collisions with a cryogenic helium buffer gas. Cold molecules are then extracted by means of an electrostatic quadrupole guide. For ND3 the source produces fluxes up to (7+/- 4(7)) x 10(10) molecules/s with peak densities up to (1.0+/- 0.6(1.0)) x 10(9) molecules/cm3. For H2CO the population of rovibrational states is monitored by depletion spectroscopy, resulting in single-state populations up to (82+/-10)%.

Electron rescattering at metal nanotips induced by ultrashort laser pulses

We report on the first investigation of plateau and cut-off structures in photoelectron spectra from nano-scale metal tips interacting with few-cycle near-infrared laser pulses. These hallmarks of electron rescattering, well-known from atom-laser interaction in the strong-field regime, appear at remarkably low laser intensities with nominal Keldysh parameters of the order of

Interaction of ultrashort laser pulses with metal nanotips: a model system for strong-field phenomena

\gtrsim 10

Probing of Optical Near-Fields by Electron Rescattering on the 1 nm Scale

. Quantum and quasi-classical simulations reveal that a large field enhancement near the tip and the increased backscattering probability at a solid-state target play a key role. Plateau electrons are by an order of magnitude more abundant than in comparable atomic spectra, reflecting the high density of target atoms at the surface. The position of the cut-off serves as an in-situ probe for the locally enhanced electric field at the tip apex.

Note: Production of sharp gold tips with high surface quality.

We discuss the interaction of ultrashort near-infrared laser pulses with sharp metal tips at moderate nominal intensities (I0 10 11 Wcm 2 ). As external electric fields are strongly enhanced at such tips (enhancement factor 10) our system turns out to be an ideal miniature laboratory to investigate strong-field effects at solid surfaces. We analyse the electron-energy spectra as a function of the strength of the laser field and the static extraction field and present an intuitive model for their interpretation. The size of the effective field acting on the metal electrons can be determined from the electron spectra. The latter are also reproduced by time-dependent density functional theory (TDDFT) simulations.

Internal-state thermometry by depletion spectroscopy in a cold guided beam of formaldehyde

We present a new method of measuring optical near-fields within ~1 nm of a metal surface based on rescattering of photoemitted electrons. With this method, we precisely measure the field enhancement factor for tungsten and gold nanotips as a function of tip radius. The agreement with Maxwell simulations is very good. Further simulations yield a field enhancement map for all materials, which shows that optical near-fields at nanotips are governed by a geometric effect under most conditions, while plasmon resonances play only a minor role. Last, we consider the implications of our results on quantum mechanical effects near the surface of nanostructures and discuss features of quantum plasmonics.

Spectroscopy of the Ã1A2 ← X1A1 transition of formaldehyde in the 30140-30790 cm-1 range: The 201403 and 202401 rovibrational bands

We present a simple method to produce sharp gold tips with excellent surface quality based on electrochemical etching with potassium chloride. Radii of curvature lie in the range of 20-40 nm and the surface roughness is measured to less than 0.8 nm. The tips are well suited for field emission, field ion microscopy, and likely for tip-enhanced Raman scattering as well as tip-enhanced near-field imaging.