Stefan Cunovic

Spatiotemporal control of nanooptical excitations

The most general investigation and exploitation of light-induced processes require simultaneous control over spatial and temporal properties of the electromagnetic field on a femtosecond time and nanometer length scale. Based on the combination of polarization pulse shaping and time-resolved two-photon photoemission electron microscopy, we demonstrate such control over nanoscale spatial and ultrafast temporal degrees of freedom of an electromagnetic excitation in the vicinity of a nanostructure. The time-resolved cross-correlation measurement of the local photoemission yield reveals the switching of the nanolocalized optical near-field distribution with a lateral resolution well below the diffraction limit and a temporal resolution on the femtosecond time scale. In addition, successful adaptive spatiotemporal control demonstrates the flexibility of the method. This flexible simultaneous control of temporal and spatial properties of nanophotonic excitations opens new possibilities to tailor and optimize the light–matter interaction in spectroscopic methods as well as in nanophotonic applications.

Single-shot timing measurement of extreme-ultraviolet free-electron laser pulses

Arrival time fluctuations of extreme-ultraviolet (EUV) pulses from the free-electron laser in Hamburg (FLASH) are measured single-pulse resolved at the experimental end-station. To this end, they are non-collinearly superimposed in space and time with visible femtosecond laser pulses on a GaAs substrate. The EUV irradiation induces changes of the reflectivity for the visible pulse. The temporal delay between the two light pulses is directly encoded in the spatial position of the reflectivity change which is captured with a CCD camera. For each single shot, the relative EUV/visible arrival-time can be measured with about 40 fs rms accuracy. The method constitutes a novel route for an improvement of future pump–probe experiments at short-wavelength free-electron lasers (FELs) by a pulse-wise correction with simultaneously measured arrival times of individual EUV pulses.

Time-resolved ion spectrometry on xenon with the jitter-compensated soft x-ray pulses of a free-electron laser

Atomic inner-shell relaxation dynamics were measured at the free-electron laser in Hamburg, FLASH, delivering 92 eV pulses. The decay of 4d core holes created in xenon was followed by detection of ion charge states after illumination with delayed 400 nm laser pulses. A timing jitter of the order of several hundred femtoseconds between laser- and accelerator-pulses was compensated for by a simultaneous delay measurement in a single-shot x-ray/laser cross-correlator. After sorting of the tagged spectra according to the measured delays, a temporal resolution equivalent to the pulse duration of the optical laser could be established. While results on ion charge states up to Xe4+ are compatible with a previous study using a high-harmonic soft x-ray source, a new relaxation pathway is opened by the nonlinear excitation of xenon atoms in the intense free-electron laser light field, leading to the formation of Xe5+.

Time-to-space mapping in a gas medium for the temporal characterization of vacuum-ultraviolet pulses

The authors introduce a method for cross correlating vacuum-ultraviolet with near-infrared femtosecond light pulses in a perpendicular geometry. Photoelectrons generated in an atomic gas by laser-assisted photoionization are used to create a two-dimensional image of the cross-correlation volume, thereby mapping time onto a space coordinate. Thus, information about pulse duration and relative timing between the pulses can be obtained without the need to scan an optical delay line. First tests using vacuum-ultraviolet pulses from the free-electron laser at the Deutsches Elektronen Synchrotron set an upper limit for their temporal jitter with respect to external optical laser pulses.

Optimal open-loop near-field control of plasmonic nanostructures

Optimal open-loop control, i.e. the application of an analytically derived control rule, is demonstrated for nanooptical excitations using polarization-shaped laser pulses. Optimal spatial near-field localization in gold nanoprisms and excitation switching is realized by applying a shift to the relative phase of the two polarization components. The achieved near-field switching confirms theoretical predictions, proves the applicability of predefined control rules in nanooptical light-matter interaction and reveals local mode interference to be an important control mechanism.

Nano-Optical Control of Hot-Spot Field Superenhancement on a Corrugated Silver Surface

Coherent control of ultrafast nano-optical excitations of a corrugated silver surface is demonstrated by means of predetermined few-parameter scans and adaptive polarization laser pulse shaping. “Hot spots” in the multiphoton photoemission signals are enhanced and manipulated with a high contrast. Switching between separated and closely spaced hot spots is shown. The latter allows controlling the shape of hot spots and yields improved nanofocusing and “purification” of the photoemission signals. Complex pulse shapes were obtained in adaptive optimizations whose features were reproducible in repeated runs. Predetermined few-parameter control scans provide insight into the interpretation of optimal pulse shapes. The results indicate the existence of long coherence lifetimes on a corrugated silver surface. This combination of collective strong nanoplasmonic near-field enhancement and long-lived coherence may be used to achieve an even stronger field enhancement (“superenhancement”) making these hot spots ideal candidates for future nanophotonic, spectroscopic, sensor and quantum information applications. In addition the observation of such long coherence lifetimes is relevant to the understanding of surface-enhanced spectroscopies such as single-molecule Raman spectroscopy.

Simultaneous Spatial and Temporal Control of Nanooptical Fields

Using time-resolved two-photon photoemission electron microscopy we demonstrate simultaneous spatial and temporal control of nanooptical fields. Cross correlation measurements reveal the ultrafast spatial switching of the local excitation on a subdiffraction length scale.

Deterministic control of subwavelength field localization in plasmonic nanoantennas

Subwavelength photoemission localization and switching in plasmonic bowtie nanoantennas is achieved experimentally. Analytic and adaptive control schemes are investigated, and agreement between both approaches is demonstrated.