Soo Hoon Chew

Time-of-flight-photoelectron emission microscopy on plasmonic structures using attosecond extreme ultraviolet pulses

We report on the imaging of plasmonic structures by time-of-flight-photoemission electron microscopy (ToF-PEEM) in combination with extreme ultraviolet (XUV) attosecond pulses from a high harmonic generation source. Characterization of lithographically fabricated Au structures using these ultrashort XUV pulses by ToF-PEEM shows a spatial resolution of ∼200 nm. Energy-filtered imaging of the secondary electrons resulting in reduced chromatic aberrations as well as microspectroscopic identification of core and valence band electronic states have been successfully proven. We also find that the fast valence band electrons are not influenced by space charge effects, which is essentially important for attosecond nanoplasmonic-field microscopy realization.

Generation of circularly polarized high harmonic radiation using a transmission multilayer quarter waveplate

High harmonic radiation is meanwhile nearly extensively used for the spectroscopic investigation of electron dynamics with ultimate time resolution. The majority of high harmonic beamlines provide linearly polarized radiation created in a gas target. However, circular polarization greatly extends the spectroscopic possibilities for high harmonics, especially in the analysis of samples with chirality or prominent spin polarization. We produced a free-standing multilayer foil as a transmission EUV quarter waveplate and applied it for the first time to high harmonic radiation. We measured a broadband (4.6 eV FWHM) ellipticity of 75% at 66 eV photon energy with a transmission efficiency of 5%. The helicity is switchable and the ellipticity can be adjusted to lower values by angle tuning. As a single element it can be easily integrated in any existing harmonic beamline without major changes.

Spatiotemporal characterization and control of lightfield nanolocalization on metallic nanostructures by nonlinear-PEEM

The excitation of surface plasmons in metallic nanostructures by resonant ultrashort femtosecond light pulses produces interesting phenomena such as optical field nanolocalization, nanoscale electric field enhancement and ultrafast sub-femtosecond beating of the plasmon eigenmodes. Nonlinear two-photon photoemission electron microscopy has proven to be a powerful tool for spatiotemporal characterization of such effects on the nanoscale below the optical diffraction limit. As a step toward using intense, few-cycle 4 femtosecond laser pulses to excite and control surface plasmons, we report on the multiphoton-photoemission electron microscopy experiments on lithographically-fabricated gold nanostructures excited by these few-cycle laser pulses. In addition, the effects of the shape and size of silver plasmonic structures, as well as the polarization of the excitation source are examined in the two-photon photoemission induced by picosecond laser pulses. Potential approaches toward spatiotemporal control of lightfield nanolocalization are described.

Development of a 10 kHz high harmonic source up to 140 eV photon energy for ultrafast time-, angle-, and phase-resolved photoelectron emission spectroscopy on solid targets

We present a newly developed high harmonic beamline for time-, angle-, and carrier-envelope phase-resolved extreme ultraviolet photoemission spectroscopy on solid targets for the investigation of ultrafast band structure dynamics in the low-fs to sub-fs time regime. The source operates at a repetition rate of 10 kHz and is driven by 5 fs few-cycle near-infrared laser pulses generating high harmonic radiation with photon energies up to 120 eV at a feasible flux. The experimental end station consists of a complementary combination of photoelectron detectors which are able to spectroscopically address electron dynamics both in real and in k-space. The versatility of the source is completed by a phase-meter which allows for tracking the carrier-envelope phase for each pulse and which is synchronized to the photoelectron detectors, thus enabling phase sensitive measurements on the one hand and the selection of single attosecond pulses for ultimate time resolution in pump-probe experiments on the other hand. We demonstrate the applicability of the source by an angle- and carrier-envelope phase-resolved photoemission measurement on a tungsten (110) surface with 95 eV extreme ultraviolet radiation.

Carrier-envelope-phase- and angle-resolved photoelectron streaking measurements on W(110)

We present an extended streaking measurement scheme by additionally detecting the electron emission angle and the carrier-envelope phase of the IR laser field. This allows a more complete and detailed insight into the electron dynamics in solids and gives access to ultrafast band structure dynamics.

Low chromatic Fresnel lens for broadband attosecond XUV pulse applications

Fresnel zone plates show a great potential in achieving high spatial resolution imaging or focusing for XUV and soft/hard X-ray radiation, however they are usually strictly monochromatic due to strong chromatic dispersion and thus do not support broad radiation spectra, preventing their application to attosecond XUV pulses. Here we report on the design and theoretical simulations based on the design of an achromatic hybrid optics combining both, a refractive and diffractive lens in one optical element. We are able to show by calculation that the chromatic dispersion along the optical axis can be greatly reduced compared to a standard Fresnel zone plate while preserving the temporal structure of the attosecond XUV pulses at focus.

X-Ray Spectroscopic Measurements from Copper and Tin Vacuum Spark Plasmas

X‐ray emissions of copper and tin plasmas produced by a low energy and low inductance vacuum spark were studied. Both the time resolved and the time integrated measurements of the x‐ray emissions were carried out using an x‐ray spectrometer (XR‐100 CR), a PIN diode and an x‐ray diode (XRD). The x‐ray spectra of the copper plasma obtained shows strong Kα and Kβ line radiations embedded in the continuum radiation. It is found that the hot spot formed has an electron temperature in the range from 2 keV to 3 keV. At this electron temperature, the dominant ionic specie is Cu27+ indicating that the plasma is not hot enough for the emission of the K line radiations. The emission of copper K lines is therefore believed to be produced by the energetic electron beam associated with sausage instability. For the case of tin plasma, strong Lα superimposed on the continuum background was observed. These tin spectra are predominantly the result of the interaction of electron beam generated from the transient hollow cath...