High-order harmonic generation

To produce high-order harmonic, laser pulses of between 100 µJ and 1 mJ are focused into a gas cell using a 600 mm focal length lens. A variable diameter iris placed before the lens provides an additional optimization parameter for harmonic generation by changing the transmitted energy and the focusing geometry, both on-axis and radially. We have used several different gas cells in order to ascertain which is most suitable. To date these have been: a glass tube closed at the ends by copper foil; thin-walled closed-ended stainless-steel tubes of 6 mm or 3 mm outer diameter; and a 3 mm tube crushed to 1.5 mm thickness in the beam propagation direction. In all cases, the focused laser burns rapidly through the metal to create a beam path with the minimum diameter. This minimizes gas leakage into the surrounding vacuum and its concomitant load on the vacuum pumps. None of the cells showed conspicuously better performance in XUV flux and these experiments continue.

The gas cell is housed in vacuum chamber pumped by a turbomolecular pump giving a base pressure of 10^-9 mbar. During operation we can increase the pressure in the gas cell to 100 mbar, at which point the surrounding pressure is 10^-2 mbar.

Beamline

The layout of the beamline is shown in Figure 1. From the gas cell, the XUV radiation co-propagates with the IR driving laser through a 4 mm diameter fixed aperture into a differential pumping stage fitted with a turbmolecular pump. It then passes through another fixed aperture of 4 mm. These improve the differential pumping between the gas cell chamber and the rest of the beamline, serve as alignment tools and reduce the IR intensity before it impinges on a 150 nm thick aluminium foil filter to avoid damaging it. The aluminium filter transmits around 70 % of the XUV radiation but only a tiny fraction of the IR, which might otherwise damage the monochromator. The XUV beam is then reflected from the first gold-coated grazing incidence toroidal mirror. This mirror, and the identical one placed after the monochromator are supported in vacuum by a six-strut arrangement that allows adjustment of all relevant axes. The design has been used extensively on beamlines at the Berlin synchrotron BESSY and has proven to be effective and reliable (see here for details). The mirror relays a 1:1 image of the harmonic source onto the entrance slit of the monochromator. The monochrometer (TGM300, Hariba-Jobyn-Yvon) is fitted with two gold-coated laminar toroidal gratings - a low energy grating with 200 lines / mm and a high energy grating with 500 lines / mm. They are mounted on a linear translator and can be exchanged under vacuum. The gratings have tuning ranges of 82.7 nm to 27.5 nm, and 31 nm to 11.2 nm, respectively, corresponding to a total tuning range from harmonic 15 to 109. The diffracted beam is imaged onto the monochromator's exit slit, from where it is imaged 1:1 onto the experimental sample by the second toroidal mirror. Each mirror chamber and the monochromator chamber are fitted with turbomolecular pumps to ensure that the experiment chamber achieves ultra-high vacuum conditions. For stability and vibrational isolation, these chambers are mounted on granite blocks each weighing approximately 1000 kg.

For pump-probe experiments, it will be possible to focus an IR beam onto the sample with variable time delay with respect to the XUV pulses. This beam will be introduced into the vacuum after the second toroidal mirror and will propagate nearly collinearly with XUV beam. This feature is currently being installed.

Ultrafast Photoelectron Spectroscopy using High-order Harmonic Radiation

As a first step towards time- and angle-resolved photoelectron spectroscopy (TR-ARPES) we have measured spectra of W(110) bulk and surface bands using different harmonics without time resolution. (First data shown below.)