The high-order harmonics beamline currently under construction at the MBI aims to provide
a facility for time-resolved laser-pump, XUV-probe experiments in surface and gas-phase
spectroscopy. The beamline is designed to deliver monochromatized Extreme Ultraviolet
(XUV) pulses ranging in photon energy from 20 eV to 100 eV with an energy resolution of
250 meV. The pulse duration is expected to lie between 100 fs and 200 fs, and the flux
between 1010 and 1011 photons / harmonic / second. The beam quality
should be sufficient to focus into a 130 µm × 130 µm spot.
Fig.1: A schematic
layout of the HHG beamline.
High-order harmonic generation is accomplished by focusing ultrafast infrared (IR, 790 nm)
pulses from a commercial titanium-sapphire (Ti:S) chirped-pulse amplifier laser (Red
Dragon, Kaptyn Murnane Labs) into a rare gas in a short gas cell in vacuum.
The laser comprises a prism-compensated Ti:S oscillator (KM Labs, Mini) pumped by 5 W at
523 nm by a Coherent Verdi V5, followed by three stages of amplification. The first stage
involves 14 passes of the IR beam through a Ti:S crystal , the second and third stages are
both double-pass arrangements. In all three amplifier stages the Ti:S crystals are He
cooled to around 55 K while in operation and are pumped at 532 nm by Q-switched Nd:YAG
lasers (Photonics Industries, DM100-532) generating around 75 W at 10 kHz. The output of
the first amplifier is typically and reliably 10 W (1 mJ / pulse) and each subsequent
stage approximately doubles the power, giving around 40 W before compression. A grating
compressor with about 60 % throughput delivers at the laser output approximately 25 W at
10 kHz (2.5 mJ / pulse) with a pulse duration of around 40 fs, as measured with
frequency-resolved optical gating (GRENOUILLE, Swamp Optics).
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
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.
Fig. 2: A typical high-order harmonic
spectrum from argon. The spectrum was recorded from an XUV photodiode while scanning the
low-energy grating of the monochromator. The XUV was generated by focusing 150 µJ, 45 fs
pulses with a centre wavelength of 790 nm into argon at 10 mbar in a 10 mm long gas cell.
Harmonic orders are shown above each line.
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
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.
Fig.3: Photograph of
the completed high-order harmonics beamline being used for initial photoemission studies
of the W(110) surface.
Ultrafast Photoelectron Spectroscopy using High-order
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.)
Fig. 4: Angle-resolved photoemission data
from W(110) using harmonic 15 (hv ~ 24 eV). The image was recorded in 10 seconds.
We intend to study ultrafast magnetization dynamics in epitaxial ferromagnetic metal films
of gadolinium and terbium on W(110). To this end we will use the tuneable XUV radiation
from the high-order harmonics beamline to map transients of the exchange-split band
structure. With this state selective and magnetically sensitive technique we expect to
identify the microscopic processes responsible for the coupling between the spin systems
and the electronic and lattice excitations, and determine their timescales. For further
reading refer to Phys. Rev.
Lett. 100 (2008) 107202 (see Recent Highlights).
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