XUV-based time- and angle-resolved photoemission spectroscopy


Time- and angle-resolved photoemission spectroscopy (trARPES)

Snapshot of a trARPES movie. Electronic Structure of WSe2 shortly after excitation of electrons in the K valley of the band structure. The bottom part of the image shows the occupied valence bands. The signal above 0.5 eV energy is rescaled by more than a factor 100 and shows the electrons excited by a near-infrared laser pulse. The red arrow indicates intervalley scattering of electrons from the K to the Sigma valley in the Brillouin zone, which occurs in less than 100 fs.

Fig.1: Screenshot of a trARPES movie. Electronic Structure of WSe2 shortly after excitation of electrons in the K valley of the band structure. The bottom part of the image shows the occupied valence bands. The signal above 0.5 eV energy is rescaled by more than a factor 100 and shows the electrons excited by a near-infrared laser pulse. The red arrow indicates intervalley scattering of electrons from the K to the Sigma valley in the Brillouin zone, which occurs in less than 100 fs.

Angle-resolved photoemission spectroscopy (ARPES) provides the most direct access to the electronic structure of solids. The measurement of energy and emission angle of electrons emitted by light with photon energy above the work function, typically extreme ultraviolet (XUV) light, allows for the determination of the material’s electronic structure.

Employing ultrashort pulses of XUV light in combination with a second ultrashort visible laser pulse adds temporal resolution to this technique: trARPES provides movies of a material’s electronic structure with a frame rate of a few tens of femtoseconds. In cooperation with Martin Wolf and the research group Dynamics of Correlated Materials headed by Laurenz Rettig, we recently advanced this technique by developing a novel XUV laser providing ultrashort pulses of ~22 eV photon energy with 0.5 MHz repetition rate.

In a trARPES experiment, a small fraction of the materials electrons is excited by a fs visible laser pulse. The excited state and its temporal evolution can be visualized by photoemission with the fs XUV laser pulse arriving at the sample at a precisely defined time after the first pulse. The excited state of the layered semiconductor material WSe2 shortly after excitation of electrons in the K valley of the electronic band structure is shown in fig.1. The signal above 0.5 eV energy shows the excited states is less than 0.1% of the signal from the valence band and is rescaled in the image. This measurement visualizes the rapid scattering of electrons from the K to the Sigma valley, which is the minimum of the material’s conduction band.

High-repetition rate XUV laser for trARPES

Until recently, there have been two branches in the implementation of trARPES, one based on high repetition rate laser systems providing high counting statistics but limited probe photon energies, which limits the accessible range in the Brillouin zone, and another based on probing with extreme ultraviolet (XUV) light generated by high harmonic generation employing high power laser systems of limited repetition rate.

In order to bridge this technology gap, we developed a 0.5 MHz XUV laser source based on high-harmonic generation (HHG) driven with the output of a femtosecond parametric laser amplifier. Its output is frequency-doubled to yield 10 µJ pulses centered at 400 nm wavelength. These pulse are focused into a jet of Ar atoms where frequency up-conversion occurs through HHG. The 7th harmonic of the 400 nm (photon energy: 3.1 eV) pulses is filtered by a combination of XUV mirror and transmission filter and focused on the sample to trigger photoemission. The XUV laser pulses (photon energy: ~21.7 eV) have a pulse duration of approximately 20 fs and a bandwidth of 100 meV.

Scheme of the trARPES experiment.

High-power OPCPA laser system

Layout of the OPCPA. CVBG = Chirped Volume Bragg Grating, FS = Fused Silica, BD = Beam Dump, Sep. = Wavelength Separator, SHG = Second Harmonic Generation. From Puppin et al., Optics Express 23, 1491 (2015).Our approach of generating XUV pulses at 0.5 MHz repetition rate requires high-average power laser system providing sufficient pulse energy for high-harmonic generation. We developed an optical parametric chirped-pulse amplifier (OPCPA) pumped with a hybrid fiber-slab laser system as front end, see Figure 1. This laser provides spectrally tunable, both in terms of central wavelength and bandwidth, femtosecond laser pulses with average output power exceeding 15 W (pulse energy: >30 µJ). The OPCPA output has a long-term power stability of 0.3% and and a spectral stability of 0.2%.

We employ two schemes for frequency up-conversion:

Deep-ultraviolet (DUV) pulses in the photon energy range 6.0-6.4 eV are generated by frequency-quadrupling in cascaded frequency-mixing in nonlinear crystals. The spectral bandwidth of these pulses support sub-30 fs pulse durations.

Extreme-ultraviolet (XUV) pulses of approximately 20 eV photon energy (wavelength: 60 nm) are generated by high harmonic generation in a jet of noble gas. This process is driven by focusing either the fundamental OPCPA output or its second harmonic to intensities in the range 1013-1014 W/cm2.

 

Further reading:

  • M. Puppin, Y. Deng, O. Prochnow, J. Ahrens, Th. Binhammer, U. Morgner, M. Krenz, M. Wolf, and R. Ernstorfer:
    500 kHz OPCPA delivering tunable sub-20 fs pulses with 15 W average power based on an all-ytterbium laser.
    Opt. Exp. 23 1491-1497 (2015) [doi: 10.1364/OE.23.001491].