Time- and angle-resolved photoemission spectroscopy of solids in the extreme ultraviolet at 500 kHz repetition rate

Abstract

Time-and angle-resolved photoemission spectroscopy (trARPES) employing a 500 kHz extreme-ultraviolet (XUV) light source operating at 21.7 eV probe photon energy is reported. Based on a high-power ytterbium laser, optical parametric chirped pulse amplification (OPCPA), and ultraviolet-driven high-harmonic generation, the light source produces an isolated high-harmonic with 110 meV bandwidth and a flux of more than 10 11 photons/second on the sample. Combined with a stateof-the-art ARPES chamber, this table-top experiment allows high-repetition rate pump-probe experiments of electron dynamics in occupied and normally unoccupied (excited) states in the entire Brillouin zone and with a temporal system response function below 40 fs. function A(k,ω) and a matrix element between the initial and final state |M k if | 2 ; here k and ω denote the electron's wavevector and angular frequency, respectively. Many-body effects are encoded in the spectral function A(k,ω) and manifest themselves in renormalization of the bare electronic bands and in the observed lineshape 1 . In a trARPES experiment, the distribution I(k,ω) is collected for a series of delays (τ ) between pump and probe pulses: after perturbation, the population distribution f(k,ω,τ ) evolves towards a quasi-thermal distribution and energetically relaxes on femto-to picosecond timescales 2 . During relaxation, the concomitant many-body interactions affect the transient spectral function A(k,ω,τ ) and even the photoemission matrix elements might change, if the final state's orbital symmetry is altered 3 . trARPES accesses at once the population dynamics, the evolution of the spectral function and the evolution of matrix elements. trARPES has found increasingly successful applications in the past few decades 4-6 : among many examples, trARPES was used to study photo-induced phase transitions 7-11 and to observe electronic states above the Fermi level, unoccupied under equilibrium conditions [12][13][14][15][16] . Energy conservation in the photoemission processes imposes that a femtosecond light source for trARPES must possess a photon energy ω ph exceeding the work function Φ, which in most materials lies in the range between 4 to 6 eV. Ultraviolet femtosecond light sources are thus required for these experiments.The conservation of the electrons' in-plane momentum ( k ) in the photoemission process allows reciprocal space resolution. The advantage of a probe with high photon energy is Journal of Electron Spectroscopy and Related Phenomena 200, 15 (2015). 79 SPECS Surface Nano Analysis GmbH, product spectrometer PHOIBOS TM 150 (2013), see

Direct measurement of key exciton properties: Energy, dynamics, and spatial distribution of the wave function

“…Although techniques such as time-resolved THz spectroscopy also allow probing optically dark excitons via internal quantum transitions [13], finite-momentum excitons that lie outside the radiative light cone remain inaccessible to such methods. This limitation is overcome by time-and angle-resolved photoemission spectroscopy (trARPES), a spectroscopic tool accessing excited states, including excitons, in energy-momentum space and on ultrafast time scales [17,[22][23][24].…”

An open-source, end-to-end workflow for multidimensional photoemission spectroscopy

“…Photoemission experiments. The measurements were conducted using the HEXTOF instrument 24 at the DESY FLASH PG-2 beamline 50 with the free-electron laser (FEL) as well as a laboratory source 21 with a METIS electron momentum microscope (SPECS METIS 1000) installed at the FHI. In the measurements at FLASH, the FEL was tuned to 36.5 eV (or 34.0 nm) and 109 eV (or 11.4 nm), the optical pump pulse had a center wavelength of 775 nm.…”

“…The algorithms involved balance physical knowledge and existing methods in image processing and computer vision. The workflow is illustrated next with data obtained at some of the electron momentum microscopes currently in operation, such as the HEXTOF (high energy X-ray time-of-flight) measurement system 24 at the free-electron laser source FLASH 32 at DESY, and the table-top high harmonic generation-based setup at the Fritz Haber Institute (FHI) 21 involving a commercial TOF and DLD (METIS 1000, SPECS GmbH). We use the material example of tungsten diselenide (WSe 2 ) measured at both experimental setups to demonstrate the workflow execution, because in momentum space, the patent features of WSe 2 band structure and the nonequilibrium dynamics initiated by optical excitation of WSe 2 have been thoroughly studied in the past (see Methods) 24 , 33 – 36 .…”

Experimental measurement of the intrinsic excitonic wave function

“…Over the past decade, the discovery of robust excitons in twodimensional (2D) systems (18)(19)(20) and advances in space-, time-, and angle-resolved photoemission spectroscopy (ARPES) techniques (21)(22)(23)(24) have created unprecedented opportunities in this regard. Analogous to collider experiments of high-energy physics, theoretical studies from the past few years have proposed using a time-resolved (TR)-ARPES framework to dissociate the exciton with a high-energy extreme ultraviolet (XUV) photon and photoemit its constituent electron (17,25,26).…”