We have investigated the magneto-optical response of Fe epitaxial films by femtosecond pump-probe polarimetry in a broad probe spectral region (1.8-2.6 eV). From the extrapolated photoinduced variation of the conductivity tensor, spin and charge dynamics have been disentangled. In particular, the analysis of the off-diagonal tensor element rules out any appreciable modification of the electronic band structure upon laser excitation and suggests that ultrafast demagnetization is determined by collective excitations, i.e., spin fluctuations. Our experimental evidence provides significant insight into the microscopic mechanisms governing the complex spin dynamics of metals in the subpicosecond time scale.

Understanding collective electronic states such as superconductivity and charge density waves is pivotal for fundamental science and applications. The layered transition metal dichalcogenide 1T-TiSe2 hosts a unique charge density wave (CDW) phase transition whose origins are still not fully understood. Here, we present ultrafast time-and angleresolved photoemission spectroscopy (TR-ARPES) measurements complemented by time-resolved reflectivity (TRR) which allows us to establish the contribution of excitonic and electron-phonon interactions to the CDW. We monitor the energy shift of the valence band (VB) and coupling to coherent phonons as a function of laser fluence. The VB shift, directly related to the CDW gap closure, exhibits a markedly slower recovery dynamics at fluences above Fth = 60 J cm -2 . This observation coincides with a shift in the relative weight of coherently coupled phonons to higher frequency modes in time-resolved reflectivity (TRR), suggesting a phonon bottleneck. Using a rate equation model, the emergence of a high-fluence bottleneck is attributed to an abrupt reduction in coupled phonon damping and an increase in exciton dissociation rate linked to the loss of CDW superlattice phonons. Thus, our work establishes the important role of both excitonic and phononic interactions in the CDW phase transition and the advantage of combining complementary femtosecond techniques to understand the complex interactions in quantum materials. Corresponding authorCorrespondence to Enrico Da Como edc25@bath.ac.uk APPENDIX A: TIME-DEPENDENCE OF FREE CARRIER POPULATION AND LINEARITY WITH PUMP FLUENCEIn order to check the linearity of the laser pumping effect in our TR-ARPES experiment, we have analysed the total intensity above the Fermi level, EF, for all spectra, which provides an indication of the transient free carrier population induced by the pump pulse. For this, we used the normalised spectra in Fig. 6 at each pump-probe delay and integrated across the high energy tail to intensity ≤ 0.25 (just below the nodal point of all spectra in the normal phase) as shown

Time- and angle-resolved photoemission spectroscopy is a powerful technique to study ultrafast electronic dynamics in solids. Here, an innovative optical setup based on a 100-kHz Yb laser source is presented. Exploiting non-collinear optical parametric amplification and sum-frequency generation, ultrashort pump (hν = 1.82 eV) and ultraviolet probe (hν = 6.05 eV) pulses are generated. Overall temporal and instrumental energy resolutions of, respectively, 85 fs and 50 meV are obtained. Time- and angle-resolved measurements on BiTeI semiconductor are presented to show the capabilities of the setup.

Black phosphorous (BP) is a layered semiconductor with high carrier mobility, anisotropic optical response and wide bandgap tunability. In view of its application in optoelectronic devices, understanding transient photo-induced effects is crucial. Here, we investigate by time- and angle-resolved photoemission spectroscopy BP in its pristine state and in the presence of Stark splitting, chemically induced by Cs ad-sorption. We show that photo-injected carriers trigger bandgap renormalization, and a concurrent valence band flattening caused by Pauli blocking. In biased samples, photo-excitation leads to a long-lived (ns) surface photovoltage of few hundreds mV that counterbalances the Cs-induced surface band bending. This allows us to disentangle bulk from surface electronic states, and to clarify the mechanism underlying the band inversion observed in bulk samples.

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