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

Black phosphorus (BP), the most stable allotrope of elemental phosphorus, is a layered semiconductor. It recently experienced a resurgence of interest after exfoliation down to few layers was accomplished [1-4]. Among two-dimensional (2D) materials, BP combines the optoelectronic properties of gapless graphene [5] and of wide-gap transition metal dichalcogenides (TMDs) [6]. The charge mobility of bulk BP is comparable to that of graphene (10 5 cm 2 V −1 s −1 ) [7]. Even if it decreases by one-to-two orders of magnitude in an isolated monolayer [3,8,9], it still surpasses that of TMDs [10-13], which proved to be sufficient to realize field-effect transistors [2]. In contrast to TMDs, in the few-layers limit ambipolar charge transport can be achieved by gating, enabling the realization of p-n junctions [14]. Also, BP-based saturable absorbers have already been successfully implemented in lasers technology [15][16][17], and polarization-sensitive photodetectors have been designed to exploit its strong in-plane absorption dichroism [4,18].BP has an orthorhombic crystal structure, with space group Cmca (Nr. 64) and point group D 18 2h [10].

The three-dimensional topological insulator BiSe presents two cone-like dispersive topological surface states centered at the [Formula: see text] point. One of them is unoccupied in equilibrium conditions and located 1.8 eV above the other one lying close to the Fermi level. In this work we employ time- and angle-resolved photoemission spectroscopy with circularly polarized pump photons to selectively track the spin dynamics of the empty topological states. We observe that spin-polarized electrons flow along the topological cone and recombine towards the unpolarized bulk states on a timescale of few tens of femtoseconds. This provides direct evidence of the capability to trigger a spin current with circularly polarized light.