Excitonic and lattice contributions to the charge density wave in 1T-TiSe2 revealed by a relaxation bottleneck

Using time- and angle-resolved photoemission spectroscopy (TR-ARPES), we have studied the femtosecond dynamics of the charge density wave (CDW) in 1T-TiSe2. This material exhibits a commensurate CDW with a (2 x 2 x 2) lattice distortion below 200 K for which the mechanism is still debated today. One of the recurring problems is disentangling the contributions from the electronic and lattice-driven order. Here, we have observed the photo-induced suppression of the CDW in real-time by tracking the valence band maximum at the ?-point following an intense laser pulse. The timescales of gap closing and band-replica unfolding (< 200 fs), and the relatively low fluence required for these processes are suggestive of an excitonic mechanism. During recovery, coherent oscillations of the CDW gap are observed relating to the A1g (3.4 THz) mode. A detailed pump fluence dependence reveals several regimes, including impeded recovery at high fluence. This bottleneck coincides with a change in the dominant frequency seen in the oscillations. Using complementary time-resolved reflectivity, we have established the threshold fluence of this phenomenon to be F > 60 ?J cm-2. Our work is supported by a rate equation model which highlights the crucial role of excitons and phonons in this complex system.