Self-amplified photo-induced gap quenching in a correlated electron material

Abstract

Capturing the dynamic electronic band structure of a correlated material presents a powerful capability for uncovering the complex couplings between the electronic and structural degrees of freedom. When combined with ultrafast laser excitation, new phases of matter can result, since far-from-equilibrium excited states are instantaneously populated. Here, we elucidate a general relation between ultrafast non-equilibrium electron dynamics and the size of the characteristic energy gap in a correlated electron material. We show that carrier multiplication via impact ionization can be one of the most important processes in a gapped material, and that the speed of carrier multiplication critically depends on the size of the energy gap. In the case of the charge-density wave material 1T-TiSe2, our data indicate that carrier multiplication and gap dynamics mutually amplify each other, which explains—on a microscopic level—the extremely fast response of this material to ultrafast optical excitation.

Enhanced ultrafast relaxation rate in the Weyl semimetal phase of MoTe2 measured by time- and angle-resolved photoelectron spectroscopy

“…There is a clear analogy between the fast dynamics at the WPs and the role played by conical intersections in the non-adiabatic relaxation of chemical systems through nonradiative transitions [22]. The bottleneck behavior associated with the formation of a local band gap has been reported in the out-of-equilibrium properties of high-temperature superconductors [23] and charge-density-wave systems [24]. It is also similar to the slowdown of the dynamics in gapped bilayer graphene with respect to gapless single layer graphene [25].…”

Observation of ionization enhancement in two-color circularly polarized laser fields

“…To help explain the observed ionization enhancement in the CE model we examine the electron energy distribution after the laser pulse has ended at a total intensity of 5x10 14 Wcm -2 [ Fig. 7].…”

“…In the simplest semi-classical picture of HHG, the electron can return to the parent ion with high kinetic energy and then any excess energy greater than the ionization potential can then be emitted as a high-harmonic photon. When the HHG process is properly phase matched, a bright coherent beam of extreme ultraviolet (EUV) or soft X-ray light is generated [6][7][8][9][10], which can be used to uncover coupled dynamics in materials with femtosecond-to-attosecond temporal resolution [11][12][13][14][15], and can also be used for high-resolution imaging [16][17][18][19]. Alternatively, if the electron does not recombine upon re-encountering the ion it may rescatter from the ion, encoding information about the sub-ångström and sub-femtosecond structure of the scattering potential into the photoelectron momentum distribution [20][21][22].…”

Ultrafast extreme ultraviolet photoemission without space charge

“…We have observed LAPE with sideband amplitudes in the 10 −3 to 10 −4 range, orders of magnitude smaller than typical 19,21,67 . To our knowledge, these are the smallest LAPE signals observed from a surface, but more importantly they mimic the response of weakly excited samples in the low-fluence regime where only a small fraction (i.e., 10 −4 of the surface state) of the sample's electrons are excited, demonstrating the feasibility of low-fluence excited state XUV ARPES experiments.…”

“…Combined, these two factors result in the need for orders of magnitude more data than ground state studies done at synchrotrons, but space charge limits the data rate to be orders of magnitude lower. Experiments have then been almost exclusively restricted to strongly excited samples using absorbed fluences on the order of ∼1 mJ/cm 2 , 19–21 such that laser excitation produces changes to the EDC visible on a linear scale. In addition to probing different physics than can be accessed in the low-fluence regime, 22 at high fluences ultrashort pump pulses also produce many electrons through multiphoton processes which add to the space-charge problem, 23–25 and pump-induced space charge can also have a non-trivial dependence on the pump-probe delay, making space charge effects difficult to separate from the dynamics of interest 26 …”