Exotic phenomena and novel functionalities emerge in solids due to the coupling between the charge carriers and collective modes. Key to exploiting this phenomenology is the ability to tune the density of carriers and the strength of their couplings to such modes. However, chemical doping often increases the disorder in the system, thus limiting the beneficial effect of modulating the charge density. Alternatively, photodoping or phonon pumping have been attempted to control the electronic density of states or the coupling between carriers and collective modes. The advantage of such an approach is that light does not induce structural disorder and can access new states of matter that only exist out of equilibrium.
Ultrafast techniques using visible light pulses have unveiled some of these phenomena by delivering excess energy to the electrons via an
intense pump pulse and subsequently monitoring the transfer of such energy to the different degrees of freedom via a delayed optical probe.
In peculiar interesting cases (e.g. graphene, graphite), when a preferential electron-boson interaction channel exists, the carrier thermalization occurs via simultaneous heating of the hot phonon modes.
In this paper, we present and analyze time-resolved pump-probe optical measurements in MgB2, characterized by a multiband electronic structure, with quasi-2D sigma bands with strong electron-phonon (el-ph) coupling, and 3D pi-bands with weak el-ph coupling. We show that the the in-plane a-axis bare plasma frequency, dominated by the sigma carriers, undergoes a blueshift during the first 170 fs after photoexcitation, followed by a slow redshift over a timescale of several ps. Such slow redshift is accompanied by a consequent blueshift of the out-of-plane c-axis bare plasma frequency.
We rationalize these observations in terms of effects of the anisotropic dynamical renormalization of Fermi areas induced by the el-ph coupling. More explicitly, the remarkable blueshift of the sigma-band is naturally explained as a consequence of the generation of hot phonons strongly
coupled to the sigma bands, permitting a photo-controlled tuning of doping. On the other hand, the subsequent charge-transfer between
sigma and pi bands is responsible for the subsequent mirrored redshift (blueshift) of the a-axis (c-axis) plasma frequencies.
This process occurs however on much longer timescale, permitting the initial photo-induced doping of the sigma bands to be detected
and suitable for further operations.