Speaker
Description
Tommaso Pincelli, Lawson T. Lloyd, Amine Wahada, Zoè de Granrut, Alexander Enders, Tania Mukhejee, Túlio de Castro, Alessandro de Vita, Samuel Beaulieu, Maciej Dendzik, Shuo Dong, Holger Oertel, Martin Wolf, Laurenz Rettig, Ralph Ernstorfer.
We present our progress in developing the new paradigm of multidimensional photoemission spectroscopy (MPES) to probe quasiparticle wavefunctions. By combining novel experimental approaches and theoretical modeling, we are able to access excitonic states, track their dynamics and map their spatial extent. Using new forms of photoemission dichroism we can access band topology and unravel subtle aspects of the electronic and excitonic wavefunctions.
We demonstrated the observation of excitonic states in WSe$_2$, a layered semiconductor with strong excitonic binding energy. Time- and angle-resolved photoemission spectroscopy (trARPES) captures the distinct behaviors of excitons and free carriers by using varying excitation energies. Moreover, owing to the momentum resolution of trARPES we can track the scattering of excitons into momentum-indirect dark states, as well as the excitonic distribution in real space. These findings demonstrate the capability of photoemission to probe excitonic states and their transitions in real-time.
Advancements in mapping band topology enable direct investigation of quasiparticle eigenfunctions rather than merely eigenvalues and populations. By exploiting photoemission matrix element effects, we can gain insight into the entanglement of spin, orbital, and valley degrees of freedom in 2D materials with strong spin-orbit coupling and broken inversion symmetry. Employing recipes such as rotating the crystal symmetry axes or the light polarization direction, we can e.g. access the orbital pseudospin in WSe2, or even a complete reconstruction of the wavefunction. Moreover, modulation of the polarization of the pump in a trARPES experiments allows tracking valley polarization in the ensuing ultrafast scattering processes.
To achieve such multidimensional experimental parameter control, the trARPES laboratory has been upgraded to significantly enhance its experimental capabilities. A new polarization control scheme allows continuous control of the angle of linear polarization, while a new circular polarization scheme is planned. The instrument is also being upgraded in order to operate with both short (<40 fs) and long (<190 fs) pulse, for high temporal or energy resolution operation respectively.
