Discovery of a Non-Equilibrium Bound State in a bulk Van der Waals Semiconductor

Time- and angle-resolved photoelectron spectroscopy (tr-ARPES) is a powerful technique for probing the electronic structure of quantum materials. By weakly exciting a material and probing the ensuing dynamics with ultrafast pulse, tr-ARPES can probe unoccupied states above the Fermi level and the transient evolution of bands, offering profound insights into the many-body interactions within materials.

In this work, we utilize tr-ARPES to investigate a bulk van der Waals semiconductor. Our polarization and angle-resolved measurements not only clearly resolve the unoccupied conduction bands above the Fermi level, but also reveal the emergence of a novel non-equilibrium in-gap bound state following photoexcitation. This bound state exhibits strongly suppressed kinetic energy, suggesting characteristics of an excitonic nature. Our findings provide new insights into the many-body interactions in this material and help elucidate its optoelectronic properties.