Momentum alignment of photoexcited carriers in graphene: The route to optovalleytronics

• Main Authors: Hartmann, Richard R., Portnoi, Mikhail E.
• Format: text
• Published: Animo Repository 2011
• Subjects: Graphene; Electronic excitation; Relaxation phenomena; Valleytronics; Physics
• Online Access: https://animorepository.dlsu.edu.ph/faculty_research/7395
• Institution: De La Salle University

A linearly polarized excitation is shown to create a strongly anisotropic distribution of photoex­cited carriers in graphene, where the momenta of photoexcited carriers are aligned preferentially normal to the polarization plane. This hitherto overlooked effect offers an experimental tool to gen­erate highly directional photoexcited carriers which could assist in the investigation of "direction­dependant phenomena" in graphene-based nanostructures. The depolarization of hot photolumi­nescence (HPL) has been used with great success to study relaxation processes in conventional 2D systems. In such systems the alignment is due to the spin-orbit interaction for photoexcited holes, whereas in graphene, it is due to the pseudo-spin. Namely, the ratio of the two components of the spinor-like graphene wavefunction depends on the electron momentum which influences the optical transition selection rules. By comparing the depolarization from successive phonon replicas, the mechanisms for phonon relaxation in graphene can be studied. Furthermore, studying the depolar­ization of HPL in a magnetic field (the Hanle effect) allows one to obtain momentum relaxation times of hot electrons. The effect of momentum alignment in graphene provides a contact-free method of characterizing energy and momentum relaxation. Our analysis of momentum alignment in the high frequency regime shows that a linearly polarized excitation allows the spatial separation of carriers belonging to different valleys, therefore opening the door to an optical means of controlling valley polarization (optovalleytronics) and quantum computing in graphene