Ultrafast Carrier Dynamics of a Photo-Excited Germanium Nanowire-Air Metamaterial

Ultrafast carrier dynamics in arrays of single crystal and relatively uniform-diameter Ge nanowires (NWs) are investigated by transient absorption measurements and effective medium simulations. We present the first quantitative analysis of a Ge NW-air metamaterial, translating the photon response of the assemblies to carrier dynamics. Three time regimes of the ultrafast recombination process are identified: Auger recombination dominant (0-5 ps), fast surface trapping and recombination dominant (5-20 ps), and a mix of fast recombination and slow surface trapping (20-200 ps). The rates of surface recombination and their dependences on pump fluence are determined, highlighting the different interactions of electrons and holes with Ge NW surface and interface states. Structural and excitation conditions can be engineered to extend the photogenerated electron and hole lifetimes. Small wire diameters and low pump powers enhance the electron lifetime because charging of defect states in the surface oxide layer produces a potential barrier for electrons to be trapped at Ge/GeOx interface. This phenomenon simultaneously causes an enhancement of hole lifetime for relatively large wire diameters and large pump powers.