Ultrafast Ballistic Hot Electron Photocurrents in Carbon Nanotubes and Metallic Nanojunctions

The integration of nanostructures into future high-speed electronics requires an extensive knowledge of the ultrafast electron dynamics. In this thesis, we fabricate devices based on single carbon nanotubes (CNTs) and metallic nanojunctions which we integrate into THz-stripline circuits. To analyze the ultrafast photocurrent dynamics, we use a unique on-chip photocurrent spectroscopy and measure the time-resolved electronic response upon the optical excitation of the nanostructure with a sub-picosecond resolution. On the one hand, we reveal the ultrafast, non-equilibrium transport properties of photogenerated electrons and holes in few to single semiconducting CNTs. By exciting either the second or first subband of the CNTs, we demonstrate that the ultrafast photocurrent in the CNTs is dominated by a ballistic transport of the photogenerated charge carriers. Moreover, we identify a thermionic emission of photogenerated charge carriers with sufficiently high kinetic energy to surpass the potential barrier in the CNTs to the contacts. This mechanism stands in contrast to the charge transport in the CNTs without laser excitation which we find to be best described by a Fowler-Nordheim tunneling. The overall slowest optoelectronic processes in the CNTs occur on a nanosecond time scale and they are consistent with a so-called lifetime-limited photocurrent. In time-averaged measurements, we observe a sign change of the photocurrent for a high bias, which we explain by an effective population inversion of the optically pumped CNT subband via charge tunneling from the metal contacts.
On the other hand, we observe an ultrafast displacement current that stems from the electron photoemission when the laser pulse impinges on the metal contacts. We investigate the photoemission process by fabricating asymmetric plasmonic nanojunctions allowing us to achieve a symmetry breaking of the spatio-temporal electron dynamics at the metal-vacuum interface and within the overall nanojunctions. We find that this symmetry breaking leads to a non-linear unipolar photoemission of hot electrons traveling ballistically across the vacuum gap in the nanojunctions, with the ultimate switching time being limited only by the laser pulse duration and the time-of-flight of the ballistic electrons. Most importantly, we verify that the non-linear photoemission response of the nanojunctions can couple into macroscopic co-planar striplines by near-field interactions and hereby drive THz transients that propagate along the striplines up to several hundreds of micrometers and with a potential bandwidth of up to 10 THz.
The results presented in this thesis give fundamental insights into the ultrafast dynamics of photogenerated charge carriers in contacted, semiconducting CNTs, which may prove essential for ultrafast optoelectronic devices and photodetectors based on semiconducting CNTs in general, but particularly on single CNTs integrated into optoelectronic high-speed circuits and THz-striplines. Moreover, our results on femtosecond electronics based on asymmetric nanoscale junctions may prove useful for telecom compatible high-frequency wafer-scale applications such as on-chip clock and synchronization devices or macroscopic on-chip signal transduction on a femtosecond time scale.