Ultrafast On-Chip Time-Resolved Photocurrents in Silicon THz Detectors and Plasmonic Nano-Emitters

Time-resolved photocurrent measurements have emerged as a versatile tool to study processes occurring in materials in the THz frequency range. In the thesis, we use on-chip THz stripline circuits to study ultrafast photocurrents in helium ion implanted silicon THz detectors and asymmetric plasmonic nanostructures. First, we analyze the impact of helium ion implantation in photoconductive silicon Auston switches on the carrier lifetime. We observe a monotonic reduction from the as-grown silicon charge carrier lifetime of 5.3 ps to a minimum charge carrier lifetime of 0.5 – 0.55 ps for a helium ion fluence of 20∙1015 ions/cm2, by implanting helium ions at an energy of 30 keV. In particular, we demonstrate that a local adjustment of the carrier lifetime can be achieved with direct-write lithography of the helium ion microscope even within a photoconductive switch.
Second, we investigate the unipolar photocurrent generated in asymmetric plasmonic and diamond-shaped nanojunctions excited with femtosecond near-infrared (NIR) pulses. We show that the pre-structured metal nanogaps with a size of ~50nm can be shaped into sub-10 nm tunneling gaps by on-chip non-thermal laser ablation. A superlinear photoemission current is observed, indicating an underlying multiphoton photoemission process with three to four absorbed photons and an associated barrier lowering due to the image charge effect. The photocurrent can be tuned toward the strong-field tunneling regime in the nanogaps, and the resulting photocurrent is well fitted by the Keldysh theory. Numerical simulations are performed to discuss how plasmonic resonance and field enhancement can be optimized by changing the length, gap spacing, and tip radius of diamond-shaped emitters. The method of tuning the gap size by laser ablation offers the possibility to explore new strongly coupled plasmonic systems.
Furthermore, we integrate asymmetric metal nanoemitters into a THz stripline circuit. We prove that excitation with 14 fs near-infrared optical pulses, corresponding to a frequency of several hundred THz, can drive an ultrafast current pulse that propagates along the striplines, reaching frequencies up to 2 THz. The observed similar superlinear time-integrated and time-resolved photocurrents are attributed to the same multiphoton photoemission with the Schottky effect supported by plasmonic field enhancement at the emitter tips. Moreover, we use an ultrafast current signal excited by a laser centered on the stripline edge to experimentally determine the dispersion characteristics of the coplanar striplines. We find excellent agreement of the effective index with the numerical simulation. The origin of the transient signal with frequencies up to 2 THz is presumably the optical rectification at the gold/ vacuum interface or tunneling from the gold into surface states of the sapphire. Finally, the improvement of the bandwidth of the coplanar stripline up to 10 THz by reducing the stripline dimensions and substrate choice is discussed. The results of the thesis show that replacing the Auston switch in the THz circuits with such plasmonic metal nano-emitters may pave the way toward future femtosecond on-chip electronics with a prospective bandwidth up to 10 THz.