On-chip THz studies of low-dimensional semiconductor systems

A wide range of novel on-chip integration techniques were developed to explore the
low-temperature perturbation of GaAs/AlGaAs based confined electron systems at
terahertz frequencies. A new methodology for pump-probe transmission measurements
of THz pulses along on-chip coplanar waveguides is demonstrated, allowing generation
of guided THz signals and subsequent detection after propagation through in-plane
integrated nanoscale devices at sub-Kelvin temperatures and under magnetic fields.
Initially, several methods that demonstrate introducing a guided-wave THz
signal into an embedded two-dimensional electron system (2DES), involve the assembly
of discrete substrate chips containing either the THz photoconductive material or a
2DES. The introduction of novel, monolithic integration of the 2DES layer and the THz
photoconductive layer into a fully integrated epitaxially grown heterostructure, allowed
successful incorporation of both the 2DES and the on-chip THz waveguides onto a
single chip. The independent characterisation measurements of both the incorporated
systems allowed optimisation of the monolithic structure, resulting in achievement of
undiminished performance for both the 2DES and the on-chip THz waveguides.
A full characterisation of THz pulses interacting with an embedded 2DES, using
both injected and capacitive, evanescent field coupling of the THz signal, are first
performed at variable temperatures from (4 K to 300 K). It is found from the
experimental observations that the temperature dependent 2DES resistivity can be
directly probed from the time-domain transmitted THz pulses. Independently, a new
method for sub-Kelvin excitation and detection of on-chip THz frequency radiation at
temperatures as low as 200 mK and magnetic fields up to 12 T is also demonstrated,
which was employed to probe the magneto conductivity of the 2DES through THz pulse
injection using on-chip waveguides.
The successful demonstration of THz pulse injection in low-dimensional
electron systems using on-chip waveguides at sub-Kelvin temperatures and under
magnetic fields paves the way for enhancing the studies of single electron transport, by
investigating the properties of 1D or 0D quantum systems in the terahertz-frequency
range.