THz On-Chip Waveguides for Ultrafast Magnetic Measurements

This thesis describes the development of on-chip terahertz (THz) waveguides
for use in ultrafast magnetic measurements. An epitaxial transfer
method is used to bond photoconductive switch material to quartz substrates.
This, combined with optical lithography methods, allows on-chip
generation and detection of THz pulses. Normally, the metal in a waveguide
has a high conductivity (eg. gold or copper) in order to reduce ohmic loss.
However, if this metal is instead a magnetic multilayer, then the THz pulse
interaction with the metal can be used to measure ultrafast magnetic effects.
This was performed using Cobalt/Copper multilayers, and a THz GMR was
seen in a transmitted pulse. Additionally, an ’inverse’ GMR was noted in
input pulses and in the pulse tail of transmitted pulses. It is believed that
this is the first time such behaviour has been documented.
Lateral spin valves offer the ability to separate charge and spin effects;
however, the dimensions of these devices is in the hundreds of nanometers
range. Coplanar waveguides were used to confine and concentrate THz
pulses into this spatial regime. Curved waveguides were fabricated to prove
confinement, and magnetoresistance properties were used to demonstrate
field concentration.
Devices were then designed to allow picosecond current injection into a
lateral spin valve geometry. Initial results are obscured due to interference
from the excitation pulse, but improvements to the design and fabrication
could pave the way for this technology in the future.