PhD Thesis Defense
Thursday, September 25, 2014 - 15:00 to 17:00
Weniger 287
Event Speaker: 
Michael Paul
Local Contact: 

This thesis will cover my work relating to the developing eld of terahertz
(THz) science and technology. It will present experimental and theoretical studies
investigating the optical and electrical properties of various material systems using
novel THz imaging and spectroscopy techniques. Due to its low photon energy,
THz imaging and spectroscopy are useful tools for non-contact, non-destructive
probing of materials. Broadband, single-cycle THz pulses are prepared using modern
THz generation technology. Using the THz detection techniques of THz raster
imaging and THz time-domain spectroscopy (THz-TDS), the local carrier dynamics
of nanomaterials such as graphene and carbon nanotubes were determined. THz
measurements on single-layer graphene grown with di erent recipes and on various
substrates exhibit sub-millimeter spatial inhomogeneity of sheet conductivity. THz
transmission data reveals that a thin plastic, polymethyl methacrylate (PMMA),
layer in contact with single-layer graphene induces a small yet noticeable reduction
in conductivity. Ulterior THz measurements performed on vertically-aligned
multi-walled carbon nanotubes (V-MWCNT) employ time-resolved THz transmission
ellipsometry. The angle- and polarization-resolved transmission measurements
reveal anisotropic characteristics of the THz electrodynamics in V-MWCNT. The
anisotropy is, however, unexpectedly weak: the ratio of the tube-axis conductivity
to the transverse conductivity, ~2.3, is nearly constant over the broad
spectral range of 0.4-1.6 THz. The relatively weak anisotropy and the strong
transverse electrical conduction indicate that THz elds readily induce electron
transport between adjacent shells within the multi-walled carbon nanotubes.
In-depth coverage of the development of a high- eld THz generation system
based on a lithium niobate prism will be presented. The evolution of techniques in
the realm of high power THz generation is ongoing. The resolved issues throughout
implementation include: magnesium doping, phase matching, and wave front distortion.
The high power, broadband THz emitter (maximum THz fi eld, Emax > 1 MV/cm) allows for
nonlinear THz spectroscopy of various material systems including
single-layer graphene and high-resistivity, bulk GaAs. THz-induced transparency
is observed in two types of single-layer graphene samples: (i) suspended
graphene-PMMA layer and (ii) graphene embedded in dielectrics. THz-induced
transparency is shown to be signi cantly higher in suspended graphene than in
graphene on a Si substrate. The experimental observation leads to a universal
nonlinear THz property of graphene that the sheet conductivity undergoes twofold
reduction when THz elds reach 0:8 MV/cm. We con rm the generality of this result by
measuring di erent graphene samples on di erent substrates. Timeresolved
THz transmission measurements show that the THz-induced transparency
in graphene is dynamic; the transient conductivity gradually decreases throughout
the pulse duration. The large THz elds induce sub-picosecond electron thermalization
and subsequent carrier-carrier scattering, transiently modulating the
electrical and optical properties, in e ect reducing the electrical conductivity of
graphene by an order of magnitude. Nonlinear THz spectroscopy methods are also
applied to the investigation of a nano-antenna patterned, high-resistivity, intrinsic
GaAs wafer. The antenna near- eld reaches 20 MV/cm due to a huge eld
enhancement in the plasmonic nanostructure. Thus, the nonlinear THz interactions
take place in the con ned nanometer-scale region adjacent to the antenna.
As a result of the huge THz elds, nano-antenna patterned GaAs demonstrates
remarkably strong nonlinear THz e ects. The elds are strong enough to generate
high density free carriers (Ne > 10^17 cm^3) via high-energy interband excitations
associated with a series of impact ionizations (n_I ~ 33-37); thus inducing large
absorption of THz radiation (> 35%).