Diffusion weighted magnetic resonance imaging (dMRI) has enabled the in vivo imaging of structures with a highly fibrous composition, such as brain white matter due to the ability to detect the hindered and restricted diffusion of water along its defined tracts. In order to increase this non-invasive technique’s sensitivity to the intricate fibrous structure and to better calibrate diffusion pulse sequences and validate fibre reconstruction modelling techniques, physical diffusion phantoms have been developed. These phantoms have a known structure and diffusion behaviour. This work aims to simplify the process of creating complex fibre-based diffusion phantoms using 3D printing material to model and mimic brain white matter fiber architecture for dMRI. We make use of a printing material consisting of a mixture of polyvinyl alcohol (PVA) and a rubber-elastomeric polymer (Gel-Lay by Poro-Lay), printed using a fused deposition modelling (FDM) printer. It is 3D printed as a rigid object but, following immersion in room-temperature water, the PVA dissolves away leaving behind the porous rubber-elastomeric polymer component to mimic the structure of brain white matter tracts. To test the validity of the methodology, two preliminary main phantoms were created: a linear 10mm × 10mm × 30mm block phantom and an orthogonal fibercrossing phantom where two blocks cross at a 90-degree angle. This was followed by creating 3 disk phantoms with fibres crossing at 30, 60 and 90 degrees. Results demonstrate reproducible high diffusion anisotropy (FA= 0.56 and 0.60) for the phantoms aligned with the fibre direction for the preliminary linear blocks. With multi-fibre ball & stick modelling in the orthogonal fibre-crossing phantom and the disk phantoms at 30, 60 and 90 degrees, image post-processing yielded crossing fibre populations that reflected the known physical architecture. These preliminary results reveal the potential of 3D-printed phantoms to validate fibre-reconstruction models and optimize acquisition protocols, paving the way for more complex phantoms and the investigation of long-term stability and reproducibility.
We demonstrate the potential utilization of a Schottky barrier in the plasmonic regime at terahertz (THz)
frequencies. Experimental evidence of local THz plasmonic field enhancement via radiation from the space-charge
distribution at a Schottky interface is shown. A 12% increase in the plasmonic-mediated transmission of THz radiation
through random, dense ensembles of Cu particles is observed when a CuxO/Au structure is introduced to the surface of
the particles. The THz electric field induces oscillations of the local charge density at the Schottky interface leading to
emission of high frequency radiation as the charges settle back into equilibrium. The non-linear THz response of the
Schottky interface introduces the physical groundwork for the implementation of plasmonic circuits that have operational
frequencies exceeding the limits of traditional semiconductor electronics.
We describe a new mechanism for ultrafast active control of plasmon propagation. By using time-domain terahertz
spectroscopy, we demonstrate that electron spin state can influence plasmon propagation. Using a random spinplasmonic
medium consisting of a dense ensemble of bimetallic ferromagnetic (F)/nonmagnetic (N) microparticles, plasmon
propagation velocity, amplitude attenuation, phase retardation and magnetic field dependence are shown to be influenced
by electron spin accumulation in the nonmagnetic layers. The observation of electron spin accumulation is attributed to
the formation of a nonequilibrium spin-dependent potential barrier at the F/N interface that acts to resist the flow of a
spin-polarized plasmon current. This phenomenon is similar to the electrically-driven spin accumulation phenomenon
resulting from current transport between F/N layers. With this first demonstration of the merger between the plasmonics
and spintronics fields, we envision the realization of a new class of ultrafast spinplasmonic devices having unique
functionalities.
Research in the terahertz regime of the electromagnetic spectrum has become increasingly prominent in
photonics for a number of reasons, including the abilities to temporally resolve ultrashort pulses (~ ps), extract
polarization information, and easily create structures with size on the order of the wavelength. In a conventional time-domain
terahertz spectroscopy system, when a pulse of radiation is incident on a sample, the system detects the radiation
transmitted along the incident beam axis. While this approach has proven to be effective for numerous applications, it is
often desirable to detect radiation scattered from a sample at off-axis angles. The realization of a pump-probe terahertz
spectroscopy system capable of time- and polarization-resolved detection at arbitrary angles is nontrivial, however, since
the sampling probe pulse must retain its spatial alignment, polarization, and timing at all detection angles. In this work,
we unveil a system capable of angular- and time-resolved detection of terahertz radiation, with detection angles spanning
360°. Moreover, the polarization of the radiation can be determined at any detection angle. The performance of the
system is demonstrated experimentally by detecting the radiation scattered from a spherical sample, which exhibits a
radiation pattern that is symmetric about the incident beam axis. To the best of our knowledge, an angularly resolved
terahertz system with such versatility has not been previously realized, and its development introduces a new platform
for explorations into angular dependent phenomena.
Ferromagnetic particle collections typically possess anisotropic terahertz (THz) transmission properties that are
sensitive to the orientation of an applied external magnetic field. Here, we show that the particle surface morphology
can have a large and dominant effect on the magnetic field orientation dependence of the THz transmission. In
particular, the THz transmission through highly porous ferromagnetic Ni particles shows isotropic dependence on the
external magnetic field orientation. This isotropic magnetic phenomenon suggests the possibility of innovative photonic
materials with tailored magnetic properties.
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