665 research outputs found
Optimization of micromachined relex klystrons for operation at terahertz frequencies
New micromachining techniques now provide us
with the technology to fabricate reflex klystron oscillators with dimensions suitable for operation in the terahertz region of the electromagnetic spectrum. For the success of these devices, accurate designs are required since the optimization of certain parameters is critical to obtaining useful amounts of ac power. Classical models for device design have long been in existence,
but these are no longer valid at terahertz frequencies. For this reason, we have developed a simulation tool, specifically aimed at the design of terahertz frequency reflex klystrons. The tool, based on the Monte Carlo algorithm, includes loss mechanisms and takes into account the main peculiarities expected for device
operation at terahertz frequencies. In this study, the tool is used to study the influence of the electron beam aperture angle and cavity dimensions (particularly the grid spacing) on ac power generation. The results demonstrate that aperture angles of less than 10 are necessary for the optimization of output power. It is
also found that the power output is highly sensitive to the distance between the grids
Investigation into intermodulation distortion in HEMTs using a quasi-2-D physical model
The need for both linear and efficient pseudomorphic high electron-mobility transistors (pHEMTs) for modern wireless handsets necessitates a thorough understanding of the origins of intermodulation distortion at the device level. For the first time, the dynamic large-signal internal physical behavior of a pHEMT is examined using a quasi-two-dimensional physical device model.
The model accounts fully for device-circuit interaction and is validated experimentally for a two-tone experiment around 5 GHz
Electrical and radiation characteristics of semilarge photoconductive terahertz emitters
We present experimental characterization of semilarge
photoconductive emitters, including their electrical/photoconductive
parameters and terahertz spectra. A range of emitters
were studied and fabricated on both LT-GaAs and SI-GaAs,
having a variety of electrode geometries. The spatial cone of terahertz
radiation was defined. The dependencies of the photocurrent
and the terahertz power on the bias voltage and the laser power
were determined. A Fourier-transform interferometer is used to
determine the terahertz spectra and to clarify the effects of the
substrate and electrode geometry
Random line tessellations of the plane: statistical properties of many-sided cells
We consider a family of random line tessellations of the Euclidean plane
introduced in a much more formal context by Hug and Schneider [Geom. Funct.
Anal. 17, 156 (2007)] and described by a parameter \alpha\geq 1. For \alpha=1
the zero-cell (that is, the cell containing the origin) coincides with the
Crofton cell of a Poisson line tessellation, and for \alpha=2 it coincides with
the typical Poisson-Voronoi cell. Let p_n(\alpha) be the probability for the
zero-cell to have n sides. By the methods of statistical mechanics we construct
the asymptotic expansion of \log p_n(\alpha) up to terms that vanish as
n\to\infty. In the large-n limit the cell is shown to become circular. The
circle is centered at the origin when \alpha>1, but gets delocalized for the
Crofton cell, \alpha=1, which is a singular point of the parameter range. The
large-n expansion of \log p_n(1) is therefore different from that of the
general case and we show how to carry it out. As a corollary we obtain the
analogous expansion for the {\it typical} n-sided cell of a Poisson line
tessellation.Comment: 26 pages, 3 figure
Optical properties of tissue measured using terahertz pulsed imaging.
The first demonstrations of terahertz imaging in biomedicine were made several years ago, but few data are available on the optical properties of human tissue at terahertz frequencies. A catalogue of these properties has been established to estimate variability and determine the practicality of proposed medical applications in terms of penetration depth, image contrast and reflection at boundaries. A pulsed terahertz imaging system with a useful bandwidth 0.5-2.5 THz was used. Local ethical committee approval was obtained. Transmission measurements were made through tissue slices of thickness 0.08 to 1 mm, including tooth enamel and dentine, cortical bone, skin, adipose tissue and striated muscle. The mean and standard deviation for refractive index and linear attenuation coefficient, both broadband and as a function of frequency, were calculated. The measurements were used in simple models of the transmission, reflection and propagation of terahertz radiation in potential medical applications. Refractive indices ranged from 1.5 ± 0.5 for adipose tissue to 3.06 ± 0.09 for tooth enamel. Significant differences (P<0.05) were found between the broadband refractive indices of a number of tissues. Terahertz radiation is strongly absorbed in tissue so reflection imaging, which has lower penetration requirements than transmission, shows promise for dental or dermatological applications
Predicting university performance in psychology: the role of previous performance and discipline-specific knowledge
Recent initiatives to enhance retention and widen participation ensure it is crucial to understand the factors that predict students' performance during their undergraduate degree. The present research used Structural Equation Modeling (SEM) to test three separate models that examined the extent to which British Psychology students' A-level entry qualifications predicted: (1) their performance in years 1-3 of their Psychology degree, and (2) their overall degree performance. Students' overall A-level entry qualifications positively predicted performance during their first year and overall degree performance, but negatively predicted their performance during their third year. Additionally, and more specifically, students' A-level entry qualifications in Psychology positively predicted performance in the first year only. Such findings have implications for admissions tutors, as well as for students who have not studied Psychology before but who are considering applying to do so at university
Water wave propagation and scattering over topographical bottoms
Here I present a general formulation of water wave propagation and scattering
over topographical bottoms. A simple equation is found and is compared with
existing theories. As an application, the theory is extended to the case of
water waves in a column with many cylindrical steps
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