Enhanced Terahertz Receiver Using a Distributed Bragg Reflector Coupled to a Photoconductive Antenna

We report the use of a terahertz (THz) receiver fabricated using a photoconductive antenna mounted upon a distributed Bragg reflector (DBR). It is shown that the relative performance of the device is in accordance with the reflectivity curves of the DBR. At maximum reflectivity the device improved upon the reference THz signal by up to 30%. In addition, we characterize the saturation of the device with laser pump power and it is shown that the device exhibits an improved optical efficiency when compared to the reference receiver.     

Exact Analysis of the Wire-Bonded Multiconductor Transmission Line

In this paper, the exact model of a multiconductor transmission line with bonding wires is presented. The model is based on the multiport admittance matrix, and is valid for any number of conductors in the structure. The model shows how to compute the two-port admittance matrix of the wire-bonded structure with direct application to the wire-bonded interdigital capacitor. The model has been validated by means of method-of-moments-based numerical simulation and by experimental work.     

GaAs-AlGaAs quantum cascade lasers: physics, technology, andprospects

In recent years, the performance of GaAs-AlGaAs-based quantum cascade (QC) lasers has improved markedly. These devices are capable of pulsed room temperature operation and can deliver respectable average powers (11 mW at λ~9 μm) operating on a Peltier cooler. This performance has been achieved by the suppression of thermally activated carrier leakage through increases in the heterobarrier band offset. We demonstrate that QC lasers, with wavelengths λ⩾9 μm, can operate using heterostructures encompassing the entire composition range of Al_xGa_1-xAs, without encountering potential problems-of the satellite X-minima for x>45%. Furthermore, we present particular characteristics of these devices, such as a phonon-limited temperature dependence, electrical and optical self-oscillations, and novel design concepts that exploit this closely lattice matched material system. Finally, we discuss improvements in device fabrication to lower the operating current through a reduction of the area of current injection. Using this technology, devices can be designed to selectively pump the fundamental lateral mode. We, therefore, observe single spatial-mode operation over the entire current range of operation     

Infrared spectroscopy of chromium-doped cadmium selenide

The maximum optical-absorption cross section of Cr²⁺ ions was evaluated from near-infrared (NIR) absorption spectroscopy and direct measurements of the chromium concentration in Cr²⁺:CdSe crystals. The emission lifetime of the excited state, ⁵E, of Cr²⁺ was measured as a function of Cr²⁺ concentration in the 2×10¹⁷ −2×10¹⁸ ions/cm³ range and as a function of temperature from 77–300 K. Lifetime values were as high as ∼6 μs in the 77–250 K range and decreased to ∼4 μs at 300 K because of nonradiative decays. Assuming that most of the Cr dopant is in the Cr²⁺ state, an optical-absorption cross section σ_a of (1.94±0.56) × 10⁻¹⁸ cm² was calculated. Implications for laser performance are discussed.     

A Recursive Algorithm for Computing Exact Reliability Measures

An algorithm is presented to find source-to-K-terminal reliability in a directed graph with independent arc failures. The algorithm is based on a discrete-time Markov chain with two absorbing states. The Markov chain has an upper triangular transition probability matrix, thus the probability of absorption in a state can be found by back-substitution. We show: 1) The source-to-K-terminal reliability is the probability of absorption in a particular absorbing state; 2) The time until absorption can be used as an alternative reliability measure; and 3) The algorithm can be used to find a third reliability measure called the degree of connectedness.     

Relative importance of growth hormone and sex steroids for the growth at puberty of trunk length, limb length, and muscle width in growth hormone-deficient children

We have followed the growth of stature, sitting height, skinfolds, muscle widths measured radiologically, and skeletal maturity in growth hormone-deficient patients in whom hGH was given and withheld in alternating three-month periods throughout puberty (referred to as "off-hGH" and "on-hGH" periods). Six boys and four girls had true isolated GH deficiency and developed puberty spontaneously. Two boys had gonadotrophin deficiency plus GH deficiency, and five boys had multiple deficiencies; in these boys the signs of puberty were induced by hormone treatment. Boys with true isolated deficiency grew about two-thirds as much in height in the off-hGH periods as in the on-hGH periods; their total gain in height during the adolescent spurt would have been about 20 cm, instead of 30 cm, if hGH had been discontinued at the beginning of puberty. The effect of hGH was entirely on growth in leg-length, however, which virtually ceased during the off-hGH periods. Growth in sitting height altered little when hGH was withdrawn. Growth in limb muscles, however, was GH dependent throughout puberty; during the majority of periods when hGH was withheld, muscle was actually lost; this occurred in the boys who were receiving large doses of testosterone as well as in those producing their own normal amounts. Subcutaneous fat diminished when hGH was given and increased when it was withdrawn; this occurred independently of administration of testosterone. There was little evidence that growth of pubic and axillary hair progressed faster during on-hGH periods, except perhaps in patients with multiple deficiencies. There was some evidence, however, that bone age progressed less rapidly during on-hGH periods than during off-hGH periods in the patients with isolated deficiency. The results in the girls agreed with those in boys so far as stature was concerned, but the relationship with sitting height and leg length appeared to be different; the reasons for this are discussed. We conclude that all children with GH deficiency should continue on treatment with hGH throughout puberty, ideally until growth ceases.