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Herein, it is demonstrated that optical excitation of InAs quantum dots (QDs) embedded directly in an InP matrix can be mediated via states in a quaternary compound constituting an InP/InGaAlAs bottom distributed Bragg reflector (DBR) and native defects in the InP matrix. It does not only change the carrier relaxation in the structure but could also lead to the imbalanced occupation of QDs with charge carriers, because the band structure favors the transfer of holes. Thermal activation of carrier transfer can be observed as an increase in the emission intensity versus temperature for excitation powers below saturation on the level of both an inhomogeneously broadened QD ensemble and single QD transitions. That increase in the QD emission is accompanied by a decrease in the emission from the InGaAlAs layer at low temperatures. Finally, carrier transfer between the InGaAlAs layer of the DBR and the InAs/InP QDs is directly proven by the photoluminescence excitation spectrum of the QD ensemble. The reported carrier transfer can increase the relaxation time of carriers into the QDs and thus be detrimental to the coherence properties of single and entangled photons. It is important to take it into account while designing QD‐based devices. [ABSTRACT FROM AUTHOR]

Spin-dependent tunneling through a quantum dot coupled to one ferromagnetic and one superconducting electrodes is studied in the Andreev reflection (AR) regime. Electrical conductance is calculated within the nonequilibrium Green function technique. Effects due to a competition between the Coulomb correlations on the dot and intradot spin-flip processes are considered in the linear transport regime and for different coupling strengths between the dot and the external electrodes. It is shown that when a coherent spin rotation is present on the dot, Coulomb interactions may lead to a significant enhancement of the AR tunneling current and even to the perfect AR transmission. Origin of occurrence of a variety of the multipeak structure of the linear conductance is also discussed.

The molecular beam epitaxy grown structures are investigated, comprising of InGaAs quantum wells (QW) separated by a thin InGaAlAs barrier from InAs quantum dots (QDs), emitting at 1.55 μm, grown on an InP substrate. To control the coupling between QW and QD parts the thickness of the barrier is changed, which commands the wave function overlap. The tuning of that parameter allows for the study of the influence of the QW potential on the energy structure of states and their wave functions in QDs, changing from an uncoupled system, where the optical response is just a sum of responses from two isolated elements, to a strongly quantum mechanically coupled system, exhibiting mixed 2D‐0D characteristics. The changes of the energy structure that are deduced from the photoreflectance and photoluminescence spectroscopy results, supported by 8‐band k · p modeling, explain the measured differences in the photoluminescence decay times between samples with different barrier thicknesses. [ABSTRACT FROM AUTHOR]

We present the results of optical measurements performed on structures consisting of an InGaAs quantum well (QW), separated by a thin barrier from a layer of self-assembled InGaAs quantum dots (QDs). Such a kind of design is called a tunnel injection structure, because its functionality is based on the tunnelling of carriers from a QW to QDs, preferably with the assistance of optical phonons. In this approach, the injector QW serves as a reservoir of the carriers (due to much higher efficiency of carrier collection) and alleviates the problem of long relaxation times needed for carriers to reach the QDs ground state. In order to investigate the structures several complementary experimental techniques are applied. Photoreflectance, an absorption-like modulation spectroscopy, gives the information about the optical transitions and the electronic structure. The temperature evolution of photoluminescence allows emission efficiency and carrier losses to be determined. Photoluminescence excitation probes directly the carrier transfer from QW to the dots. The interpretation of the results is supported by the calculations in the envelope function formalism. It has been found out that the wavefunction position of the lowest lying levels depends on the QW parameters and thus different regimes of tunnelling are proposed. [ABSTRACT FROM AUTHOR]

There is presented an experimental setup for the measurements of photomodulated reflectivity spectra of low-dimensional semiconductor structures with a micrometer spatial (plane) resolution. The setup has been developed as an extended and improved version of a standard bright configuration, i.e., where the probe beam is provided directly by a broad band light source (e.g., halogen lamp) and then it is dispersed after being reflected off the sample. It gives typically the plane resolution, expressed by the spot size of the beams on the sample surface, on the level of single millimetres. Introducing optics, based on a long working distance and a high numerical aperture microscope objective, has allowed decreasing the spot size by three orders of magnitude into the micrometer range for both the probe and the pump beams. The optimization of microphotoreflectance signal to noise ratio has made it possible to detect the normalized reflectivity coefficient changes (ΔR/R) from an ultrathin single quantum well formed of the wetting layer in the structure with self-assembled InAs/GaAs quantum dots and from single pillar microresonators of the lateral sizes in the range of single micrometers. [ABSTRACT FROM AUTHOR]

To maintain adequate cerebral blood flow despite frequent changes in systemic arterial blood pressure and to constantly adjust blood supply to the current metabolic demand dictated by neuronal electrical activity, brain developed a myriad of mechanisms. These are designed to protect central nervous system from fatal consequences of hypoxia and energy deficit and are collectively called “cerebral autoregulation”. Despite years of research mechanisms responsible for regulation of CBF functioning under physiologic and pathologic conditions are still not clear. When these mechanisms are damaged or exhausted, patients life is in danger, as even slight, negligible under normal conditions, systemic hemodynamic disturbances might lead to cerebral infarct. Even perfect imaging of the irreversible brain damage with MR for the particular patient is too late action. Thus, detection of cerebral blood flow disturbances and impaired autoregulation, which are known to be associated with high risk of stroke, are extremely important in clinical practice. Several methods have been developed to quantify this process and thus evaluate risk of cerebral ischemia and guide therapeutic process. This review focuses on current knowledge on physiology of regulation of cerebral blood flow, mechanisms responsible for brain damage resulted from cerebral ischemia and reviews noninvasive diagnostic tests to assess cerebral autoregulation. [ABSTRACT FROM AUTHOR]

Integrating light emitters based on III-V materials with silicon-based electronics is crucial for further increase in data transfer rates in communication systems since the indirect bandgap of silicon prevents its direct use as a light source. We investigate here InAs/InGaAlAs quantum dot (QD) structures grown directly on 5° off-cut Si substrate and emitting light at 1.5 μm, compatible with established telecom platform. Using different dislocation defect filtering layers, exploiting strained superlattices, and supplementary QD layers, we mitigate the effects of lattice constant and thermal expansion mismatches between III-V materials and Si during growth. Complementary optical spectroscopy techniques, i.e. photoreflectance and temperature-, time- and polarization-resolved photoluminescence, allow us to determine the optical quality and application potential of the obtained structures by comparing them to a reference sample-state-of-the-art QDs grown on InP. Experimental findings are supported by calculations of excitonic states and optical transitions by combining multiband k • p and configuration-interaction methods. We show that our design of structures prevents the generation of a considerable density of defects, as intended. The emission of Si-based structures appears to be much broader than for the reference dots, due to the creation of different QD populations which might be a disadvantage in particular laser applications, however, could be favorable for others, e.g., in broadly tunable devices, sensors, or optical amplifiers. Eventually, we identify the overall most promising combination of defect filtering layers and discuss its advantages and limitations and prospects for further improvements.

In this work, we determine the temperature dependence of refractive indices of In 0.53 Al 0.1 Ga 0.37 As and Al 0.9 Ga 0.1 As semiconductor alloys at telecommunication wavelengths in the range from room temperature down to 10 K. For that, we measure the temperature-dependent reflectance of two structures: with an Al 0.9 Ga 0.1 As/GaAs distributed Bragg reflector (DBR) designed for 1.3 µm and with an In 0.53 Al 0.1 Ga 0.37 As/InP DBR designed for 1.55 µm. The obtained experimental results are compared to DBR reflectivity spectra calculated within the transfer matrix method to determine refractive index values. We further show that changes due to the thermal expansion of the DBR layers are negligible for our method.

Hypothermia has long been employed for therapeutic purposes but its use has been limited because of the potential life-threatening side-effects. In late eighties a neuroprotective effect of bold hypothermia was demonstrated and this implied that the method could be used more safely. It has then been shown in a lot of animal experiments that post-ischaemic mild hypothermia significantly limits damage to the brain caused by cardiac arrest. Similar results have been obtained for local brain ischaemia and for experimental head trauma. Molecular basis for this neuroprotection with mild hypothermia has been found to be complex, involving attenuation of the excitotoxic effects of glutamate and diminishing the synthesis of free radicals. In the last decade some clinical series and multicenter randomised trials have shown that mild hypothermia is safe and effective in global brain ischaemia due to cardiac arrest. Clinical data suggest also its effectiveness in ischaemic stroke though no multicenter randomised study has been published to date. At present there are conflicting results of clinical trials concerning brain injuries. Although some authors have reported up to 38% improvement in the outcome, a recently published multicenter randomised trial has failed to demonstrate any practical benefit of mild hypothermia after acute brain injury. There is however virtually no data in the literature on mild hypothermia in spinal cord injury.

Transcranial Doppler sonography is commonly used for diagnosis of cerebral vasospasm. However, the overall diagnostic performance of this method in detection of arterial narrowing has not been established. Blood velocity threshold, diagnostic for vasospasm, has been proposed for conventional, "blind" transcranial Doppler sonography (TCD). Nevertheless it cannot be used for transcranial color Doppler sonography (TCCD), because in this method the obtained blood velocities in the major cerebral arteries are higher than are in TCD. The aim of this study was to estimate the accuracy of transcranial color Doppler sonography in detection of middle cerebral artery (MCA) narrowing by means of receiver operating characteristic curve analysis (ROC). One hundred thirty four patients were studied with TCCD immediately before cerebral angiography. There were 75 men and 59 women, age range from 18 to 74 years, mean age 49 years. Of the 268 MCAs examined, 227 arteries were finally included in the construction of the ROC curve. Angiographic vasospasm was graded as none, mild (equal to or less than 25% of vessel caliber reduction--16 patients) and moderate-to-severe (more than 25% of vessel caliber reduction--29 patients). The overall diagnostic performance of transcranial color Doppler sonography in the detection of moderate-to-severe vasospasm of middle cerebral artery was found to be very high. The value of associated area under the ROC curve was 0.94. The value of this area for all vasospasms was 0.85, and this indicates good accuracy of a test. The best performing TCCD parameter for the detection of MCA narrowing was revealed to be peak-systolic velocity. The optimal trade-off between sensitivity and specificity was achieved with a peak systolic velocity of 182 cm/s. Therefore this velocity threshold is proposed as most universal for the diagnosis of vasospasm in the middle cerebral artery.

Numerous biological effects of oestrogens within the brain support the argument that their influence is not restricted to the reproductive system. Specific estrogen receptors were found in the central nervous system as early as in the 70's, but until recently little was known about the molecular background of oestrogens activity in the brain. The last decade witnessed a great effort aimed at better understanding of the tangled molecular mechanisms underlying subtle liaisons between brain metabolism and cerebral blood flow and particularly their disturbances triggered by brain ischaemia. These investigations had an ultimate goal--an effective method of preventing an expanding biochemical "catastrophe" within and around the ischaemic brain regions (the so-called "neuroprotection"). Molecular mechanisms by which oestrogens increase both metabolism and blood flow through certain regions of the brain became only recently better understood. Now there is a consensus as to that these hormones increase secretion of neuromediators, stimulate formation of new synapses and can activate certain genes, responsible for production of anti-apoptotic proteins and growth factors. Oestrogens can also dilate cerebral vessels, here acting through increased synthesis of nitric oxide and by stimulating such compounds as prostacycline and a potent vasodilator-epoxyeikosotrienoic acid. There is a body of recent evidence which suggest that during brain ischaemia the physiological estrogen stimulation, of both, brain metabolism and cerebral blood flow, becomes biased towards increased release of vasodilating substances. As the metabolism is not spurred accordingly, the net effect of oestrogens is neuroprotection. Other protective properties of oestrogens within the brain are related to attenuation of the excitotoxic effects of glutamate and to the activation of enzymes scavenging free oxygen radicals. Moreover, oestrogens can diminish free radicals synthesis and act as free radicals scavengers themselves. They can also activate synthesis of bcl-2 protein, which prevents cell apoptosis in the ischaemic regions. The relevant evidence, which continue to accumulate, may suggest that it is time now to consider launching a clinical investigation on the role of oestrogens as potentially important neuroprotective factors.