We report a new full vector finite element model for analyzing the optical properties of azimuthally symmetric oxide-apertured vertical-cavity surface emitting lasers (VCSELs). Our model allows for ...quasi-exact calculation of the lasing mode blueshift, threshold gain, and field profile. Through a detailed analysis of a sample VCSEL, we ascertain the physical effects which determine diffractive or parasitic mode loss. They are: 1) the background density of parasitic modes and 2) the coupling strength between the lasing mode and the parasitic mode continuum. The coupling strength is in turn determined by the relative alignment between the lasing and parasitic mode propagation vectors and the lasing mode penetration into the oxide region. This analysis improves our understanding of the optical physics of apertured VCSELs and should enable the next leap down in lasing threshold.
1. We studied the effects of low temperature on the action potentials and membrane currents of guinea-pig ventricular myocytes,
using a tight-seal whole-cell clamp technique. 2. The action potential ...duration at 95% repolarization was prolonged from 146
+/- 33 ms (mean +/- S.D., n = 6) at 33-34 degrees C (control temperature) to 314 +/- 83 ms at 24-25 degrees C (low temperature).
3. In whole-cell clamp experiments, low temperature decreased the calcium current (ICa), the delayed rectifier potassium current
(IK), and the inwardly rectifying potassium current (IK1) with 'apparent' Q10 (temperature coefficient) values of 2.3 +/-
0.6 for ICa, 4.4 +/- 1.2 for IK tail current and 1.5 +/- 0.3 for IK1 (n = 7). 4. The effect of low temperature on IK was further
studied in the presence of 0.6 microM nicardipine to block ICa. The decay phase of the IK tail consisted of two exponential
components. The fast but not the slow component was highly sensitive to the temperature change with an apparent Q10 of 4.5.
5. We found that a component of time-independent current is also sensitive to the temperature. The current had a linear I-V
relationship and remained almost unchanged after inhibition of Na(+) -K+ pump in K(+)-free external solution. 6. Using our
mathematical model of the ventricular action potential (a modification from the DiFrancesco-Noble model), we simulated the
action potential at low temperature by modifying some of the membrane currents, namely IK, IK1, ICa and a component of background
current. It was shown that simultaneous changes in these currents could reproduce approximately 75% of the action prolongation
induced by low temperature.
Ultralight axionlike particles (ALPs) are compelling dark matter candidates because of their potential to resolve small-scale discrepancies between \(\Lambda\)CDM predictions and cosmological ...observations. Axion-photon coupling induces a polarization rotation in linearly polarized photons traveling through an ALP field; thus, as the local ALP dark matter field oscillates in time, distant static polarized sources will appear to oscillate with a frequency proportional to the ALP mass. We use observations of the cosmic microwave background from SPT-3G, the current receiver on the South Pole Telescope, to set upper limits on the value of the axion-photon coupling constant \(g_{\phi\gamma}\) over the approximate mass range \(10^{-22} - 10^{-19}\) eV, corresponding to oscillation periods from 12 hours to 100 days. For periods between 1 and 100 days (\(4.7 \times 10^{-22} \text{ eV} \leq m_\phi \leq 4.7 \times 10^{-20} \text{ eV}\)), where the limit is approximately constant, we set a median 95% C.L. upper limit on the amplitude of on-sky polarization rotation of 0.071 deg. Assuming that dark matter comprises a single ALP species with a local dark matter density of \(0.3\text{ GeV/cm}^3\), this corresponds to \(g_{\phi\gamma} < 1.18 \times 10^{-12}\text{ GeV}^{-1} \times \left( \frac{m_{\phi}}{1.0 \times 10^{-21} \text{ eV}} \right)\). These new limits represent an improvement over the previous strongest limits set using the same effect by a factor of ~3.8.
The formation of CO{sub 2} in quiescent regions of molecular clouds is not yet fully understood, despite CO{sub 2} having an abundance of around 10%-34% H{sub 2}O. We present a study of the formation ...of CO{sub 2} via the nonenergetic route CO + OH on nonporous H{sub 2}O and amorphous silicate surfaces. Our results are in the form of temperature-programmed desorption spectra of CO{sub 2} produced via two experimental routes: O{sub 2} + CO + H and O{sub 3} + CO + H. The maximum yield of CO{sub 2} is around 8% with respect to the starting quantity of CO, suggesting a barrier to CO + OH. The rate of reaction, based on modeling results, is 24 times slower than O{sub 2} + H. Our model suggests that competition between CO{sub 2} formation via CO + OH and other surface reactions of OH is a key factor in the low yields of CO{sub 2} obtained experimentally, with relative reaction rates of k{sub CO+H}<<k{sub CO+OH}< k{sub H{sub 2}O{sub 2}+H}< k{sub OH+H},k{sub O{sub 2}+H}. Astrophysically, the presence of CO{sub 2} in low A{sub V} regions of molecular clouds could be explained by the reaction CO + OH occurring concurrently with the formation of H{sub 2}O via the route OH + H.
Icy grain mantles are the main reservoir of the volatile elements that link chemical processes in dark, interstellar clouds with the formation of planets and composition of their atmospheres. The ...initial ice composition is set in the cold, dense parts of molecular clouds, prior to the onset of star formation. With the exquisite sensitivity of JWST, this critical stage of ice evolution is now accessible for detailed study. Here we show the first results of the Early Release Science program "Ice Age" that reveal the rich composition of these dense cloud ices. Weak ices, including, \(^{13}\)CO\(_2\), OCN\(^-\), \(^{13}\)CO, OCS, and COMs functional groups are now detected along two pre-stellar lines of sight. The \(^{12}\)CO\(_2\) ice profile indicates modest growth of the icy grains. Column densities of the major and minor ice species indicate that ices contribute between 2 and 19% of the bulk budgets of the key C, O, N, and S elements. Our results suggest that the formation of simple and complex molecules could begin early in a water-ice rich environment.
The UV photoreactivity of polycyclic aromatic hydrocarbons (PAHs) in porous amorphous solid water has long been known to form both oxygenated photoproducts and photofragments. The aim of this study ...was to examine the influence of the ice structure upon reactivity under soft UV irradiation conditions. Mixtures of PAHs with amorphous solid water (porous and compact) and crystalline (cubic and hexagonal) ices were prepared in a high vacuum chamber and irradiated using a mercury lamp for up to 2.5 hours. The results show that the production of oxygenated PAHs is efficient only in amorphous water ice, while fragmentation can occur in both amorphous and crystalline ices. We conclude that the reactivity is driven by PAH-water interactions in favourable geometries, notably where dangling bonds are available at the surface of pores. These results suggest that the formation of oxygenated PAH molecules is most likely to occur in interstellar environments with porous (or compact) amorphous solid water and that this reactivity could considerably influence the inventory of aromatics in meteorites.
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