Tight focal spots using azimuthally polarised light from a Fresnel cone

When focusing a light beam at high numerical aperture, the resulting electric field profile in the focal plane depends on the transverse polarisation profile, as interference between different parts of the beam needs to be taken into account. It is well known that radial polarised light produces a longitudinal polarisation component and can be focused below the conventional diffraction limit for homogeneously polarised light, and azimuthally polarised light that carries one unit of angular momentum can achieve even tighter focal spots. This is of interest for example for enhancing resolution in scanning microscopy. There are numerous ways to generate such polarisation structures, however, setups can be expensive and usually rely on birefringent components, hence prohibiting broadband operation. We have recently demonstrated a passive, low-cost technique using a simple glass cone (Fresnel cone) to generate beams with structured polarisation. We show here that the polarisation structure generated by Fresnel cones focuses better than radial polarised light at all numerical apertures. Furthermore, we investigate in detail the application of polarised light structures for two-photon microscopy. Specifically we demonstrate a method that allows us to generate the desired polarisation structure at the back aperture of the microscope by pre-compensating any detrimental phase shifts using a combination of waveplates

Quantal calculations on the rotational excitation of NH(_3) and OH in collisions with H(_2)

Results are presented for quantal close coupled calculations of the rotational excitation of NH(_3) and OH in collisions with both ortho and para-H(_2). For the latter, these are the first calculations to include the rotational structure of the H(_2) molecule, whilst for the former, previous NH(_3) - ortho-H(_2) calculations have been subject to subsidiary approximations. The results from the NH3-H2 calculation show substantial qualitative changes in the cross-sections when ground state ortho-H(_2) (j = 1) replaces ground state para-H(_2) (j = 0) as the collision partner. In particular, cross-sections which were very small for NH(_3) - para-H(_2) collisions can be of a comparable magnitude with the other rotationally inelastic cross-sections for NH(_3) - ortho-H(_2) collisions. The changes in cross-sections are discussed in relation to the collisional pumping scheme for an astrophysical maser in the (jk = 33) inversion lines. From the OH-H(_2) calculations it is found that the propensities towards preferential excitation of a given component of the A doublets are reduced in strength when ortho-H(_2) replaces ground state para-H(_2) as the collision partner, similarly when (j = 2) para-H(_2) replaces ground state para-H(_2) the propensities are weakened. In both cases, the results are discussed in the context of crossed beam measurements at energies of 605cm(^-1)(NH(_3)-H(_2)) and 680cm(^-1) (OH-H(_2)). It is found that discrepancies between the experimental results and theoretical calculations for ground state para-H(_2) collisions can be explained, at least in part, by the neglect of the (j > 0) H(_2) rotational states in the latter

Happiness and the Human Development Index : the paradox of Australia

According to the well-being measure known as the U.N. Human Development Index, Australia now ranks 3rd in the world and higher than all other English-speaking nations. This paper questions that assessment. It reviews work on the economics of happiness, considers implications for policymakers, and explores where Australia lies in international subjective well-being rankings. Using new data on approximately 50,000 randomly sampled individuals from 35 nations, the paper shows that Australians have some of the lowest levels of job satisfaction in the world. Moreover, among the sub-sample of English-speaking nations, where a common language should help subjective measures to be reliable, Australia performs poorly on a range of happiness indicators. The paper discusses this paradox. Our purpose is not to reject HDI methods, but rather to argue that much remains to be understood in this area

Battling the Coronavirus Infodemic Among Social Media Users in Africa

How can we induce social media users to be discerning when sharing information during a pandemic? An experiment on Facebook Messenger with users from Kenya and Nigeria tested interventions designed to decrease intentions to share COVID-19 misinformation without decreasing intentions to share factual posts. The initial stage of the study incorporated: (i) a factorial design with 40 intervention combinations; and (ii) a contextual adaptive design, increasing the probability of assignment to treatments that worked better for previous subjects with similar characteristics. The second stage evaluated the best-performing treatments and a targeted treatment assignment policy estimated from the data. We precisely estimate null effects from warning flags and related article suggestions, tactics used by social media platforms. However, tips to identify misinformation and nudges to consider information's accuracy reduced misinformation sharing by 4.2% and 4.9% respectively. Such low-cost scalable interventions may improve the quality of information circulating online.Comment: 54 pages including appendix, 10 figure

Depression

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