In this Letter, we show that it is possible to structure the longitudinal polarization component of light. We illustrate our approach by demonstrating linked and knotted longitudinal vortex lines ...acquired upon nonparaxially propagating a tightly focused subwavelength beam. The remaining degrees of freedom in the transverse polarization components can be exploited to generate customized topological vector beams.
We measure the near-resonant transmission of light through a dense medium of potassium vapor confined in a cell with nanometer thickness in order to investigate the origin and validity of the ...collective Lamb shift. A complete model including the multiple reflections in the nanocell reproduces accurately the observed line shape. It allows the extraction of a density-dependent shift and width of the bulk atomic medium resonance, deconvolved from the cavity effect. We observe an additional, unexpected dependence of the shift with the thickness of the medium. This extra dependence demands further experimental and theoretical investigations.
Laminae I-III of the spinal dorsal horn contain many inhibitory interneurons that use GABA and/or glycine as a neurotransmitter. Distinct neurochemical populations can be recognised among these ...cells, and these populations are likely to have differing roles in inhibiting pain or itch. Quantitative studies in rat have shown that inhibitory interneurons account for 25-40% of all neurons in this region. The sst2A receptor is expressed by around half the inhibitory interneurons in laminae I-II, and is associated with particular neurochemically-defined populations. Although much of the work on spinal pain mechanisms has been performed on rat, the mouse is now increasingly used as a model, due to the availability of genetically altered lines. However, quantitative information on the arrangement of interneurons is lacking in the mouse, and it is possible that there are significant species differences in neuronal organisation. In this study, we show that as in the rat, nearly all neurons in laminae I-III that are enriched with glycine also contain GABA, which suggests that GABA-immunoreactivity can be used to identify inhibitory interneurons in this region. These cells account for 26% of the neurons in laminae I-II and 38% of those in lamina III. As in the rat, the sst2A receptor is only expressed by inhibitory interneurons in laminae I-II, and is present on just over half (54%) of these cells. Antibody against the neurokinin 1 receptor was used to define lamina I, and we found that although the receptor was concentrated in this lamina, it was expressed by many fewer cells than in the rat. By estimating the total numbers of neurons in each of these laminae in the L4 segment of the mouse, we show that there are around half as many neurons in each lamina as are present in the corresponding segment of the rat.
We experimentally demonstrate the heralded generation of bichromatic single photons from an atomic collective spin excitation (CSE). The photon arrival times display collective quantum beats, a novel ...interference effect resulting from the relative motion of atoms in the CSE. A combination of velocity-selective excitation with strong laser dressing and the addition of a magnetic field allows for exquisite control of this collective beat phenomenon. The present experiment uses a diamond scheme with near-IR photons that can be extended to include telecommunications wavelengths or modified to allow storage and retrieval in an inverted-Y scheme.
We present an experimental measurement of the cooperative Lamb shift and the Lorentz shift using a nanothickness atomic vapor layer with tunable thickness and atomic density. The cooperative Lamb ...shift arises due to the exchange of virtual photons between identical atoms. The interference between the forward and backward propagating virtual fields is confirmed by the thickness dependence of the shift, which has a spatial frequency equal to twice that of the optical field. The demonstration of cooperative interactions in an easily scalable system opens the door to a new domain for nonlinear optics.
Abstract
The spectroscopy of hot atomic vapours is a hot topic. Many of the work-horse techniques of contemporary atomic physics were first demonstrated in hot vapours. Alkali-metal atomic vapours ...are ideal media for quantum-optics experiments as they combine: a large resonant optical depth; long coherence times; and well-understood atom–atom interactions. These features aid with the simplicity of both the experimental set up and the theoretical framework. The topic attracts much attention as these systems are ideal for studying both fundamental physics and has numerous applications, especially in sensing electromagnetic fields and quantum technology. This tutorial reviews the necessary theory to understand the Doppler broadened absorption spectroscopy of alkali-metal atoms, and explains the data taking and processing necessary to compare theory and experiment. The aim is to provide a gentle introduction to novice scientists starting their studies of the spectroscopy of thermal vapours while also calling attention to the application of these ideas in the contemporary literature. In addition, the work of expert practitioners in the field is highlighted, explaining the relevance of three extensively-used software packages that complement the presentation herein.
This paper reviews the factors affecting the fatty acid composition of adipose tissue and muscle in pigs, sheep and cattle and shows that a major factor is the total amount of fat. The effects of ...fatty acid composition on meat quality are also reviewed. Pigs have high levels of polyunsaturated fatty acids (PUFA), including the long chain (C20-22) PUFA in adipose tissue and muscle. The full range of PUFA are also found in sheep adipose tissue and muscle whereas cattle ‘conserve’ long chain PUFA in muscle phospholipid. Linoleic acid (18:2
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6) is a major ingredient of feeds for all species. Its incorporation into adipose tissue and muscle in relation to the amount in the diet is greater than for other fatty acids. It is deposited in muscle phospholipid at a high level where it and its long chain products eg aracidonic acid (20:4
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6) compete well for insertion into phospholipid molecules. Its proportion in pig adipose tissue declines as fat deposition proceeds and is an index of fatness. The same inverse relationships are not seen in ruminant adipose tissue but in all species the proportion of 18:2
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6 declines in muscle as fat deposition increases. The main reason is that phospholipid, where 18:2
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6 is located, declines as a proportion of muscle lipid and the proportion of neutral lipid, with its higher content of saturated and monounsaturated fatty acids, increases. Oleic acid (18:1
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9), formed from stearic acid (18:0) by the enzyme stearoyl Co-A desaturase, is a major component of neutral lipid and in ruminants the same enzyme forms conjugated linoleic acid (CLA), an important nutrient in human nutrition. Like 18:2
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6,
α-linolenic acid (18:3
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3) is an essential fatty acid and is important to ruminants since it is the major fatty acid in grass. However it does not compete well for insertion into phospholipid compared with 18:2
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6 and its incorporation into adipose tissue and muscle is less efficient. Greater biohydrogenation of 18:3
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3 and a long rumen transit time for forage diets also limits the amount available for tissue uptake compared with 18:2
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6 from concentrate diets. A positive feature of grass feeding is that levels of the nutritionally important long chain
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3 PUFA are increased ie EPA (20:5
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3) and DHA (22:6
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3). Future research should focus on increasing
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3 PUFA proportions in lean carcasses and the use of biodiverse pastures and conservation processes which retain the benefits of fresh leafy grass offer opportunities to achieve this. The varying fatty acid compositions of adipose tissue and muscle have profound effects on meat quality. Fatty acid composition determines the firmness/oiliness of adipose tissue and the oxidative stability of muscle, which in turn affects flavour and muscle colour. Vitamin E is an essential nutrient, which stabilises PUFA and has a central role in meat quality, particularly in ruminants.
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