Statistical mechanics of semiflexible ribbon polymers

The statistical mechanics of a ribbon polymer made up of two semiflexible chains is studied using both analytical techniques and simulation. The system is found to have a crossover transition at some finite temperature, from a type of short range order to a fundamentally different sort of short range order. In the high temperature regime, the 2-point correlation functions of the object are identical to worm-like chains, while in the low temperature regime they are different due to a twist structure. The crossover happens when the persistence length of individual strands becomes comparable to the thickness of the ribbon. In the low temperature regime, the ribbon is observed to have a novel ``kink-rod'' structure with a mutual exclusion of twist and bend in contrast to smooth worm-like chain behaviour. This is due to its anisotropic rigidity and corresponds to an {\it infinitely} strong twist-bend coupling. The double-stranded polymer is also studied in a confined geometry. It is shown that when the polymer is restricted in a particular direction to a size less than the bare persistence length of the individual strands, it develops zigzag conformations which are indicated by an oscillatory tangent-tangent correlation function in the direction of confinement. Increasing the separation of the confining plates leads to a crossover to the free behaviour, which takes place at separations close to the bare persistence length. These results are expected to be relevant for experiments which involve complexation of two or more stiff or semiflexible polymers.Comment: 20 pages, 11 figures. PRE (in press

Nanoscale waveguiding methods

While 32 nm lithography technology is on the horizon for integrated circuit (IC) fabrication, matching the pace for miniaturization with optics has been hampered by the diffraction limit. However, development of nanoscale components and guiding methods is burgeoning through advances in fabrication techniques and materials processing. As waveguiding presents the fundamental issue and cornerstone for ultra-high density photonic ICs, we examine the current state of methods in the field. Namely, plasmonic, metal slot and negative dielectric based waveguides as well as a few sub-micrometer techniques such as nanoribbons, high-index contrast and photonic crystals waveguides are investigated in terms of construction, transmission, and limitations. Furthermore, we discuss in detail quantum dot (QD) arrays as a gain-enabled and flexible means to transmit energy through straight paths and sharp bends. Modeling, fabrication and test results are provided and show that the QD waveguide may be effective as an alternate means to transfer light on sub-diffraction dimensions

Sentinel lymph node mapping with type-II quantum dots

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Absorption by water increases fluorescence image contrast of biological tissue in the shortwave infrared.

Recent technology developments have expanded the wavelength window for biological fluorescence imaging into the shortwave infrared. We show here a mechanistic understanding of how drastic changes in fluorescence imaging contrast can arise from slight changes of imaging wavelength in the shortwave infrared. We demonstrate, in 3D tissue phantoms and in vivo in mice, that light absorption by water within biological tissue increases image contrast due to attenuation of background and highly scattered light. Wavelengths of strong tissue absorption have conventionally been avoided in fluorescence imaging to maximize photon penetration depth and photon collection, yet we demonstrate that imaging at the peak absorbance of water (near 1,450 nm) results in the highest image contrast in the shortwave infrared. Furthermore, we show, through microscopy of highly labeled ex vivo biological tissue, that the contrast improvement from water absorption enables resolution of deeper structures, resulting in a higher imaging penetration depth. We then illustrate these findings in a theoretical model. Our results suggest that the wavelength-dependent absorptivity of water is the dominant optical property contributing to image contrast, and is therefore crucial for determining the optimal imaging window in the infrared

Author Correction: Non-invasive monitoring of chronic liver disease via near-infrared and shortwave-infrared imaging of endogenous lipofuscin (Nature Biomedical Engineering, (2020), 4, 8, (801-813), 10.1038/s41551-020-0569-y).

A Correction to this paper has been published: https://doi.org/10.1038/s41551-020-0569-y

Enhanced charge carrier mobility and lifetime suppress hysteresis and improve efficiency in planar perovskite solar cells

Perovskite solar cells PSCs are very promising lab scale technologies to deliver inexpensive solar electricity. Low temperature, planar PSCs are of particularly interest for large scale deployment due to their inherent suitability for flexible substrates and potential for silicon perovskite tandems. So far, planar PSCs have been prone to large current voltage hysteresis and low stabilized power output due to a number of issues associated with this kind of device configuration. We find that the suppression of the yellow phase impurity amp; 8706; FAPbI3 present in formamidium based perovskites, by RbI addition, contributes to low hysteresis, higher charge carrier mobility, long lived carrier lifetimes and a champion stabilized power output of 20.3 using SnOx as the electron selective contact. We study the effects of these impurities on the transient behavior that defines hysteresis and its relation to ionic movement. In addition, we find that the formation of a RbPbI3 phase does not significantly affect the charge carrier lifetimes and consequently the performance of the devices. This brings new physical insights onto the role of different impurities in perovskite solar cells, which make these materials so remarkabl