The analysis of fluorophore orientation by multiphoton fluorescence microscopy

The accessibility of tunable, ultrafast laser sources has spurred the development and wide application of specialized microscopy techniques based on chromophore fluorescence following two- and three-photon absorption. The attendant advantages of such methods, which have led to a host of important applications including three-dimensional biological imaging, include some features that have as yet received relatively little attention. In the investigation of cellular or subcellular processes, it is possible to discern not only on the location, concentration, and lifetime of molecular species, but also the orientations of key fluorophores. Detailed information can be secured on the degree of orientational order in specific cellular domains, or the lifetimes associated with the rotational motions of individual fluorophores; both are accessible from polarization-resolved measurements. This paper reports the equations that are required for any such investigation, determined by robust quantum electrodynamical derivation. The general analysis, addressing a system of chromophores oriented in three dimensions, determines the fluorescence signal produced by the nonlinear polarizations that are induced by multiphoton absorption, allowing for any rotational relaxation. The results indicate that multiphoton imaging can be further developed as a diagnostic tool, either to selectively discriminate micro-domains in vivo, or to monitor dynamical changes in intracellular fluorophore orientation

Controlling nanoscale optical emission with off-resonant laser light

In the optical excitation of many nanoscale systems, the primary result of photon absorption is an electronic excitation that is typically followed by ultrafast relaxation processes. The losses associated with such relaxation generally produce a partial degradation of the optical energy acquired, before any ensuing photon emission occurs. Recent work has shown that the intensity and directional character of such emission may be significantly influenced through engagement with a completely off-resonant probe laser beam of sufficient intensity: the mechanism for this optical coupling is a secondorder nonlinearity. It is anticipated that the facility to actively control fluorescent emission in this way may lead to new opportunities in a variety of applications where molecular chromophores or quantum dots are used. In the latter connection it should prove possible to exploit the particle size dependence of the nonlinear optical dispersion, as well as that of the emission wavelength. Specific characteristics of the effect are calculated, and suitable experimental implementations of the mechanism are proposed. We anticipate that this all-optical control device may introduce significant new perspectives to fluorescence imaging techniques and other analytical applications

How do sound waves in a Bose-Einstein condensate move so fast?

Low-momentum excitations of a dilute Bose-Einstein condensate behave as phonons and move at a finite velocity v_s. Yet the atoms making up the phonon excitation each move very slowly; v_a = p/m --> 0. A simple "cartoon picture" is suggested to understand this phenomenon intuitively. It implies a relation v_s/v_a = N_ex, where N_ex is the number of excited atoms making up the phonon. This relation does indeed follow from the standard Bogoliubov theory.Comment: 6 pages, 2 figures (.eps), LaTeX2e. More introductory discussion adde

Resonance energy transfer and interface forces:Quantum electrodynamical analysis

On the propagation of radiation with a suitably resonant optical frequency through a dense chromophoric system - a doped solid for example - photon capture is commonly followed by one or more near-field transfers of the resulting optical excitation, usually to closely neighboring chromophores. Since the process results in a change to the local electronic environment, it can be expected to also shift the electromagnetic interactions between the participant optical units, producing modified inter-particle forces. Significantly, it emerges that energy transfer, when it occurs between chromophores or particles with electronically dissimilar properties (such as differing polarizabilities), engenders hitherto unreported changes in the local potential energy landscape. This paper reports the results of quantum electrodynamical calculations which cast a new light on the physical link between these features. The theory also elucidates a significant relationship with Casimir-Polder forces; it transpires that there are clear and fundamental links between dispersion forces and resonance energy transfer. Based on the results, we highlight specific effects that can be anticipated when laser light propagates through an interface between two absorbing media. Both steady-state and pulsed excitation conditions are modeled and the consequences for interface forces are subjected to detailed analysis

Optomechanical control of molecular motors

The majority of mechanisms that can be deployed for optical micromanipulation are not especially amenable for extension into the nanoscale. At the molecular level, the rich variety of schemes that have been proposed to achieve mechanical effect using light commonly exploit specific chemical structures; familiar examples are compounds that can fold by cis-trans isomerization, or the mechanically interlocked architectures of rotaxanes. However, such systems are synthetically highly challenging, and few of them can realistically form the basis for a true molecular motor. Developing the basis for a very different strategy based on programmed electronic excitation, this paper explores the possibility of producing controlled mechanical motion through optically induced modifications of intermolecular force fields, not involving the limitations associated with using photochemical change, nor the high intensities required to produce and manipulate optical binding forces between molecules. Calculations reveal that significant, rapidly responsive effects can be achieved in relatively simple systems. By the use of suitable laser pulse sequences, the possibilities include the generation of continuous rotary motion, the ultimate aim of molecular motor design

Semiparametric theory and empirical processes in causal inference

In this paper we review important aspects of semiparametric theory and empirical processes that arise in causal inference problems. We begin with a brief introduction to the general problem of causal inference, and go on to discuss estimation and inference for causal effects under semiparametric models, which allow parts of the data-generating process to be unrestricted if they are not of particular interest (i.e., nuisance functions). These models are very useful in causal problems because the outcome process is often complex and difficult to model, and there may only be information available about the treatment process (at best). Semiparametric theory gives a framework for benchmarking efficiency and constructing estimators in such settings. In the second part of the paper we discuss empirical process theory, which provides powerful tools for understanding the asymptotic behavior of semiparametric estimators that depend on flexible nonparametric estimators of nuisance functions. These tools are crucial for incorporating machine learning and other modern methods into causal inference analyses. We conclude by examining related extensions and future directions for work in semiparametric causal inference

Hopping Conduction in Disordered Carbon Nanotubes

We report electrical transport measurements on individual disordered carbon nanotubes, grown catalytically in a nanoporous anodic aluminum oxide template. In both as-grown and annealed types of nanotubes, the low-field conductance shows as exp[-(T_{0}/T)^{1/2}] dependence on temperature T, suggesting that hopping conduction is the dominant transport mechanism, albeit with different disorder-related coefficients T_{0}. The field dependence of low-temperature conductance behaves an exp[-(xi_{0}/xi)^{1/2}] with high electric field xi at sufficiently low T. Finally, both annealed and unannealed nanotubes exhibit weak positive magnetoresistance at low T = 1.7 K. Comparison with theory indicates that our data are best explained by Coulomb-gap variable range hopping conduction and permits the extraction of disorder-dependent localization length and dielectric constant.Comment: 10 pages, 5 figure

Advance telephone calls ahead of reminder questionnaires increase response rate in non-responders compared to questionnaire reminders only : The RECORD phone trial

Peer reviewedPublisher PD

Efficacy of polyacrylamide hydrogel for female urinary incontinence: outcome of a single centre

Introduction: Periurethral injection with polyacrylamide hydrogel (PAHG, Bulkamid®) is a minimally invasive treatment option to be considered for women with stress urinary incontinence. The manufacturer recommends injecting between 1.5 ml and 2 ml periurethrally. This study aims to evaluate the long-term efficacy of PAHG, and to determine whether there is a correlation between the volume of PAHG injected and the outcome in terms of symptoms. Methods: A retrospective study was conducted between 2011 and 2018. Patients were contacted by telephone and the International Consultation on Incontinence Questionnaire – Urinary Incontinence Short Form (ICIQ-UI SF) was used to assess their symptoms. A linear regression analysis test was performed to assess the correlation between the outcome and the volume of PAHG injected. Results: One hundred and fifteen PAHG injections were performed on 101 patients. The volume of PAHG injected ranged from 0.8 ml to 3 ml. Two patients reported procedure-related complications. Of the patients that attended their three-month follow-up, 62 (58.5%) patient-episodes reported an improvement. 62 patients were contacted by telephone and the median length of follow-up was 37.5 months. An improvement in the ICIQ-UI SF score was observed in 45.8% of patients with a mean improvement of 4 points. The volume of PAHG injected did not affect the outcome. 31% also reported a benefit with PAHG five years after their injection following previous incontinence surgery. Conclusions: PAHG injection is safe and improves symptoms of urinary incontinence at up to 7.5 years in 45.8% of women. PAHG is also useful after previous incontinence surgery. The volume of PAHG injected did not influence the outcome

Explosive dome eruptions modulated by periodic gas-driven inflation

Volcan Santiaguito (Guatemala) "breathes" with extraordinary regularity as the edifice's conduit system accumulates free gas, which periodically vents to the atmosphere. Periodic pressurization controls explosion timing, which nearly always occurs at peak inflation, as detected with tiltmeters. Tilt cycles in January 2012 reveal regular 26 ± 6 min inflation/deflation cycles corresponding to at least ~101 kg/s of gas fluxing the system. Very long period (VLP) earthquakes presage explosions and occur during cycles when inflation rates are most rapid. VLPs locate ~300 m below the vent and indicate mobilization of volatiles, which ascend at ~50 m/s. Rapid gas ascent feeds pyroclast-laden eruptions lasting several minutes and rising to ~1 km. VLPs are not observed during less rapid inflation episodes; instead, gas vents passively through the conduit producing no infrasound and no explosion. These observations intimate that steady gas exsolution and accumulation in shallow reservoirs may drive inflation cycles at open-vent silicic volcanoes. Key Points Regular 26 min inflation/deflation cycles are observed at silicic volcanoInflation rates control whether volcano explodes or passively degassesLocation of gas reservoir and flux of gas through the volcano are quantifie