1,720,980 research outputs found
Exploiting optical non-linearities for super-resolution label-free optical microscopy
No full textOptical microscopy uniquely provides non-invasive imaging of biological specimens and has
become an essential tool in life sciences. So far, fluorescence microscopy techniques have been
the most widely used due to their ability to visualize the molecules of interest with high
contrast, high specificity, and high spatial and temporal resolution. In the last decades, much
effort was put into the development of advanced super-resolution fluorescence microscopy
techniques to circumvent the diffraction limit and image with unprecedented spatial
resolution. The non-linearities contained in saturation phenomena were exploited for this
purpose, and led to the development of techniques like stimulated emission depletion (STED)
and saturated excitation (SAX) microscopy, which proved to have the potentiality to really
break the diffraction barrier and tune the resolution down to infinitely small focal volumes
(Chapter 1). Despite their well-established benefits, (super-resolution) fluorescence
microscopy techniques rely on the photophysical properties of fluorescent molecules to obtain
the desired contrast and resolution, and the labeling procedures may alter the physical
properties of the specimen and may come at the cost of photobleaching and photodamage
effects.
With the development of ultrashort pulsed laser sources, new types of non-linear optical
interactions became accessible (Chapter 1) and started to acquire a central role in optical
microscopy for label-free imaging, providing novel non-fluorescence-based contrast
mechanisms which purely rely on intrinsic properties of the molecules of interest. In this way,
the invasiveness and the phototoxicity can be reduced, and the degradation of the fluorescence
signal due to photobleaching can be avoided. Moreover, non-linear optical microscopy
techniques allow for three-dimensional imaging due to the intrinsic optical sectioning
capabilities of non-linear phenomena, while the use of longer wavelengths in the near-infrared
part of the spectrum results in lower absorption and scattering and permits to image deeper
inside tissues (Chapter 1). The main drawback of these non-linear optical microscopy
techniques is their relatively poor spatial resolution, especially when using longer excitation
wavelengths, and the fact that the imaging of non-fluorescent species with sub-diffraction
resolution is still a challenging task.
In this framework, this work aims at extending the super-resolution approaches to
label-free microscopy techniques, based on the fact that, in principle, any saturable optical
process between molecular states, not necessarily involving fluorescent transitions, is a
potential candidate for breaking the diffraction barrier.
In particular, non-linear optical processes are here exploited in near-infrared pump-probe
microscopy techniques, such as transient absorption microscopy (TAM) and stimulated Raman scattering (SRS) microscopy (Chapter 3). In these techniques, two synchronized femto/picosecond
pulsed laser beams are used to investigate ultrafast electronic and vibrational
dynamical properties of the sample with high spatial and temporal resolution, high sensitivity
and high chemical specificity. The interaction with the sample is recorded as an intensity
variation of one of the two beams, which is extracted from the background adding a fast
intensity modulation and filtering with a lock-in amplifier. New dynamical and chemical
information can be accessed at the molecular level and in a label-free way, using intrinsic
biomolecules as natural contrast agents.
Absorption-based pump-probe microscopy is here optimized to retrieve structural and
dynamical information from graphene-based samples with high sensitivity, and its
conventional configuration is combined with an additional doughnut-shaped pump beam,
which allows for the reduction of the effective focal volume exploiting transient absorption
saturation. By optimizing the experimental parameters, such as power and temporal overlap of
the saturation beam, single layer graphene deposited on a glass surface can be imaged at the
nanoscale (Chapter 4).
Moreover, the saturation of the vibrational excitation in SRS microscopy is here theoretically
and experimentally assessed, in order to evaluate the applicability of the SAX approach to
achieve isotropic sub-diffraction imaging capabilities (Chapter 5)
Label-Free Pump–Probe Nanoscopy
No full textIn the last few decades fluorescence microscopy has been the most widely used microscopy technique and much effort has been put into the development of advanced super-resolution fluorescence microscopy techniques to circumvent the diffraction limit. Despite their well-established benefits, these techniques have to rely on the photo-physical properties of fluorescent molecules to obtain the desired contrast and spatial resolution. The labeling procedure may cause unwanted alterations in the sample. With the advent of ultrashort-pulsed laser sources, it became possible to better explore novel non-fluorescent-based contrast mechanisms that rely solely on intrinsic properties of the molecules of interest and which led to the development of label-free microscopy approaches. In this chapter, the imaging capabilities of absorption-based pump–probe microscopy are presented. This technique explores the ultrafast dynamic properties of the sample with high spatial and temporal resolution, as well as high sensitivity and chemical specificity. Two pulses, a pump and a probe, with a proper spatial and temporal overlap are used. The pump is absorbed, inducing a measurable change in the sample carrier population, which is then monitored by a delayed probe pulse. The development of new label-free approaches also represents a key challenge for the exploration of super-resolution approaches in non-fluorescence-based methods
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
“The first difficulty is time”: The impact of gestational age limits on reproductive health and justice in the context of cross-border travel for abortion care in Europe
Full text linkDrawing on qualitative and quantitative data collected during a 5-year multi-disciplinary European research project, in this article we show how restrictions on access to legal abortion, and particularly gestational age (GA) limits at the end of the first trimester of pregnancy, negatively affect women and pregnant people living in European countries where abortion is legal on request or on broad grounds. First, we examine why most Eu-ropean legislations establish GA limits, and illustrate how abortion is framed in national laws and in the current national and international legal and political debates on abortion rights. We then show, based on research data we collected during our 5-year project and contextualized with existing data and statistics, how these restrictions force thousands of people to travel across borders from European countries where abortion is legal, delaying access to care, and increasing pregnant people's health risks. Finally, we explore, from an anthropological perspective, how pregnant people who travel across borders for abortion care conceptualize abortion access, and the relationship between the right to abortion care and the GA restrictions that limit this right. Our study par-ticipants criticize the time restrictions established by the laws in their countries of residence as failing to meet pregnant people's needs, highlight the crucial importance of easy, timely access to abortion care even beyond the first trimester of pregnancy, and suggest a more relational approach to the right to access safe, legal abortion. Abortion travel is also a matter of reproductive justice because access to care depends on specific resources including finances, information, support, citizenship status, and social networks. Our work contributes to scholarly and public debates about reproductive governance and justice, by shifting the locus of attention to GA limits and its impact on women and pregnant people, particularly in geopolotical settings where abortion laws are deemed liberal
Variations on the Author
Full text link“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
Appropriate Similarity Measures for Author Cocitation Analysis
Full text linkWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
Label-Free Chromatin-DNA Imaging by Circular Polarized Light Scattering Scanning Microscopy
No full textCircular Dichroism (CD) resolved scanning microscopy has proven to be an interesting label free technique to study chiral biological systems. This technique is based on the measurement of the difference of intensity between circular left and right polarization states after interaction with the sample. It was demonstrated in early works that far from the absorption bands, the CD signal is attributed only to scattering effects and is thus referred to as Circular Intensity Differential Scattering (CIDS), which is sensitive to the higher-order chiral conformation of biopolymers.We have developed a scanning microscope based on the measurements of the CIDS signal, using a 50 kHz Photoelastic Modulator (PEM) and two channels detection via a lock-in amplifier. In this way, a differential image extracted from the Mueller Matrix formalism gives access to information at the single molecular level using a high-numerical aperture objective (Nikon 100X oil objective) in a time compatible with the scan time (a few seconds for a complete 512x512 image). As a proof of principle of the technique, the CIDS configuration has been coupled with a modified confocal scanning microscope allowing a multimodal acquisition of the CIDS and the fluorescence contrasts simultaneously. We have studied the chromatin-DNA organization of fixed isolated Hek-cell nuclei, which has shown our capability to distinguish the different order of compaction in DNA, i.e. the more compacted heterochromatin areas from the euchromatin areas
Label-Free Super-Resolution Microscopy By Means of Transient Absorption Saturation
No full textn recent decades, super-resolution optical microscopy has become an attractive research topic that typically has been developed based on fluorescent labelling that becomes the essential tool for breaking diffraction-limited in biological imaging. However, deep tissue and label-free imaging remain challenging, especially for tick and highly scattering biological specimens [1]. With the development of ultrashort pulse laser sources, non-linear optical (NLO) light-matter interaction has acquired a central role in optical microscopy for label-free imaging due to efficient signal generation in the non-linear process [2]. The advantages of NLO approaches include the reduction of scattering due to the use of near-infrared radiation, which enables high penetration depth and reduces the aberrations introduced by the specimen [1]. Non-linear interaction comprises the generation of sum frequencies, high-harmonic generation ..
Barriers and delays in access to abortion care: a cross-sectional study of people traveling to obtain care in England and the Netherlands from European countries where abortion is legal on broad grounds
No full textIntroduction This study characterized the extent to which (1) financial barriers and (2) abortion care-seeking within a person's country of residence were associated with delays in abortion access among those travelling to England and the Netherlands for abortion care from European countries where abortion is legal on broad grounds in the first trimester but where access past the first trimester is limited to specific circumstances.Methodology We drew on cross-sectional survey data collected at five abortion clinics in England and the Netherlands from 2017 to 2019 (n = 164). We assessed the relationship between difficulty paying for the abortion/travel, acute financial insecurity, and in-country care seeking on delays to abortion using multivariable discrete-time hazards models.Results Participants who reported facing both difficulty paying for the abortion procedure and/or travel and difficulty covering basic living costs in the last month reported longer delays in accessing care than those who had no financial difficulty (adjusted hazard odds ratio: 0.39 95% CI 0.21-0.74). This group delayed paying other expenses (39%) or sold something of value (13%) to fund their abortion, resulting in similar to 60% of those with financial difficulty reporting it took them over a week to raise the funds needed for their abortion. Having contacted or visited an abortion provider in the country of residence was associated with delays in presenting abroad for an abortion.Discussion These findings point to inequities in access to timely abortion care based on socioeconomic status. Legal time limits on abortion may intersect with individuals' interactions with the health care system to delay care
- …
