20,279 research outputs found
Shot noise limited characterization of femtosecond light pulses
Probing the evolution of physical systems at the femto- or attosecond
timescale with light requires accurate characterization of ultrashort optical
pulses. The time profiles of such pulses are usually retrieved by methods
utilizing optical nonlinearities, which require significant signal powers and
operate in a limited spectral
range\cite{Trebino_Review_of_Scientific_Instruments97,Walmsley_Review_09}. We
present a linear self-referencing characterization technique based on time
domain localization of the pulse spectral components, operated in the
single-photon regime. Accurate timing of the spectral slices is achieved with
standard single photon detectors, rendering the technique applicable in any
spectral range from near infrared to deep UV. Using detection electronics with
about ps response, we retrieve the temporal profile of a picowatt pulse
train with fs resolution, setting a new scale of sensitivity in
ultrashort pulse characterization.Comment: Supplementary information contained in raw dat
ORIGAMIX, a CdTe-based spectro-imager development for nuclear applications
The Astrophysics Division of CEA Saclay has a long history in the development
of CdTe based pixelated detection planes for X and gamma-ray astronomy, with
time-resolved imaging and spectrometric capabilities. The last generation,
named Caliste HD, is an all-in-one modular instrument that fulfills
requirements for space applications. Its full-custom front-end electronics is
designed to work over a large energy range from 2 keV to 1 MeV with excellent
spectroscopic performances, in particular between 10 and 100 keV (0.56 keV FWHM
and 0.67 keV FWHM at 13.9 and 59.5 keV). In the frame of the ORIGAMIX project,
a consortium based on research laboratories and industrials has been settled in
order to develop a new generation of gamma camera. The aim is to develop a
system based on the Caliste architecture for post-accidental interventions or
homeland security, but integrating new properties (advanced spectrometry,
hybrid working mode) and suitable for industry. A first prototype was designed
and tested to acquire feedback for further developments. In this study, we
particularly focused on spectrometric performances with high energies and high
fluxes. Therefore, our device was exposed to energies up to 700 keV (133Ba,
137Cs) and we measured the evolution of energy resolution (0.96 keV at 80 keV,
2.18 keV at 356 keV, 3.33 keV at 662 keV). Detection efficiency decreases after
150 keV, as Compton effect becomes dominant. However, CALISTE is also designed
to handle multiple events, enabling Compton scattering reconstruction, which
can drastically improve detection efficiencies and dynamic range for higher
energies up to 1408 keV (22Na, 60Co, 152Eu) within a 1-mm thick detector. In
particular, such spectrometric performances obtained with 152Eu and 60Co were
never measured before with this kind of detector.Comment: Nuclear Instruments and Methods in Physics Research Section A:
Accelerators, Spectrometers, Detectors and Associated Equipment. Available
online 9 January 2015, ISSN 0168-9002
(http://www.sciencedirect.com/science/article/pii/S0168900215000133).
Keywords: CdTe; X-ray; Gamma-ray; Spectrometry; Charge-sharing; Astrophysics
Instrumentation; Nuclear Instrumentation; Gamma-ray camera
Quantum Nature of Plasmon-Enhanced Raman Scattering
We report plasmon-enhanced Raman scattering in graphene coupled to a single
plasmonic hotspot measured as a function of laser energy. The enhancement
profiles of the G peak show strong enhancement (up to ) and narrow
resonances (30 meV) that are induced by the localized surface plasmon of a gold
nanodimer. We observe the evolution of defect-mode scattering in a defect-free
graphene lattice in resonance with the plasmon. We propose a quantum theory of
plasmon-enhanced Raman scattering, where the plasmon forms an integral part of
the excitation process. Quantum interferences between scattering channels
explain the experimentally observed resonance profiles, in particular, the
marked difference in enhancement factors for incoming and outgoing resonance
and the appearance of the defect-type modes.Comment: Keywords: plasmon-enhanced Raman scattering, SERS, graphene, quantum
interferences, microscopic theory of Raman scattering. Content: 22 pages
including 5 figures + 11 pages supporting informatio
Confocal and multiphoton imaging of intracellular Ca<sup>2+</sup>
This chapter compares the imaging capabilities of a range of systems including multiphoton microscopy in regard to measurements of intracellular Ca<sup>2+</sup> within living cells. In particular, the excitation spectra of popular fluorescent Ca<sup>2+</sup> indicators are shown during 1P and 2P excitation. The strengths and limitations of the current indicators are discussed along with error analysis which highlights the value of matching the Ca<sup>2+</sup> affinity of the dye to a particular aspect of Ca<sup>2+</sup> signaling. Finally, the combined emission spectra of Ca<sup>2+</sup> and voltage sensitive dyes are compared to allow the choice of the optimum combination to allow simultaneous intracellular Ca<sup>2+</sup> and membrane voltage measurement
Observation of a single atom in a magneto-optical trap
Fluorescence from Cs atoms in a magneto-optical trap is detected under conditions of very low atomic density. Discrete steps are observed in the fluorescent signal versus time and are associated with the arrival and departure of individual trapped atoms. Histograms of the frequency of occurrence of a given level of fluorescence exhibit a series of uniformly spaced peaks that are attributed to the presence of N = 0, 1, 2 atoms in the trap
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