157 research outputs found
Generation of microwave radiation by nonlinear interaction of a high-power, high-repetition rate, 1064-nm laser in KTP crystals
We report measurements of microwave (RF) generation in the centimeter band
accomplished by irradiating a nonlinear KTiOPO (KTP) crystal with a
home-made, infrared laser at nm as a result of optical rectification
(OR). The laser delivers pulse trains of duration up to s. Each train
consists of several high-intensity pulses at an adjustable repetition rate of
approximately GHz. The duration of the generated RF pulses is
determined by that of the pulse trains. We have investigated both microwave-
and second harmonic (SHG) generation as a function of the laser intensity and
of the orientation of the laser polarization with respect to the
crystallographic axes of KTP.Comment: 5 pages, 5 figures, to appear in Optics Letters, vol. 38 (2013
Microwave emission by nonlinear crystals irradiated with a high-intensity, mode-locked laser
We report on the experimental investigation of the efficiency of some
nonlinear crystals to generate microwave (RF) radiation as a result of optical
rectification (OR) when irradiated with intense pulse trains delivered by a
mode-locked laser at nm. We have investigated lithium triborate (LBO),
lithium niobate (LiNbO), zinc selenide (ZnSe), and also potassium titanyl
orthophosphate (KTP) for comparison with previous measurements. The results are
in good agreement with the theoretical predictions based on the form of the
second-order nonlinear susceptibility tensor. For some crystals we investigated
also the second harmonic generation (SHG) to cross check the theoretical model.
We confirm the theoretical prediction that OR leads to the production of higher
order RF harmonics that are overtones of the laser repetition rate.Comment: accepted for publication in Journal of Optics, in pres
Cathodo- and radioluminescence of Tm:YAG and Nd:YAG in an extended wavelength range
We have studied the cathodo- and radioluminescence of Nd:YAG and of Tm:YAG
single crystals in an extended wavelength range up to m in view
of developing a new kind of detector for low-energy, low-rate energy deposition
events. Whereas the light yield in the visible range is as large as photons/MeV, in good agreement with literature results, in the
infrared range we have found a light yield photons/MeV, thereby proving that ionizing radiation is particularly
efficient in populating the low lying levels of rare earth doped crystals.Comment: submitted for publication in Journal of Luminescenc
Generation of microwave fields in cavities with laser-excited nonlinear media: competition between the second- and third-order optical nonlinearities
We discuss a scheme for the parametric amplification of the quantum fluctuations of the
electromagnetic vacuum in a three-dimensional microwave resonator, and report the preliminary
measurements to test its feasibility. In the present experimental scheme, the fundamental mode of
a microwave cavity is nonadiabatically perturbed by modulating the index of refraction of the
nonlinear optical crystal enclosed therein. Intense, multi-GHz laser pulses, such as those
delivered by a mode-locked laser source, impinge on the crystal to accomplish the n-index
modulation. We theoretically analyze the process of parametric generation, which is related to
the third-order nonlinear coefficient \u3c7(3) of the nonlinear crystal, and assess the suitable
experimental conditions for generating real photons from the vacuum. Second-order nonlinear
processes are first analyzed as a possible source of spurious photons in quantum vacuum
experiments when an ideal, mode-locked laser source is considered. The combination of a crystal
non-null \u3c7(2) coefficient and a real mode-locked laser system\u2014i.e. one featuring offset-fromcarrier
noise and unwanted secondary oscillations\u2014is also experimentally investigated, paving
the way for future experiments in three-dimensional cavities
A new technique for infrared scintillation measurements
We propose a new technique to measure the infrared scintillation light yield
of rare earth (RE) doped crystals by comparing it to near UV-visible
scintillation of a calibrated Pr:(LuY)AlO
sample. As an example, we apply this technique to provide the light yield in
visible and infrared range up to \SI{1700}{nm} of this crystal.Comment: submitted to NIM
Injection of photoelectrons into dense argon gas
The injection of photoelectrons in a gaseous or liquid sample is a widespread
technique to produce a cold plasma in a weakly--ionized system in order to
study the transport properties of electrons in a dense gas or liquid. We report
here the experimental results of photoelectron injection into dense argon gas
at the temperatureT=142.6 K as a function of the externally applied electric
field and gas density. We show that the experimental data can be interpreted in
terms of the so called Young-Bradbury model only if multiple scattering effects
due to the dense environment are taken into account when computing the
scattering properties and the energetics of the electrons.Comment: 18 pages, 10 figures, figure nr. 10 has been redrawn, to be submitted
to Plasma Sources Science and Technolog
Particle detection through the quantum counter concept in YAG:Er
We report about a novel scheme for particle detection based on the infrared
quantum counter concept. Its operation consists of a two-step excitation
process of a four level system, that can be realized in rare earth-doped
crystals when a cw pump laser is tuned to the transition from the second to the
fourth level. The incident particle raises the atoms of the active material
into a low lying, metastable energy state, triggering the absorption of the
pump laser to a higher level. Following a rapid non-radiative decay to a
fluorescent level, an optical signal is observed with a conventional detectors.
In order to demonstrate the feasibility of such a scheme, we have investigated
the emission from the fluorescent level S (540 nm band) in an
Er-doped YAG crystal pumped by a tunable titanium sapphire laser when it
is irradiated with 60 keV electrons delivered by an electron gun. We have
obtained a clear signature this excitation increases the
metastable level population that can efficiently be exploited to generate a
detectable optical signal
Infrared emission spectrum and potentials of and states of Xe excimers produced by electron impact
We present an investigation of the Xe excimer emission spectrum
observed in the near infrared range about 7800 cm in pure Xe gas and in
an Ar (90%) --Xe (10%) mixture and obtained by exciting the gas with energetic
electrons. The Franck--Condon simulation of the spectrum shape suggests that
emission stems from a bound--free molecular transition never studied before.
The states involved are assigned as the bound state with atomic limit and the dissociative state with limit. Comparison with the spectrum simulated by using theoretical
potentials shows that the dissociative one does not reproduce correctly the
spectrum features.Comment: 4 pages, 3 figures, submitted to Phys. Rev. Let
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“Looks familiar, but I do not know who she is”:The role of the anterior right temporal lobe in famous face recognition
Processing a famous face involves a cascade of steps including detecting the presence of a face, recognizing it as familiar, accessing semantic/biographical information about the person, and finally, if required, production of the proper name. Decades of neuropsychological and neuroimaging studies have identified a network of occipital and temporal brain regions ostensibly comprising the 'core' system for face processing. Recent research has also begun to elucidate upon an 'extended' network, including anterior temporal and frontal regions. However, there is disagreement about which brain areas are involved in each step, as many aspects of face processing occur automatically in healthy individuals and rarely dissociate in patients. Moreover, some common phenomena are not easily induced in an experimental setting, such as having a sense of familiarity without being able to recall who the person is. Patients with the semantic variant of Primary Progressive Aphasia (svPPA) often recognize a famous face as familiar, even when they cannot specifically recall the proper name or biographical details. In this study, we analyzed data from a large sample of 105 patients with neurodegenerative disorders, including 43 svPPA, to identify the neuroanatomical substrates of three different steps of famous face processing. Using voxel-based morphometry, we correlated whole-brain grey matter volumes with scores on three experimental tasks that targeted familiarity judgment, semantic/biographical information retrieval, and naming. Performance in naming and semantic association significantly correlates with grey matter volume in the left anterior temporal lobe, whereas familiarity judgment with integrity of the right anterior middle temporal gyrus. These findings shed light on the neuroanatomical substrates of key components of overt face processing, addressing issues of functional lateralization, and deepening our understanding of neural substrates of semantic knowledge
Studies of a three-stage dark matter and neutrino observatory based on multi-ton combinations of liquid xenon and liquid argon detectors
We study a three stage dark matter and neutrino observatory based on
multi-ton two-phase liquid Xe and Ar detectors with sufficiently low
backgrounds to be sensitive to WIMP dark matter interaction cross sections down
to 10E-47 cm^2, and to provide both identification and two independent
measurements of the WIMP mass through the use of the two target elements in a
5:1 mass ratio, giving an expected similarity of event numbers. The same
detection systems will also allow measurement of the pp solar neutrino
spectrum, the neutrino flux and temperature from a Galactic supernova, and
neutrinoless double beta decay of 136Xe to the lifetime level of 10E27 - 10E28
y corresponding to the Majorana mass predicted from current neutrino
oscillation data. The proposed scheme would be operated in three stages G2, G3,
G4, beginning with fiducial masses 1-ton Xe + 5-ton Ar (G2), progressing to
10-ton Xe + 50-ton Ar (G3) then, dependent on results and performance of the
latter, expandable to 100-ton Xe + 500-ton Ar (G4). This method of scale-up
offers the advantage of utilizing the Ar vessel and ancillary systems of one
stage for the Xe detector of the succeeding stage, requiring only one new
detector vessel at each stage. Simulations show the feasibility of reducing or
rejecting all external and internal background levels to a level <1 events per
year for each succeeding mass level, by utilizing an increasing outer thickness
of target material as self-shielding. The system would, with increasing mass
scale, become increasingly sensitive to annual signal modulation, the agreement
of Xe and Ar results confirming the Galactic origin of the signal. Dark matter
sensitivities for spin-dependent and inelastic interactions are also included,
and we conclude with a discussion of possible further gains from the use of
Xe/Ar mixtures
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