2,011 research outputs found
Carrier dynamics in ion-implanted GaAs studied by simulation and observation of terahertz emission
We have studied terahertz (THz) emission from arsenic-ion implanted GaAs both
experimentally and using a three-dimensional carrier dynamics simulation. A
uniform density of vacancies was formed over the optical absorption depth of
bulk GaAs samples by performing multi-energy implantations of arsenic ions (1
and 2.4MeV) and subsequent thermal annealing. In a series of THz emission
experiments the frequency of peak THz power was found to increase significantly
from 1.4 to 2.2THz when the ion implantation dose was increased from 10^13 to
10^16 cm-3. We used a semi-classical Monte-Carlo simulation of ultra-fast
carrier dynamics to reproduce and explain these results. The effect of the
ion-induced damage was included in the simulation by considering carrier
scattering at neutral and charged impurities, as well as carrier trapping at
defect sites. Higher vacancy concentrations and shorter carrier trapping times
both contributed to shorter simulated THz pulses, the latter being more
important over experimentally realistic parameter ranges.Comment: 6 pages, 7 figure
Three-dimensional carrier-dynamics simulation of terahertz emission from photoconductive switches
A semi-classical Monte Carlo model for studying three-dimensional carrier
dynamics in photoconductive switches is presented. The model was used to
simulate the process of photoexcitation in GaAs-based photoconductive antennas
illuminated with pulses typical of mode-locked Ti:Sapphire lasers. We analyzed
the power and frequency bandwidth of THz radiation emitted from these devices
as a function of bias voltage, pump pulse duration and pump pulse location. We
show that the mechanisms limiting the THz power emitted from photoconductive
switches fall into two regimes: when illuminated with short duration (<40 fs)
laser pulses the energy distribution of the Gaussian pulses constrains the
emitted power, while for long (>40 fs) pulses, screening is the primary
power-limiting mechanism. A discussion of the dynamics of bias field screening
in the gap region is presented. The emitted terahertz power was found to be
enhanced when the exciting laser pulse was in close proximity to the anode of
the photoconductive emitter, in agreement with experimental results. We show
that this enhancement arises from the electric field distribution within the
emitter combined with a difference in the mobilities of electrons and holes.Comment: 7 pages, 7 figure
Charge trapping in polymer transistors probed by terahertz spectroscopy and scanning probe potentiometry
Terahertz time-domain spectroscopy and scanning probe potentiometry were used
to investigate charge trapping in polymer field-effect transistors fabricated
on a silicon gate. The hole density in the transistor channel was determined
from the reduction in the transmitted terahertz radiation under an applied gate
voltage. Prolonged device operation creates an exponential decay in the
differential terahertz transmission, compatible with an increase in the density
of trapped holes in the polymer channel. Taken in combination with scanning
probe potentionmetry measurements, these results indicate that device
degradation is largely a consequence of hole trapping, rather than of changes
to the mobility of free holes in the polymer.Comment: 4 pages, 3 figure
Polarisation-sensitive terahertz detection by multicontact photoconductive receivers
We have developed a terahertz radiation detector that measures both the
amplitude and polarization of the electric field as a function of time. The
device is a three-contact photoconductive receiver designed so that two
orthogonal electric-field components of an arbitrary polarized electromagnetic
wave may be detected simultaneously. The detector was fabricated on Fe+
ion-implanted InP. Polarization-sensitive detection is demonstrated with an
extinction ratio better than 100:1. This type of device will have immediate
application in studies of birefringent and optically active materials in the
far-infrared region of the spectrum.Comment: 3 pages, 3 figure
Spectral fluctuations of Schr\"odinger operators generated by critical points of the potential
Starting from the spectrum of Schr\"odinger operators on , we
propose a method to detect critical points of the potential. We argue
semi-classically on the basis of a mathematically rigorous version of
Gutzwiller's trace formula which expresses spectral statistics in term of
classical orbits. A critical point of the potential with zero momentum is an
equilibrium of the flow and generates certain singularities in the spectrum.
Via sharp spectral estimates, this fluctuation indicates the presence of a
critical point and allows to reconstruct partially the local shape of the
potential. Some generalizations of this approach are also proposed.\medskip
keywords : Semi-classical analysis; Schr\"odinger operators; Equilibriums in
classical mechanics.Comment: 18 pages, Final versio
The Diabolo photometer and the future of ground-based millimetric bolometer devices
The millimetric atmospheric windows at 1 and 2 mm are interesting targets for
cosmological studies. Two broad areas appear leading this field: 1) the search
for high redshift star-forming galaxies and 2) the measurement of
Sunyaev-Zel'dovich (SZ) effect in clusters of galaxies at all redshifts. The
Diabolo photometer is a dual-channel photometer working at 1.2 and 2.1 mm and
dedicated to high angular resolution measurements of the Sunyaev--Zel'dovich
effect towards distant clusters. It uses 2 by 3 bolometers cooled down to 0.1 K
with a compact open dilution cryostat. The high resolution is provided by the
IRAM 30 m telescope. The result of several Winter campaigns are reported here,
including the first millimetric map of the SZ effect that was obtained by
Pointecouteau et al. (2001) on RXJ1347-1145, the non-detection of a millimetric
counterpart to the radio decrement towards PC1643+4631 and 2 mm number count
upper limits. We discuss limitations in ground-based single-dish millimetre
observations, namely sky noise and the number of detectors. We advocate the use
of fully sampled arrays of (100 to 1000) bolometers as a big step forward in
the millimetre continuum science. Efforts in France are briefly mentionned.Comment: 7 pages, 6 figures, to appear in the Proceedings of the 2K1BC
``Experimental Astronomy at millimeter wavelengths'', Breuil-Cervinia (AO)
Italy - July 9 - 13, 2001, Eds. M. De Petris et a
Influence of surface passivation on ultrafast carrier dynamics and terahertz radiation generation in GaAs
The carrier dynamics of photoexcited electrons in the vicinity of the surface
of (NH4)2S-passivated GaAs were studied via terahertz (THz) emission
spectroscopy and optical-pump THz-probe spectroscopy. THz emission spectroscopy
measurements, coupled with Monte Carlo simulations of THz emission, revealed
that the surface electric field of GaAs reverses after passivation. The
conductivity of photoexcited electrons was determined via optical-pump
THz-probe spectroscopy, and was found to double after passivation. These
experiments demonstrate that passivation significantly reduces the surface
state density and surface recombination velocity of GaAs. Finally, we have
demonstrated that passivation leads to an enhancement in the power radiated by
photoconductive switch THz emitters, thereby showing the important influence of
surface chemistry on the performance of ultrafast THz photonic devices.Comment: 4 pages, 3 figures, to appear in Applied Physics Letter
Factors associated with repetitive violent behavior of psychiatric inpatients.
A small number of psychiatric inpatients displays a large proportion of Violent Behaviors (VB). These can have a major impact on both victims and patients themselves. This study explored personal, situational and institutional risk factors and their combined effects, which could lead to repetitive VB (three or more assaults). Data from 4518 patients, aged 18 to 65, admitted to an acute psychiatric care facility, were included in the analysis. VB, defined as physical aggressions against another person, were assessed by the Staff Observation Aggression Scale-Revised. 414 VB were reported during the study period, involving 199 patients. 0.75 % of all patients were repetitively violent and committed 43% of all VB. Factors that were linked to repetitive VB were living in sheltered housing before hospitalization, suffering from schizophrenia with substance abuse comorbidity, cumulating hospitalization days and some situational factors, like the fact of being in nursing offices and pharmacies. When all personal, situational and institutional factors were considered together, the combined effects of length of stay and living in sheltered housing increased the risk of repetitive VB. We have identified a small group of vulnerable patients for whom new modalities of inter-institutional networking should be developed to prevent repetitive VB
Evolution of dopant-induced helium nanoplasmas
Two-component nanoplasmas generated by strong-field ionization of doped
helium nanodroplets are studied in a pump-probe experiment using few-cycle
laser pulses in combination with molecular dynamics simulations. High yields of
helium ions and a pronounced, droplet size-dependent resonance structure in the
pump-probe transients reveal the evolution of the dopant-induced helium
nanoplasma. The pump-probe dynamics is interpreted in terms of strong inner
ionization by the pump pulse and resonant heating by the probe pulse which
controls the final charge states detected via the frustration of electron-ion
recombination
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