4,145 research outputs found
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
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
Simulation and optimisation of terahertz emission from InGaAs and InP photoconductive switches
We simulate the terahertz emission from laterally-biased InGaAs and InP using
a three-dimensional carrier dynamics model in order to optimise the
semiconductor material. Incident pump-pulse parameters of current Ti:Sapphire
and Er:fibre lasers are chosen, and the simulation models the semiconductor's
bandstructure using parabolic Gamma, L and X valleys, and heavy holes. The
emitted terahertz radiation is propagated within the semiconductor and into
free space using a model based on the Drude-Lorentz dielectric function. As the
InGaAs alloy approaches InAs an increase in the emitted power is observed, and
this is attributed to a greater electron mobility. Additionally,
low-temperature grown and ion-implanted InGaAs are modelled using a finite
carrier trapping time. At sub-picosecond trapping times the terahertz bandwidth
is found to increase significantly at the cost of a reduced emission power.Comment: 9 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
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
The immune environment of the mammary gland fluctuates during post-lactational regression and correlates with tumour growth rate
Post-lactational mammary gland regression encompasses extensive programmed cell death and removal of milk-producing epithelial cells, breakdown of extracellular matrix components and redifferentiation of stromal adipocytes. This highly regulated involution process is associated with a transient increased risk of breast cancer in women. Using a syngeneic tumour model, we show that tumour growth is significantly altered depending on the stage of involution at which tumour cells are implanted. Tumour cells injected at day 3 involution grew faster than those in nulliparous mice, whereas tumours initiated at day 6 involution grew significantly slower. These differences in tumour progression correlate with distinct changes in innate immune cells, in particular among F4/80-expressing macrophages and among TCRδ(+) unconventional T cells. Breast cancer post-pregnancy risk is exacerbated in older first-time mothers and, in our model, initial tumour growth is moderately faster in aged mice compared with young mice. Our results have implications for breast cancer risk and the use of anti-inflammatory therapeutics for postpartum breast cancers
The effect of varying plyometric volume on stretch-shortening cycle capability in collegiate male rugby players
The purpose of this study was to identify the effectiveness of low and high volume plyometric loads on developing stretch shortening cycle capability in collegiate rugby players. A between- group repeated measures design was used. Thirty six subjects (age 20.3 ±1.6 yrs, mass 91.63 ±10.36kg, stature 182.03 ±5.24cm) were randomly assigned to one of three groups, a control group (CG), a low volume plyometric group (LPG) or a high volume plyometric group (HPG). Data were collected from a force plate, and measures of reactive strength index (RSI) and leg stiffness were calculated from jump height, contact time and flight time. A significant between group × time (F = 4.01, P <0.05) interaction effect for RSI was observed. Bonferroni post hoc analysis indicated that both the LPG training group (P = 0.002) and HPG training group (P = 0.009) were significantly higher than the control group. No significant interaction effect between time × group were observed for leg stiffness (F = 1.39, P = 0.25). The current study has demonstrated that it is possible to improve reactive strength capabilities via the use of a low volume plyometric program. The low volume program elicited the same performance improvement in RSI as a high volume program whilst undertaking a lower dose. This suggests that strength and conditioning coaches may be able to benefit from the ability to develop more time efficient and effective plyometric programs
"Until the trial is complete you can't really say whether it helped you or not, can you?": exploring cancer patients' perceptions of taking part in a trial of acupressure wristbands.
BACKGROUND: Nested qualitative studies within clinical trials provide data on patients' experiences of receiving trial interventions and can inform and improve trial designs. The present study explored patients' experiences of participating in a randomised controlled trial of acupressure wristbands for chemotherapy related nausea. METHODS: A randomised three-group sham-controlled trial was carried out to evaluate the effectiveness of acupressure wristbands in the management of chemotherapy-related nausea. A convenience sample of 26 patients volunteered to participate in a qualitative study to explore their experiences of using acupressure wristbands, and taking part in the clinical trial. Participants were recruited from each of the three UK geographical sites from which the trial was conducted: Manchester, Liverpool and Plymouth. In-depth semi-structured interviews were conducted with the participants in their own homes or other location convenient for participating patients. Interviews were audio-taped, transcribed verbatim and analysed using Framework methodology. RESULTS: The main motivational factors influencing participants to take part in the trial were a desire to 'give something back' and limit their own experience of nausea. Participants were largely satisfied with the organisation and running of the acupressure wristband trial. Many participants experienced positive outcomes as a result of taking part in the trial. Lapses in memory, or poor health as a result of their chemotherapy treatment, led to some participants failing to complete trial paperwork on designated days. Two sham wristband participants reported wearing the bands inappropriately resulting in pressure being applied to the acupoint. Almost all of the participants interviewed had only experienced mild nausea or vomiting during the trial. Participants were pragmatic on the extent to which the wristbands were responsible for this lack of nausea and vomiting during the trial. However, many participants, including some patients receiving sham acupressure, believed the wristbands to have had a positive impact on their nausea and vomiting; there was a perception that the wristbands were, at least in part, responsible for the lack of nausea and vomiting they had experienced. CONCLUSIONS: Participants perceive acupressure wristbands as reducing the level of nausea and vomiting experienced during chemotherapy treatment. Reports that some participants wore wristbands inappropriately, and/or delayed completion of trial paperwork could represent confounding variables and have implications for the trial results, and the design of clinical trials within the field of cancer
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