450 research outputs found
Dynamics of ripple formation on silicon surfaces by ultrashort laser pulses in sub-ablation conditions
An investigation of ultrashort pulsed laser induced surface modification due
to conditions that result in a superheated melted liquid layer and material
evaporation are considered. To describe the surface modification occurring
after cooling and resolidification of the melted layer and understand the
underlying physical fundamental mechanisms, a unified model is presented to
account for crater and subwavelength ripple formation based on a synergy of
electron excitation and capillary waves solidification. The proposed
theoretical framework aims to address the laser-material interaction in
sub-ablation conditions and thus minimal mass removal in combination with a
hydrodynamics-based scenario of the crater creation and ripple formation
following surface irradiation with single and multiple pulses, respectively.
The development of the periodic structures is attributed to the interference of
the incident wave with a surface plasmon wave. Details of the surface
morphology attained are elaborated as a function of the imposed conditions and
results are tested against experimental data
Designing for comfort in shared and automated vehicles (SAV): a conceptual framework
To date, automotive design and research is heavily biased towards the driver. However, with the rapid advance of vehicle automation, the driving task will increasingly being taken over by a machine. Automation by itself, however, will not be able to tackle the transport challenges we are facing and the need for shared mobility is now widely recognized. Future mobility solutions are therefore expected to consist of Shared and Automated Vehicles (SAV). This means that the passenger experience will take center stage in the design of future road vehicles. Whereas at first sight this may not appear to be different to the experience in other modes of transport, automation and shared mobility introduce different psychological, physical and physiological challenges. These are related to the fact that the occupant is no longer in control, has to put his or her life in the hands of a computer, while at the same time expects such future vehicles to render travel time more efficient or pleasurable and engage in so-called non-driving related tasks. Taking inspiration from work conducted in the field of aircraft passenger comfort experience, we discuss major comfort factors in the context of SAV and highlight both similarities and differences between transport modes. We present a human centered design framework to assist both the research agenda and the development of safe, usable, comfortable, and desirable future mobility solutions
Ultrafast dynamics of coherent optical phonons and nonequilibrium electrons in transition metals
The femtosecond optical pump-probe technique was used to study dynamics of
photoexcited electrons and coherent optical phonons in transition metals Zn and
Cd as a function of temperature and excitation level. The optical response in
time domain is well fitted by linear combination of a damped harmonic
oscillation because of excitation of coherent phonon and a
subpicosecond transient response due to electron-phonon thermalization. The
electron-phonon thermalization time monotonically increases with temperature,
consistent with the thermomodulation scenario, where at high temperatures the
system can be well explained by the two-temperature model, while below
50 K the nonthermal electron model needs to be applied. As the
lattice temperature increases, the damping of the coherent phonon
increases, while the amplitudes of both fast electronic response and the
coherent phonon decrease. The temperature dependence of the damping of
the phonon indicates that population decay of the coherent optical
phonon due to anharmonic phonon-phonon coupling dominates the decay process. We
present a model that accounts for the observed temperature dependence of the
amplitude assuming the photoinduced absorption mechanism, where the signal
amplitude is proportional to the photoinduced change in the quasiparticle
density. The result that the amplitude of the phonon follows the
temperature dependence of the amplitude of the fast electronic transient
indicates that under the resonant condition both electronic and phononic
responses are proportional to the change in the dielectric function.Comment: 10 pages, 9 figures, to appear in Physical Review
Intrinsic response time of graphene photodetectors
Graphene-based photodetectors are promising new devices for high-speed
optoelectronic applications. However, despite recent efforts, it is not clear
what determines the ultimate speed limit of these devices. Here, we present
measurements of the intrinsic response time of metal-graphene-metal
photodetectors with monolayer graphene using an optical correlation technique
with ultrashort laser pulses. We obtain a response time of 2.1 ps that is
mainly given by the short lifetime of the photogenerated carriers. This time
translates into a bandwidth of ~262 GHz. Moreover, we investigate the
dependence of the response time on gate voltage and illumination laser power
Frequency Characteristics of Visually Induced Motion Sickness
This article was published in the journal, Human Factors [Sage Publications / © Human Factors and Ergonomics Society.]. The definitive version is available at: http://dx.doi.org/10.1177/0018720812469046Objective: The aim of this study was to explore
the frequency response of visually induced motion
sickness (VIMS) for oscillating linear motion in the foreand-
aft axis.
Background: Simulators, virtual environments,
and commercially available video games that create an
illusion of self-motion are often reported to induce
the symptoms seen in response to true motion. Often
this human response can be the limiting factor in the
acceptability and usability of such systems. Whereas
motion sickness in physically moving environments
is known to peak at an oscillation frequency around
0.2 Hz, it has recently been suggested that VIMS peaks
at around 0.06 Hz following the proposal that the
summed response of the visual and vestibular selfmotion
systems is maximized at this frequency. Methods: We exposed 24 participants to random
dot optical flow patterns simulating oscillating foreand-
aft motion within the frequency range of 0.025 to
1.6 Hz. Before and after each 20-min exposure, VIMS was
assessed with the Simulator Sickness Questionnaire.
Also, a standard motion sickness scale was used to rate
symptoms at 1-min intervals during each trial.
Results: VIMS peaked between 0.2 and 0.4 Hz with
a reducing effect at lower and higher frequencies.
Conclusion: The numerical prediction of the
âcrossover frequencyâ hypothesis, and the design
guidance curve previously proposed, cannot be accepted
when the symptoms are purely visually induced.
Application: In conditions in which stationary
observers are exposed to optical flow that simulates
oscillating fore-and-aft motion, frequencies around 0.2
to 0.4 Hz should be avoided
Optimizing sowing density-based management decisions with different nitrogen rates on smallholder maize farms in northern Nigeria
Open Access Article; Published online: 18 Jan 2021In this study, the CERES-Maize model was calibrated and evaluated using data from 60 farmersâ fields across Sudan (SS) and Northern Guinea (NGS) Savannas of Nigeria in 2016 and 2017 rainy seasons. The trials consisted of 10 maize varieties sown at three different sowing densities (2.6, 5.3, and 6.6 plants mâ2) across farmersâ field with contrasting agronomic and nutrient management histories. Model predictions in both years and locations were close to observed data for both calibration and evaluation exercises as evidenced by low normalized root mean square error (RMSE) (â€15%), high modified d-index (> 0.6), and high model efficiency (>0.45) values for the phenology, growth, and yield data across all varieties and agro-ecologies. In both years and locations and for both calibration and evaluation exercises, very good agreements were found between observed and model-simulated grain yields, number of days to physiological maturity, above-ground biomass, and harvest index. Two separate scenario analyses were conducted using the long-term (26 years) weather records for Bunkure (representing the SS) and Zaria (representing the NGS). The early and extra-early varieties were used in the SS while the intermediate and late varieties were used in the NGS. The result of the scenario analyses showed that early and extra-early varieties grown in the SS responds to increased sowing density up to 8.8 plants mâ2 when the recommended rate of N fertilizers (90 kg N haâ1) was applied. In the NGS, yield responses were observed up to a density of 6.6 plants mâ2 with the application of 120 kg N haâ1 for the intermediate and late varieties. The highest mean monetary returns to land (US957.7 haâ1) was simulated for scenarios with 6.6 plants mâ2 and 90 Kg N haâ1 in the SS. In the NGS, monetary return per hectare was highest with a planting density of 6.6 plants mâ2 with the application of 120 kg N, while the return to labor was highest for sowing density of 5.3 plants mâ2 at the same N fertilizer application rates. The results of the long-term simulations predicted increases in yield and economic returns to land and labor by increasing sowing densities in the maize belts of Nigeria without applying N fertilizers above the recommended rates
Homodyne detection for measuring internal quantum correlations of optical pulses
A new method is described for determining the quantum correlations at
different times in optical pulses by using balanced homodyne detection. The
signal pulse and sequences of ultrashort test pulses are superimposed, where
for chosen distances between the test pulses their relative phases and
intensities are varied from measurement to measurement. The correlation
statistics of the signal pulse is obtained from the time-integrated difference
photocurrents measured.Comment: 7 pages, A4.sty include
Photon wave functions, wave-packet quantization of light, and coherence theory
The monochromatic Dirac and polychromatic Titulaer-Glauber quantized field
theories (QFTs) of electromagnetism are derived from a photon-energy wave
function in much the same way that one derives QFT for electrons, that is, by
quantization of a single-particle wave function. The photon wave function and
its equation of motion are established from the Einstein energy-momentum-mass
relation, assuming a local energy density. This yields a theory of photon wave
mechanics (PWM). The proper Lorentz-invariant single-photon scalar product is
found to be non-local in coordinate space, and is shown to correspond to
orthogonalization of the Titulaer-Glauber wave-packet modes. The wave functions
of PWM and mode functions of QFT are shown to be equivalent, evolving via
identical equations of motion, and completely describe photonic states. We
generalize PWM to two or more photons, and show how to switch between the PWM
and QFT viewpoints. The second-order coherence tensors of classical coherence
theory and the two-photon wave functions are shown to propagate equivalently.
We give examples of beam-like states, which can be used as photon wave
functions in PWM, or modes in QFT. We propose a practical mode converter based
on spectral filtering to convert between wave packets and their corresponding
biorthogonal dual wave packets.Comment: 34 pages, 3 figures, minor correction
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