301 research outputs found
Efficient microwave-induced optical frequency conversion
Frequency conversion process is studied in a medium of atoms with a
configuration of levels, where transition between two lower states is driven by
a microwave field. In this system, conversion efficiency can be very high by
virtue of the effect of electromagnetically induced transparency (EIT).
Depending on intensity of the microwave field, two regimes of EIT are realized:
''dark-state'' EIT for the weak field, and Autler-Townes-type EIT for the
strong one. We study both cases via analytical and numerical solution and find
optimum conditions for the conversion.Comment: 15 pages, 5 figure
Generation of continuous-wave THz radiation by use of quantum interference
We propose a scheme for generation of continuous-wave THz radiation. The
scheme requires a medium where three discrete states in a
configuration can be selected, with the THz-frequency transition between the
two lower metastable states. We consider the propagation of three-frequency
continuous-wave electromagnetic (e.m.) radiation through a medium.
Under resonant excitation, the medium absorption can be strongly reduced due to
quantum interference of transitions, while the nonlinear susceptibility is
enhanced. This leads to very efficient energy transfer between the e.m. waves
providing a possibility for THz generation. We demonstrate that the photon
conversion efficiency is approaching unity in this technique.Comment: 18 pages, 4 figure
Efficient Raman Sideband Generation in a Coherent Atomic Medium
We demonstrate the efficient generation of Raman sidebands in a medium
coherently prepared in a dark state by continuous-wave low-intensity laser
radiation. Our experiment is performed in sodium vapor excited in
configuration on the D line by two laser fields of resonant frequencies
and , and probed by a third field .
First-order sidebands for frequencies , and up to the
third-order sidebands for frequency are observed. The generation
starts at a power as low as 10 microwatt for each input field. Dependencies of
the intensities of both input and generated waves on the frequency difference
(), on the frequency and on the optical
density are investigated.Comment: 7 pages, 6 figure
Transverse fatigue behaviour and residual stress analyses of double sided FSW aluminium alloy joints
Friction stir welding (FSW) since its invention has been attracting relevant interest for joining aluminium alloys. Due to the nature of this process, the materials can be joint without melting. Thanks to this peculiar characteristic, the issues associated with the cooling from liquid phase are avoided or considerably reduced, such as cracking, porosity, and defects. However, as well as other well-established welding techniques, the FSW process gives rise to formation of residual stress in the welding region and surrounding volume: heat and thermo-mechanical affected zones. Presence of residual stress in a mechanical component is well-known to affect its performance, particularly regarding fatigue at high number of cycles. Another aspect that influences the fatigue life is the underlying microstructure. In this work, we firstly study the residual stress field and the underlying microstructural features arising in FSW butt joints and their effect on the fatigue performance of this type of weldments. The evaluation of residual stress field is carried out by means of modern experimental techniques. In the first instance, synchrotron X-ray powder diffraction was employed for two-dimensional full field maps of residual stress. Corroboration of these measurements was done by exploiting the capability of focused ion beam and digital image correlation (FIB-DIC), which is able to deliver pointwise absolute measurement of residual stress. A set of FSW samples were then tested under uniaxial fatigue loading at several loading ranges, in the high cycle fatigue regime, in order to understand whether the severity of loads affects the crack path and life endurance. Fractographic and electron backscattered diffraction (EBSD) analysis then revealed crack nucleation site and propagation mechanisms with the respect of the underlying microstructure. Outcome of these experimental studies is then thoroughly discussed
Phase correlation of laser waves with arbitrary frequency spacing
The theoretically predicted correlation of laser phase fluctuations in
Lambda-type interaction schemes is experimentally demonstrated. We show, that
the mechanism of correlation in a Lambda scheme is restricted to high frequency
noise components, whereas in a double- scheme, due to the laser phase
locking in closed-loop interaction, it extends to all noise frequencies. In
this case the correlation is weakly sensitive to coherence losses. Thus the
double-Lambda scheme can be used to correlate e.m. fields with carrier
frequency differences beyond the GHz regime.Comment: 4 pages, 4 figure
Enhanced Nonlinear Generation in a Three-Level Medium with Spatially Dependent Coherence
We consider a method for efficient parametric generation of a laser pulse. A
single laser field is injected to a three-level medium which has two lower
states and one excited state. The lower states are prepared initially in a
position-dependent coherent superposition state. It is shown that by proper
choice of the position dependence of the superposition along the direction of
propagation, the incoming field can be converted completely to a new field.Comment: Revtex4 document, 3 pages, 2 figure
Temporal build-up of electromagnetically induced transparency and absorption resonances in degenerate two-level transitions
The temporal evolution of electromagnetically induced transparency (EIT) and
absorption (EIA) coherence resonances in pump-probe spectroscopy of degenerate
two-level atomic transition is studied for light intensities below saturation.
Analytical expression for the transient absorption spectra are given for simple
model systems and a model for the calculation of the time dependent response of
realistic atomic transitions, where the Zeeman degeneracy is fully accounted
for, is presented. EIT and EIA resonances have a similar (opposite sign) time
dependent lineshape, however, the EIA evolution is slower and thus narrower
lines are observed for long interaction time. Qualitative agreement with the
theoretical predictions is obtained for the transient probe absorption on the
line in an atomic beam experiment.Comment: 10 pages, 9 figures. Submitted to Phys. Rev.
Synchrotron x-ray scattering analysis of nylon-12 crystallisation variation depending on 3D printing conditions
7Nylon-12 is an important structural polymer in wide use in the form of fibres and bulk structures. Fused filament fabrication (FFF) is an extrusion-based additive manufacturing (AM) method for rapid prototyping and final product manufacturing of thermoplastic polymer objects. The resultant microstructure of FFF-produced samples is strongly affected by the cooling rates and thermal gradients experienced across the part. The crystallisation behaviour during cooling and solidification influences the micro- and nano-structure, and deserves detailed investigation. A commercial Nylon-12 filament and FFF-produced Nylon-12 parts were studied by differential scanning calorimetry (DSC) and wide-angle X-ray scattering (WAXS) to examine the effect of cooling rates under non-isothermal crystallisation conditions on the microstructure and properties. Slower cooling rates caused more perfect crystallite formation, as well as alteration to the thermal properties.openopende Jager B.; Moxham T.; Besnard C.; Salvati E.; Chen J.; Dolbnya I.P.; Korsunsky A.M.de Jager, B.; Moxham, T.; Besnard, C.; Salvati, E.; Chen, J.; Dolbnya, I. P.; Korsunsky, A. M
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