451,225 research outputs found
Dynamics of fast pattern formation in porous silicon by laser interference
Patterns are fabricated on 290 nm thick nanostructured porous silicon layers by phase-mask laser interference using single pulses of an excimer laser (193 nm, 20 ns pulse duration). The dynamics of pattern formation is studied by measuring in real time the intensity of the diffraction orders 0 and 1 at 633 nm. The results show that a transient pattern is formed upon melting at intensity maxima sites within a time 1-µs) upon melting induced by homogeneous beam exposure and related to the different scenario for releasing the heat from hot regions. The diffraction efficiency of the pattern is finally controlled by a combination of laser fluence and initial thickness of the nanostructured porous silicon layer and the present results open perspectives on heat release management upon laser exposure as well as have potential for alternative routes for switching applications.Postprint (published version
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Laser-assisted photothermal imprinting of nanocomposite
We report on a laser-assisted photothermal imprinting method for directly patterning carbon
nanofiber-reinforced polyethylene nanocomposite. A single laser pulse from a solid state
Nd:YAG laser (10 ns pulse, 532 nm and 355 nm wavelengths) is used to melt/soften a thin skin
layer of the polymer nanocomposite. Meanwhile, a fused quartz mold with micro-sized surface
relief structures is pressed against the surface of the composite. Successful pattern transfer is
realized upon releasing the quartz mold. Although polyethylene is transparent to the laser beam,
the carbon nanofibers in the high density polyethylene (HDPE) matrix absorb the laser energy
and convert it into heat. Numerical heat conduction simulation shows the HDPE matrix is
partially melted or softened, allowing for easier imprinting of the relief pattern of the quartz
mold.Mechanical Engineerin
Ionization of atoms by few-cycle EUV laser pulses: carrier-envelope phase dependence of the intra-pulse interference effects
We have investigated the ionization of the H atom by intense few-cycle laser
pulses, in particular the intra-pulse interference effects, and their
dependence on the carrier-envelope phase (CEP) of the laser pulse. In the final
momentum distribution of the continuum electrons the imprint of two types of
intra-pulse interference effects can be observed, namely the temporal and
spatial interference. During the spatial interference electronic wave packets
emitted at the same time, but following different paths interfere leading to an
interference pattern measurable in the electron spectra. This can be also
interpreted as the interference between a direct and a scattered wave, and the
spatial interference pattern as the holographic mapping (HM) of the target.
This HM pattern is strongly influenced by the carrier-envelope phase through
the shape of the laser pulse. Here, we have studied how the shape of the HM
pattern is modified by the CEP, and we have found an optimal CEP for the
observation of HM
Laser-induced radial birefringence and spin-to-orbital optical angular momentum conversion in silver-doped glasses
Samples of Ag/Na ion-exchanged glass that have been subject to
intense laser irradiation may develop novel optical properties, as a
consequence of the formation of patterns of silver nanoparticles and other
structures. Here, we report the observation of a laser-induced permanent
transverse birefringence, with the optical axis forming a radial pattern, as
revealed by the spin-to-orbital angular momentum conversion occurring in a
probe light beam. The birefringence pattern can be modeled well as resulting
from thermally-induced stresses arising in the silver-doped glass during laser
exposure, although the actual mechanism leading to the permanent anisotropy is
probably more complex.Comment: 3 pages, 3 figure
Plasma wake inhibition at the collision of two laser pulses in an underdense plasma
An electron injector concept for laser-plasma accelerator was developed in
ref [1] and [2] ; it relies on the use of counter-propagating ultrashort laser
pulses. In [2], the scheme is as follows: the pump laser pulse generates a
large amplitude laser wakefield (plasma wave). The counter-propagating
injection pulse interferes with the pump laser pulse to generate a beatwave
pattern. The ponderomotive force of the beatwave is able to inject plasma
electrons into the wakefield. We have studied this injection scheme using 1D
Particle in Cell (PIC) simulations. The simulations reveal phenomena and
important physical processes that were not taken into account in previous
models. In particular, at the collision of the laser pulses, most plasma
electrons are trapped in the beatwave pattern and cannot contribute to the
collective oscillation supporting the plasma wave. At this point, the fluid
approximation fails and the plasma wake is strongly inhibited. Consequently,
the injected charge is reduced by one order of magnitude compared to the
predictions from previous models.Comment: 4 pages, 4 figure
Experimental observation of speckle instability in nonlinear disordered media
Temporal fluctuations of the speckle pattern formed upon backscattering of a
laser beam from an interface between gold and nonlinear polymer film have been
observed as a function of optical power. The instability can be explained by
coupling of laser light to surface plasmons and other guided modes, which
experience multiple scattering while propagating in the film along the
interface. The speckle pattern produced in this process is extremely sensitive
to fluctuations of the scattering potential near the interface.Comment: 4 pages, 5 figure
Coupling mechanism of gain-guided integrated semiconductor laser arrays
It is shown that a gain-guided laser array couples via propagating fields rather than the evanescent mode coupling typically responsible for directional coupling in passive (directional couplers) and active (laser array) devices. We show that these phase-locked modes exhibit an interference pattern, in the junction plane, which arises from the curvature of the phase fronts of optical fields of the interacting lasers. The experimental results are interpreted with the aid of a simple theoretical model, and the effect of the observed mode pattern on the coupling of gain-guided lasers is discussed
Determination of beam incidence conditions based on the analysis of laser interference patterns
Beam incidence conditions in the formation of two-, three- and four-beam laser interference patterns are presented and studied in this paper. In a laser interference lithography (LIL) process, it is of importance to determine and control beam incidence conditions based on the analysis of laser interference patterns for system calibration as any slight change of incident angles or intensities of beams will introduce significant variations of periods and contrasts of interference patterns. In this work, interference patterns were captured by a He-Ne laser interference system under different incidence conditions, the pattern period measurement was achieved by cross-correlation with, and the pattern contrast was calculated by image processing. Subsequently, the incident angles and intensities of beams were determined based on the analysis of spatial distributions of interfering beams. As a consequence, the relationship between the beam incidence conditions and interference patterns is revealed. The proposed method is useful for the calibration of LIL processes and for reverse engineering applications
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