The self-channeling of extremely high power laser beams permits the ignition of plasma filaments in dielectrics, such as air and glasses. If no constraints are imposed by the geometry of the material, the plasma appears as a straight bright line. Here, we show that plasma filaments may be ignited in the vicinity of the interface between two materials in optical fibers, i.e., either the core-cladding or the cladding-air interface. In the latter case, helical plasma filaments lead to the emission of rainbow spiral beams. In perspective, our results may pave the way to a novel approach for micro-structuring optical fibers, and for generating light beams with orbital angular momentum.
We study multiphoton absorption-induced damages to standard silica multimode optical fibers, induced by means of femtosecond infrared laser beams. During the damaging process, the dynamics of beam propagation turns out to non-trivially evolve over a time scale of several hours. Such a long term evolution produces an irreversible drop of the optical transmission, which is accompanied by a drastic change of the output supercontinuum spectrum. A microscopic analysis of the damages was carried out by means of both optical microscopy and absorption-contrast computed X-ray tomography. This has permitted us to obtain information about the sign of the refractive index variation which is induced by the optical breakdown. Our results will find application in a wide array of emerging technologies employing high-power fiber optic beams, such as fiber lasers and micromachining.
We report on ultrafast opto-acoustic modulation of light reflectance in artificial epsilon-near-zero metamaterials made of two layers of Ag separated by an Al2O3 layer. By means of non-degenerate two color pump-probe experiments we demonstrate an optically induced acoustic modulation of the reflectance up to 20% via generation of acoustic waves inside the cavity upon mechanical expansion of the metal due to hot electron-phonon coupling nonlinearity in the Ag layers. The presented architecture opens the pathway towards novel routes to exploit light-matter interactions for opto-acoustic modulation at GHz frequencies. Moreover, our system can be designed to work in transmission geometry and is very versatile in terms of shifting the presented properties along a broad range of wavelengths, from UV to mid-IR. Our approach, beyond light-driven information processing, might impact also opto-mechanics, light-driven phonon induced up conversion mechanisms, non-linear optical and acoustic properties of materials, energy harvesting, and heat-assisted ultrafast magneto-optical recording.
The first experimental evidence of random laser action in a partially ordered, dye doped nematic liquid crystal
with long-range dielectric tensor fluctuations is reported. Above a given pump power the fluorescence curve
collapses and discrete sharp peaks emerge above the residual spontaneous emission spectrum. The spectral
linewidth of these emission peaks is narrow banded, typically around 0.5nm. The unexpected surviving of
interference effects in recurrent multiple scattering of the emitted photons provide the required optical feedback
for lasing in nematic liquid crystalline materials. Light waves coherent backscattering in orientationally ordered
nematics manifests a weak localization, strongly supporting the diffusive laser action phenomenon in the presence
of a gain medium. Unlike distributed feedback mirror-less laser, this system can be considered as a cavity-less
microlaser where the disorder unexpectedly plays the most important role, behaving as randomly distributed
feedback laser. The far field spatial distribution of the emission intensity shows a huge number of bright tiny
spots spatially overlapped and the intensity of each pulse strongly fluctuates in time and space. Here, we report
the main characteristics of this novel systems for various confinement geometries and under different conditions.
A brief presentation of boundary-less systems such as free standing and freely suspended dye doped nematic films
and droplets is also introduced, revealing unique emission features because of the complete absence of confining borders.
The study of optical solitons and light filaments steering in liquid crystals requires utilization of particular
cells designed for top view investigation and realized with an input interface which enables both to control the
molecular director configuration and to prevent light scattering. Up to now, the director orientation imposed by
this additional interface has been only estimated by experimental observations. In this paper, we report on the
design and characterization of liquid crystal cells for investigation of optical spatial solitons as well as on a simple
model describing the configuration of the molecular director orientation under the anchoring action of multiple
interfaces. The model is based on the elastic continuum theory and only strong anchoring is considered for
boundary conditions. Controlling of the director orientation at the input interface, as well as in the bulk, allows
to obtain configurations that can produce distinct optical phenomena in a light beam propagating inside the cell.
For a particular director configuration, it is possible to produce two waves: the extraordinary and the ordinary
one. With a different director configuration, the extraordinary wave only is obtained, which propagates inside
the cell at an angle of more than 7° with respect to the impinging wave vector direction. Under this peculiar
configuration and by applying an external voltage, it is possible to have a good control of the propagation
direction of the optical spatial soliton.
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