892 research outputs found
Spontaneously generated X-shaped light bullets
We observe the formation of an intense optical wavepacket fully localized in
all dimensions, i.e. both longitudinally (in time) and in the transverse plane,
with an extension of a few tens of fsec and microns, respectively. Our
measurements show that the self-trapped wave is a X-shaped light bullet
spontaneously generated from a standard laser wavepacket via the nonlinear
material response (i.e., second-harmonic generation), which extend the soliton
concept to a new realm, where the main hump coexists with conical tails which
reflect the symmetry of linear dispersion relationship.Comment: 5 pages, 4 figures, submitted for publicatio
Modulational instability in dispersion-kicked optical fibers
We study, both theoretically and experimentally, modulational instability in
optical fibers that have a longitudinal evolution of their dispersion in the
form of a Dirac delta comb. By means of Floquet theory, we obtain an exact
expression for the position of the gain bands, and we provide simple analytical
estimates of the gain and of the bandwidths of those sidebands. An experimental
validation of those results has been realized in several microstructured fibers
specifically manufactured for that purpose. The dispersion landscape of those
fibers is a comb of Gaussian pulses having widths much shorter than the period,
which therefore approximate the ideal Dirac comb. Experimental spontaneous MI
spectra recorded under quasi continuous wave excitation are in good agreement
with the theory and with numerical simulations based on the generalized
nonlinear Schr\"odinger equation
Place-based innovation ecosystems : Boston-Cambridge innovation districts
This report focuses on the case study of the Boston area and identifies the key success factors in the Boston regional innovation ecosystem. It discusses how the macro-innovation eco-ecosystem is composed by a variety of interconnected micro-innovation ecosystems, mutually reinforcing each other and making the entire âterritorialâ system successful. The spatial configuration of these micro-innovation ecosystems at the urban scale has been specifically investigated, thus leading the authors to theorize that the Innovation District may act as an enabler for place-based innovation.
Evidence from the Boston case study shows that there is not a single magic recipe for the successful implementation of place-based and social innovation-driven strategies. On the contrary, the variety of place-grounded combinations of micro and macro initiatives, embedded in the social and spatial fine grain of places and encompassing a diversity of actors, can create the conditions that enable places to thrive and local economic activities to grow in a sustainable way
Heteroclinic structure of parametric resonance in the nonlinear Schr\"odinger equation
We show that the nonlinear stage of modulational instability induced by
parametric driving in the {\em defocusing} nonlinear Schr\"odinger equation can
be accurately described by combining mode truncation and averaging methods,
valid in the strong driving regime. The resulting integrable oscillator reveals
a complex hidden heteroclinic structure of the instability. A remarkable
consequence, validated by the numerical integration of the original model, is
the existence of breather solutions separating different Fermi-Pasta-Ulam
recurrent regimes. Our theory also shows that optimal parametric amplification
unexpectedly occurs outside the bandwidth of the resonance (or Arnold tongues)
arising from the linearised Floquet analysis
Spatial solitary-wave optical memory
We consider some features of spatial solitary-wave switching in a unidirectional ring cavity that is partially filled with a fast and saturably self-focusing nonlinear medium. Large (part-beam switched) solitary arrays are considered. It is found that prescribed binary patterns may be encoded in the duration of a single cavity transit and subsequently remain stable over thousands of transits. Beam interrupt allows pixels to be switched off in fewer than ten cavity transits. Pixel instabilities on an unpixelated beam are shown to arise from spatial solitary attractive forces and intensity gradients
Tunneling mediated by conical waves in a 1D lattice
The nonlinear propagation of 3D wave-packets in a 1D Bragg-induced band-gap
system, shows that tranverse effects (free space diffraction) affect the
interplay of periodicity and nonlinearity, leading to the spontaneous formation
of fast and slow conical localized waves. Such excitation corresponds to
enhanced nonlinear transmission (tunneling) in the gap, with peculiar features
which differ on the two edges of the band-gap, as dictated by the full
dispersion relationship of the localized waves.Comment: 5 pages, 6 figure
Programmed assembly of polymer-DNA conjugate nanoparticles with optical readout and sequence-specific activation of biorecognition
Soft micellar nanoparticles can be prepared from DNA conjugates designed to assemble via base pairing
such that strands containing a polymer corona and a cholesterol tail generate controlled supramolecular
architecture. Functionalization of one DNA conjugate strand with a biorecognition ligand results in
shielding of the ligand when in the micelle, while encoding of the DNA sequences with overhangs allows
supramolecular unpacking by addition of a complementary strand and sequence-specific unshielding of
the ligand. The molecular assembly/disassembly and âonâoffâ switch of the recognition signal is
visualized by FRET pair signalling, PAGE and a facile turbidimetric binding assay, allowing direct and
amplified readout of nucleic acid sequence recognition
Laser beam filamentation in fractal aggregates
We investigate filamentation of a cw laser beam in soft matter such as
colloidal suspensions and fractal gels. The process, driven by
electrostriction, is strongly affected by material properties, which are taken
into account via the static structure factor, and have impact on the statistics
of the light filaments.Comment: 4 pages, 5 figures. Revised version with corrected figure 5. To be
published in Phys. Rev. Let
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