79 research outputs found
Holographic capture of femtosecond pulse propagation
We have implemented a holographic system to study the propagation of femtosecond laser pulses with high temporal (150 fs) and spatial resolutions (4 µm). The phase information in the holograms allows us to reconstruct both positive and negative index changes due to the Kerr nonlinearity (positive) and plasma formation (negative), and to reconstruct three-dimensional structure. Dramatic differences were observed in the interaction of focused femtosecond pulses with air, water, and carbon disulfide. The air becomes ionized in the focal region, while in water long plasma filaments appear before the light reaches a tight focus. In contrast, in carbon disulfide the optical beam breaks up into multiple filaments but no plasma is measured. We explain these different propagation regimes in terms of the different nonlinear material properties
A Dynamical Model of Harmonic Generation in Centrosymmetric Semiconductors
We study second and third harmonic generation in centrosymmetric
semiconductors at visible and UV wavelengths in bulk and cavity environments.
Second harmonic generation is due to a combination of symmetry breaking, the
magnetic portion of the Lorentz force, and quadrupolar contributions that
impart peculiar features to the angular dependence of the generated signals, in
analogy to what occurs in metals. The material is assumed to have a non-zero,
third order nonlinearity that gives rise to most of the third harmonic signal.
Using the parameters of bulk Silicon we predict that cavity environments can
significantly modify second harmonic generation (390nm) with dramatic
improvements for third harmonic generation (266nm). This occurs despite the
fact that the harmonics may be tuned to a wavelength range where the dielectric
function of the material is negative: a phase locking mechanism binds the pump
to the generated signals and inhibits their absorption. These results point the
way to novel uses and flexibility of materials like Silicon as nonlinear media
in the visible and UV ranges
Resonant, broadband and highly efficient optical frequency conversion in semiconductor nanowire gratings at visible and UV wavelengths
Using a hydrodynamic approach we examine bulk- and surface-induced second and
third harmonic generation from semiconductor nanowire gratings having a
resonant nonlinearity in the absorption region. We demonstrate resonant,
broadband and highly efficient optical frequency conversion: contrary to
conventional wisdom, we show that harmonic generation can take full advantage
of resonant nonlinearities in a spectral range where nonlinear optical
coefficients are boosted well beyond what is achievable in the transparent,
long-wavelength, non-resonant regime. Using femtosecond pulses with
approximately 500 MW/cm2 peak power density, we predict third harmonic
conversion efficiencies of approximately 1% in a silicon nanowire array, at
nearly any desired UV or visible wavelength, including the range of negative
dielectric constant. We also predict surface second harmonic conversion
efficiencies of order 0.01%, depending on the electronic effective mass,
bistable behavior of the signals as a result of a reshaped resonance, and the
onset fifth order nonlinear effects. These remarkable findings, arising from
the combined effects of nonlinear resonance dispersion, field localization, and
phase-locking, could significantly extend the operational spectral bandwidth of
silicon photonics, and strongly suggest that neither linear absorption nor skin
depth should be motivating factors to exclude either semiconductors or metals
from the list of useful or practical nonlinear materials in any spectral range.Comment: 12 pages, 4 figure
Third harmonic generation at 223 nm in the metallic regime of GaP
We demonstrate second and third harmonic generation from a GaP substrate 500 ÎĽm thick. The second harmonic field is tuned at the absorption resonance at 335 nm, and the third harmonic signal is tuned at 223 nm, in a range where the dielectric function is negative. These results show that a phase locking mechanism that triggers transparency at the harmonic wavelengths persists regardless of the dispersive properties of the medium, and that the fields propagate hundreds of microns without being absorbed even when the harmonics are tuned to portions of the spectrum that display
metallic behavior.Peer ReviewedPostprint (published version
Gap solitons in a nonlinear quadratic negative index cavity
By integrating the full Maxwell's equations we predict the existence of gap solitons in a quadratic, Fabry-Perot negative index cavity. An intense, fundamental pump pulse shifts the band structure that forms when magnetic and electric plasma frequencies are different so that a weak, second harmonic pulse initially tuned inside the gap is almost entirely transmitted. The process is due cascading, which occurs far from phase matching conditions, and causes pulse compression. A nonlinear polarization spawns a dark soliton, while a nonlinear magnetization produces a bright soliton
Second and Third Harmonic Generation in Metal-Based Nanostructures
We present a new theoretical approach to the study of second and third
harmonic generation from metallic nanostructures and nanocavities filled with a
nonlinear material, in the ultrashort pulse regime. We model the metal as a
two-component medium, using the hydrodynamic model to describe free electrons,
and Lorentz oscillators to account for core electron contributions to both the
linear dielectric constant and to harmonic generation. The active nonlinear
medium that may fill a metallic nanocavity, or be positioned between metallic
layers in a stack, is also modeled using Lorentz oscillators and surface
phenomena due to symmetry breaking are taken into account. We study the effects
of incident TE- and TM-polarized fields and show that a simple re-examination
of the basic equations reveals additional exploitable dynamical features of
nonlinear frequency conversion in plasmonic nanostructures.Comment: 33 pages, including 11 figures and 74 references; corrected
affiliations and some typo
Graphene-based perfect optical absorbers harnessing guided mode resonances
We numerically and experimentally investigate graphene-based optical
absorbers that exploit guided mode resonances (GMRs) achieving perfect
absorption over a bandwidth of few nanometers (over the visible and
near-infrared ranges) with a 40-fold increase of the monolayer graphene
absorption. We analyze the influence of the geometrical parameters on the
absorption rate and the angular response for oblique incidence. Finally, we
experimentally verify the theoretical predictions in a one-dimensional,
dielectric grating and placing it near either a metallic or a dielectric
mirror
Graphene-based absorber exploiting guided mode resonances in one-dimensional gratings
A one-dimensional dielectric grating, based on a simple geometry, is proposed
and investigated to enhance light absorption in a monolayer graphene exploiting
guided mode resonances. Numerical findings reveal that the optimized
configuration is able to absorb up to 60% of the impinging light at normal
incidence for both TE and TM polarizations resulting in a theoretical
enhancement factor of about 26 with respect to the monolayer graphene
absorption (about 2.3%). Experimental results confirm this behaviour showing
CVD graphene absorbance peaks up to about 40% over narrow bands of few
nanometers. The simple and flexible design paves the way for the realization of
innovative, scalable and easy-to-fabricate graphene-based optical absorbers
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