38 research outputs found
Harmonic generation and energy transport in dielectric and semiconductors at visible and UV wavelengths: the case of GaP
We study inhibition of absorption, transparency, energy and momentum
transport of the inhomogeneous component of harmonic pulses in dielectrics and
semiconductors, at visible and UV wavelengths, focusing on materials like GaP.
In these spectral regions GaP is characterized by large absorption, metallic
behavior or a combination of both. We show that phase locking causes the
generated inhomogeneous signals to propagate through a bulk metallic medium
without being absorbed, that is occurs even in centrosymmetric materials via
the magnetic Lorentz force, and that the transport of energy and momentum is
quite peculiar and seemingly anomalous. These results make it clear that there
are new opportunities in ultrafast nonlinear optics and nano-plasmonics in new
wavelength ranges.Comment: 16 pages, 5 figures, 1 vide
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
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
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
Enhanced second harmonic generation from resonant GaAs gratings
We study second harmonic generation in nonlinear, GaAs gratings. We find
large enhancement of conversion efficiency when the pump field excites the
guided mode resonances of the grating. Under these circumstances the spectrum
near the pump wavelength displays sharp resonances characterized by dramatic
enhancements of local fields and favorable conditions for second harmonic
generation, even in regimes of strong linear absorption at the harmonic
wavelength. In particular, in a GaAs grating pumped at 1064nm, we predict
second harmonic conversion efficiencies approximately five orders of magnitude
larger than conversion rates achievable in either bulk or etalon structures of
the same material.Comment: 8 page
Dynamical Model of Harmonic Generation in Centrosymmetric Semiconductors at Visible and UV Wavelengths
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 spatial symmetry breaking, the magnetic portion of the Lorentz force, and quadrupolar contributions from inner core electrons. The material is assumed to have a nonzero, third-order nonlinearity that gives rise to most of the third harmonic signal. Using the parameters of bulk silicon we predict that cavity environments modify the dependence of second harmonic generation on incident angle, while improving third harmonic conversion efficiency by several orders of magnitude relative to bulk silicon. This occurs despite the fact that the harmonic signals may be tuned to a wavelength range where the dielectric function of the material is negative: A phase-locking mechanism binds the generated signals to the pump and inhibits their absorption. These results point the way to alternative uses and flexibility of materials such as silicon as nonlinear media in the visible and UV ranges
Intellectual Property, Open Science and Research Biobanks
In biomedical research and translational medicine, the ancient war between exclusivity (private control over information) and access to information is proposing again on a new battlefield: research biobanks. The latter are becoming increasingly important (one of the ten ideas changing the world, according to Time magazine) since they allow to collect, store and distribute in a secure and professional way a critical mass of human biological samples for research purposes. Tissues and related data are fundamental for the development of the biomedical research and the emerging field of translational medicine: they represent the “raw material” for every kind of biomedical study. For this reason, it is crucial to understand the boundaries of Intellectual Property (IP) in this prickly context. In fact, both data sharing and collaborative research have become an imperative in contemporary open science, whose development depends inextricably on: the opportunities to access and use data, the possibility of sharing practices between communities, the cross-checking of information and results and, chiefly, interactions with experts in different fields of knowledge. Data sharing allows both to spread the costs of analytical results that researchers cannot achieve working individually and, if properly managed, to avoid the duplication of research. These advantages are crucial: access to a common pool of pre-competitive data and the possibility to endorse follow-on research projects are fundamental for the progress of biomedicine. This is why the "open movement" is also spreading in the biobank's field. After an overview of the complex interactions among the different stakeholders involved in the process of information and data production, as well as of the main obstacles to the promotion of data sharing (i.e., the appropriability of biological samples and information, the privacy of participants, the lack of interoperability), we will firstly clarify some blurring in language, in particular concerning concepts often mixed up, such as “open source” and “open access”. The aim is to understand whether and to what extent we can apply these concepts to the biomedical field. Afterwards, adopting a comparative perspective, we will analyze the main features of the open models – in particular, the Open Research Data model – which have been proposed in literature for the promotion of data sharing in the field of research biobanks.
After such an analysis, we will suggest some recommendations in order to rebalance the clash between exclusivity - the paradigm characterizing the evolution of intellectual property over the last three centuries - and the actual needs for access to knowledge. We argue that the key factor in this balance may come from the right interaction between IP, social norms and contracts. In particular, we need to combine the incentives and the reward mechanisms characterizing scientific communities with data sharing imperative