1,189 research outputs found
Photonic quasicrystals for general purpose nonlinear optical frequency conversion
We present a general method for the design of 2-dimensional nonlinear
photonic quasicrystals that can be utilized for the simultaneous phase-matching
of arbitrary optical frequency-conversion processes. The proposed scheme--based
on the generalized dual-grid method that is used for constructing tiling models
of quasicrystals--gives complete design flexibility, removing any constraints
imposed by previous approaches. As an example we demonstrate the design of a
color fan--a nonlinear photonic quasicrystal whose input is a single wave at
frequency and whose output consists of the second, third, and fourth
harmonics of , each in a different spatial direction
Multistep cascading and fourth-harmonic generation
We apply the concept of multistep cascading to the problem of fourth-harmonic
generation in a single quadratic crystal. We analyze a new model of parametric
wave mixing and describe its stationary solutions for two- and three-color
plane waves and spatial solitons. Some applications to the optical frequency
division as well as the realization of the double-phase-matching processes in
engineered QPM structures with phase reversal sequences are also discussed.Comment: 3 pages, 3 figure
A theory of nonvertical triplet energy transfer in terms of accurate potential energy surfaces: The transfer reaction from π,π∗ triplet donors to 1,3,5,7-cyclooctatetraene
Triplet energy transfer (TET) from aromatic donors to 1,3,5,7-cyclooctatetraene (COT) is an extreme case of “nonvertical” behavior, where the transfer rate for low-energy donors is considerably faster than that predicted for a thermally activated (Arrhenius) process. To explain the anomalous TET of COT and other molecules, a new theoretical model based on transition state theory for nonadiabatic processes is proposed here, which makes use of the adiabatic potential energy surfaces (PES) of reactants and products, as computed from high-level quantum mechanical methods, and a nonadiabatic transfer rate constant. It is shown that the rate of transfer depends on a geometrical distortion parameter γ = (2g2/κ1)1/2 in which g stands for the norm of the energy gradient in the PES of the acceptor triplet state and κ1 is a combination of vibrational force constants of the ground-state acceptor in the gradient direction. The application of the model to existing experimental data for the triplet energy transfer reaction to COT from a series of π,π∗ triplet donors, provides a detailed interpretation of the parameters that determine the transfer rate constant. In addition, the model shows that the observed decrease of the acceptor electronic excitation energy is due to thermal activation of C�C bond stretchings and C–C bond torsions, which collectively change the ground-state COT bent conformation (D2d) toward a planar triplet state (D8h)[email protected]
Saturation effects in the sub-Doppler spectroscopy of Cesium vapor confined in an Extremely Thin Cell
Saturation effects affecting absorption and fluorescence spectra of an atomic
vapor confined in an Extremely Thin Cell (cell thickness ) are
investigated experimentally and theoretically. The study is performed on the
line ( of and concentrates on the two
situations and , the most contrasted ones with
respect to the length dependence of the coherent Dicke narrowing. For , the Dicke-narrowed absorption profile simply broadens and
saturates in amplitude when increasing the light intensity, while for , sub-Doppler dips of reduced absorption at line-center appear on the
broad absorption profile. For a fluorescence detection at ,
saturation induces narrow dips, but only for hyperfine components undergoing a
population loss through optical pumping. These experimental results are
interpreted with the help of the various existing models, and are compared with
numerical calculations based upon a two-level modelling that considers both a
closed and an open system.Comment: 11 pages, 12 figure
Spatial quadratic solitons guided by narrow layers of a nonlinear material
We report analytical solutions for spatial solitons supported by layers of a
quadratically nonlinear material embedded into a linear planar waveguide. A
full set of symmetric, asymmetric, and antisymmetric modes pinned to a
symmetric pair of the nonlinear layers is obtained. The solutions describe a
bifurcation of the subcritical type, which accounts for the transition from the
symmetric to asymmetric modes. The antisymmetric states (which do not undergo
the bifurcation) are completely stable (the stability of the solitons pinned to
the embedded layers is tested by means of numerical simulations). Exact
solutions are also found for nonlinear layers embedded into a nonlinear
waveguide, including the case when the uniform and localized nonlinearities
have opposite signs (competing nonlinearities). For the layers embedded into
the nonlinear medium, stability properties are explained by comparison to the
respective cascading limit.Comment: J. Opt. Soc. Am. B, in pres
Parametric localized modes in quadratic nonlinear photonic structures
We analyze two-color spatially localized modes formed by parametrically
coupled fundamental and second-harmonic fields excited at quadratic (or chi-2)
nonlinear interfaces embedded into a linear layered structure --- a
quasi-one-dimensional quadratic nonlinear photonic crystal. For a periodic
lattice of nonlinear interfaces, we derive an effective discrete model for the
amplitudes of the fundamental and second-harmonic waves at the interfaces (the
so-called discrete chi-2 equations), and find, numerically and analytically,
the spatially localized solutions --- discrete gap solitons. For a single
nonlinear interface in a linear superlattice, we study the properties of
two-color localized modes, and describe both similarities and differences with
quadratic solitons in homogeneous media.Comment: 9 pages, 8 figure
Virtually Augmented Self-Hypnosis applied to endovascular interventions (VA-HYPO): Randomized Controlled Trial Protocol.
Endovascular interventions (EVI) are increasingly performed as minimally-invasive alternatives to surgery and have many advantages, including a decreased need for general anesthesia. However, EVI can be stressful for patients and often lead to anxiety and pain related to the procedure. The use of local anesthetics, anxiolytics, and analgesic drugs can help avoid general anesthesia. Nevertheless, these drugs have potential side effects. Alternative nonpharmacological therapies can improve patients' experience during conscious interventions and reduce the need for additional medications. The added value of virtually augmented self-hypnosis (VA-HYPO) and its potential to reduce pain and anxiety during peripheral and visceral arterial and venous EVI is unknown. This is a prospective two-arm trial designed to randomize 100 patients in two groups according to the use or not of VA-HYPO during peripheral EVI as a complementary nonpharmacological technique to improve patient comfort. The main objective is to compare per-procedural anxiety, and the secondary aim is to compare the rated per-procedural pain in both groups. The potential significance is that VA-HYPO may improve patients' experience during peripheral and visceral arterial and venous EVI and other minimally invasive interventions performed under local anesthesia. Trial registration: Our study is registered on clinicaltrials.gov, with trial registration number: NCT04561596
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