98 research outputs found
Decoupling frequencies, amplitudes and phases in nonlinear optics
In linear optics, light fields do not mutually interact in a medium. However, they do mix when their field strength becomes comparable to electron binding energies in the so-called nonlinear optical regime. Such high fields are typically achieved with ultra-short laser pulses containing very broad frequency spectra where their amplitudes and phases are mutually coupled in a convolution process. Here, we describe a regime of nonlinear interactions without mixing of different frequencies. We demonstrate both in theory and experiment how frequency domain nonlinear optics overcomes the shortcomings arising from the convolution in conventional time domain interactions. We generate light fields with previously inaccessible properties by avoiding these uncontrolled couplings. Consequently, arbitrary phase functions are transferred linearly to other frequencies while preserving the general shape of the input spectrum. As a powerful application, we introduce deep UV phase control at 207ânm by using a conventional NIR pulse shaper
Linearizing nonlinear optics
In the framework of linear optics, light fields do not interact with each
other in a medium. Yet, when their field amplitude becomes comparable to the
electron binding energies of matter, the nonlinear motion of these electrons
emits new dipole radiation whose amplitude, frequency and phase differ from the
incoming fields. Such high fields are typically achieved with ultra-short,
femtosecond (1fs = 10-15 sec.) laser pulses containing very broad frequency
spectra. Here, the matter not only couples incoming and outgoing fields but
also causes different spectral components to interact and mix through a
convolution process. In this contribution, we describe how frequency domain
nonlinear optics overcomes the shortcomings arising from this convolution in
conventional time domain nonlinear optics1. We generate light fields with
previously inaccessible properties because the uncontrolled coupling of
amplitudes and phases is turned off. For example, arbitrary phase functions are
transferred linearly to the second harmonic frequency while maintaining the
exact shape of the input power spectrum squared.
This nonlinear control over output amplitudes and phases opens up new avenues
for applications based on manipulation of coherent light fields. One could
investigate c.f. the effect of tailored nonlinear perturbations on the
evolution of discrete eigenmodes in Anderson localization2. Our approach might
also open a new chapter for controlling electronic and vibrational couplings in
2D-spectroscopy3 by the geometrical optical arrangement
Many Body Diffusion and Interacting Electrons in a Harmonic Confinement
We present numerically exact energy estimates for two-dimensional electrons
in a parabolic confinement. By application of an extension of the recently
introduced many-body diffusion algorithm, the ground-state energies are
simulated very efficiently. The new algorithm relies on partial
antisymmetrization under permutation of particle coordinates. A comparison is
made with earlier theoretical results for that system.Comment: Revised version, submitted to Phys. Rev. B E-mail adresses:
[email protected]; [email protected]; [email protected]
Extracellular N-Acetylaspartate in Human Traumatic Brain Injury.
N-acetylaspartate (NAA) is an amino acid derivative primarily located in the neurons of the adult brain. The function of NAA is incompletely understood. Decrease in brain tissue NAA is presently considered symptomatic and a potential biomarker of acute and chronic neuropathological conditions. The aim of this study was to use microdialysis to investigate the behavior of extracellular NAA (eNAA) levels after traumatic brain injury (TBI). Sampling for this study was performed using cerebral microdialysis catheters (M Dialysis 71) perfused at 0.3 ÎŒL/min. Extracellular NAA was measured in microdialysates by high-performance liquid chromatography in 30 patients with severe TBI and for comparison, in radiographically "normal" areas of brain in six non-TBI neurosurgical patients. We established a detailed temporal eNAA profile in eight of the severe TBI patients. Microdialysate concentrations of glucose, lactate, pyruvate, glutamate, and glycerol were measured on an ISCUS clinical microdialysis analyzer. Here, we show that the temporal profile of microdialysate eNAA was characterized by highest levels in the earliest time-points post-injury, followed by a steady decline; beyond 70 h post-injury, average levels were 40% lower than those measured in non-TBI patients. There was a significant inverse correlation between concentrations of eNAA and pyruvate; eNAA showed significant positive correlations with glycerol and the lactate/pyruvate (L/P) ratio measured in microdialysates. The results of this on-going study suggest that changes in eNAA after TBI relate to the release of intracellular components, possibly due to neuronal death or injury, as well as to adverse brain energy metabolism.We gratefully acknowledge financial support as follows. Research support: the Medical Research Council (MRC, Grant Nos. G0600986 ID79068 and G1002277 ID98489) and the National Institute for Health Research Biomedical Research Centre (NIHR BRC) Cambridge (Neuroscience Theme; Brain Injury and Repair Theme). Authorsâ support: R.J.S. - NIHR BRC (Neuroscience Theme; Brain Injury and Repair Theme); SV - NIHR BRC (Neuroscience Theme; Brain Injury and Repair Theme); I.J. â MRC (Grant no. G1002277 ID 98489) and NIHR BRC Cambridge; D.K.M. - NIHR Senior Investigator Award; P.J.H. â NIHR Research Professorship, Academy of Medical Sciences/Health Foundation Senior Surgical Scientist Fellowship; K.L.H.C. â NIHR BRC Cambridge (Neuroscience Theme; Brain Injury and Repair Theme).This is the final version of the article. It first appeared from Mary Ann Liebert via http://dx.doi.org/10.1089/neu.2015.395
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Tracking ultrafast solid-state dynamics using high harmonic spectroscopy
WWe establish time-resolved high harmonic generation (tr-HHG) as a powerful spectroscopy method for tracking photoinduced dynamics in strongly correlated materials through a detailed investigation of the insulator-to-metal phase transitions in vanadium dioxide. We benchmark the technique by comparing our measurements to established momentum-resolved ultrafast electron diffraction, and theoretical density functional calculations. Tr-HHG allows distinguishing of individual dynamic channels, including a transition to a thermodynamically hidden phase. In addition, the HHG yield is shown to be modulated at a frequency characteristic of a coherent phonon of the equilibrium monoclinic phase over a wide range of excitation fluences. These results demonstrate that tr-HHG is capable of tracking complex dynamics in solids through its sensitivity to the band structure
Tabletop imaging of structural evolutions in chemical reactions
The introduction of femto-chemistry has made it a primary goal to follow the
nuclear and electronic evolution of a molecule in time and space as it
undergoes a chemical reaction. Using Coulomb Explosion Imaging we have shot the
first high-resolution molecular movie of a to and fro isomerization process in
the acetylene cation. So far, this kind of phenomenon could only be observed
using VUV light from a Free Electron Laser [Phys. Rev. Lett. 105, 263002
(2010)]. Here we show that 266 nm ultrashort laser pulses are capable of
initiating rich dynamics through multiphoton ionization. With our generally
applicable tabletop approach that can be used for other small organic
molecules, we have investigated two basic chemical reactions simultaneously:
proton migration and C=C bond-breaking, triggered by multiphoton ionization.
The experimental results are in excellent agreement with the timescales and
relaxation pathways predicted by new and definitively quantitative ab initio
trajectory simulations
Tracking ultrafast solid-state dynamics using high harmonic spectroscopy
We establish time-resolved high harmonic generation (tr-HHG) as a powerful
spectroscopy for photoinduced dynamics in strongly correlated materials through
a detailed investigation of the insulator-to-metal transitions in vanadium
dioxide. We benchmark our technique by comparing our measurements to
established momentum-resolved ultrafast electron diffraction, and theoretical
density functional calculations. Tr-HHG allows distinguishing of individual
dynamic channels, including a transition to a thermodynamically hidden phase.
In addition, the HHG yield is shown to be modulated at a frequency
characteristic of a coherent phonon in the equilibrium monoclinic phase over a
wide range of excitation fluences. These results demonstrate that tr-HHG is
capable of tracking complex dynamics in solids through its sensitivity to the
band structure.Comment: 20 pages and 4 figures main text, 8 pages and 4 figures supplementary
informatio
Focally perfused succinate potentiates brain metabolism in head injury patients.
Following traumatic brain injury, complex cerebral energy perturbations occur. Correlating with unfavourable outcome, high brain extracellular lactate/pyruvate ratio suggests hypoxic metabolism and/or mitochondrial dysfunction. We investigated whether focal administration of succinate, a tricarboxylic acid cycle intermediate interacting directly with the mitochondrial electron transport chain, could improve cerebral metabolism. Microdialysis perfused disodium 2,3-13C2 succinate (12âmmol/L) for 24âh into nine sedated traumatic brain injury patients' brains, with simultaneous microdialysate collection for ISCUS analysis of energy metabolism biomarkers (nine patients) and nuclear magnetic resonance of 13C-labelled metabolites (six patients). Metabolites 2,3-13C2 malate and 2,3-13C2 glutamine indicated tricarboxylic acid cycle metabolism, and 2,3-13C2 lactate suggested tricarboxylic acid cycle spinout of pyruvate (by malic enzyme or phosphoenolpyruvate carboxykinase and pyruvate kinase), then lactate dehydrogenase-mediated conversion to lactate. Versus baseline, succinate perfusion significantly decreased lactate/pyruvate ratio (pâ=â0.015), mean difference -12%, due to increased pyruvate concentration (+17%); lactate changed little (-3%); concentrations decreased for glutamate (-43%) (pâ=â0.018) and glucose (-15%) (pâ=â0.038). Lower lactate/pyruvate ratio suggests better redox status: cytosolic NADH recycled to NAD+ by mitochondrial shuttles (malate-aspartate and/or glycerol 3-phosphate), diminishing lactate dehydrogenase-mediated pyruvate-to-lactate conversion, and lowering glutamate. Glucose decrease suggests improved utilisation. Direct tricarboxylic acid cycle supplementation with 2,3-13C2 succinate improved human traumatic brain injury brain chemistry, indicated by biomarkers and 13C-labelling patterns in metabolites.The author(s) disclosed receipt of the following financial support for the research, authorship, and/or publication of this article: Medical Research Council (Grant Nos. G0600986 ID79068 and G1002277 ID98489) and National Institute for Health Research Biomedical Research Centre, Cambridge (Neuroscience Theme; Brain Injury and Repair Theme). Authorsâ support: IJ â Medical Research Council (Grant no. G1002277 ID 98489) and National Institute for Health Research Biomedical Research Centre, Cambridge; KLHC â National Institute for Health Research Biomedical Research Centre, Cambridge (Neuroscience Theme; Brain Injury and Repair Theme); CG â the Canadian Institute of Health Research; AH â Medical Research Council/Royal College of Surgeons of England Clinical Research Training Fellowship (Grant no. G0802251) and Raymond and Beverly Sackler Fellowship; DKM and JDP â National Institute for Health Research Senior Investigator Awards; PJH â National Institute for Health Research Professorship, Academy of Medical Sciences/Health Foundation Senior Surgical Scientist Fellowship and the National Institute for Health Research Biomedical Research Centre, Cambridge
Roadmap of ultrafast x-ray atomic and molecular physics
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