16 research outputs found
Ultrafast Plasmonic Control of Second Harmonic Generation
Efficient frequency conversion techniques are crucial to the development of
plasmonic metasurfaces for information processing and signal modulation. In
principle, nanoscale electric-field confinement in nonlinear materials enables
higher harmonic conversion efficiencies per unit volume than those attainable
in bulk materials. Here we demonstrate efficient second-harmonic generation
(SHG) in a serrated nanogap plasmonic geometry that generates steep electric
field gradients on a dielectric metasurface. An ultrafast pump is used to
control plasmon-induced electric fields in a thin-film material with inversion
symmetry that, without plasmonic enhancement, does not exhibit an an even-order
nonlinear optical response. The temporal evolution of the plasmonic near-field
is characterized with ~100as resolution using a novel nonlinear interferometric
technique. The ability to manipulate nonlinear signals in a metamaterial
geometry as demonstrated here is indispensable both to understanding the
ultrafast nonlinear response of nanoscale materials, and to producing active,
optically reconfigurable plasmonic device
Instantaneous band gap collapse in photoexcited monoclinic VO due to photocarrier doping
Using femtosecond time-resolved photoelectron spectroscopy we demonstrate
that photoexcitation transforms monoclinic VO quasi-instantaneously into a
metal. Thereby, we exclude an 80 femtosecond structural bottleneck for the
photoinduced electronic phase transition of VO. First-principles many-body
perturbation theory calculations reveal a high sensitivity of the VO
bandgap to variations of the dynamically screened Coulomb interaction,
supporting a fully electronically driven isostructral insulator-to-metal
transition. We thus conclude that the ultrafast band structure renormalization
is caused by photoexcitation of carriers from localized V 3d valence states,
strongly changing the screening \emph{before} significant hot-carrier
relaxation or ionic motion has occurred
Finishing the euchromatic sequence of the human genome
The sequence of the human genome encodes the genetic instructions for human physiology, as well as rich information about human evolution. In 2001, the International Human Genome Sequencing Consortium reported a draft sequence of the euchromatic portion of the human genome. Since then, the international collaboration has worked to convert this draft into a genome sequence with high accuracy and nearly complete coverage. Here, we report the result of this finishing process. The current genome sequence (Build 35) contains 2.85 billion nucleotides interrupted by only 341 gaps. It covers ∼99% of the euchromatic genome and is accurate to an error rate of ∼1 event per 100,000 bases. Many of the remaining euchromatic gaps are associated with segmental duplications and will require focused work with new methods. The near-complete sequence, the first for a vertebrate, greatly improves the precision of biological analyses of the human genome including studies of gene number, birth and death. Notably, the human enome seems to encode only 20,000-25,000 protein-coding genes. The genome sequence reported here should serve as a firm foundation for biomedical research in the decades ahead
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Mass Spectrometric Fingerprinting of Tank Waste Using Tunable, Ultrafast Infrared Lasers
During the past year, we have initiated systematic studies of mass spectrometry of model tank-waste materials using both conventional nanosecond, single-frequency and tunable, subpicosecond mid-infrared lasers. In addition to making initial measurements, we have also constructed several new items of equipment for the experiment and begun to develop a model of the processes which lead to efficient desorption and ionization of organic molecules (e.g., toluene, benzene, crown ethers) from sodium nitrate. Comparisons of nanosecond and picosecond lasers, and of infrared and ultraviolet lasers, have been especially instructive. These accomplishments are detailed in the following paragraphs
Switchable polarization rotation of visible light using a plasmonic metasurface
A metasurface comprising an array of silver nanorods supported by a thin film of the phase change material vanadium dioxide is used to rotate the primary polarization axis of visible light at a pre-determined wavelength. The dimensions of the rods were selected such that, across the two phases of vanadium dioxide, the two lateral localized plasmon resonances (in the plane of the metasurface) occur at the same wavelength. Illumination with linearly polarized light at 45° to the principal axes of the rod metasurface enables excitation of both of these resonances. Modulating the phase of the underlying substrate, we show that it is possible to reversibly switch which axis of the metasurface is resonant at the operating wavelength. Analysis of the resulting Stokes parameters indicates that the orientation of the principal linear polarization axis of the reflected signal is rotated by 90° around these wavelengths. Dynamic metasurfaces such as these have the potential to form the basis of an ultra-compact, low-energy multiplexer or router for an optical signal
Efficient forward second-harmonic generation from planar archimedean nanospirals
The enhanced electric field at plasmonic resonances in nanoscale antennas can
lead to efficient harmonic generation, especially when the plasmonic geometry is
asymmetric on either inter-particle or intra-particle levels. The planar
Archimedean nanospiral offers a unique geometrical asymmetry for second-harmonic
generation (SHG) because the SHG results neither from arranging centrosymmetric
nanoparticles in asymmetric groupings, nor from non-centrosymmetric
nanoparticles that retain a local axis of symmetry. Here, we report forward SHG
from planar arrays of Archimedean nanospirals using 15 fs pulses from a
Ti:sapphire oscillator tuned to 800 nm wavelength. The measured
harmonic-generation efficiencies are 2.6·10−9, 8·10−9 and
1.3·10−8 for left-handed circular, linear, and right-handed
circular polarizations, respectively. The uncoated nanospirals are stable under
average power loading of as much as 300 μWper nanoparticle. The nanospirals also
exhibit selective conversion between polarization states. These experiments show
that the intrinsic asymmetry of the nanospirals results in a highly efficient,
two-dimensional harmonic generator that can be incorporated into metasurface
optics
Quantitative hyperspectral coherent diffractive imaging spectroscopy of a solid-state phase transition in vanadium dioxide
Solid-state systems can host a variety of thermodynamic phases that can be controlled with magnetic fields, strain, or laser excitation. Many phases that are believed to exhibit exotic properties only exist on the nanoscale, coexisting with other phases that make them challenging to study, as measurements require both nanometer spatial resolution and spectroscopic information, which are not easily accessible with traditional x-ray spectromicroscopy techniques. Here, we use coherent diffractive imaging spectroscopy (CDIS) to acquire quantitative hyperspectral images of the prototypical quantum material vanadium oxide across the vanadium L2,3 and oxygen K x-ray absorption edges with nanometer-scale resolution. We extract the full complex refractive indices of the monoclinic insulating and rutile conducting phases of VO2 from a single sample and find no evidence for correlation-driven phase transitions. CDIS will enable quantitative full-field x-ray spectromicroscopy for studying phase separation in time-resolved experiments and other extreme sample environments where other methods cannot operate.EC/H2020/758461/EU/Probing nanoscale and femtosecond fluctuations in high temperature superconductors/SeeSuperEC/H2020/754510/EU/COFUND BIST POSTDOCTORAL FELLOWSHIP PROGRAMME/PROBIS