26,687 research outputs found
Index to NASA Tech Briefs, 1975
This index contains abstracts and four indexes--subject, personal author, originating Center, and Tech Brief number--for 1975 Tech Briefs
Index to 1984 NASA Tech Briefs, volume 9, numbers 1-4
Short announcements of new technology derived from the R&D activities of NASA are presented. These briefs emphasize information considered likely to be transferrable across industrial, regional, or disciplinary lines and are issued to encourage commercial application. This index for 1984 Tech B Briefs contains abstracts and four indexes: subject, personal author, originating center, and Tech Brief Number. The following areas are covered: electronic components and circuits, electronic systems, physical sciences, materials, life sciences, mechanics, machinery, fabrication technology, and mathematics and information sciences
Ultrafast Insulator-Metal Phase Transition in VO2 Studied by Multiterahertz Spectroscopy
The ultrafast photoinduced insulator-metal transition in VO2 is studied at
different temperatures and excitation fluences using multi-THz probe pulses.
The spectrally resolved mid-infrared response allows us to trace separately the
dynamics of lattice and electronic degrees of freedom with a time resolution of
40 fs. The critical fluence of the optical pump pulse which drives the system
into a long-lived metallic state is found to increase with decreasing
temperature. Under all measurement conditions we observe a modulation of the
eigenfrequencies of the optical phonon modes induced by their anharmonic
coupling to the coherent wave packet motion of V-V dimers at 6.1 THz.
Furthermore, we find a weak quadratic coupling of the electronic response to
the coherent dimer oscillation resulting in a modulation of the electronic
conductivity at twice the frequency of the wave packet motion. The findings are
discussed in the framework of a qualitative model based on an approximation of
local photoexcitation of the vanadium dimers from the insulating state.Comment: 10 pages, 8 figures submitted to Physical Review
Time-Domain Separation of Optical Properties From Structural Transitions in Resonantly Bonded Materials
The extreme electro-optical contrast between crystalline and amorphous states
in phase change materials is routinely exploited in optical data storage and
future applications include universal memories, flexible displays,
reconfigurable optical circuits, and logic devices. Optical contrast is
believed to arise due to a change in crystallinity. Here we show that the
connection between optical properties and structure can be broken. Using a
unique combination of single-shot femtosecond electron diffraction and optical
spectroscopy, we simultaneously follow the lattice dynamics and dielectric
function in the phase change material Ge2Sb2Te5 during an irreversible state
transformation. The dielectric function changes by 30% within 100 femtoseconds
due to a rapid depletion of electrons from resonantly-bonded states. This
occurs without perturbing the crystallinity of the lattice, which heats with a
2 ps time constant. The optical changes are an order-of-magnitude larger than
those achievable with silicon and present new routes to manipulate light on an
ultrafast timescale without structural changes
Dynamics and control of gold-encapped gallium arsenide nanowires imaged by 4D electron microscopy
Eutectic related reaction is a special chemical/physical reaction involving
multiple phases, solid and liquid. Visualization of phase reaction of composite
nanomaterials with high spatial and temporal resolution provides a key
understanding of alloy growth with important industrial applications. However,
it has been a rather challenging task. Here we report the direct imaging and
control of the phase reaction dynamics of a single, as-grown free-standing
gallium arsenide nanowire encapped with a gold nanoparticle, free from
environmental confinement or disturbance, using four-dimensional electron
microscopy. The non-destructive preparation of as-grown free-standing nanowires
without supporting films allows us to study their anisotropic properties in
their native environment with better statistical character. A laser heating
pulse initiates the eutectic related reaction at a temperature much lower than
the melting points of the composite materials, followed by a precisely
time-delayed electron pulse to visualize the irreversible transient states of
nucleation, growth and solidification of the complex. Combined with theoretical
modeling, useful thermodynamic parameters of the newly formed alloy phases and
their crystal structures could be determined. This technique of dynamical
control and 4D imaging of phase reaction processes on the nanometer-ultrafast
time scale open new venues for engineering various reactions in a wide variety
of other systems
Proceedings of the Thirteenth International Conference on Time-Resolved Vibrational Spectroscopy
The thirteenth meeting in a long-standing series of âTime-Resolved Vibrational Spectroscopyâ (TRVS) conferences was held May 19th to 25th at the Kardinal Döpfner Haus in Freising, Germany, organized by the two Munich Universities - Ludwig-Maximilians-UniversitĂ€t and Technische UniversitĂ€t MĂŒnchen. This international conference continues the illustrious tradition of the original in 1982, which took place in Lake Placid, NY. The series of meetings was initiated by leading, world-renowned experts in the field of ultrafast laser spectroscopy, and is still guided by its founder, Prof. George Atkinson (University of Arizona and Science and Technology Advisor to the Secretary of State). In its current format, the conference contributes to traditional areas of time resolved vibrational spectroscopies including infrared, Raman and related laser methods. It combines them with the most recent developments to gain new information for research and novel technical applications. The scientific program addressed basic science, applied research and advancing novel commercial applications.
The thirteenth conference on Time Resolved Vibrational Spectroscopy promoted science in the areas of physics, chemistry and biology with a strong focus on biochemistry and material science. Vibrational spectra are molecule- and bond-specific. Thus, time-resolved vibrational studies provide detailed structural and kinetic information about primary dynamical processes on the picometer length scale. From this perspective, the goal of achieving a complete understanding of complex chemical and physical processes on the molecular level is well pursued by the recent progress in experimental and theoretical vibrational studies.
These proceedings collect research papers presented at the TRVS XIII in Freising, German
Coherent ultrafast spin-dynamics probed in three dimensional topological insulators
Topological insulators are candidates to open up a novel route in spin based
electronics. Different to traditional ferromagnetic materials, where the
carrier spin-polarization and magnetization are based on the exchange
interaction, the spin properties in topological insulators are based on the
coupling of spin- and orbit interaction connected to its momentum. Specific
ways to control the spin-polarization with light have been demonstrated: the
energy momentum landscape of the Dirac cone provides spin-momentum locking of
the charge current and its spin. The directionality of spin and momentum, as
well as control with light has been demonstrated. Here we demonstrate a
coherent femtosecond control of spin-polarization for states in the valence
band at around the Dirac cone.Comment: 14 pages, 4 figure
Characterization of Cs vapor cell coated with octadecyltrichlorosilane using coherent population trapping spectroscopy
We report the realization and characterization using coherent population
trapping (CPT) spectroscopy of an octadecyltrichlorosilane (OTS)-coated
centimeter-scale Cs vapor cell. The dual-structure of the resonance lineshape,
with presence of a narrow structure line at the top of a Doppler-broadened
structure, is clearly observed. The linewidth of the narrow resonance is
compared to the linewidth of an evacuated Cs cell and of a buffer gas Cs cell
of similar size. The Cs-OTS adsorption energy is measured to be (0.42
0.03) eV, leading to a clock frequency shift rate of K in
fractional unit. A hyperfine population lifetime, , and a microwave
coherence lifetime, , of 1.6 and 0.5 ms are reported, corresponding to
about 37 and 12 useful bounces, respectively. Atomic-motion induced Ramsey
narrowing of dark resonances is observed in Cs-OTS cells by reducing the
optical beam diameter. Ramsey CPT fringes are detected using a pulsed CPT
interrogation scheme. Potential applications of the Cs-OTS cell to the
development of a vapor cell atomic clock are discussed.Comment: 33 pages, 13 figure
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