2,047 research outputs found
Relationships Between Atomic Diffusion Mechanisms and Ensemble Transport Coefficients in Crystalline Polymorphs
Ionic transport in conventional ionic solids is generally considered to
proceed via independent diffusion events or "hops''. This assumption leads to
well-known Arrhenius expressions for transport coefficients, and is equivalent
to assuming diffusion is a Poisson process. Using molecular dynamics
simulations of the low-temperature B1, B3, and B4 AgI polymorphs, we have
compared rates of ion-hopping with corresponding Poisson distributions to test
the assumption of independent hopping in these common structure-types. In all
cases diffusion is a non-Poisson process, and hopping is strongly correlated in
time. In B1 the diffusion coefficient can be approximated by an Arrhenius
expression, though the physical significance of the parameters differs from
that commonly assumed. In low temperature B3 and B4 diffusion is characterised
by concerted motion of multiple ions in short closed loops. Diffusion
coefficients can not be expressed in a simple Arrhenius form dependent on
single-ion free-energies, and intrinsic diffusion must be considered a
many-body process
Molecular Dynamics Simulation of Coherent Interfaces in Fluorite Heterostructures
The standard model of enhanced ionic conductivities in solid electrolyte
heterostructures follows from a continuum mean-field description of defect
distributions that makes no reference to crystalline structure. To examine
ionic transport and defect distributions while explicitly accounting for
ion-ion correlations and lattice effects, we have performed molecular dynamics
simulations of a model coherent fluorite heterostructure without any extrinsic
defects, with a difference in standard chemical potentials of mobile fluoride
ions between phases induced by an external potential. Increasing the offset in
fluoride ion standard chemical potentials across the internal interfaces
decreases the activation energies for ionic conductivity and diffusion and
strongly enhances fluoride ion mobilities and defect concentrations near the
heterostructure interfaces. Non-charge-neutral "space-charge" regions, however,
extend only a few atomic spacings from the interface, suggesting a continuum
model may be inappropriate. Defect distributions are qualitatively inconsistent
with the predictions of the continuum mean-field model, and indicate strong
lattice-mediated defect-defect interactions. We identify an atomic-scale
"Frenkel polarisation" mechanism for the interfacial enhancement in ionic
mobility, where preferentially oriented associated Frenkel pairs form at the
interface and promote local ion mobility via concerted diffusion processes
Supercontinuum generation in dispersion engineered highly nonlinear (y=10/W/m) As2S3 chalcogenide planar waveguide
We demonstrate supercontinuum generation in a highly
nonlinear As2S3 chalcogenide planar waveguide which is dispersion
engineered to have anomalous dispersion at near-infrared wavelengths.
This waveguide is 60 mm long with a cross-section of 2 μm by 870 nm,
resulting in a nonlinear parameter of 10 /W/m and a dispersion of
+29 ps/nm/km. Using pulses with a width of 610 fs and peak power of
68 W, we generate supercontinuum with a 30 dB bandwidth of 750 nm, in
good agreement with theory
Highly sensitive, broadband microwave frequency identification using a chip-based Brillouin optoelectronic oscillator
Detection and frequency estimation of radio frequency (RF) signals are critical in modern RF systems, including wireless communication and radar. Photonic techniques have made huge progress in solving the problem imposed by the fundamental trade-off between detection range and accuracy. However, neither fiber-based nor integrated photonic RF signal detection and frequency estimation systems have achieved wide range and low error with high sensitivity simultaneously in a single system. In this paper, we demonstrate the first Brillouin opto-electronic oscillator (B-OEO) based on on-chip stimulated Brillouin scattering (SBS) to achieve RF signal detection. The broad tunability and narrowband amplification of on-chip SBS allow for the wide-range and high-accuracy detection. Feeding the unknown RF signal into the B-OEO cavity amplifies the signal which is matched with the oscillation mode to detect low-power RF signals. We are able to detect RF signals from 1.5 to 40 GHz with power levels as low as −67 dBm and a frequency accuracy of ± 3.4 MHz. This result paves the way to compact, fully integrated RF detection and channelization.Australian Research Council (ARC) Linkage grant (LP170100112) with Harris Corporation. U.S. Air Force (USAF) through AFOSR/AOARD (FA2386-16-1-4036); U.S. Office of Naval Research Global (ONRG) (N62909-18-1-2013)
On-chip multi-stage optical delay based on cascaded Brillouin light storage
Storing and delaying optical signals plays a crucial role in data centers, phased array antennas, communication, and future computing architectures. Here, we show a delay scheme based on cascaded Brillouin light storage that achieves multi-stage delay at arbitrary positions within a photonic integrated circuit. Importantly these multiple resonant transfers between the optical and acoustic domain are controlled solely via external optical control pulses, allowing cascading of the delay without the need of aligning multiple structural resonances along the optical circuit
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