98 research outputs found
Dynamic recurrent neural networks for stable adaptive control of wing rock motion
Wing rock is a self-sustaining limit cycle oscillation (LCO) which occurs as the result of nonlinear coupling between the dynamic response of the aircraft and the unsteady aerodynamic forces. In this thesis, dynamic recurrent RBF (Radial Basis Function) network control methodology is proposed to control the wing rock motion. The concept based on the properties of the Presiach hysteresis model is used in the design of dynamic neural networks. The structure and memory mechanism in the Preisach model is analogous to the parallel connectivity and memory formation in the RBF neural networks. The proposed dynamic recurrent neural network has a feature for adding or pruning the neurons in the hidden layer according to the growth criteria based on the properties of ensemble average memory formation of the Preisach model. The recurrent feature of the RBF network deals with the dynamic nonlinearities and endowed temporal memories of the hysteresis model. The control of wing rock is a tracking problem, the trajectory starts from non-zero initial conditions and it tends to zero as time goes to infinity. In the proposed neural control structure, the recurrent dynamic RBF network performs identification process in order to approximate the unknown non-linearities of the physical system based on the input-output data obtained from the wing rock phenomenon. The design of the RBF networks together with the network controllers are carried out in discrete time domain. The recurrent RBF networks employ two separate adaptation schemes where the RBF's centre and width are adjusted by the Extended Kalman Filter in order to give a minimum networks size, while the outer networks layer weights are updated using the algorithm derived from Lyapunov stability analysis for the stable closed loop control. The issue of the robustness of the recurrent RBF networks is also addressed. The effectiveness of the proposed dynamic recurrent neural control methodology is demonstrated through simulations to suppress the wing rock motion in AFTI/F-16 testbed aircraft having the delta wing configuration. The potential implementation as well as the practicality of the control methodology are also discusse
Smith-Purcell Radiation from Low-Energy Electrons
Recent advances in the fabrication of nanostructures and nanoscale features
in metasurfaces offer a new prospect for generating visible, light emission
from low energy electrons. In this paper, we present the experimental
observation of visible light emission from low-energy free electrons
interacting with nanoscale periodic surfaces through the Smith-Purcell (SP)
effect. SP radiation is emitted when electrons pass in close proximity over a
periodic structure, inducing collective charge motion or dipole excitations
near the surface, thereby giving rise to electromagnetic radiation. We
demonstrate a controlled emission of SP light from nanoscale gold gratings with
periodicity as small as 50 nm, enabling the observation of visible SP radiation
by low energy electrons (1.5 to 6 keV), an order of magnitude lower than
previously reported. We study the emission wavelength and intensity dependence
on the grating pitch and electron energy, showing agreement between experiment
and theory. Further reduction of structure periodicity should enable the
production of SP-based devices that operate with even slower electrons that
allow an even smaller footprint and facilitate the investigation of quantum
effects for light generation in nanoscale devices. A tunable light source
integrated in an electron microscope would enable the development of novel
electron-optical correlated spectroscopic techniques, with additional
applications ranging from biological imaging to solid-state lighting.Comment: 16 pages, 4 figure
Rapid fabrication of 3D terahertz split ring resonator arrays by novel single-shot direct write focused proximity field nanopatterning
For the next generation of phoXonic, plasmonic, optomechanical
and microfluidic devices, the capability to create 3D
microstructures is highly desirable. Fabrication of such structures by
conventional top-down techniques generally requires multiple timeconsuming
steps and is limited in the ability to define features spanning
multiple layers at prescribed angles. 3D direct write lithography (3DDW)
has the capability to draw nearly arbitrary structures, but is an inherently
slow serial writing process. Here we present a method, denoted focused
proximity field nanopatterning (FPnP), that combines 3DDW with single or
multiphoton interference lithography (IL). By exposing a thick photoresist
layer having a phase mask pattern imprinted on its surface with a tightly
focused laser beam, we produce locally unique complex structures. The
morphology can be varied based on beam and mask parameters. Patterns
may be written rapidly in a single shot mode with arbitrary positions
defined by the direct write, thus exploiting the control of 3DDW with the
enhanced speed of phase mask IL. Here we show the ability for this
technique to rapidly produce arrays of “stand-up” far IR resonators
Photoluminescent Energy Transfer from Poly(phenyleneethynylene)s to Near-Infrared Emitting Fluorophores
Photoluminescent energy transfer was investigated in conjugated polymer-fluorophore blended thin films. A pentiptycene-containing poly(phenyleneethynylene) was used as the energy donor, and 13 fluorophores were used as energy acceptors. The efficiency of energy transfer was measured by monitoring both the quenching of the polymer emission and the enhancement of the fluorophore emission. Near-infrared emitting squaraines and terrylenes were identified as excellent energy acceptors. These results, where a new fluorescent signal occurs in the near-infrared region on a completely dark background, offer substantial possibilities for designing highly sensitive turn-on sensors.National Institute of General Medical Sciences (U.S.) (F32GM086044)Massachusetts Institute of Technology. Institute for Soldier Nanotechnologies (DAAD-19-02-0002
Interferometric analysis of laser-driven cylindrically focusing shock waves in a thin liquid layer
Shock waves in condensed matter are of great importance for many areas of science and technology ranging from inertially confined fusion to planetary science and medicine. In laboratory studies of shock waves, there is a need in developing diagnostic techniques capable of measuring parameters of materials under shock with high spatial resolution. Here, time-resolved interferometric imaging is used to study laser-driven focusing shock waves in a thin liquid layer in an all-optical experiment. Shock waves are generated in a 10 µm-thick layer of water by focusing intense picosecond laser pulses into a ring of 95 µm radius. Using a Mach-Zehnder interferometer and time-delayed femtosecond laser pulses, we obtain a series of images tracing the shock wave as it converges at the center of the ring before reemerging as a diverging shock, resulting in the formation of a cavitation bubble. Through quantitative analysis of the interferograms, density profiles of shocked samples are extracted. The experimental geometry used in our study opens prospects for spatially resolved spectroscopic studies of materials under shock compression.Massachusetts Institute of Technology. Institute for Soldier Nanotechnologies (Contract W911NF-13-D-0001
Multi-frame Interferometric Imaging with a Femtosecond Stroboscopic Pulse Train for Observing Irreversible Phenomena
We describe a high-speed single-shot multi-frame interferometric imaging
technique enabling multiple interferometric images with femtosecond exposure
time over a 50 ns event window to be recorded following a single laser-induced
excitation event. The stroboscopic illumination of a framing camera is made
possible through the use of a doubling cavity which produces a femtosecond
pulse train that is synchronized to the gated exposure windows of the
individual frames of the camera. The imaging system utilizes a Michelson
interferometer to extract phase and ultimately displacement information. We
demonstrate the method by monitoring laser-induced deformation and the
propagation of high-amplitude acoustic waves in a silicon nitride membrane. The
method is applicable to a wide range of fast irreversible phenomena such as
crack branching, shock-induced material damage, cavitation and dielectric
breakdown
Fullwave Maxwell inverse design of axisymmetric, tunable, and multi-scale multi-wavelength metalenses
We demonstrate new axisymmetric inverse-design techniques that can solve
problems radically different from traditional lenses, including
\emph{reconfigurable} lenses (that shift a multi-frequency focal spot in
response to refractive-index changes) and {\emph{widely separated}}
multi-wavelength lenses (m and m). We also present
experimental validation for an axisymmetric inverse-designed monochrome lens in
the near-infrared fabricated via two-photon polymerization. Axisymmetry allows
fullwave Maxwell solvers to be scaled up to structures hundreds or even
thousands of wavelengths in diameter before requiring domain-decomposition
approximations, while multilayer topology optimization with degrees
of freedom can tackle challenging design problems even when restricted to
axisymmetric structures.Comment: 13 pages, 6 figure
Airborne LIDAR Measurements of Water Vapor, Ozone, Clouds, and Aerosols in the Tropics Near Central America During the TC4 Experiment
Large scale distributions of ozone, water vapor, aerosols, and clouds were measured throughout the troposphere by two NASA Langley lidar systems on board the NASA DC-8 aircraft as part of the Tropical Composition, Cloud, and Climate Coupling Experiment (TC4) over Central and South America and adjacent oceans in the summer of 2007. Special emphasis was placed on the sampling of convective outflow and transport, sub-visible cirrus clouds, boundary layer aerosols, Saharan dust, volcanic emissions, and urban and biomass burning plumes. This paper presents preliminary results from this campaign, and demonstrates the value of coordinated measurements by the two lidar systems
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