1,721,163 research outputs found
Replication Data for: Fragile proxies: Explaining rebel defection against their state sponsors
No full textForeign governments frequently intervene in armed conflicts by sponsoring rebels against their adversaries. A sponsorship is less costly than a direct military intervention, but rebels often defy orders, desert fighting, or turn guns against their sponsors. Under what conditions do rebels defect against their sponsors? Drawing on organizational theory, I argue that as rebel organizations become less centralized and formalized, the rebels are likely to defect against their sponsors. This occurs because non-centralized organizations have weak central leadership and allow for dispersed decision-making, both of which narrow the manipulative capacity of sponsors. Due to these disadvantages, non-centralized rebel movements are less accountable to their sponsors, cannot credibly commit to rapidly change their policies in response to changes in the sponsor’s demands, and suffer from frequent and destructive quarrels between the top and lower echelons. Using multilevel logistic models for panel data, I test my argument on a novel dataset. My quantitative analysis shows that rebel structure is a robust predictor of defection
Engineering Optical Forces in Waveguides and Cavities Based on Optical Response
No full textWe present a new treatment of optical forces, revealing that the forces in virtually all optomechanically variable systems can be computed exactly and simply from only the optical phase and amplitude response of the system. This treatment, termed the response theory of optical forces (or RTOF), provides conceptual clarity to the essential physics of optomechanical systems, which computationally intensive Maxwell stress-tensor analyses leave obscured, enabling the construction simple models with which optical forces and trapping potentials can be synthesized based on the optical response of optomechanical systems. A theory of optical forces, based on the optical response of systems, is advantageous since the phase and amplitude response of virtually any optomechanical system (involving waveguides, ring resonators or photonic crystals) can be derived, with relative ease, through well-established analytical theories. In contrast, conventional Maxwell stress tensor methods require the computation of complex 3-dimensional electromagnetic field distributions; making a theory for the synthesis of optical forces exceedingly difficult. Through numerous examples, we illustrate that the optical forces generated in complex waveguide and microcavity systems can be computed exactly through use of analytical scattering-matrix methods. When compared with Maxwell stress-tensor methods of force computation, perfect agreement is found.Sandia CorporationUnited States. National Nuclear Security Administration (contract DE-AC04-94AL85000)United States. Dept. of the Air Force. Office of the Director of Defense Research and Engineering (contract FA8721-05-C-0002
Optonanomechanical self-adaptive photonic devices based on light forces: A path to robust high-index-contrast nanophotonic circuits
No full textWe describe a proposed new class of optonanomechanical integrated photonic devices that can have self-adaptive behavior and self-adaptive optical frequency response, through the use of optical forces to manipulate their movable parts. We propose applications for this technology, and show how such devices can address the enormous dimensional and thermal sensitivity present in nanophotonic structures. Through synthesis of the optomechanical potential, we propose to design and control either the effective optical, or the mechanical, properties of the nanostructure, such as a giant effective optical nonlinear response, nonlinear dynamics and memory. We show device designs that can trap desired states at picometer resolution. We also describe the design of a novel, self-tuning microcavity design whose moving parts adjust in response to light forces alone to always place the resonance at the wavelength of the incident light over a wide wavelength range. This device concept provides an athermal resonator design (temperature-independent resonance frequency), without use of materials with negative thermooptic coefficients. It could also address a major challenge with conventional strong-confinement (high-index-contrast) integrated photonics - their extreme sensitivities - through a self-locking filter bank and optical cross-connect proposal, that in principle can use arbitrarily low power to trim resonant filter passbands to a wavelength channel grid.Director of Defense Research and Engineering (Air Force contract FA8721-05-C-0002
Dynamic cortical participation during bilateral, cyclical ankle movements: Effects of Parkinson's disease.
No full textParkinson's disease (PD) is known to increase asymmetry and variability of bilateral movements. However, the mechanisms of such abnormalities are not fully understood. Here, we aimed to investigate whether kinematic abnormalities are related to cortical participation during bilateral, cyclical ankle movements, which required i) maintenance of a specific frequency and ii) bilateral coordination of the lower limbs in an anti-phasic manner. We analyzed electroencephalographic and electromyographic signals from nine men with PD and nine aged-matched healthy men while they sat and cyclically dorsi- and plantarflexed their feet. This movement was performed at a similar cadence to normal walking under two conditions: i) self-paced and ii) externally paced by a metronome. Participants with PD exhibited reduced range of motion and more variable bilateral coordination. However, participants with and without PD did not differ in the magnitude of corticomuscular coherence between the midline cortical areas and tibialis anterior and medial gastrocnemius muscles. This finding suggests that either the kinematic abnormalities were related to processes outside linear corticomuscular communication or PD-related changes in neural correlates maintained corticomuscular communication but not motor performance
Average Weighted Monthly SPEI values for NUTS2016 Dataset
No full textThis dataset provides average weighted values of the Standardised Precipitation-Evapotranspiration Index(SPEI) for various NUTS2016 levels. SPEI is a multi-scalar measure of drought severity frequently used in multi-disciplinary scholarly research to detect, observe and examine the onset and termination of drought episodes. The SPEI dataset used here originates from The SPEI Global Drought Monitor], which offers near real-time information about drought conditions worldwide, with a 1 degree spatial resolution and a monthly coverage from January 1950 until March 2021. It is based on the Thortnthwaite equation for estimating potential evapotranspiration or PET. Using this equation, SPEI is calculated as the the difference between the precipitation (P) and PET for every available month.While the Thortnthwaite equation is less superior to the FAO-56 Penman-Monteith estimation of potential evapotranspiration when it comes to long-term climatological analysis, there are several advantages of using the estimations from the SPEI Global Drought Monitor. The main advantage is its near real-time character, which is best suited for drought monitoring and early warning purposes. This dataset also relies on less robust methods of PET computation, allowing for a more efficient processing of the dataset. Finally, this dataset is updated during the first days of the following month based on the most reliable and updated sources of climatic data. Average temperature data are obtained from the NOAA NCEP CPC GHCN_CAMS gridded dataset while monthly precipitation sums data are obtained from the 'first guess' Global Precipitation Climatology Centre(GPCC). The SPEI data was downloaded from the https://soton.eead.csic.es/spei/10/nc/spei01.nc in the 1-month scale and imported as stacked raster file with 855 layers in total (each layer is a month-year ranging from January 1950 until March 2021). Missing values were assigned based on the value suggested by the authors of the original dataset. Next, the stacked raster was processed in R using exactextractr package for zonal statistics of multiband raster files, which summarizes pixel values over polygonal areas. The NUTS2016 polygons were obtained from Eurostat. Using one of the most common methods in zonal statistics, the mean value of every monthly SPEI cell was intersected by the polygon units for each NUTS2016 shapefile separately (NUTS0, NUTS1, NUTS2 and NUTS3) and then weighted by the fraction of the cell that is covered. The resulting values range from negative (higher drought levels) to positive (lower drought levels)
Hebbian Neuroplasticity in the Human Corticospinal Tract as Induced by Specific Electrical and Magnetic Stimulation Protocols
Get PDFConventional functional electrical stimulation (FES) therapy, if provided shortly after an
incomplete spinal cord injury, is able to help an individual to restore voluntary hand
function. This is thought to occur through the induction of neuroplasticity. However,
conventional FES therapy employs a push-button-based control scheme, which does not
fully require the recipient to generate volitional movements. The first study in this thesis
therefore sought to determine, in an early proof-of-concept test with able-bodied
participants, whether control strategies which are triggered by volitional activity
(including an electroencephalography-based brain-machine interface (BMI-FES) and an
electromyogram-based control scheme (EMG-FES)) might provide greater benefits to
hand function. The results offer relatively weak evidence to suggest that BMI-FES, and
especially EMG-FES, were able to induce greater neuroplasticity than conventional
treatments in the corticospinal tract leading to the hands, but that this did not
immediately translate to more functional improvements such as maximum grip force.
ii
The second study in this thesis focussed on spinal associative stimulation (SAS), which
involves paired stimulation pulses at both the head (via transcranial magnetic
stimulation), and the wrist (via peripheral nerve stimulation). The purpose of this, as with
the first study, was to induce neuroplasticity and upregulate the corticospinal tract
leading to the hands. While limited research has suggested that it is possible to produce
neuroplasticity through SAS, all such studies have provided stimulation at a fixed
frequency of 0.1 or 0.2 Hz. The present study therefore sought to compare the
effectiveness of a typical 0.1 Hz paradigm with a 1 Hz paradigm, and a paradigm which
provided stimulation in 5 Hz “bursts”. None of the paradigms were able to successfully
induce neuroplasticity in a consistent manner. The increased variability in this study as
compared to the previous one, despite the nearly identical assessment methodology,
suggests that responses to the SAS treatment may have been highly individual. This
serves to highlight a potential limitation of the treatment, which is that its effectiveness
may not be universal, but rather dependent on each specific recipient. This may be a
challenge faced by SAS should it continue to be tested as a novel therapy.Ph
Going Beyond Counting First Authors in Author Co-citation Analysis
Get PDFThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Rapid adiabatic devices enabling integrated electronic-photonic quantum systems on chip
Get PDFQuantum systems’ integration in chip-scale photonic circuits is the most promising way to succeed in scaling up complex systems for applications ranging from quantum computation to secure communications.
Large systems with many components, especially for scaled all-optical quantum or classical processors, will require improved building blocks with greatly reduced loss, and enhanced bandwidth and robustness to fabrication uncertainties, temperature, etc. In this work, we introduce the concept of rapid adiabatic mode evolution that is the basis of a new family of passive devices with fundamentally improved performance, that we refer to as rapid adiabatic devices. In conventional adiabatic devices, a concept well known in photonics, the waveguide cross-section slowly evolves along the propagation direction, with no particular attention paid to transverse positioning of the cross-section. In contrast, in rapid adiabatic devices, we control the transverse position evolution (taking a tailored off-axis path while advancing along the direction of propagation).
This has a major impact on the dominant crosstalk mechanism, the limiting factor to all performance metrics. By judicious synthesis and design, the dominant crosstalk coupling mechanism can be minimized or even set to zero everywhere along the structure. This concept brings a new paradigm to photonic passives that we stand the test of time as an important tool in the integrated photonics tool-box. We experimentally demonstrate a new integrated 2×2 beam splitter design we call a Rapid Adiabatic Coupler (RAC) in different fabrication platforms. The design is implemented in state-of-the art, field-leading CMOS photonics platforms pioneered in our group, taking into account foundry-imposed limitations on design. It nevertheless shows field-leading, very low-loss and extremely broadband 50:50 splitting ratio over hundreds of nanometers of optical bandwidth. In addition, we also demonstrate other photonic passives based on the concept – Rapid Adiabatic Crossings (RAX), a Rapid Adiabatic Mode Splitter (RAMS) as well as a Polarization Splitter Rotator based on the RAMS. These new high performance, compact components will enable larger-scale systems on chip with a higher number of components, not only for quantum photonics applications but also for other types of systems for sensing, optical AI accelerators, optical “FPGAs”, optical switches and routers, optical communication links and others.
Another key building block for quantum photonic systems is integrated single photon sources. Following the first demonstration of a pair source integrated with pump filters by our group, here we demonstrate a monolithically integrated tunable photon pair source and pump filter on chip in a commercial, advanced 45nm CMOS microelectronics process. Next, we propose electronic-photonic quantum systems on chip, that contain monolithically integrated electronics and photonic components, as a platform to further scale up complexity in, and modularize, quantum systems on chip. As a first demonstration concept, we propose and demonstrate the first experimental step toward a “wall-plug” photon pair source implemented as an electronic-photonic monolithic chiplet. The idea is a CMOS die (or electronic-photonic block on the chip) that takes only electrical DC power, optical CW laser “DC power”, and control signals, and generates high quality photon pairs. The system contains a thermally tunable second-order filter with heater drivers integrated in the chiplet electronics to clean the input pump laser, a self-locking source ring with integrated electronic circuits that allow the ring resonance to automatically align to the pump laser and low-loss, high extinction, high-order thermally tunable filters.
These results taken together show that monolithic integration in CMOS micro-electronics processes does allow high performance photonics, while also supporting scalable complex circuits with electronic control to account for the extreme sensitivity of photonic components and impart reconfigurability and tunability; showing it as a viable approach to build large-scale electronic-photonic systems with a realistic path to commercial technologies.
This work was supported in part by the NSF RAISE-EQuIP program (Award 1842692) and by the Packard Foundation (Award 2012-38222).2023-05-23T00:00:00
Towards scalable quantum technologies: monolithically integrated electronic-photonic quantum light sources
No full textQuantum technologies are at the forefront of scientific advancements, with the potential to profoundly alter our lives. These quantum applications depend on a consistent, large-scale supply of qubits to realize their potential and achieve quantum advantage. The development of practical quantum technologies is currently hindered by the absence of a scalable quantum platform. Integrated silicon photonics, realized within state-of-the-art CMOS foundries, emerges as a promising solution to this challenge, enabling a scalable quantum source of light via the monolithic integration of thousands of photonic components alongside complex control electronics.
In this dissertation I introduce a scalable implementation of the electronic-photonic quantum source of light on chip. This system can be used as a fundamental block in more complex systems used to realize quantum computing, communication, or sensing. The physical realization of our quantum light source on chip is derived strictly from the first principles, while accounting for the imperfections associated with the manufacturing variability observed in CMOS foundries. By designing all critical elements of our system to offer active control over their frequency spectrum, we reinforce the system against all sources of technical noise and enable a long-term stable operation. This document details our comprehensive design approach and validates the scalability and functionality of our electronic-photonic quantum light source through experimental demonstrations.
Additionally, I present a theoretical design of a classical passive device that can achieve unprecedented laser linewidth narrowing at high efficiency. Through utilization of resonators defined by guiding material with a strong chi(2) nonlinearity and Q-engineering, we predict that the optical parametric oscillation (OPO) can efficiently convert the input laser light into two output waves, while all the laser noise is transferred in one of the output waves. I present analytical expression relating the conversion efficiency of the OPO process to the achieved linewidth narrowing, while taking into the account the underlying material platform and the quality of the optical resonator. This part of my research opens new avenues for precision laser applications, enhancing the prospects for simpler and inexpensive low-noise lasers.2026-09-11T00:00:00
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