12,749 research outputs found
Paraxial ray optics cloaking
Despite much interest and progress in optical spatial cloaking, a
three-dimensional (3D), transmitting, continuously multidirectional cloak in
the visible regime has not yet been demonstrated. Here we experimentally
demonstrate such a cloak using ray optics, albeit with some edge effects. Our
device requires no new materials, uses isotropic off-the-shelf optics, scales
easily to cloak arbitrarily large objects, and is as broadband as the choice of
optical material, all of which have been challenges for current cloaking
schemes. In addition, we provide a concise formalism that quantifies and
produces perfect optical cloaks in the small-angle (`paraxial') limit
Paraxial full-field cloaking
We complete the `paraxial' (small-angle) ray optics cloaking formalism
presented previously [Choi and Howell, Opt. Express 22, 29465 (2014)], by
extending it to the full-field of light. Omnidirectionality is then the only
relaxed parameter of what may be considered an ideal, broadband, field cloak.
We show that an isotropic plate of uniform thickness, with appropriately
designed refractive index and dispersion, can match the phase over the whole
visible spectrum. Our results support the fundamental limits on cloaking for
broadband vs. omnidirectionality, and provide insights into when anisotropy may
be required
Control of critical coupling in a ring resonator fiber configuration: application to wavelength-selective switching, modulation, amplification, and oscillation
By controlling the internal loss of a ring resonator near critical coupling, we demonstrate control of the transmitted power in a fiber that is coupled to the resonator. We also demonstrate wavelength-selective optical amplification and oscillation
One-dimensional wave equations defined by fractal Laplacians
We study one-dimensional wave equations defined by a class of fractal
Laplacians. These Laplacians are defined by fractal measures generated by
iterated function systems with overlaps, such as the well-known infinite
Bernoulli convolution associated with the golden ratio and the 3-fold
convolution of the Cantor measure. The iterated function systems defining these
measures do not satisfy the post-critically finite condition or the open set
condition. By using second-order self-similar identities introduced by
Strichartz et al., we discretize the equations and use the finite element and
central difference methods to obtain numerical approximations to the weak
solutions. We prove that the numerical solutions converge to the weak solution,
and obtain estimates for the rate of convergence
Engineering transverse Bragg resonance waveguides for large modal volume lasers
We recently analyzed a new class of laser amplifier based on transverse Bragg reflection. We show that the unique properties of Bragg confinement make it possible through modal loss discrimination to achieve single-transverse-mode operation with transverse modal size that is an order of magnitude larger than in lasers that depend on total internal reflection for transverse confinement
Effect of Socio-Demographics, Health-Related Problems, and Family Structure on Chronic Absenteeism Among Children
Purpose/Background: From 5 to 7.5 million school children are chronically absent, defined as missing ≥15 days of school within a year. Students miss schools due to various reasons such as health, socioeconomic status, and environmental factors. We examined child’s health and behavior, family structure, and socio-demographics to understand chronic absenteeism.
Materials & Methods: The population included children ages 6 to 17 years from the Medical Expenditure Panel Survey (MEPS) years 2008-2013. Multivariable logistic regressions were used to identify the risk factors of chronic absenteeism, adjusting for the complex sampling design.
Results: Among socio-demographic variables, age ≥14 years, race/ethnicity, lower-income family, public health insurance, US-born, and speaking English at home were associated with chronic absenteeism. Asians, Mexican Hispanics, and blacks have lower chronic absenteeism than whites. Among health-related variables, children using an inhaler for asthma, having behavioral problems, and less healthy than other children were more likely to be chronically absent. Among family variables, a smaller family size was a risk factor for chronic absenteeism.
Discussion/Conclusion: Asthma and behavioral problems were highly associated with chronic absenteeism. The identification of children at risk for chronic absenteeism will help the educational professionals identify the barriers to academic achievements and develop integrated educational interventions and policies to support disadvantaged children
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