1,592 research outputs found
Active-matrix GaN micro light-emitting diode display with unprecedented brightness
Displays based on microsized gallium nitride light-emitting diodes possess extraordinary brightness. It is demonstrated here both theoretically and experimentally that the layout of the n-contact in these devices is important for the best device performance. We highlight, in particular, the significance of a nonthermal increase of differential resistance upon multipixel operation. These findings underpin the realization of a blue microdisplay with a luminance of 10⁶ cd/m²
Critical points of Wang-Yau quasi-local energy
In this paper, we prove the following theorem regarding the Wang-Yau
quasi-local energy of a spacelike two-surface in a spacetime: Let be a
boundary component of some compact, time-symmetric, spacelike hypersurface
in a time-oriented spacetime satisfying the dominant energy
condition. Suppose the induced metric on has positive Gaussian
curvature and all boundary components of have positive mean curvature.
Suppose where is the mean curvature of in and
is the mean curvature of when isometrically embedded in .
If is not isometric to a domain in , then 1. the Brown-York mass
of in is a strict local minimum of the Wang-Yau quasi-local
energy of , 2. on a small perturbation of in
, there exists a critical point of the Wang-Yau quasi-local energy of
.Comment: substantially revised, main theorem replaced, Section 3 adde
X-ray diffraction measurements of the c-axis Debye-Waller factors of YBa2Cu3O7 and HgBa2CaCu2O6
We report the first application of x-rays to the measurement of the
temperature dependent Bragg peak intensities to obtain Debye-Waller factors on
high-temperature superconductors. Intensities of (0,0,l) peaks of YBa2Cu3O7 and
HgBa2CaCu2O6 thin films are measured to obtain the c-axis Debye-Waller factors.
While lattice constant and some Debye-Waller factor measurements on high Tc
superconductors show anomalies at the transition temperature, our measurements
by x-ray diffraction show a smooth transition of the c-axis Debye-Waller
factors through T. This suggests that the dynamic displacements of the
heavy elements along the c-axis direction in these compounds do not have
anomalies at Tc. This method in combination with measurements by other
techniques will give more details concerning dynamics of the lattice.Comment: 4 pages, 2 figures. To be published in Physical Review B (Brief
Report
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Relationships Between Archimedean Copulas and Morgenstern Utility Functions
The (additive) generator of an Archimedean copula is a strictly decreasing and convexfunction, while Morgenstern utility functions (applying to risk aversion decision makers) arenondecreasing and concave. In this presentation, relationships between generators and utilityfunctions are established. For some well known Archimedean copula families, links betweenthe generator and the corresponding utility function are demonstrated.Some new copulafamilies are derived from classes of utility functions which appeared in the literature, andtheir properties are discussed. It is shown how dependence properties of an Archimedeancopula translate into properties of the utility function from whichthey are constructed
Effects of LED device size on UV-C short-range LoS optical wireless communication
We report systematic investigation of the device-size-dependent performance of ultraviolet C (UV-C) light emitting diodes (LEDs) for optical wireless communication (OWC). Utilizing 273 nm-wavelength devices with diameters in the range of 40 μ m to 300 μ m, the size-dependent electrical, optical and frequency response characteristics of AlGaN UV-C LEDs are analyzed. As the junction area scales down, the smaller devices present lower optical power but faster modulation speed. Based on a 1-m point-to-point OWC system, this study further explores the LED size effect on the communication performance including channel gain, signal-to-noise ratio (SNR), theoretical Shannon capacity, achievable data transmission rate, relevant ratio, and spectral efficiency (SE). The system employing a 60 μ m diameter (micro) LED transmitter achieves the highest average SNR and SE accompanying a data transmission rate up to a 5.53 Gbps at the forward error correction floor of 3.8 ×10−3 . These results suggest an optimal device diameter of ∼ 60 μ m for further development of high-performance UV-C short-range line-of-sight (LoS) OWC
Deep ultraviolet CMOS-controlled micro light-emitting diode array
We report a Deep Ultraviolet (DUV) AlGaN micro-light emitting diode (micro-LED) array driven by a matching array of electronic drivers implemented in Complementary Metal-Oxide-Semiconductor (CMOS) technology. This 40 × 10 pixel integrated device required improvements in micro-LED fabrication combined with control over the LED ground level in the custom designed CMOS chip. It allows each of the micro-LEDs, with a measured peak emission wavelength of 271 nm, to be addressed independently in continuous wave (CW) or nanosecond pulsed operation, with optical output powers and pulse energies per pixel of 80 μW and 0.2 pJ, respectively. Performance of this high-resolution electronically-driven array for multi-channel UV-C wireless communications is given as an example of its potentially wide-ranging uses
High-speed optical camera communication using CMOS-driven micro-LED projector
We demonstrate a 7.76-Mb/s optical camera communication system using a CMOS-driven micro-LED projector and a high-speed camera. PAM4 signal was transmitted based on the imaging multiple-input-multiple-output system. The results predict a potential data rate of over 1 Gb/s using the full micro-LED projector chip
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