High uniform illumination of light-emitting diodes lighting with applying the multiple-curvature lens

Light-emitting-diode (LED) sources will play a very important role in the future. Nowadays, there are many traditional light sources gradually being replaced by LED sources. From the viewpoint of illumination, different environments have the same requirement uniformity of illumination. The basic reason for this is to insure human comfort. Therefore, the promotion of illumination uniformity is an especially important key issue. In this article, we propose one effective design to improve the uniformity of illumination of LEDs with Lambertian radiation profiles by inserting into the lighting system, multiple-curvature lens. The enhanced illumination uniformity of LEDs, in this system, is due to energy redistribution by controlling the direction of incident light, using geometrical optics theory. Ray tracing was applied for the simulations required. Finally, the uniformity of illumination is 76.17% in LEDs using multiple-curvature lens, which is twice the uniformity of conventional LEDs. Furthermore, the phenomenon of overload in the multiple-curvature lens is reduced by increasing the numbers of rings on the lens. The overload is less than 0.02 lm in the case of lens designed with 23 rings.     

Distribution of oxidation states of Cr ions in Ca or Ca/Mg co-doped Cr:Y3Al5O12 single-crystal fibers with nitrogen or oxygen annealing environments

The valence states of Cr ions in Ca or Ca/Mg co-doped Cr:Y₃Al₅O₁₂ (YAG) single-crystal fibers are studied. The fibers were grown using the laser-heated pedestal growth method, followed by annealing treatments up to 1500 °C. The concentrations of the Cr³⁺ and Cr⁴⁺ ions in octahedral and tetrahedral sites in oxygen or nitrogen environments were characterized. Above 700 °C, migration of Cr⁴⁺ between octahedral and tetrahedral sites takes place; its relative stabilization energy was estimated. For Ca,Cr:YAG annealed in an oxygen or nitrogen environment, it was 0.25 and 0.3 eV, respectively. For Mg,Ca,Cr:YAG annealed in oxygen or nitrogen, it was 0.47 and 0.49 eV, respectively. For the Ca,Cr:YAG crystal fiber (Ca/Cr=113.1%) with oxygen annealing, about 35% and 2.5% of Ca ions took part in charge compensation for Cr⁴⁺ in the octahedral and tetrahedral sites, respectively. The density of oxygen vacancies depends on the concentration of Ca ions. The estimated ratios of the unreacted oxygen vacancies to total oxygen vacancies were about 63% and 88% for oxygen and nitrogen annealing, respectively. The main limitation on the concentration of Cr⁴⁺ in the tetrahedral site of YAG is the presence of unreacted oxygen vacancies.     

Properties of CVD alumina-titania composite films grown at different CO2/H2 inputs

Amorphous alumina-titania (Al₂O₃-TiO₂) films were prepared on silicon substrates by low-pressure chemical vapor deposition (CVD) using a mixture of aluminum tri-sec-butoxide (ATSB) and titanium tetrachloride (TiCl₄) at different CO₂/H₂ inputs (the ATSB/TiCl₄/CO₂/H₂ system). The films had increased Al contents at higher temperatures and CO₂/H₂ inputs. The `splotchy' deposits were observed. The higher compressive internal stress at higher temperature was attributed to the films with a thinner thickness. Higher compressive internal stress and more Al-O bonding resulted in higher specific critical load. Films deposited at low temperature of 350 °C have a defected structure and a higher dielectric property, due to the non-stoichiometric nature at the Ti-rich composition. Resistivity decreased from 10¹¹ to 10⁸-10⁹ Ω cm after annealing. Breakdown voltages increased slightly with substrate temperature and were in the range of 2.3-6.4 MV/cm. Refractive indices were in the range of 1.71-2.28. Greater than 60% transmittance was observed at visible range for all films.     

Synthesis and Characterization of Two‐Photon Active Chromophores Based on Tetrathienoacene (TTA) and Dithienothiophene (DTT)

Three new donor–π–donor (D‐π‐D) tetrathienoacene (thieno[2′,3′:4,5]thieno[3,2‐b]thieno[2,3‐d]thiophene (TTA))‐cored chromophores, end‐functionalized with electron‐donating triphenylamine (TPA) groups, were developed and characterized for their two‐photon‐related properties by using both nano‐ and femtosecond laser pulses as the probing tools. TTA‐based chromophores exhibit stronger and more widely dispersed two‐photon absorption (2PA) than those of dithienothiophene (DTT)‐based congeners. As a consequence, the bithiophene‐conjugated TTA chromophore exhibits the highest maximum 2PA cross‐section value (up to 2500 GM) with good thermal stability, and thus, it is the best performing two‐photon chromophore among the studied model compounds. The bithiophene‐conjugated DTT analogue exhibits the second highest maximum two‐photon absorptivity of 1950 GM, which is nearly 7 times larger than that of previously reported DTT‐based chromophores.     

FDTD analysis of point-defected periodical dielectric waveguides

In this paper, we utilize the finite-difference time-domain method to analyze the transmission characteristics of the point-defected periodical dielectric waveguides (point-defected PDWGs). Each point-defected PDWG contains a defected cylinder distinct from the others. It is found that the optical transmittance becomes worse due to the shorter defected cylinder but it is not affected by the longer one. Moreover, the optical transmission is very sensitive to the radius of the defected cylinder whatever it is larger or smaller than that of the other cylinders.