Doo Hee Cho

Influence of 4f absorption transitions of Dy3+ on the emission spectra of Tm3+-doped tellurite glasses

Abstract We have demonstrated that a fluorescence emission centered at ∼810 nm responsible for Tm 3+ : 3 H 4 → 3 H 6 transition is effectively captured via re-absorption by an absorption transition corresponding to Dy 3+ : 6 H 15/2 → 6 F 7/2 in tellurite glasses. It is known that Tm3+-doped fiber amplifiers operating at the 1450 nm band suffer from amplified spontaneous emission (ASE) at around 810 nm. Therefore, our experimental results imply that a Dy3+-doped layer introduced in the vicinity of the Tm3+-doped layer in an optical fiber, as a representative example, can efficiently absorb the ∼810 nm emission from thulium and consequently reduce the ASE power.

Energy transfer from Tm3+: to Dy3+: in oxyfluoride tellurite glasses

Abstract Thermal analysis and optical properties indicate that Tm 3+ and Dy 3+ co-doped oxyfluoride tellurite glasses can be excellent candidates for fiber amplifiers at 1.47 μm region. Substitution of GeO 2 and In 2 O 3 in tellurite glasses increased the glass stability without decreasing the lifetime of the upper Tm 3+ : 3 H 4 level, and the fluoride substitution increased lifetime of excited energy levels of Tm 3+ ion. Dy 3+ co-doping in the Tm 3+ -doped oxyfluo⋮appride tellurite glasses enhanced the depopulation of the Tm 3+ : 3 F 4 level through an energy transfer Tm 3+ : 3 F 4 →Dy 3+ : 6 H 11/2 . We report the measurements of crystallization, glass transition temperatures, and optical properties of Tm 3+ , Dy 3+ co-doped oxyfluoride tellurite glasses.

A Light Scattering Layer for Internal Light Extraction of Organic Light-Emitting Diodes Based on Silver Nanowires

We propose and fabricate a random light scattering layer for light extraction in organic light-emitting diodes (OLEDs) with silver nanodots, which were obtained by melting silver nanowires. The OLED with the light scattering layer as an internal light extraction structure was enhanced by 49.1% for the integrated external quantum efficiency (EQE). When a wrinkle structure is simultaneously used for an external light extraction structure, the total enhancement of the integrated EQE was 65.3%. The EQE is maximized to 65.3% at a current level of 2.0 mA/cm(2). By applying an internal light scattering layer and wrinkle structure to an OLED, the variance in the emission spectra was negligible over a broad viewing angle. Power mode analyses with finite difference time domain (FDTD) simulations revealed that the use of a scattering layer effectively reduced the waveguiding mode while introducing non-negligible absorption. Our method offers an effective yet simple approach to achieve both efficiency enhancement and spectral stability for a wide range of OLED applications.

Simultaneously enhanced device efficiency, stabilized chromaticity of organic light emitting diodes with lambertian emission characteristic by random convex lenses

An optical functional film applicable to various lighting devices is demonstrated in this study. The phase separation of two immiscible polymers in a common solvent was used to fabricate the film. In this paper, a self-organized lens-like structure is realized in this manner with optical OLED functional film. For an OLED, there are a few optical drawbacks, including light confinement or viewing angle distortion. By applying the optical film to an OLED, the angular spectra distortion resulting from the designed organic stack which produced the highest efficiency was successfully stabilized, simultaneously enhancing the efficiency of the OLED. We prove the effect of the film on the efficiency of OLEDs through an optical simulation. With the capability to overcome the main drawbacks of OLEDs, we contend that the proposed film can be applied to various lighting devices.

Large area organic light emitting diodes with multilayered graphene anodes

In this work, we demonstrate fully uniform blue fluorescence graphene anode OLEDs, which have an emission area of 10×7 mm2. Catalytically grown multilayered graphene films have been used as the anode material. In order to compensate the current drop, which is due to the graphene’s electrical resistance, we have furnished metal bus lines on the support. Processing and optical issues involved in graphene anode OLED fabrications are presented. The fabricated OLEDs with graphene anode showed comparable performances to that of ITO anode OLEDs. Our works shows that metal bus furnished graphene anode can be extended into large area OLED lighting applications in which flexibility and transparency is required.

Piezoelectric and electrostatic microelectromechanical system actuator

Provided is a microelectromechanical system (MEMS) actuator in which a cantilever piezoelectric actuator and a comb actuator are combined to perform dual shaft drive. The MEMS includes: a stationary comb fixed on a substrate; a movable comb disposed separately from the substrate; and a spring connected to the movable comb and the substrate to resiliently support the movable comb, wherein the movable comb includes a piezoelectric material layer in a laminated manner to be perpendicularly moved by a piezoelectric phenomenon and laterally moved by an electrostatic force to the stationary comb, whereby the MEMS actuator can be used in a driving apparatus of an ultra-slim optical disk drive since the movable comb is made of a piezoelectric material to simultaneously perform focusing actuation to a Z-axis as well as planar actuation.

Trivalent holmium-activated fluoride fiber for potential 1340 /spl sim/ 1400 nm optical amplifiers

In summary, we propose that Ho/sup 3+/ and Ho/sup 3+//Yb/sup 3+/-doped fluoride glasses are a good candidate material for fiber amplifiers operating at the OH-band of silica fiber for optical transmission. Upconversion excitation in the wavelengths of 880-920 nm and 970-990 nm is achieved for Ho/sup 3+/ single-doped glasses and 910-1000 nm for Ho/sup 3+//Yb/sup 3+/-codoped glasses.