Michael M. Morrell

Highly efficient and bright organic electroluminescent devices with an aluminum cathode

The electron injection process, which limits the electroluminescent performance of organic devices, has been enhanced tremendously by inserting a layer of LiF with appropriate thickness between the cathode and a quinacridone doped organic layer. Devices with an Al/LiF cathode demonstrated a luminance in excess of 20 000 cd/m2 and an external quantum efficiency of 3%, which is comparable to devices with a Mg/LiF cathode. These devices show maximum luminance of 45 000 cd/m2 prior to failure in continuous bias operation. For the same LiF thickness, the operating voltage for devices with Al/LiF was lower than the corresponding operating voltage for devices with Mg/LiF or Mg alone. Tunneling theory is used to explain this enhancement.

Bright blue organic light-emitting diode with improved color purity using a LiF/Al cathode

We report a two-layer, blue organic light-emitting diode with a 4,4′-bis(2,2-diphenylvinyl)-1,1′-biphenyl emission layer and a LiF/Al cathode which has an external quantum efficiency of 1.4% and a maximum luminance of 3000 cd/m2. Insertion of the thin LiF layer results in a 50-fold increase in the device efficiency compared to a device with an aluminum only cathode, and eliminates the need for an electron-transporting layer, such as tris(8-hydroxyquinoline)aluminum. This results in a device with excellent color purity with an emission peak at 476 nm and a full width at half maximum of 78 nm. Using ultraviolet photoelectron spectroscopy, we find that the effective work-function of aluminum decreases dramatically with sub-monolayer amounts of LiF deposited on the surface.

Waveguide distributed Bragg reflector laser arrays in erbium doped glass made by dry Ag film ion exchange

We demonstrate a distributed Bragg reflector (DBR) waveguide laser array by using an Ag film ion-exchange technique. We achieve an output power of 11 mW at 1540 nm for a coupled pump power of 145 mW, with a threshold of 60 mW and slope efficiency of 13%. The thin film ion exchange produces the highest index change possible at the surface, due to the ion exchange technique. Hence a wide array of wavelengths can be implemented in a single chip. We demonstrate a lasing wavelength range of 2.1 nm (1548.6 to 1550.7 nm) in an array with a single grating. Since the index change due to our process is large, we can fine tune the wavelengths of the array to fall on International Telecommunication Union (ITU) grid wavelengths by annealing. We demonstrate fine tuning of the wavelength for a channel from 1540.2 to 1540 nm, ITU specified wavelength, by annealing.

Short-length microstructured phosphate glass fiber lasers with large mode areas

We report fabrication and testing of the first phosphate glass microstructured fiber lasers with large Er–Yb-codoped cores. For an 11-cm-long cladding-pumped fiber laser, more than 3 W of continuous wave output power is demonstrated, and near single-mode beam quality is obtained for an active core area larger than 400 μm2.

Short cladding-pumped Er/Yb phosphate fiber laser with 1.5 W output power

We report experimental results on a high-power, cladding-pumped, heavily Er∕Yb co-doped phosphate fiber laser of very short length. Up to 1.5W cw laser power was obtained from an11-cm-long multimode-core active fiber with optimized input and output couplers, when pumped by a 15W diode laser at 975nm. The fiber laser was demonstrated at 1535nm with a linewidth <1.2nm, and a good beam quality of M2<3.

Near diffraction-limited laser emission from a polymer in a high finesse planar cavity

We report near diffraction-limited laser emission from the conjugated polymer BEH:PPV in a cavity made with two dielectric mirrors providing a high finesse planar cavity. The laser has a sharp intensity threshold, a strong directionality, and a high degree of polarization. These emission characteristics are compared with those of a single polymer layer without optical feedback.

Ultracompact cladding-pumped 35-mm-short fiber laser with 4.7-W single-mode output power

We report on ultracompact cladding-pumped fiber lasers, fabricated from single-mode phosphate glass microstructured optical fibers, with several watts of cw output at 1.5μm. A maximum cw output power of 4.7W has been achieved from a fiber laser that is only 35mm in length, corresponding to a yield of 1.34W∕cm of active microstructured fiber.

3-Dimensional thermal analysis and active cooling of short-length high-power fiber lasers

A fully 3-dimensional finite element model has been developed that simulates the internal temperature distribution of short-length high-power fiber lasers. We have validated the numerical model by building a short, cladding-pumped, Er-Yb-codoped fiber laser and measuring the core temperature during laser operation. A dual-end-pumped, actively cooled, fiber laser has generated >11 W CW output power at 1535 nm from only 11.9 cm of active fiber. Simulations indicate power-scaling possibilities with improved fiber and cooling designs.

Buried ion-exchanged glass waveguides:burial-depth dependence on waveguide width

A detailed theoretical and experimental study of the depth dependence of buried ion-exchanged waveguides on waveguide width is reported. Modeling, which includes the effect of nonhomogeneous time-dependent electric field distribution, agrees well with our experiments showing that burial depth increases linearly with waveguide width. These results may be used in the proper design of integrated optical circuits that need waveguides of different widths at different sections, such as arrayed waveguide gratings.

A numerical study of operational characteristics of organic light-emitting diodes

We propose a simple model for the device characteristics of organic single layer molecular or polymeric light-emitting diodes. The model is based on Poisson’s equation and the conservation law of charges. A bimolecular recombination process is incorporated phenomenologically, and boundary conditions are given by carrier injection functions. Equations for the electric field and carrier concentrations are formulated for single-carrier and double-carrier injection cases. The equations are solved for different parameters including carrier mobility and energy barrier height at the electrodes. Current–voltage characteristics, relative quantum efficiency, and emission distribution are obtained. The results show that (1) at least one barrier height should be low for device operation at low voltage, (2) high mobility is essential for devices with high brightness, (3) low electron mobility confines the emission region near the cathode and should be avoided to prevent electrode quenching. We also discuss the effects...