Jiafu Wang

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.

Energy and charge transfer in organic light-emitting diodes: A soluble quinacridone study

A soluble derivative of quinacridone, N,N′-di-isoamyl quinacridone (DIQA), has been synthesized and used to study the mechanisms of Forster energy transfer and charge transfer in organic light-emitting diodes (OLEDs) based on 8-hydroxyquinoline (Alq3). Quantum efficiencies and spectra were measured for both photoluminescence (PL) and electroluminescence (EL) for films of poly(9-vinylcarbazole) (PVK) doped with Alq3 and DIQA. Both PL and EL showed an efficiency enhancement in films of PVK:Alq3:DIQA compared to films of PVK:Alq3. However, the optimal DIQA doping concentration was found to be lower for EL than for PL. Examination of the spectra revealed that more emission originated from DIQA for EL than for PL at a given doping level. We conclude that Forster energy transfer from Alq3 to DIQA occurs in both cases of PL and EL, but that charge transfer to DIQA occurs in the operation of the OLED resulting in additional pathways to DIQA emission. Ultraviolet photoelectron spectroscopy measurements showed that...

High-efficiency organic electrophophorescent devices through balance of charge injection

Balancing the charge injection plays an important role not only in fluorescence-based organic light-emitting devices, but also in the performance of the recently introduced organic electrophosphorescent devices. By achieving a better balance in the injected charges, it is possible to fabricate devices based on the pure red emitter 2, 3, 7, 8, 12, 13, 17, 18-octaethyl-21H, 23H-porphine, PtOEP, with external quantum efficiency close to 9%.

Phase stability of guest/host photorefractive polymers studied by light scattering experiments

We report on light scattering measurements in guest/host photorefractive polymers doped with different polar dyes as a function of temperature and sample composition. Crystallization processes of the polar dye are found to follow a nucleation and growth mechanism. The structure and melting point of the polar dye, the storage temperature, and the amount of plasticizer are shown to exert a major influence on the shelf lifetime of the samples. New composites with high efficiency and long shelf lifetime are presented.

Compact multimode pumped erbium-doped phosphate fiber amplifiers

The performance of compact multimode pumped erbium-doped phosphate fiber amplifiers is presented. A fiber amplifier with a small signal net gain of 41 dB at 1535 nm and 21 dB over the full C-band is demonstrated using a newly developed 8-cm-long erbium-doped phosphate fiber excited with a 1-W, 975-nm multimode laser diode. A theoretical model is developed for the multimode pumped amplifier based on modified rate equations and an effective beam propagation method. Close agreement between experimental and modeling results is observed.

Stabilization of the response time in photorefractive polymers

The optical and photoconductive fatigue of fast photorefractive polymers have been studied in a family of C60-sensitized polymer composites containing styrene-based chromophores with varying ionization potential. Changes in response time and in photoconductivity were studied for exposures up to 104 J/cm2. Increasing the chromophore ionization potential beyond that of the polyvinylcarbazole host was found to stabilize the response time. Studies of the electric-field dependence of the steady-state diffraction efficiency in various samples confirm the role of C60 anions as possible traps.

Linear and nonlinear optical properties of Co 3 O 4 nanoparticle-doped polyvinyl-alcohol thin films

Thin films made from a composite of the polymer polyvinyl-alcohol and cobalt oxide (Co3O4) nanoparticles were fabricated by spin coating. Linear and nonlinear optical properties of thin films with thicknesses of hundreds of nanometers were investigated. The refractive index and absorption coefficient were measured and two direct band gaps (Eg = 1.38 eV and 2.0 eV) were determined from the absorption spectrum. Reversed saturable absorption and saturable absorption were observed when the films were illuminated with the different fluences. Optical nonlinearities corresponding to reverse saturable absorption were measured by the z-scan technique. A nonlinear refractive index (n2) of ~10−10 cm2/W and nonlinear absorption (β) of ~103 cm/GW have been measured from 425 nm to 675 nm. The experimental results show that the Co3O4 nanoparticle/PVA composite is a promising material for nonlinear optical devices in the visible, since it takes advantages of the high optical nonlinearities of transition metal oxides and the superior mechanical properties and convenient fabrication properties of polymers.

All-fiber wavelength-swept laser near 2 μm

We report, for the first time to our knowledge, an all-fiber wavelength-swept Tm-doped laser based on a fiber Fabry-Perot tunable filter in the 2 μm spectral region. The laser wavelength can be continuously tuned over 200 nm from 1840 to 2040 nm in a short period of time. The demonstrated tuning speed was 12.5 μm/s with a tuning efficiency of 17.5 nm/V. The spectral linewidth of the laser was measured to be approximately 300 MHz or 0.01 cm(-1). This kind of laser can find potential applications in both high-resolution laser spectroscopy and tunable mid-IR generation via nonlinear frequency conversion.

Photorefractive polymers sensitized by two-photon absorption

We demonstrate the recording of holograms and their nondestructive readout in a photorefractive polymer, using two-photon absorption. Sensitivity is provided by the excitation of the electroactive chromophore with femtosecond pulses, followed by charge injection into the photoconducting poly(N -vinylcarbazole) matrix. The holograms can be fully erased with a pulsed laser source but are insensitive to cw laser beams with the same wavelength. Studies of the field and intensity dependence of the diffraction efficiency indicate that the holograms are formed through the photorefractive effect.

Ellipsometric determination of the electric-field-induced birefringence of photorefractive dyes in a liquid carbazole derivative

Abstract We have used ellipsometric measurements on solutions of 2-dihexylamino-7-dicyanomethylidene-3,4,4a,5,6-pentahydronaphthalene and related polyenes in liquid 9-(2′-ethylhexyl)carbazole to measure the electric-field-induced birefringence at wavelengths of 690 nm and 830 nm. This electric-field-induced birefringence is related to the product of dipole moment squared and the polarizability anisotropy, μ 2 Δ α , and to a large extent determines the dyes photorefractive figure-of-merit (FOM). We show that μ 2 Δ α increases with the conjugation length and report one of the highest μ 2 Δ α values observed so far.