Showing papers by "Alex K.-Y. Jen published in 2002"

Polymer-based optical waveguides: Materials, processing, and devices

Abstract: Polymer optical waveguide devices will play a key role in several rapidly developing areas of broadband communications, such as optical networking, metropolitan/access communications, and computing systems due to their easier processibility and integration over inorganic counterparts. The combined advantages also makes them an ideal integration platform where foreign material systems such as YIG (yttrium iron garnet) and lithium niobate, and semiconductor devices such as lasers, detectors, amplifiers, and logic circuits can be inserted into an etched groove in a planar lightwave circuit to enable full amplifier modules or optical add/drop multiplexers on a single substrate. Moreover, the combination of flexibility and toughness in optical polymers makes it suitable for vertical integration to realize 3D and even all-polymer integrated optics. In this review, a survey of suitable optical polymer systems, their processing techniques, and the integrated optical waveguide components and circuits derived from these materials is summarized. The first part is focused on discussing the characteristics of several important classes of optical polymers, such as their refractive index, optical loss, processibility/mechanical properties, and environmental performance. Then, the emphasis is placed on the discussion of several novel passive and active (electro-optic and thermo-optic) polymer systems and versatile processing techniques commonly used for fabricating component devices, such as photoresist-based patterning, direct lithographic patterning, and soft lithography. At the end, a series of compelling polymer optical waveguide devices including optical interconnects, directional couplers, array waveguide grating (AWG) multi/demultiplexers, switches, tunable filters, variable optical attenuators (VOAs), and amplifiers are reviewed. Several integrated planar lightwave circuits, such as tunable optical add/drop multiplexers (OADMs), photonic crystal superprism waveguides, digital optical switches (DOSs) integrated with VOAs, traveling-wave heterojunction phototransistors, and three-dimensionally (3D) integrated optical devices are also highlighted.

Highly efficient fluorene- and benzothiadiazole-based conjugated copolymers for polymer light-emitting diodes

Abstract: A series of highly efficient conjugated polymers have been synthesized for application in polymer light-emitting diodes (PLEDs). These polymers were based on the copolymerization between 9,9-dihexylfluorene and benzothiadiazole (BT). In several cases, a third comonomer was added to adjust the charge injecting and transporting properties of the polymers. All of these copolymers exhibited strong green emission at around 540 nm which can be attributed to either the charge transfer between an electron-rich segment and an electron-deficient BT-containing segment of the polymers or the Forster energy transfer between different polymer chains. These copolymers also exhibited very high photoluminescence quantum efficiencies up to 55%. A double-layer device using one of the copolymers as the emissive layer exhibited a low turn-on voltage (3.4 V), a very high external quantum efficiency (6.0%), and a high brightness of 59 400 cd/cm2.

Divalent osmium complexes: synthesis, characterization, strong red phosphorescence, and electrophosphorescence.

Abstract: We report new divalent osmium complexes that feature strong red metal-to-ligand-charge-transfer (MLCT) phosphorescence and electrophosphorescence. The general formula of the complexes is Os(II)(N−N)2L−L, where N−N is either a bipyridine or a phenanthroline and L−L is either a phosphine or an arsine. New polypyridyl ligands synthesized are 4,4‘-di(biphenyl)-2,2‘-bipyridine (15) and 4,4‘-di(diphenyl ether)-2,2‘-bipyridine (16), and the 1,10-phenanthroline derivatives synthesized are 4,7-bis(p-methoxyphenyl)-1,10-phenanthroline (17), 4,7-bis(p-bromophenyl)-1,10-phenanthroline (18), 4,7-bis(4‘-phenoxybiphen-4-yl)-1,10-phenanthroline (19), and 4,7-bis(4-naphth-2-ylphenyl)-1,10-phenanthroline (20). 4,4‘-Diphenyl-2,2‘-bipyridine (21) and 4,7-diphenyl-1,10-phenanthroline (22) were also used in these studies. Strong π-acid ligands used were 1,2-bis(diphenylarseno)ethane (23), cis-1,2-bis(diphenylphosphino)ethylene (24), and cis-1,2-vinylenebis(diphenylarsine) (25). Ligand 25 is used for the first time in these typ...

Highly Efficient and Thermally Stable Electro‐Optical Dendrimers for Photonics

Abstract: After a brief review on electro-optical (EO) polymers, the recent development of EO dendrimers is summarized. Both single- and multiple-dendron-modified nonlinear optical (NLO) chromophores in the guest–host polymer systems showed a very significant enhancement of poling efficiency (up to a three-fold increase) due to the minimization of intermolecular electrostatic interactions among large dipole moment chromophores through the dendritic effect. Moreover, multiple NLO chromophore building blocks can also be placed into a dendrimer to construct a precise molecular architecture with a predetermined chemical composition. The site-isolation effect, through the encapsulation of NLO moieties with dendrons, can greatly enhance the performance of EO materials. A very large EO coefficient (r33 = 60 pm/V at 1.55 μm) and high temporal stability (85 °C for more than 1000 h) were achieved in a NLO dendrimer (see Figure) through the double-end functionalization of a three-dimensional phenyl-tetracyanobutadienyl (Ph-TCBD)-containing NLO chromophore with thermally crosslinkable trifluorovinylether-containing dendrons.

Effect of Cyano Substituents on Electron Affinity and Electron-Transporting Properties of Conjugated Polymers

Abstract: A series of cyano-containing distyrylbenzenes were synthesized as the model compounds to systematically study the effect of cyano substituents on the redox behaviors of conjugated molecules. By introducing the electron-withdrawing functional groups (cyano and dicyanovinyl) onto the phenylene ring, both electron affinity and electrochemical stability of the resulting distyrylbenzenes are greatly enhanced. The results enabled us to design and synthesize a new class of highly electron affinitive, fluorene-based copolymers with these cyano-containing chromophores as comonomers. The effects of acceptor strength and side chain on electron-transporting properties of these polymers were also investigated. By properly adjusting copolymer compositions, a combined high electron affinity and transport was achieved in a statistic copolymer, poly(fluorenebenzothiadiazole−cyanophenylenevinylene) (PFB−CNPV). An external quantum efficiency up to 0.88% and brightness as high as 4730 cd/m2 were achieved in a double-layer li...

Perfluorocyclobutane-Based Arylamine Hole-Transporting Materials for Organic and Polymer Light-Emitting Diodes

Abstract: A series of in-situ thermally polymerizable hole-transporting materials based on the triarylamine-containing perfluorocyclobutanes (PFCBs) have been developed and characterized. Electrochemical studies reveal that the energy level of the highest occupied molecular orbital (HOMO) for these materials is between –5.1 and –5.3 eV, which is a good match for the work function of indium tin oxide (ITO), a commonly used anode for organic and polymeric light-emitting diodes. UV-vis absorption, photo- and electro-luminescence spectral studies indicate that aggregates are formed during the thermal polymerization. The PFCB-based materials are very robust after polymerization, which enables the fabrication of multilayer polymer light-emitting diodes. Highly efficient organic and polymer LEDs are demonstrated using these materials as the hole-transporting layer, illustrating the superb performance of the PFCB-based polymers.

Red electrophosphorescence from osmium complexes

Abstract: We report red electrophosphorescence from light-emitting diodes based on osmium (Os) complexes. Efficient red emission was achieved using an in situ polymerized tetraphenyldiaminobiphenyl-containing polymer as the hole-transporting layer and Os complexes doped blend of poly(N-vinylcarbazole) and 2-tert-butylphenyl-5-biphenyl-1,3,4-oxadiazole as the emitting layer. The emission peaks of the reported Os complexes, ranging from 620 to 650 nm, can be tuned by changing the structures of the ligands because the emission originates from triplet metal-to-ligand-charge-transfer excited state. The Os complexes trap both electrons and holes, which facilitates the direct recombination of holes and electrons on the complex sites. The peak external quantum efficiency and brightness achieved from the complexes were 0.82% and 970 cd/m2, respectively. The Commission Internationale de I’Eclairage chromaticity coordinates (x,y) for the best red emission from the complexes are (0.65, 0.33).

Red-emitting electroluminescent devices based on osmium-complexes-doped blend of poly(vinylnaphthalene) and 1,3,4-oxadiazole derivative

Abstract: Efficient red electrophosphorescence was achieved from double-layer light-emitting devices using osmium (Os)-complexes-doped blend of poly(vinylnaphthalene) (PVN) and 2-(4-tert-butylphenyl)-5-(4-biphenylyl)-1,3,4-oxadiazole (PVN:PBD) as the emitting layer. Blending PVN with PBD greatly suppresses the electromer emission of PVN. The PVN:PBD blend emanates a short-wavelength electroluminescence emission peaking at around 375 nm, which overlaps well with the absorption spectra of the Os complexes and ensures very efficient energy transfer to the Os-complex dopants. The best external quantum efficiency of the double-layer devices was 2.2%, with a photometric efficiency of 1.9 cd/A.

Bright and efficient exciplex emission from light-emitting diodes based on hole-transporting amine derivatives and electron-transporting polyfluorenes

Abstract: We report highly efficient and bright emission from exciplexes generated between a series of hole-transporting amine derivatives and two electron-transporting fluorene–dicyanophenyl (FCNP) copolymers. These exciplexes were formed at either the interface between tetraphenyldiamine-containing perfluorocyclobutane polymers and the FCNP copolymers, or in the blends of the FCNP copolymers with small molecule amine derivatives such as triphenylamine, N,N′-diphenyl-N,N′-bis(3-methylphenyl)-[1,1′-biphenyl]-4,4′-diamine, and N,N′-diphenyl-N,N′-bis(1-naphthyl)-[1,1′-biphenyl]-4,4′-diamine. The exciplex emission is largely dependent on the composition of the hole-transporting materials. The best device derived from these exciplexes demonstrated a very low turn-on voltage (2.8 V), a high external quantum efficiency (0.91%), and a high brightness of 3370 cd/m2. The desirable properties of these devices were attributed to the excellent electron transport ability of the FCNP copolymers.

A Binaphthyl–Bithiophene Copolymer for Light‐Emitting Devices

Abstract: We report the properties of a binaphthyl- bithiophene copolymer (PHBNT 2 ) used either as an emitter or as a hole transporter in light-emitting diodes. The polymer has a well-defined conjugation length between two binaphthyl units that is independent of its polymer chain length. It is easily soluble in common organic solvents and can form high quality thin films by a spin coating technique. The HOMO and LUMO energy levels of the PHBNT 2 were -5.42 and -2.50 eV respectively, as determined by cyclic voltammetry. A single layer device of ITO/PHBNT 2 /Al using PHBNT 2 as an emitter, had a turn-on voltage of 5.7 V and had an orange emission peak at 568 nm. The luminous efficiency for a double-layer device of ITO/PHBNT 2 /Alq 3 /Al using PHBNT 2 as the hole transporting layer is 0.57 lm.W -1 .

Optimized cladding materials for nonlinear-optic polymer-based devices

Abstract: We have investigated various conductive and nonconductive polymer materials suitable for use as cladding layers in nonlinear optic (NLO) polymer based opto-electronic devices. Our goal was to maximize the nonlinearity of the NLO core materials, while minimizing the total poling voltage and minimizing the absorption loss. Using a cladding material that is more conductive than the NLO core material, the majority of the applied poling voltage is dropped across the core, realizing a maximum EO coefficient with minimum applied poling voltage. We found, however, that there are tradeoffs between absorption loss, conductivity, refractive index, materials processability and materials compatibility when using off-the-shelf materials. Results are presented for a 3-layer device structure using a conductive polymer material for both the top and bottom cladding layers.

Nonlinear optic polymer electro-optic modulators for space applications

Abstract: Optoelectronic devices based on nonlinear optic (NLO) polymers, with electro-optic (EO) coefficients in excess of 100 pm/V at 1.06 μm and dielectric constants of < 3, have demonstrated 100+ GHz data rates with less than 4 volt operating voltages. This has gained interest from the space based applications community, since in addition to being tolerant to a space environment, electro-optic devices for space applications will also need to operate at high data rates and at low operational powers. We have investigated various NLO polymers for core materials as well as passive polymers with various conductivities, both ionic and electronic, suitable for use as optical cladding layers in NLO polymer based opto-electronic devices. Our goal was to find materials that would be tolerant to irradiation as well as maximizing the nonlinearity of the NLO core material, thus minimizing the total applied poling voltage, and minimize the optical absorption loss. Using a cladding material that is more conductive than the NLO core material, the majority of the applied poling voltage is dropped across the core, thus maximizing the EO coefficient with minimum applied voltage or power. We found, however, that it is necessary to balance the optical and electromagnetic properties of the materials with their processability and compatibility.

Novel europium and osmium complexes for pure red light emitting diode applications

Abstract: Pure and efficient red light-emitting diodes based on novel europium (Eu) and osmium (Os) complexes were demonstrated. The Eu complex, with dendron substituted diketone ligands, exhibits high photoluminescence efficiency of 45%. When a copolymer containing carbazole and 1,3,4-oxadiazole groups was used as the host, narrow electroluminescence at 617 nm was achieved, with a 'full width at half maximum of 4 nm and a maximum external quantum efficiency (η) of 0.80%, The Os complex shows pure red emission peaking at 650 nm. The Commission Internationale de l'Eclairage (CIE) chromaticity coordinates (x, y) are (0.65, 0.33). Maximum η and brightness achieved were 0.82% and 590 cd/m 2 , respectively.

Organic electro-optics: from molecules to devices

Abstract: Employing guidance from quantum and statistical mechanics, the electro-optic activity of organic materials has been increased to values greater than 100 pm/V at telecommunication wavelengths (e.g., 130 pm/V at 1.3 microns). Electro-optic materials now afford significant advantages in terms of bandwidth and electro-optic activity over competitive inorganic materials such as lithium niobate. Organic materials have also been found to be quite processable permitting the fabrication by reactive ion etching and photolithographic techniques of 3-D active waveguide structures and integration with both VLSI semiconductor electronics and silica fiber optics. Both stripline and microresonator structures have been fabricated, as have low-optical-loss coupling structures. A number of prototype devices demonstrating superior performance have been demonstrated; however, the long-term, in-field performance of such devices still remains to be evaluated. This article focuses on statistical mechanical theoretical methods that have aided the design of improved materials.

Efficient Two-Photon Absorbing Chromophores with Fine-Tuned л-Bridges

Abstract: A new series of symmetric two-photon absorbing chromophores with various л-bridges were synthesized. It has been shown that the spectra of two-photon absorption and two-photon induced fluorescence can be fined tuned by structural variations in the electronic nature and twist angle of л-bridges of D-л-D type chromophores. These chromophores exhibited large twophoton absorption cross-sections (up to 1800 GM), which were determined by two-photon induced fluorescence technique using femtosecond pulse excitation. The colors of two-photon pumped emission of these chromophores varied from blue to red (433 ∼ 604 nm) as the structure of л-bridges changed.

Effects of side-chain modification on poling efficiency of highly polarizable nonlinear optical chromophores in electro-optical polymers

Abstract: Monte Carlo simulations suggest that the functionalization of bulky side groups on highly efficient nonlinear optical chromophores will improve the poling efficiency of the electro-optic polymers by reducing the intermolecular electrostatic interactions from these large dipole moments (μ) chromophores. However, very little information has been provided from theoretical simulation to describe the optimal functionality of the bulky side group needed on individual chromophore in order to be compatible with its environment, e.g. neighboring chromophores and polymer matrix. To further understand the influence of side-chain modification of chromophore on both chromophore-chromophore and chromophore-polymer matrix interactions, we have synthesized a series of highly polarizable nonlinear optical chromophores with various side-chain modifications in terms of shape, rigidity and functionality. Linear E-O coefficients ( r 33 ) of these functionalized chromophores in amorphous poly(carbonate) were evaluated using the contact poling technique. Several important chromophore and polymer parameters, such as, steric hindrance and free volume were used to explain the overall results from chromophore-chromophore and chromophore-polymer matrix interactions on E-O property.

Polymer molecular environment effects of Disperse Red Vis-NIR absorption behavior

Abstract: Spectral absorption behavior of Disperse Red-1 and Disperse Red-19 dyes incorporated into a series of polymers by covalent attachment, representing various chemical structures, is characterized by photothermal deflection spectroscopy. Of particular interest are the spectral characteristics of the red edge of the main dye electronic absorption peak, and the fine structure in the near-IR, dominated by overtones of fundamental C-H and O-H stretching modes. The spectral structure in these key regions can be influenced by inter- and intramolecular interactions, or conformational or configurational changes in the dye. The NIR structure, in turn, will dictate absorption loss in optical devices prepared from these materials at key transmission wavelengths (1.3 and 1.55 um) for waveguide devices. A well characterized dye-polymer system, DR1-PMMA, is compared with two other polymer systems. Differences in spectral absorption behavior is assessed in terms of polymer host structure and bonding environment.

Novel perfluorocyclobutane (PFCB)-containing polymers and dendrimers for photonic devices

Abstract: A wide variety of aromatic trifluorovinyl ether monomers and highly fluorinated crosslinkable dendrimers have been developed via novel synthetic strategies. Through the thermal dimerization of trifluorovinyl ether moieties on the monomers or on the periphery of dendrimers, these monomers or dendrimers can be melt or solution polymerized to form perfluorocyclobutane(PFCB)-containing prepolymers with good processability for optical waveguide fabrication. By further thermal crosslinking, the resulting thermoset materials possess low optical loss (0.3-0.4 dB/cm at 1310 nm with 1% of DR-1 or DCM doping), high thermal stability (T g : 100-400 °C), good thermo-optic property, high solvent and humid resistance, and excellent mechanical flexibility. The combination of processability and performance in these PFCB-containing thermoset materials make them as ideal candidates for the fabrication of high-performance polymeric planar lightwave circuit components with the applications in the telecom and datacom optical networks.

Nanoscale tailoring of dendrimers and polymers for photonic and optoelectronic applications

Abstract: Recent development of dendron-containing NLO chromophores and polymers is summarized. By modifying the chromophore shape or applying the site isolation principle to these materials, we have systematically build up our understanding of how to molecular engineer the NLO materials. In this process, we have introduced the dentritic structures to these materials, varied from 3-D shaped dendritic chromophore, to fully-functionalized dendrimers with the center cores of NLO chromophores and crosslinkable periphery, and to side-chain dendronized NLO polymers. Compared to the conventional designed organic NLO materials, these nanoscale tailored NLO chromophores and macromolecules provide great opportunities for the simultaneous optimization of macroscopic electro-optic activity, thermal stability, and optical loss.

Organic light-emitting diodes incorporating nanometer thick films of europium-cored complexes

Abstract: Europium cored complexes may be used as a source of red emission in light emitting diodes. Novel europium cored complexes have been synthesized and incorporated into organic light emitting diodes (OLED's). These complexes emit red light at 615 nm with a full width half maximum (FWHM) of less than 5 nm. The europium complexes consist of one equivalent of europium chelated to three equivalents of a nonsymmetrical β-diketone ligand. The Claissen condensation of a polycyclic aromatic sensitizer and an ester of a fluorinated carboxylic acid create the ligands. The use of a sensitizer such as phenanthrene results in a ligand that has an emission band that directly overlaps with the absorption band of europium. The use of fluorinated chains improves the overall processibility as well as the charge transfer capability of the resulting metal cored complex. The europium core is further encapsulated by the inclusion of an additional polycyclic aromatic compound such as 4, 7 diphenyl - 1, 10 phenanthroline. Emission of 615 nm light is accomplished through excitation of the ligand and efficient Forrester energy transfer to the europium complex. A multiple layer device consisting of a substrate of indium tin oxide, followed by thin layers of BTPD-PFCB (with a thickness of 20nm), a polymer blend containing the europium complex (30 nm), followed by a layer of calcium (50nm) and finally a protective layer of silver (120 nm). The polymer blends were either poly(n-vinyl carbazole)(PVK) or poly vinyl naphthalene (PVN). The device performance was further improved by the incorporation of another lanthanide metal complex. These complexes were based upon similar ligands surrounding gadolinium. In these devices, there is a Dexter energy transfer as well as the Forster energy transfer. For the devices that are based on a PVN:PBD as a polymer host, the lowest turn on voltage was 12.0 volts. The devices that use PVK:TPD devices was 178 cd/m2 with an external quantum efficiency of 0.61%.For PVK:TKD the brightness was 116 cd/m2 with an external quantum efficiency of 0.048%. Devices that incorporate the gadolinium complexes have the turn on voltage of 5.6 volts. We report a maximum brightness of 201 cd/m2 with an external quantum efficiency of 1.0%.

Dendritic NLO chromophore with fluorinated dendrons for improving poling efficiency in electro-optic devices

Abstract: Encapsulated by highly-fluorinated dendrons, a nonlinear optical chromophore core, which is based on the phenyl-tetracyanobutadienyl (Ph-TCBD) thiophene-stilbene-based NLO chromophore, exhibits a large ~30-40 nm blue shift of the charge-transfer absorption maximum, 20 oC higher decomposition temperature, and most impressively, three times higher E-O coefficient. The combination of these appreciable improvements makes the molecular design of dendritic modification as a very promising molecular-engineering for next generation of E-O materials.

Highly Efficient Electron-Transporting Polymers for Light-Emitting Diodes

Abstract: A series of well-defined oligomers were synthesized to provide insight on the effect of the position of cyano substituents on the electronic properties of conjugated molecules and polymers. Two copolymers PF3CNP1 and PF1CNP1 composed of 9,9-di-n-hexylfluorene and 2,5-dicyanobenzene have been synthesized based on this model study. By attaching two electron-withdrawing cyano groups onto the phenylene ring, the electron affinity and electron transporting ability of polyfluorenes are greatly enhanced. The devices employing a conducting polymer, poly(3,4-ethylene dioxythiophene) (PEDOT) as the hole injecting layer, and the cyano- containing copolymers as the emitting and electron-transporting layer showed a much improved electroluminescence (EL) efficiency and brightness compared to that of the homopolymer, poly(9,9-di-n-hexylfluorene) (PHF) with the same device configuration. It is ascribed to the smaller energy barrier for electron injection and higher electron mobility due to increased electron affinity of the polymers through the cyano substituents. The device based on PF3CNP1 has the best performance with a low turn-on voltage (3.4 V), bright blue emission (5430 cd/m2 at a bias voltage of 7.4 V), and a maximum external quantum efficiency of 0.50 % due to both optimized electron affinity and charge carrier mobility.

Synthesis and Characterization of Novel Conjugated Light-Emitting Polymers

Abstract: Novel fluorene-based conjugated light-emitting polymers have been designed and synthesized. By varying the compositions of the polymer backbone, the charge-injecting and -transporting properties of the polymers were significantly improved. The light-emitting diodes (LEDs) using these polymers as the emissive layers exhibited low turn-on voltage, a high external quantum efficiency, and high brightness due to balanced electron and hole conductivity.

Organic light-emitting devices based upon divalent osmium complexes: II. Design, fabrication, and characterization of osmium nanoscale devices

Abstract: Efficient red electrophosphorescence was achieved from double-layer light emitting devices using osmium (Os) complex doped blends of either poly(vinylcarbazole) and 2- tert -butylphenyl-5-biphenyl-1,3,4-oxadiazole (PVK:PBD), or poly(vinyl naphthalene) and 2- tert -butylphenyl-5-biphenyl-1,3,4-oxadiazole (PVN:PBD) as the emitting layer. Blending PVN with PBD greatly suppresses the electromer emission of PVN. The PVN:PBD blend emanates a short wavelength EL emission peaking at around 375 nm, which well overlaps with the absorption spectra of the Os complexes and ensures very efficient energy transfer to the Os complex dopants. PVK:PBD has and EL emission around 450 nm which does not overlap the absorption bands of the osmium complexes as well and produces devices of lower efficiency, but PVK is a better transport layer and produces brighter devices. The best external quantum efficiency of the double-layer devices was 2.2%, with a photometric efficiency of 1.9 cd/A. The brightest device achieved was 1,400 cd/m 2 .