In this paper, we report on progress in the fabrication of a high speed optical phase modulator based on a nonlinear
electro-optic (EO) polymer with an in-plane slotline radio frequency (RF) electrode structure. Compared to microstrip, a
slotline RF electrode design has several potential advantages such as easy fabrication, high poling efficiency, low Vπ,
and suppression of current drift. The guest-host nonlinear polymer system AJL49/APC was used in the device. Design,
fabrication, and characterization are presented.
We investigate the effects of interfacial layers on the photovoltaic properties of bilayer organic heterojunction photovoltaic devices. The devices were fabricated using aluminum phthalocyanine chloride (AlPcCl) as electron donor and fullerene (C60) as electron acceptor. Two types of interfacial layers inserted between the transparent indium-tin-oxide anode and the AlPcCl layer were investigated: PEDOT:PSS and MoO3. We find that these interfacial layers have a strong influence on the device open-circuit voltage (VOC). The effects of temperature and illumination intensity on VOC were explored.
A major breakthrough in the area of organic electro-optic (EO) materials has been recently achieved. To go beyond the
oriented gas model limit for organic EO materials, new approaches of using nanoscale architecture control and
supramolecular self-assembly have been proved as a very effective method to create a new paradigm for materials with
very exciting properties. High-performance EO polymers were demonstrated by a facile and reliable Diels-Alder "click"
reaction for postfunctionalization and lattice hardening to improve EO activity and thermal stability. This type of "click"
chemistry paves the way to systematically study the relationship among EO activity, chromophore shape, and number
density of the chromophores. Reversible supramolecular interactions were also introduced to a new generation of EO
dendrimers and polymers to create self-assembled nano-objects, overcome strong intermolecular electrostatic
interaction, and improve their poling efficiency and stability. These self-organized EO materials were used as hosts in a
binary chromophore system to further improve chromophore number density and r33 value. With these novel approaches,
we succeeded in enlarging the full potential of organic NLO materials by a factor of 3~5 and developing a variety of
nano-structured organic EO materials with ultrahigh r33 values (>300 pm/V at the wavelengths of 1310 and 1550 nm,
more than 10 times that of LiNbO3) and excellent auxiliary property, such as thermal stability and optical transparency.
The success of these material developments has inspired the exploration of new device concepts to take full advantage of
organic EO materials with ultrahigh r33 values.
We investigate the dependence of the photovoltaic characteristics of organic photocells on the relative concentration of the donor-acceptor molecular complex. The devices were fabricated using a new [MEH-PPV] - co - [phenylene vinylene] blend with C60. We find that the morphology and device performance are strongly influenced by the molar fraction (x) of C60 in the electroactive layer of the device. The best device was obtained with x = 0.6 and manifested VOC = 0.85 V, JSC = 2.65 mA/cm2, FF = 0.42, and ηPext = 1%.
Using modified Teng-Man reflection ellipsometry, very high linear electro-optic coefficients (r33 = 250 - 300 pm/V) have been measured in thin films of poled organic glasses. The glasses consist of two chromophores designed to yield synergistically enhanced orientation during the poling process. The chromophores were ordered by the contact poling method under moderate electric fields of ~ 0.44 MV/cm. Compared to measurements made 1-4 hours after poling, the electro-optic coefficient relaxed to a value about 15% lower in a period of one week and thereafter remained relatively stable at room temperature. We report both standard Teng-Man reflection type measurements made at a 45° angle of incidence as well as a more complete analysis of nonlinear reflection ellipsometric data as a function of angle of incidence and optical bias. The more complete analysis takes into account the properties of the multilayer stack structure of the test samples consisting of glass/ITO/NLO-organic/gold. Limitations of a simple model to analyze Teng-Man reflection data will be discussed, as well as contributions of electrochromism.
Recent breakthroughs in developing exceptional organic electro-optic (EO) materials are reviewed. Whole series of guest-host polymers furnished with high μβ chromophores have shown large electro-optic coefficients around 100~160 pm/V @ 1.31μm. Moreover, new generation of NLO chromophores based on pyrroline and pyrrolizine acceptors have been designed and synthesized. To go beyond the typical oriented gas model limit for poled polymers, new approach of
using nanoscale architecture control and supramoleaular self-assembly has been proved as a very effective method to create a new paradigm for materials with very exciting properties. The approaches of employing Diels-Alder reactions for postfunctionalization and lattice hardening also provide a facile and reliable way to generate high-performance EO polymers and dendrimers. This type of "click" chemistry paves the way to systematically study the relationships between chromophore shape and number density, controlled self-assembly, in addition to provide the material properties needed for multi-layer device fabrication. Finally, a new generation of binary monolithic glasses has been developed that exhibit unprecedented high EO activities through careful manipulation of intricate supramolecular interactive forces for self-assembly. The results obtained from these poled binary organic glass materials (r33 as high as 310 pm/V at 1.31μm) are the highest values ever reported which are >10 times of the commercial lithium niobate crystals. The success of these material developments has recently inspired the exploration of new device concepts trying to take full advantage of the organic EO materials with ultrahigh r33 values.
Warren Herman, Wei-Yen Chen, Younggu Kim, Glenn Hutchinson, Wei Lou Cao, Yongzhang Leng, Victor Yun, Hongye Liang, Yi-Hsing Peng, Min Du, Lisa Lucas, Ping-Tong Ho, Julius Goldhar, Chi Lee
We report progress in the development of polymer waveguides and devices for photonic applications in three areas: non-photolithographic techniques for polymer waveguide fabrication, bistability in laterally-coupled polymer microring resonators, and ultrafast photoconductive switches fabricated from semiconducting polymers. The non-photolithographic techniques for waveguide fabrication under development include laser milling with an excimer laser and programmable automatic dispensing of multimode polymer waveguides using an Essemtech automatic dispenser. Asymmetric diffraction gratings fabricated using phase masks and the interference of two excimer laser beams have exhibited concentration of optical power into the 1st diffraction order. Polymer micro-ring resonators laterally coupled to a bus line were fabricated by lithography from benzocyclobutene with radii as small as 10 μm and free spectral ranges on the order of 20 nm. These devices exhibit bistability in the frequency domain which can arise from thermal or nonlinear optical changes in refractive index and that may have application for all-optical switching. Metal-polymer-metal switches fabricated with interdigitated electrodes in an inverted structure exhibited fast transient photoconductive pulsewidths under 20 ps in response to femtosecond pump laser pulses, but the measurement was bandwidth limited by the oscilloscope. Here we report pump-probe measurements that indicate carrier lifetimes on the order of 2 ps.
Methods that successfully predict the refractive index at near-infrared wavelengths of negatively birefringent polymer films for optical waveguide applications are presented. The starting point for these methods is a correlation based on connectivity indexes originally developed by Bicerano for the refractive index of isotropic polymers at visible wavelengths. This correlation is applied to a set of polyimides at near infrared wavelengths with modifications in order to improve its predictive power. The polyimides were synthesized by condensation of monomers to form the precursor poly(amic acid)s followed by imidization in solution. Solutions of the polyimides were then spin coated onto glass substrates and baked to produce films of 2-3 microns in thickness with a variable negative birefringence. The refractive index profiles of these films near 1320 nm were then measured in both the TE- and TM- modes using a prism-coupling technique. The average refractive index of these films was then compared to the prediction generated by the model. The agreement between the predicted and observed values has been sufficient to enable the rapid development of materials for optical waveguides without the need for many rounds of trial-and-error investigation. These techniques facilitate the development of specialized polymers for optical waveguide applications.
Alternating polyelectrolyte deposition (APD) is a room temperature process that can produce noncentrosymmetric ordered films of nonlinear optical polymers (NLOP). Previous studies using a stilbazolium-substituted polyepichlorohydrin (SPECH) as the NLO-active polycation exhibited a saturation of the SHG signal independent of the number of deposited layers. This saturation effect was thought to be due a change in the charge density in the outermost layers. The charge density is the main driving force for chromophore alignment. In this study, NLO-inactive spacer layers were used to regenerate the charge density before continuing with deposition of NLO-active layers. The overall SHG signal from these 'multi-deck sandwich' films was higher than previously obtainable with SPECH as the NLO-active material. The surface morphology, roughness, and film thickness have been characterized by Tapping Mode Atomic Force Microscopy.
Electrooptic films based on a new class of porphyrin chromophores with exceptionally large molecular first hyperpolarizabilities are begin developed. These chromophores feature donor and acceptor groups linked to the porphyrin macrocycle at the 5- and 15-meso positions through intervening ethynyl moieties. We report herein preliminary result that describe the ambient temperature assembly of ordered, electrooptic films using the Langmuir-Blodgett-Kuhn method. The facile formation of such films was mae possible by the preparation of amphiphilic chromophores containing hydrophobic aliphatic substituents to enhance the polar alignment at the air-water interface. Second harmonic generation and UV-Vis absorption were used to characterize the films as a function of the number of layers deposited. Polar films containing up to 10 layers were made by simple Y-type homolayer deposition, and up to six layers by the Y- type heterolayer method in which the porphyrin layers were interleaved with poly(t-butyl methacrylate). The polar order in these films relaxed exponentially. Ionic structures are recommended for improved stability.
Acentric films of stilbazolium-substituted polyepichlorohydrin (SPECH) and poly(sodium 4- styrenesulfonate) (PSS) were formed using alternating polyelectrolyte deposition (APD) on hydrophobic glass substrates. APD is a layer-by-layer technique for the formation of polymer films by alternately immersing a substrate in aqueous solutions of a polyanion and a polycation. APD provides precise control of the overall film thickness that through automated processing may exceed a thousand layers. The peak maximum UV-visible absorbance in transmission through the SPECH/PSS films was linear as a function of the number of bilayers. Second harmonic generation (SHG) was used as a tool to indicate acentric order to polarizable side-chain chromophores within the APD films. The SHG exhibited the expected quadratic intensity increase with film thickness after 30 bilayers had been deposited. During slow temperature ramping of the SPECH/PSS APD films on hydrophobic glass, in situ SHG measurements revealed that 90 percent of the polar order is retained at temperature well over 120 degrees C. The additional minima in the Maker fringe data, created by interference of the second harmonic waves generated at the two sides of a double-coated substrate, were found to go to zero, which is indicative of high quality films. The observed high thermal stability of the polar order makes the APD films attractive for device applications.
Geoffrey Lindsay, Andrew Chafin, Roy Gratz, Richard Hollins, Melvin Nadler, Eric Nickel, John Stenger-Smith, Rena Yee, Warren Herman, Peter Zarris, Paul Ashley
New thermally stable, spin-castable, electro-optic (EO) polymers designed for high frequency optical modulators are reported (the third generation accordion polymers). The softening temperature (the glass transition temperature) is about 240 degrees Celsius, and the upper limit on short term thermal baking stability is about 320 degrees Celsius. The refractive index at 1.3 microns is about 1.73 and fairly birefringent. The second-order nonlinear optic coefficient, d33, of a second generation accordion polymer containing essentially the same chromophore, measured by second- harmonic generation at 1.06 microns, is 120 pm/V (resonance enhanced by the 495 nm absorption). Measurement of the electro-optic coefficient, r33, is in progress. The added thermal stability in these polymers is due to the all- aryl amine electron donor. The molecular topology of the polymer backbone makes it possible for over 85 weight percent of the bulk material to be comprised of EO-active chromophore. The chromophores are configured in a head-to- head mainchain topology. The films are completely amorphous (no microcrystalline scattering sites).
Progress on second-order nonlinear optical polymers for high speed optical signal modulation is reported. New accordion polymers containing head-to-head backbone chromophores extended with a styryl-thienyl-vinyl linkage are described. The maximum absorption of the electronic transition (ground state yields excited state) for these polymers occurs at a wavelength of 494 nm. A corona-poled film of the ortho-xylyl bridged accordion polymer has a resonance-enhanced second- order nonlinear optical coefficient of 120 pm/V measured at 1.06 microns, and a glass transition temperature of about 187 degree(s)C. Along with linear polymer, polar cyclic dimers were formed during the polymerization. An extraction process was developed to remove the cyclic dimers from the polymer.
We report accelerated aging tests of nonlinear optical polymer planar waveguides subjected to electron irradiation at doses up to 25 MRad(Si). The decay of the second-order optical nonlinearity of dye-doped poly(methyl methacrylate) films was monitored by second-harmonic generation with the temperature maintained near the glass transition temperature. The results indicate a decreased orientational stability of the chromophores in irradiated PMMA films.
We report on the effects of electron irradiation at doses up to 44 MRad on planar waveguides fabricated from nonlinear optical polymers. The second-order optical nonlinearity was measured by second-harmonic generation before and after irradiation of two types of polymeric waveguides: one fabricated from a guest-host polymer and the other from a main- chain polymer. Mechanisms for observed effects are discussed.
An optical transmitter is described which uses a commercial LED coupled to an external modulator to achieve a 1.25 Gb/s data rate. Both a Mach-Zehnder modulator and a reverse ?? directional coupler were evaluated for modulation of the optical signal. Analytical expressions are derived for the extinction ratio in each device as a function of spectral width of the source. Experimentally, both devices produced an extinction ratio > 20 dB with peak output powers of —20.1 and —23.9 dBm respectively. We estimate that by minimising losses in the system, peak output powers of —12.5 and —15.1 dBm respectively can be achieved with the two devices using the available LED. We also discuss design improvements which can reduce the wavelength sensitivity of the ?? switch and describe a drive circuit capable of extending the upper frequency limit of the current switch from 200 to 1250 MHz.
Numerous studies have shown that the orientational decay rate of nonlinear optical (NLO) polymers increases as the glass transition temperature is approached. A high glass transition temperature is thus beneficial to long-term poling stability. Here we report on the synthesis and nonlinear optical properties of poly(N-MNA Acrylamide), which as a glass transition temperature of 205 degree(s)C. This side-chain polymer has the NLO organic molecule 2- methyl-4-nitroaniline (MNA) attached through a carbonyl group to an acrylic polymer backbone. Second harmonic Maker fringe measurements used to evaluate the second-order optical nonlinearity in corona-poled thin films resulted in a second harmonic coefficient d33 approximately equals 3 pm/V at 1064 nm. Thin films were quite transparent with an absorption maximum for poled samples occurring near approximately 320 nm.
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