Evaluation of polymer based third order nonlinear integrated optics devices

Nonlinear polymers are promising materials for high speed active integrated optics devices. In this paper we evaluate the perspectives polymer based nonlinear optical devices can offer. Special attention is directed to the materials aspects. In our experimental work we applied mainly Akzo Nobel DANS side-chain polymer that exhibits large second and third order coefficients. This material has been characterized by third harmonic generation, z-scan and pump-probe measurements. In addition, various waveguiding structures have been used to measure the nonlinear absorption (two photon absorption) on a ps time-scale. Finally an integrated optics Mach Zehnder interferometer has been realized and evaluated. It is shown that the DANS side-chain polymer has many of the desired properties: the material is easily processable in high-quality optical waveguiding structures, has low linear absorption and its nonlinearity has a pure electronic origin. More materials research has to be done to arrive at materials with higher nonlinear coefficients to allow switching at moderate light intensity ( < 1 W peak power) and also with lower nonlinear absorption coefficients.     

Enhanced electrooptical active materials based on n-hexyl group flexible isolation in NLO chromophores

Two principally novel organic nonlinear optical chromophores (1 and 2) with flexible (n-hexyl group) or rigid isolated (benzyl group) group are designed and successfully synthesized. The prepared chromophores were characterized by MS, ¹H-NMR and UV–Vis spectra. Their thermal stability was studied by thermal gravimetric analyzer and differential scanning calorimetry. Poled films of the chromophores doped in amorphous polycarbonate afford the maximum electro-optic tensor coefficient (r₃₃) equal to 39 pm/V, 63 pm/V for chromophore 1 and chromophore 2, respectively at the wavelength 1,064 nm. The reason of so large differences between these two chromophores’ linear electrooptics coefficients were explained within a framework of performed quantum chemical calculations and it is crucially dependent on distances between the chromophore molecules and the polymer chains.     

Synthesis and characterization of novel AZO chromophore with multi hydroxyl groups

Novel organic second order nonlinear optical chromophore with 4,4′-(5-(bis(2-hydroxyethyl)amino)-1,3-phenylene)bis(oxy)dibutan-1-ol as electron donor and azo thiophene as π-electronic bridge was synthesized and characterized by ¹H NMR spectra and mass spectra. After the introduction of four hydroxyl groups to the electron donor, the solubility of chromophore Z1 in water was improved greatly (0.5 g). Else, the solubility of chromophore Z1 in common organic solvents was still very good. The thermal decomposition temperature was above 250 °C, which was high enough for its application in both the preparation of optoelectronic devices and Second harmonic generation imaging. Electro-optical films prepared by chromophore Z1 showed us the largest second-order nonlinear optical coefficient was about 85 pm/V, which was large enough for the detection of second harmonic generation signal in water solution. Else, the photochemical stability of chromophore Z1 was also studied; the advantage of chromophore with azo bond was very obvious for improving the photochemical stability of organic second order nonlinear optical chromophores.     

Preparation of sol-gel films containing thiazole-based chromophore and their nonlinear optical performances

A push-pull thiazolylazo chromophore and alkoxysilane-terminated chromophore have been synthesized. Their structures were verified by elemental analysis, FTIR, UV-Visible spectra and ¹H NMR. Followed by a hydrolysis and copolymerization process of the alkoxysilane with poly(3-(trimethoxysilyl)propyl methacrylate) (PTMSPM), transparent hybrid films were obtained by spin-coating. From TGA thermogram the initial decomposition temperature of the hybrid film was determined to be 231 °C. The molecular hyperpolarizability of thiazolylazo chromophore was evaluated by solvatochromic method and the nonlinear optical coefficient value of the hybrid film was also calculated to be 56.8 pm/V by second harmonic generation (SHG) measurements. The poled film exhibits fairly high stability of optical nonlinearity in depoling experiment, implying its suitability for device applications.     

Structural, optical and non-linear optics properties of highly doped molybdenum indium oxide thin film

Molybdenum (Mo)-doped In₂O₃ thin film with 10 wt% was successfully prepared by evaporation method. After annealing at 600 °C the film changes it colour from very dark to a clear transparent film. SEM and AFM analysis reveal that the film is continuous with high metallic coverage >98 % and exhibits a granular structure with typical grain size of 50 nm. More interestingly, the film shows more than 90 % transparency from visible to near infrared region and with wide optical band gap of 4.26 eV. The widening of the band gap is due to the Burstein–Möss (BM) effect as Mo will occupy In sites within the structure of the film thus increasing the carrier concentration thus enhancing its electrical properties. The nonlinear optical properties of Mo-doped In₂O₃ film with glass substrate were investigated using z-scan technique. Under cw excitation the film exhibits large reverse saturation absorption and negative nonlinearities. The real and imaginary parts of third order susceptibility of the film were measured and found that the imaginary part which arises from the change in absorption is dominant.     

Design and preparation of novel Diels–Alder crosslinking polymer and its application in NLO materials

Principally novel crosslinking nonlinear optical and optoelectronics system based on Diels–Alder reaction was designed. The copolymer of methyl methacrylate and anthracen-9-ylmethyl methacrylate (PMMA-AMA) was used as a host polymer; chromophore ETO was used as guest chromophore; chromophore ETO and N,N-(methylenediphenyl)bismaleimide were used as crosslinker. The thermodynamic property of crosslinking system studied by differential scanning calorimeter (DSC) showed us that the glass transition temperature was about 65?°C and the crosslinking temperature was varied between 80 and 120?°C The crosslinking reactive speed and effectiveness were studied by ultraviolet absorption and infrared absorption spectroscopy with spectral resoluiotn 1 cm^?1. These results have indicated that the cross linking process could be finished at 110?°C for 20 min. Surprisingly, such EO polymer showed us a large EO coefficient of about 96.3 pm/V at wavelength 1 µm and excellent long term stability about 85% with respect to its initial value and can be kept after 250 h of heating at 80?°C.     

Organic materials for nonlinear optics

This review discusses nonlinear optical (NLO) polymeric materials. As an example of a second-order NLO material, a novel copolymer with a diazo dye attached is investigated. The second-order NLO coefficient χ⁽²⁾ of the copolymer reaches 1.0 × 10^?6 esu, which is 7 times larger than that of LiNbO₃. A third-order NLO coefficient χ⁽³⁾ larger than 10^?10 esu is obtained for polymers where NLO dyes are introduced into the polymer backbone. The optical transmission loss of these polymers is revealed to be around 2 dB/cm. As these polymers can be formed into channel waveguides using the photo-bleaching technique, they show promise for use in NLO devices because of their processability, transparency and large optical nonlinearity.     

Study of nonlinear optical properties of organic dye by Z-scan technique using He–Ne laser

Laser induced nonlinear absorption coefficient of Brilliant Green solution was measured by single beam open aperture Z-scan technique using a continuous wave He–Ne laser at the wavelength of 632.8 nm. It was found that the material exhibits multiphoton absorption type optical nonlinearity. Significant optical nonlinearity is an indicative that Brilliant Green dye is prominent material for low power nonlinear applications. Ultraviolet–visible and photoluminescence (PL) spectra of Brilliant Green solutions are also recorded. A strong linear absorption band with a peak at 625 nm has been observed, while the PL intensity was found to decrease due to quenching effect on increasing the concentration.     

Synthesis and properties of sulfonated polyimide–polybenzimidazole copolymers as proton exchange membranes

Chemical stability of polymer electrolyte membranes (PEMs) is the key factor affecting the lifetime of fuel cells. It is greatly desirable to develop the PEMs with both high proton conductivity and excellent chemical stability. In this study, a series of sulfonated polyimide–polybenzimidazole copolymers (SPI-co-PBIs) are synthesized via random condensation polymerization of 1,4,5,8-naphthalene tetracarboxylic dianhydride, 4,4′-bis(4-aminophenoxy)biphenyl-3,3′-disulfonic acid, and an amine-terminated polybenzimidazole oligomer. The ion exchange capacities of the resulting SPI-co-PBIs are in the range 1.90–2.47 meq g^?1. Under fully hydrated condition, the SPI-co-PBI membranes show higher proton conductivities than Nafion112. It is found that the incorporation of a small fraction of PBI moiety into the polyimide structure resulted in significant improvement in radical oxidative stability. For example, the SPI-co-PBI-19/1 containing 5 mol % PBI moiety shows only 0.6 wt% weight loss after being soaked in the Fenton’s reagent (3 % H₂O₂ + 3 ppm FeSO₄) at 80 °C for 150 min, whereas the corresponding benzimidazole group-free sulfonated polyimide is completely dissolved in the Fenton’s reagent at 80 °C for 140 min. The SPI-co-PBI membranes also show excellent hydrolytic stability due to the highly stable ladder structure of the benzimidazobenzisoquinolinone linkages.     

Low-temperature synthesis of ZnO nanoparticles for inverted polymer solar cell application

Zinc oxide (ZnO) nanoparticles were synthesized by a simple wet chemical method at low temperature. Morphologies, crystalline structure, and optical transmission of ZnO nanoparticles were investigated. The results showed that the average diameter of as-synthesized ZnO nanoparticles was about 4.9 nm, the nanoparticles were wurtzite-structured (hexagonal) ZnO and had optical band gap of 3.28 eV. Very high optical transmission (>80 %) in visible light region of ZnO nanoparticulate thin films was achieved. Furthermore, an inverted polymer solar cell consisted of ZnO nanoparticles and polymer were fabricated. The device exhibited an open circuit voltage (V_oc) of 0.50 V, a short circuit current density (J_sc) of 1.76 mA/cm², a fill-factor of 38 %, and a power conversion efficiency of 0.42 %.     

Design of an As2Se3-based photonic quasi-crystal fiber with highly nonlinear and dual zero-dispersion wavelengths

We propose an As₂Se₃-based highly nonlinear photonic quasi-crystal fiber with dual zero-dispersion wavelengths (ZDWs). Using a full-vector finite element method, the proposed fiber is optimized to obtain high nonlinear coefficient, low confinement loss and two zero-dispersion points by optimizing the structure parameters. Numerical results demonstrate that the proposed photonic quasi-crystal fiber (PQF) has dual ZDWs and the nonlinear coefficient up to 2600 W^?1 km^?1 within the wavelength range from 2 to 5.5 μm. Due to the introduction of the large air holes in the third ring of the proposed fiber, the ability of confining the fundamental mode field can be improved effectively and thus the low confinement loss can be obtained. The proposed PQF with high nonlinearity and dual ZDWs will have a number of potential applications in four-wave mixing, super-continuum generation, and higher-order dispersion effects.     

Synthesis and NLO properties of hybrid inorganic-organic films containing thiophene ring

A family of second-order nonlinear optical sol-gel films with a pendent thiophene chromophore has been developed. The key step in preparation these hybrids was the syntheses of alkoxysilane dye containing thiophene ring, which was accomplished by utilizing the urethane forming reaction. Molecular structural characterization for the resultant was achieved by FTIR, ¹H NMR and UV-Visible spectra. The microscopic optical nonlinearity of the thiophene chromophore was evaluated to be 7646 × 10^− 30 esu D by solvatochromic method and second harmonic coefficients (d₃₃) of the hybrid film was also calculated to be 41.2 pm/V by second harmonic generation (SHG) measurements, respectively. Furthermore, the stability of optical nonlinearity in poled film was also investigated through a depoling experiment.     

Surface morphology and thermal figure of merit of a new compound thin film

The third nonlinear optical properties of a new compound 4,4′-bis(3-methoxy benzylidene amino) biphenyl doped poly-methyl methacrylate (PMMA) have been studied using Z-scan technique. Experiments are performed using a continuous waveguide (cw) diode laser at 532 nm wavelength and 0.68 kW/cm² laser intensity. The optical power limiting behavior of sample doped PMMA was also investigated. It also shows a very good optical limiting behavior with a limiting threshold of 4.7 mW. We attribute the nonlinear absorption and optical limiting property of the sample film to two photon absorption effect at 532 nm. The experimental evidences of observing diffraction pattern in compound 4,4′-bis(3-methoxybenzylideneamino) biphenyl doped PMMA has been present. The refractive index change, Δn, and nonlinear refractive index, n ₂ determined from the number of observed ring. We obtained good values of Δn = 105.154 × 10^?4and n ₂ = 154.154 × 10^?7 cm²/W. Variation of refractive index with temperature, dn/dT, and figure of merit, H, are found to be 8.858 × 10^?6 1/°C and 5.316 × 10^?6, respectively. This large nonlinearity is attributed to a thermal effect resulting from linear absorption. Theoretical diffraction pattern that agree well with experimental one are generated using a wave theory.     

Bulk crystal growth,spectral, optical,thermal, electrical and third-order NLO properties of benzylidene malononitrile derivative single crystal: a promising material for nonlinear optical device applications

Bulk organic intramolecular charge transfer nonlinear optical single crystal of 2-(2,4-dimethoxybenzylidene) malononitrile (DMM) with the sizes of 24?×?18?×?13 mm³ have been successfully grown by slow evaporation solution growth technique at 35 °C using acetone as the solvent. The lattice parameters of the grown DMM crystal was evaluated by single crystal X-ray diffraction analysis. The optical transmittance (T) data was taken from the well-polished crystal of DMM, and cut-off wavelength (λ?=?483 nm) was identified by UV–Vis spectral studies. Thermal stability and melting point (145.78 °C) were studied with TGA–DSC analysis. The low value of the dielectric constant (ε_r) of DMM suggests that the crystal can be used in the microelectronics industry. The laser-induced damage threshold experiment shows that the grown DMM bulk crystal possess an excellent resistance to laser radiation with a high threshold up to 1.75 GW/cm², much larger than those of several known organic and inorganic NLO materials. The DMM crystal exhibits positive optical non-linearity and reverse saturation absorption. It also exhibited a nonlinear refractive index (n₂) in the order of 10^?11 m²/W, a nonlinear absorption coefficient (β) in the order of 10^?5 m/W, third order non-linear susceptibility (χ⁽³⁾) in the order of 10^?5 esu and a second-order molecular hyperpolarizability (γ) in the order of 10^?33 esu. All the above results indicate that DMM has a potential application as a useful NLO candidate.     

Preparation and characteristics of polyimide/CaMoO<Subscript>4</Subscript> nanocomposites with enhanced dielectric performance and thermal stability

The polyimide/CaMoO₄ nanocomposites were successfully prepared by ex-situ polymerization. (3-Aminopropyl) triethoxysilane (KH550) was used as coupling agent to disperse the CaMoO₄ nanoparticles in polyimide matrix homogeneously. To characterize the structure and properties of the obtained nanocomposites, Scanning electron microscope, Fourier transform infrared spectroscopy, X-ray diffraction, dielectric properties and thermal stability were studied. It was observed that the CaMoO₄ nanoparticles were dispersed homogeneously in the polyimide matrix without obvious aggregation. The results show that nano-sized CaMoO₄ will improve dielectric constant and decrease dielectric loss of the nanocomposites in the relatively high-frequency region (>10 kHz). And the nanocomposite with excellent dielectric properties of ε?=?3.04, δ?=?8.0?×?10^?3 was obtained after doping 2.5 wt% nano-sized CaMoO₄ into polyimides. Dielectric loss of the nanocomposites is reduced in low frequency domain (<10 kHz) and enhanced in high frequency domain (>10 kHz). In addition, the thermal stability of the nanocomposites was enhanced from 544 to 651?°C compared to pure polyimide.     

Side-chain second-order nonlinear optical poly(urethane-imide)/photosensitive polyimide blends with the improved dipole orientation stability by photo-crosslinking

A negative tone photosensitive polyimide (PSPI) was incorporated into a side-chain nonlinear optical poly(urethane-imide) (PUI) in an attempt to improve the dipole orientation stability of PUI. Upon UV irradiation after electric poling, the relaxation of the order parameter determined by the UV–Vis spectroscopy of the blend film was much suppressed, and the stability of dipole orientation was effectively improved. The improvement in the stability of dipole orientation results mainly from the photo-crosslinking of PSPI, which restricts the movement of the PUI molecular chains.     

Linear and nonlinear optical properties of RF sputtered (Pb,La)(Zr,Ti)O<Subscript>3</Subscript> ferroelectric thin films

Linear and nonlinear optical properties of (Pb,La)(Zr,Ti)O₃ (PLZT) ferroelectric thin films were presented in this paper. The PLZT ferroelectric thin films have been in situ grown on quartz substrates by radio-frequency (RF) magnetron sputtering at 650 °C. Their crystalline structure and surface morphologies were examined by X-ray diffraction and atomic force microscopy, respectively. It can be found that the PLZT thin films exhibit well-crystallized perovskite structure and good surface morphology. The fundamental optical constants (the band gap energy, linear refractive index, and linear absorption coefficient) were obtained through the optical transmittance measurements. A Z-scan technique was used to investigate the optical nonlinearity of the PLZT thin films on quartz substrates. The films display the strong third-order nonlinear optical effect. A large and negative nonlinear refractive index n ₂ is determined to be 1.21 × 10⁻⁶ esu for the PLZT thin films. All results show that the PLZT ferroelectric thin films have potential applications in optical limiting, switching, and modulated-type optical devices.     

Tensile properties of polyacrylonitrile- and pitch-based hybrid carbon fiber/polyimide composites with some nanoparticles in the matrix

The tensile properties and fracture behavior of polyacrylonitrile (PAN)- and pitch-based hybrid carbon fiber/polyimide composites with several types of nanoparticles (25 nm C, 20–30 nm β-SiC, 130 nm β-SiC, 80 nm SiO₂, and 300 nm SiO₂) added to the matrix were investigated. The tensile stress–strain curves of PAN- and pitch-based hybrid carbon fiber/polyimide composites with 25 nm C, 20–30 nm β-SiC, and 80 nm SiO₂ nanoparticles have complex shapes (jagged trace), whereas the tensile response of hybrid carbon fiber/polyimide composites with 130 nm β-SiC and 300 nm SiO₂ nanoparticles indicates an instantaneous failure. The stress after the initial failure in hybrid carbon fiber/polyimide composites improves by adding 25 nm C, 20–30 nm β-SiC, and 80 nm SiO₂ nanoparticles to the matrix and correlates with the fracture toughness of the polyimide matrix.     

Single‐Junction Binary‐Blend Nonfullerene Polymer Solar Cells with 12.1% Efficiency

A new fluorinated nonfullerene acceptor, ITIC‐Th1, has been designed and synthesized by introducing fluorine (F) atoms onto the end‐capping group 1,1‐dicyanomethylene‐3‐indanone (IC). On the one hand, incorporation of F would improve intramolecular interaction, enhance the push–pull effect between the donor unit indacenodithieno[3,2‐b]thiophene and the acceptor unit IC due to electron‐withdrawing effect of F, and finally adjust energy levels and reduce bandgap, which is beneficial to light harvesting and enhancing short‐circuit current density (J _SC). On the other hand, incorporation of F would improve intermolecular interactions through C? F···S, C? F···H, and C? F···π noncovalent interactions and enhance electron mobility, which is beneficial to enhancing J _SC and fill factor. Indeed, the results show that fluorinated ITIC‐Th1 exhibits redshifted absorption, smaller optical bandgap, and higher electron mobility than the nonfluorinated ITIC‐Th. Furthermore, nonfullerene organic solar cells (OSCs) based on fluorinated ITIC‐Th1 electron acceptor and a wide‐bandgap polymer donor FTAZ based on benzodithiophene and benzotriazole exhibit power conversion efficiency (PCE) as high as 12.1%, significantly higher than that of nonfluorinated ITIC‐Th (8.88%). The PCE of 12.1% is the highest in fullerene and nonfullerene‐based single‐junction binary‐blend OSCs. Moreover, the OSCs based on FTAZ:ITIC‐Th1 show much better efficiency and better stability than the control devices based on FTAZ:PC₇₁BM (PCE = 5.22%).     

Nonlinear optical investigations on Al doping ratio in ZnO thin film under pulsed Nd:YAG laser irradiation

In the present study, it has been reported on the effect of Al doping on linear and nonlinear optical properties of ZnO thin films synthesized by spray pyrolysis method. The structural properties of ZnO thin films with different Al doping levels (0–4 wt%) were analyzed using X-ray diffraction (XRD). The results obtained from XRD analysis indicated that the grain size decreased as the Al doping value increased. The UV–Vis diffused refraction spectroscopy was used for calculation of band gap. The optical band gap of Al-doped ZnO (AZO) thin films is increased from 3.26 to 3.31 eV with increasing the Al content from 0 to 4 wt%. The measurements of nonlinear optical properties of AZO thin films have been performed using a nanosecond Nd:YAG pulse laser at 532 nm by the Z-scan technique. The undoped ZnO thin film exhibits reverse saturation absorption (RSA) whereas the AZO thin films exhibit saturation absorption (SA) that shows RSA to SA process with adding Al to ZnO structure under laser irradiation. On the other hand, all the films showed a self-defocusing phenomenon because the photons of laser stay on below the absorption edge of the ZnO and AZO films. The third-order nonlinear optical susceptibility, χ⁽³⁾, of AZO thin films, was varied from of the order of 10^?5–10^?4 esu. The results suggest that AZO thin films may be promising candidates for nonlinear optical applications.