Cascaded wavelength conversion as favorable application of nonlinear optical polymers

This work is part of a continuing project that is investigating the use of integrated optical (IO) waveguides and optical fibers coated with nonlinear optical (NLO) polymers as electric field sensing devices. The IO waveguides were fabricated using three different techniques: (1) in the NLO polymer itself (on glass and silicon substrates), (2) in glass microscope slides using thin film silver diffusion and silver ion exchange methods, and (3) in thin film optical epoxies. The Ag+ glass and UV cured optical epoxy waveguides were subsequently coated with NLO polymers; the 3-D patterned waveguides provided lateral confinement of the optical wave that propagated in the polymer. A low index buffer layer of polysiloxane glass resin (Owens Illinois GR650E) was needed on the silicon substrates to decrease the propagation loss. The NLO polymers were dip coated or spun on these substrates and were poled with a corona poling technique with fields of the order of 3 MV/cm. Interferometric and polarization rotation methods were used to measure the effects of applied electric fields on the NLO devices at wavelengths of 633, 780, and 830 nm. Electrooptic coefficients were determined and compared with similar coefficients obtained using second harmonic generation methods on the same NLO polymers.

This paper presents a novel class of nonlinear optical (NLO) polymer for second harmonic generation (SHG). This new class of NLO polymer consists of the linear polyurethane, T-polymer, of tolylene 2,4-diisocyanate (TDI) with 3-[(2-hydroxyethyl)amino]-5-(hydroxymethyl)-4‘-nitroazobenzene (T-AZODIOL), whose dipole moment is aligned transverse to the main chain backbone. For comparison, the linear polyurethane L-polymer, whose NLO chromophore is incorporated into the main chain, is synthesized from TDI with 4-[N-(2-hydroxyethyl)-N-methylamino]-3‘-(hydroxymethyl)azobenzene (AZODIOL). T-polymer poled at optimum conditions of a corona poling voltage 8.0 kV, a temperature of 95 °C, and a time of 60 min shows a large second-order nonlinearity of d33 = 1.6 × 10-7 esu (67 pm/V) with good thermal stability at room temperature. The oriented NLO dipole moments for T-polymer do not show significant relaxation at ambient conditions after 60 days except for small depression a few days after poling, whereas, the SHG activity of L-polymer is largely decayed at room temperature. The better thermal stability of this new class of T-polymer is related to the smaller free volume in T-polymer.

The investigation of dipole relaxation processes in nonlinear optical (NLO) polymers containing chromophore dipoles with large hyperpolarizabilities is important for optimizing the poling process and for predicting the long-term stability of the poling-induced order. The primary or α relaxation is difficult to assess by dielectric spectroscopy in polymers with high glass transition temperature due to thermally induced chromophore degradation. A fast experimental procedure is developed for the investigation of dielectric relaxation processes in NLO polymers, without severely inducing chromophore degradation. The procedure is based on the measurement of the dielectric function ε̃(T)=ε′(T)−iε″(T) at a few frequencies from 30 Hz to 30 kHz, while heating the polymer at a constant rate. The complex plane representation of the temperature-dependent dielectric function is used to determine the distribution of relaxation times, while the temperature-dependent mean relaxation time τα(t) is numerically determined from the dielectric loss ε″(T). With only three decades in frequency, information on five decades in time or 90 K in temperature is gained above the glass transition temperature. The strong α and the weak β relaxation below the glass transition are separately investigated by thermally stimulated depolarization after a suitable two-step poling procedure. The method has been applied to a typical polyimidelike side-chain nonlinear optical polymer with modified Disperse Red 1 chromophores. Even below room temperature, a γ-relaxation process is observed, demonstrating significant mobility of the chromophores much below the glass transition. From the results of thermally stimulated depolarization it is concluded that the initial fast decay of the electro-optical response to a temporally stable value is related to the partial depolarization caused by the β relaxation.

We have synthesized and evaluated nonlinear optical (NLO) properties of azo dye attached novel NLO polymers with high optical nonlinearity and/or high thermal stability. An azo-dye attached NLO polymer with a furan derivative as an acceptor was synthesized. The second order NLO susceptibility (χ(2) 33) at 1.3 μm fundamental wavelength was more than two times larger than that of the NLO polymer with dicyanovinyl (DCV) acceptor and the same azobenzene structure. A χ (2) 33 of 43pm/V was obtained. Polymethacrylic acid reacted with a typical azo dye, Disperse Red 1 dye (DR-1), in its side-chain was also synthesized using a new approach, i.e., via intermediate reaction of the acid and N,N'- dicyclohexylcarbodiimide. Glass transition temperatures Tg of 170°C were obtained for the dye attached polymethacrylic acid with the DR- 1 contents of 29 mol%. A χ (2) 33 of 53pm/V at 1.3 μm was obtained. This polymer can be further improved to have more stablility by heat treatment. Through this treatment, the methacryl imide structure was formed and the polymer has Tg of 165°C. This imidized polymer exhibited a χ (2) 33 of 45 pm/V at 1.3 μm and maintained about 90% of its initial value after 230 hours or more at 100°C. Novel NLO polyarylethers where electron-withdrawing groups for NLO activity were introduced into the polymers by post-functionalization were developed. These NLO polyarylethers have high Tgs, typically higher than 200°C. A 100 mol% DR-1 dye containing NLO polymer with the fluorene structure has very low optical propagation loss of 0.42dB/cm at 1.3 μm wavelength and shows low polarized light dependence of the refractive indices. The polymer has χ (2) 33 of about 57pm/V at 1.3|um fundamental wavelength and the nonlinearity was maintained at 100°C for more than 200 hrs Key words: second order nonlinear optics, nonlinear optical polymers, thermal stability, furan derivative, polymethacrylic acid, azo dye, imidization, polyarylether

The purpose of this work is to investigate the effect of corona poling on the optical functions of the nonlinear-optical (NLO) polymers. We measured the complex refractive indices of NLO polymers deposited on glasses before and after corona poling using spectroscopic ellipsometry at room temperature at various incidence angles. In addition, we observed the absorption spectral change using ultraviolet-visible spectroscopy after corona poling, which is attributed to the chromophore orientation. In order to make NLO polymers, we attached a NLO active dye and photocrosslinkable moieties as side chains of poly(4-hydroxystyrene), deposited the polymer thin films on glass substrates by spin-coating, and corona-poled it along the surface-normal direction to align the dipoles of the chromophores along the electric field direction. In detail, PSDR1-25 designates a poly(4-hydroxystyrene) where 25% of hydroxyl groups in the side chain are substituted by disperse red 1 (DR1) and PSDR1-50 means the same except for 50% substitution. PSDR1 has the second-order NLO property, which is expected to show an acentric ordering by poling electric field. We used a parametric optical constant model to estimate the complex refractive index of the polymers. We found that the poling induced the uniaxial property of the complex refractive index of the polymers by aligning the chromophores along the poling direction. With increased poling voltage, we found that the oscillator strength of the major peak near the 2.4eV (designated as Ea) peak decreases in the surface-parallel direction (x), whereas that of the surface-normal direction (z) increases. Moreover, a detailed analysis of the dielectric functions using the standard critical point model showed that the poling induced a new peak Ec (∼2.6eV) between dominant Ea (∼2.3eV) and weak Eb (∼2.9eV) peaks for PSDR1-50 polymers. With increased poling voltage, the Ec peak strength increased compared to those of Ea and Eb peaks. We tentatively attribute Ec to lower symmetry, possibly Stark effect, induced in the chromophores by the corona poling.

The role of introduced isolation groups in PVK-based nonlinear optical polymers: Enlarged nonlinearity, improved processibility, and enhanced thermal stability

Poled nonlinear optical (NLO) polymer films are usually supported by substrates. This leaves two possibilities for investigating polarization and electric-field profiles in such samples with the thermal-pulse-technique: Thermal-pulse generation in the front electrode on the film or in the rear electrode between film and substrate. The influence of the substrate on the amplitude and on the long-time behavior of the thermal-pulse signal is theoretically treated. Experiments were performed on a poled double-layer sample consisting of a polymer film with NLO chromophores adhered to a similar polymer without chromophores, the latter film adhered to the substrate. The results demonstrate the validity of the proposed experimental approach.

Side-chain polymer containing the carbazolyldinitrophenylhydrazone moiety was synthesized for nonlinear optical (NLO) applications. Because free radical polymerization resulted in molecular weight limitation due to the chain transfer nature of the NLO active chromophore, to prepare a high molecular weight polymer, a polymer reaction was chosen to functionalize a preformed aldehyde prepolymer. This prepolymer (P1) was synthesized by the reaction of 6-(3-formyl-carbazol-9-yl)-hexyl methacrylate (M1) with methyl methacrylate (MMA). Successful polymerization was confirmed by 1H and 13C NMR spectroscopy and the molecular weight of the polymer was determined by gel permeation chromatography analysis. Polymer reaction of P1 with 2,4-dinitrophenyl hydrazine was carried out to prepare the NLO polymer (P2) and the reaction was monitored by NMR spectroscopy. This NLO polymer was soluble in common solvent and high optical quality thin film could be easily prepared for NLO activity measurement. The electro-optic coefficient (r33) of the polymer was determined 1.4 pm V−1 at poling field of 80 V μm−1.

Presented is the effect of using various cladding layers with different dielectric constants on the applied modulation voltage for nonlinear optic (NLO) polymer based integrated OE devices. The dielectric constants of the core and cladding materials used for NLOs polymer based integrated optoelectronic devices are typically very similar in magnitude. This suggests that even for low modulation rates, only 20% to 25% of the applied modulation voltage (voltage between the electrodes) is being dropped across the core region. With this small percentage of applied voltage reaching the NLO core layer, it becomes necessary to apply 4 to 5 times higher modulation voltage in order to achieve the desired (pi) phase change through the core.

Epoxy polymers with donor–acceptor type side groups were synthesized for application in nonlinear optics. The stability of the Pockels coefficient was measured in thin films after poling. The relaxation times and their temperature dependence seem to be correlated with dielectric measurements. Guest–host systems (polystyrene and polymethylmethacrylate with dimethylaminonitrostilbene) were investigated for comparison.

Chromophore Poling in Thin Films of Organic Glasses. 2. Two-Electrode Corona Discharge Setup In Part 1 of the article we provided description of the corona discharge physics and overview of the methods used for corona poling in thin organic films. Subsequent sections describe comparatively simple technical methods for poling the organic nonlinear optical polymers using a two-electrode (point-to-plate or wire-to-plate) technique. The polarization build-up was studied by the DC positive corona method for poling the nonlinear optical (NLO) polymers. The experimental setup provides the corona discharge current from 0.5 μA up to 3 μA by applying 3 kV - 12 kV voltage to the corona electrode and makes possible selection among the types of corona electrodes (needle, multi-needle, wire, etc.). The results of experimental testing of the poling setup show that at fixed optimal operational parameters of poling - the sample orientation temperature and the discharge current - the corona charging of polymeric materials can successfully be performed applying the two-electrode technique. To study the dynamics of both poling and charge transport processes the three-electrode charging system - a corona triode - should be applied.

In our previous work we introduced charge carrier blocking layers to realize an increase in the poling field and, hence, an increase in the nonlinearity, or electro-optic (EO) coefficient, r33, of the nonlinear optic (NLO) polymer disperse red 1:polymethylmethacrylate (DR1:PMMA). In addition, we not only achieved higher poling voltages, which resulted in higher r33s at these higher poling voltages, but we also observed higher r33s when both the samples with and without the charge carrier blocking layers were poled at the same poling voltage. We attributed that primarily to a decrease in the surface resistance. Here we provide a more detailed analysis and propose that the increase may be attributed not only to surface resistance but a combination of lower surface resistance, more uniform electric field distribution and charge carrier blocking, provided by the charge carrier blocking layers.

A compact and easy-to-use experimental set-up for pyroelectric investigations of poled nonlinear optical (NLO) polymers is reported. The arrangement allows for the investigation of the NLO dipole relaxation under isothermal or nonisothermal (e.g., linear heating of the polymer) conditions. Furthermore, polarization patterns in the plane of a polymer film and polarization profiles in its thickness direction can be measured in the same experimental set-up. In waveguide structures, it is possible to determine the thicknesses of an active guiding layer and of the claddings, if their thermal parameters are known. The isothermal pyroelectric relaxation data are discussed within the framework of phenomenological relaxation functions. It is found that they cannot be fitted with the stretched exponential function, a result which is in agreement with broadband dielectric-relaxation measurements. Pyroelectric experiments are the only method for investigations of all these problems in one and the same easy-to-use set-up.

A full‐scale analysis of the absorption edges by modified Tauc‐Lorentz models is essential in determining the optical bandgap and Urbach energy of semiconductors, transparent conductors, ionic compounds, and dielectric materials. This technique has not yet been applied to analyzing organic nonlinear optical (NLO) materials. This problem is tackled by preparing high‐quality films of guest–host NLO polymers with a wide thickness range from sub‐micron to 200 microns, allowing accurate measurement of full‐spectral absorption coefficients of NLO materials over four orders of magnitude by the UV‐VIS‐NIR spectroscopy. The Tauc model and a new Monolog–Lorentz model are used to study the optical absorption edge of guest‐host NLO polymers containing various push‐pull chromophores and the dependence of optical bandgap and Urbach edge on the structure and composition of materials is analyzed. The results reveal the critical transition of the Urbach exponential tail to a low energy tail that overlaps with vibrational overtones of materials at the telecom wavelengths. Determining the fundamental absorption region of organic NLO films in this study provides quantitative insight into the research to harness the resonance‐enhanced nonlinear coefficients of materials by operating at the wavelengths near the band edge with the control of optical loss.

Introduction. Organic Electro-optic Compounds. Nonlinear Optics and Electro-optics. Crystal Growth of Organic Materials. Langmuir-Bloddgett Films. Electro-optic and Nonlinear Optical Polymers. Experimental Methods. Nonlinear Optical and Electro-optic Properties of Organic Compounds. Electro-optics and Nonlinear Optics in Waveguides. Photorefractive Effects in Organic Materials. References.