Organic Electro-optic Research Articles

Synthesis and characterization of a julolidine-based electro-optic molecular glass.

Organic electro-optic (EO) materials have recently gained considerable attention owing to their advantages compared to inorganic EO materials. Among different kinds of organic EO materials, organic EO molecular glass exhibits desired prospect because of its high chromophore loading density and large macroscopic EO activity. The objective of this study is to design and synthesize a novel organic EO molecular glass JMG utilizing julolidine moiety as the electron donor, thiophene moiety as the conjugated bridge, trifluoromethyl substituted tricyanofuran derivate (Ph-CF3-TCF) as the electron acceptor. The JMG's structure was characterized through NMR and HRMS. The photophysical property, glass transition temperature, first hyperpolarizability (β) and dipole moment (μ) of JMG were determined through UV-vis spectra, DSC test and DFT calculation. JMG's Tg reached to 79 °C and it can form high-quality optical film. The theoretical calculation shows that the first hyperpolarizability (β) and dipole moment (μ) of JMG were calculated to 730×10-30 esu and 21.898 D. After connecting poling with the poling voltage of 49 V/μm at 90 ℃for 10 min, the highest EO coefficient (r33) of the poled JMG films reached to 147 pm/V. A novel julolidine-based NLO chromophore with two tert-butyldiphenylsilyl (TBDPS) groups was successfully prepared and characterized. TBDPS group is introduced as the film-forming group, and it also plays the role of isolation group, which can suppress the electrostatic interaction between chromophores, improve the poling efficiency and further enhance the EO activity. The excellent performances endow JMG with potential applications in device fabrication.

Fine-tuning of organic optical double-donor NLO chromophores with DA-supported functional groups.

New strategic chromophores with updated fine-tuning of previously reported BLD1 and BLD3 chromophores were designed. BLD1 and BLD3 have silicon functional groups on the donor unit, and the bridge has a good chance of self-assembling, so in the present study we fine-tuned the isolating groups to the bulky cyclic alkene to improve their dipole moment and organic electro-optic (OEO) properties as well. To demonstrate the impact of cyclic alkenes on the electron-donating groups in sensible NLO chromophore designs, a thorough analysis and comparison of the chromophore synthesis, UV-Vis calculations, solvatochromic behavior of the chromophore, DFT quantum mechanical calculations, thermal stabilities, and much lower dipole moments was conducted.

Polymer micro-ring modulator on silicon nitride platform

The development of silicon nitride (SiN) waveguide platforms has enabled the realization of photonic circuits with unprecedented performance. Despite the exciting progress, SiN suffers from intrinsic material properties that block it for high-speed electro-optic (EO) modulators. Here, we present an approach to heterogeneously integrate organic EO polymer on SiN for high-performance modulators. We utilize the high resistivity of SiN to achieving a strong overlap between the optical and electrical field. Furthermore, sol–gel SiO2 side-claddings inserted in waveguides shorten the inter-electrode distance and increase the EO activity to a much higher level. As a result, the fabricated micro-ring modulator exhibits an electrical tunability of 10 pm/V, corresponding to an in-device EO coefficient as 60 pm/V. High frequency tests in the device show a 3-dB bandwidth of 32 GHz and a data rate of 40 Gbps. The proposed approach will enable efficient mass-production of the micro-footprint modulators and promote the development of integrated SiN photonics.

Low half-wave voltage polymeric electro-optic modulator using CLD-1/PMMA for electrocardiogram (ECG) signal acquisition.

Electro-optic (EO) modulators are typically made of inorganic materials such as lithium niobate; the replacement of these modulators with organic EO materials is a promising alternative due to their lower half-wave voltage (Vπ), ease of handling, and relatively low cost. We propose the design and fabrication of a push-pull polymer electro-optic modulator with voltage-length parameters (VπL) of 1.28 V·cm. The device uses a Mach-Zehnder structure and is made of a second-order nonlinear optical host-guest polymer composed of a CLD-1 chromophore and PMMA polymer. The experimental results show that the loss is 1.7 dB, Vπ drops to 1.6 V, and the modulation depth is 0.637 dB at 1550 nm. The results of a preliminary study show that the device is capable of efficiently detecting electrocardiogram (ECG) signals with performance on par with that of commercial ECG devices.

Recent Progress in Design of Organic Electro‐optic Materials with Ultrahigh Electro‐optic Activities†

Comprehensive SummaryRationally designed organic electro‐optic (OEO) materials demonstrate ultra‐large electro‐optic (EO) activities, affording inorganic‐organic hybrid photonic devices with low drive voltage, large bandwidth, low energy consumption, and small footprint. OEO materials hold the potential to achieve EO coefficients (r33) over 1000 pm/V. Over the past decade, however, the best performance of OEO materials is limited to 300—600 pm·V−1. This is partly because of the concern of increasing dipole moment and optical loss due to the redshifted absorbance of high hyperpolarizability chromophores. Recent advance of theory‐guided design enables the OEO materials to achieve greatly enhanced hyperpolarizability and EO activity with dipole moment and propagation loss within acceptable constraints. Simultaneously, progress in hybrid device designs has greatly shortened the length of modulating waveguide, which resulted in significantly reduced sensitivity to propagation loss from redshifted absorption of OEO materials. Driven by theory‐guided design method, several high‐performance OEO materials have been presented with greatly enhanced EO coefficients beyond 1000 pm·V−1. This brief review summarizes the strategies to improve the EO activity including molecular engineering and hyperpolarizability, highlights the recent great progress in design of high‐performance OEO materials, and discusses the problems needed to be solved in application for current OEO materials.

Organic Electro-Optics and Optical Rectification: From Mesoscale to Nanoscale Hybrid Devices and Chip-Scale Integration of Electronics and Photonics

Organic Electro-Optics and Optical Rectification: From Mesoscale to Nanoscale Hybrid Devices and Chip-Scale Integration of Electronics and Photonics

Electro-Optic Activity in Excess of 1000 pm V-1 Achieved via Theory-Guided Organic Chromophore Design.

High performance organic electro-optic (OEO) materials enable ultrahigh bandwidth, small footprint, and extremely low drive voltage in silicon-organic hybrid and plasmonic-organic hybrid photonic devices. However, practical OEO materials under device-relevant conditions are generally limited to performance of ≈300 pm V-1 (10× the EO response of lithium niobate). By means of theory-guided design, a new series of OEO chromophores is demonstrated, based on strong bis(4-dialkylaminophenyl)phenylamino electron donating groups, capable of EO coefficients (r33 ) in excess of 1000 pm V-1 . Density functional theory modeling and hyper-Rayleigh scattering measurements are performed and confirm the large improvement in hyperpolarizability due to the stronger donor. The EO performance of the exemplar chromophore in the series, BAY1, is evaluated neat and at various concentrations in polymer host and shows a nearly linear increase in r33 and poling efficiency (r33 /Ep , Ep is poling field) with increasing chromophore concentration. 25 wt% BAY1/polymer composite shows a higher poling efficiency (3.9± 0.1 nm2 V-2 ) than state-of-the-art neat chromophores. Using a high-ε charge blocking layer with BAY1, a record-high r33 (1100± 100 pm V-1 ) and poling efficiency (17.8± 0.8nm2 V-2 ) at 1310nm are achieved. This is the first reported OEO material with electro-optic response larger than thin-film barium titanate.

Self‐Aligned Non‐Centrosymmetric Conjugated Molecules Enable Electro‐Optic Perovskites

AbstractSolution‐processed organic electro‐optic (EO) chromophores are well suited for integration in optical modulators on silicon photonics chips. They contain highly conjugated molecules with polar functional groups offering large dipoles and a large nonlinear optical response. However, these molecules form centrosymmetric aggregates that hinder poling efficiency and the resultant macroscopic EO response. Furthermore, at elevated temperatures, the alignment of poled molecules can be lost, leading to a reduction in the EO response. Inorganic EO materials exhibit excellent thermal stability but lower performance. Here 2D metal halide perovskite scaffolds that align EO molecules are reported. The approach heralds a material design strategy that combines the features of organic and inorganic EO materials. The EO material exhibits promising thermal stability with a performance approaching that of organic EO materials. Anchor diammonium non‐centrosymmetric molecules are anchored inside a 2D metal halide perovskite scaffold, thereby avoiding aggregation. The authors lever the thermal stability of this compound and pole the organic molecules under an electric field at 533 K. The 2D perovskites exhibit a macroscopic EO coefficient of 68 pm V−1—a twofold increase over LiNbO3. Density functional theory calculations show that the in‐plane alignment of the molecular dipole moments can account for the EO response.

Electro-Optic Modulation Using Metal-Free Perovskites.

Electro-optic (EO) modulation is of interest to impart information onto an optical carrier. Inorganic crystals-most notably LiNbO3 and BaTiO3-exhibit EO modulation and good stability, but are difficult to integrate with silicon photonic technology. Solution-processed organic EO materials are readily integrated but suffer from thermal degradation at the temperatures required in operating conditions for accelerated reliability studies. Hybrid organic-inorganic metal halide perovskites have the potential to overcome these limitations; however, these have so far relied on heavy metals such as lead and cadmium. Here, we report linear EO modulation using metal-free perovskites, which maintain the crystalline features of the inorganic EO materials and incorporate the flexible functionality of organic EO chromophores. We find that, by introducing a deficiency of cations, we reduce the symmetry in the perovskite crystal and produce thereby an increased EO response. The best-engineered perovskites reported herein showcase an EO coefficient of 14 pm V-1 at a modulation frequency of 80 kHz, an order of magnitude higher than in the nondefective materials. We observe split peaks in the X-ray diffraction and neutron diffraction patterns of the defective sample, indicating that the crystalline structure has been distorted and the symmetry reduced. Density functional theory (DFT) studies link this decreased symmetry to NH4+ deficiencies. This demonstration of EO from metal-free perovskites highlights their potential in next-generation optical information transmission.

Bis(4-dialkylaminophenyl)heteroarylamino donor chromophores exhibiting exceptional hyperpolarizabilities

Theory-guided design enables new organic electro-optic (OEO) materials with record hyperpolarizability and high electro-optic activity.

Systematic study of the structure-property relationship of a series of near-infrared absorbing push-pull heptamethine chromophores for electro-optics

This study reports the facile synthesis, characterization and quantitative structure-property relationship analysis of molecular and material properties of tricyanofuran-based (TCF) dipolar heptamethines with different electron donors of indoline ( F1 ), benzo[ e ]- indoline ( F2 ), benz[ cd ]indoline ( F3 ), and Michler’s base derivatives ( M1 and M2 ). The linear and nonlinear optical (NLO) properties of these chromophores have been thoroughly investigated, and the relationship between molecular and bulk response has been analyzed and compared with dipolar tetraene AJLZ53 as one of the best chromophores for electro-optic (EO) devices. In particular, we provide responsible data collection and analysis of optical and EO properties for poled thin films using a widely-recognized and accredited methodology of prism coupling system with the help of rigid oriented gas model. We found that these push-pull heptamethines with synthetic efficacy exhibit high near-infrared absorption, excellent chemical stability and large hyperpolarizabilities ( β ) varied from 1,023×10−30 esu for F1 , 3,047×10−30 esu for M1 , and 3,547×10−30 esu for F3 at 1,304 nm in poled films, respectively. The β values of these molecules are among the highest ones for TCF-based dipolar chromophores, and also agree well with reported analytical results in the solutions. In poled polymers with a modest chromophoric loading density of 1.3× 1020 cm−3, M1 and M2 give a high poled-induced noncentrosymmetric order and relatively large r 33 values around 40 pm V−1 at 1,304 nm, indicative of large μβ values and suitable structural modification for high poling efficiency. Furthermore, a binary EO polymer based on the co-loading of M1 and AJLZ53 achieve a large r 33 value of 143.3 pm V−1 at 1,304 nm. Our studies suggest that concise synthesis and molecular design of push-pull polymethines can be well guided by the tabulation of their linear and NLO properties in bulk materials, and streamline future development of high performance organic EO materials for photonic applications.

A Twist on Nonlinear Optics: Understanding the Unique Response of π-Twisted Chromophores.

Materials with large nonlinear optical (NLO) response have the ability to manipulate the frequency and phase of incident light and exhibit phenomena that form the basis of modern telecommunication systems. In molecule-based materials, the second- and third-order NLO performance is related to the hyperpolarizability (β) and second hyperpolarizability (γ) of the constituent molecules. The search for higher β materials is driven by the desire to keep pace with expanding demand for high speed data transmission, while discovery of high γ chromophores is crucial for the development of emergent photonic technologies reliant on manipulation of "light-with-light". For decades, it was believed that for highest performance, organic NLO materials must be composed of planar π-system chromophores, and much exploratory research focused on subtle molecular modifications, which generally yielded incremental increases in μβ, where μ is the molecular dipole moment. The surprising recent discovery that twisted π-system chromophores can exhibit dramatically higher β values than their planar analogues has revealed a new design paradigm and stimulated the development of high performance twisted intramolecular charge transfer (TICT) chromophores, which are composed of electron-donating and electron-accepting π-substituents joined by a sterically constrained twisted biaryl fragment. In such chromophores, the twisting of the π-system enforces charge separation in the electronic ground state, leading to large dipole moments and low-lying charge-transfer excitations. This unique electronic structure forms the basis for enhanced NLO response, with an archetypal TICT chromophore, TMC-2, exhibiting very large second- ( μβ = 24 000 × 10-48 esu) and third-order (γ = 1.4 × 10-33 esu) metrics in dilute low-polarity solutions. This Account summarizes several approaches to enhance μβ in various environments, including (1) manipulating the biaryl torsional angle, (2) modifying the electron accepting fragment, (3) extending conjugation, (4) adding multiple twisted fragments, (5) modifying chromophore side chains, and (6) tuning the chromophore environment. Another set of modifications is explored to enhance γ, including (1) coupling to a cyanine dye to hybridize the cyanine and TICT orbitals, (2) manipulating the donor and acceptor group identity. The extensive modifications described above yield a detailed understanding of TICT chromophore molecular NLO response and unambiguous evidence that such chromophores have the potential to revolutionize organic electro-optics.

Optimization of Plasmonic-Organic Hybrid Electro-Optics

Plasmonic-organic hybrid technology affords the potential for exceptional bandwidth, extremely small footprint, and very low drive voltages resulting in substantially improved energy efficiency for devices. Optical loss is a well-recognized problem for plasmonic technologies but is currently addressed with some notable success. Thereby, the optimization of electrically poled organic electro-optic (OEO) materials is most critical since a large electro-optical coefficient allows implementation of short active device structures that result in lower insertion losses and lower voltage-length products. Most importantly, short structures also guarantee largest bandwidths and best energy efficiencies. Yet, an efficient optimization of in-device performance of OEO materials requires the development of novel computational simulation methods, especially as waveguide width dimensions reach tens of nanometers in plasmonic waveguides and as electrode surface/material interfacial effects become more and more dominant. The focus of this communication is on novel multi-scale modeling methods, including coarse-grained Monte Carlo statistical mechanical simulations combined with quantum mechanical methods to simulate and analyze the linear and nonlinear optical properties for high chromophore number density solid-state OEO materials. New chromophores are developed with the assistance of theory and may lead to an order of magnitude improvement in device performance.

Monolithic NLO chromophores with different pendant groups and matrix-assisted-poling effect: Synthesis and characterization

Realization of the large electro-optic (EO) activity for organic nonlinear optical (NLO) chromopores with high first order hyperpolarizability requires simultaneous optimization for molecular structure. In this work, we investigated the influences of different pendant groups in chromophore's side chains on the performance of EO materials. A series of NLO chromophores F1-F3 based on the same conjugated structure but with different pendant groups have been synthesized and systematically investigated. Due to the disperse-red type pendant groups, chromophore F3 shows excellent film-forming ability and relatively high glass transition temperature without blending with optically inactive polymer matrix. The Variable Angle Spectroscopic Ellipsometry (VASE) measurement proves the neat F3 film displays a higher index of refraction value and weak absorption at 1310 nm and 1550 nm, which is beneficial for reducing the operation voltage and optical loss of modulator. Due to the effective isolating ability of pendant groups, the best EO coefficient of the poled F3 film reaches to 81 pm/V, which is two times larger than the value of the same type chromophore in guest-host material. This result, together with high index of refraction and low optical loss, will shed light on a new perspective for the design of new monolithic organic EO materials.

Facile preparation of crosslinkable organic EO polymers based on AZO nonlinear optical chromophore: great indemnification for long term of stability

Due to the simple preparation technology and appropriate crosslinking process, polyurethane was selected as the host polymer for the preparation of organic electro-optics (EO) materials. The synthesis and crosslinking process of polyurethane demanded the nonlinear optical (NLO) chromophore possessing strong stability in acid environment. Fortunately, after the introduction of azo group in the electronic bridge, the stability of this kind of chromophores in acidic environment was improved greatly. Polyurethane EO polymers and EO films based on this kind of chromophore were prepared successfully. The crosslinking process was monitored by infrared spectroscopy clearly. The crosslinking process has direct relationship with the molecular weight of the crosslinkers. Crosslinker of triethanolamine with small molecular weight showed low crosslinking temperature (80 °C) and short crosslinking time (20 min). The crosslinking temperature and time was improved to 140 °C and 60 min based on Poly(bisphenol A glycidyl ether aniline) (BPAN) crosslinker with large molecular weight. More importantly, this kind of EO polymers showed us large second order nonlinear optical coefficients (76.1 pm/V) and good long term stability (130 °C).

Benefits of the use of auxiliary donors in the design and preparation of NLO chromophores

Organic electro-optic (EO) materials have been widely studied and used in microwave photonic devices, due to their advantages in half-wave voltage, bandwidth, low cost and ease of integration. Nonlinear optical (NLO) chromophores, as the core of organic EO materials, can directly decide the characters of organic EO materials, including large EO activity, long term stability, solubility and optical loss. Auxiliary donors have attracted great attentions in NLO chromophores for their strong electron donating ability and significant isolation effect. This feature article describes the application of auxiliary donor in NLO chromophores, including the EO activities, optical characters, stability, crystal structures and machinability.

Optimum Exchange for Calculation of Excitation Energies and Hyperpolarizabilities of Organic Electro-optic Chromophores.

Organic electro-optic (OEO) materials integrated into silicon-organic hybrid devices afford significant improvements in size, weight, power, and bandwidth performance of integrated electronic/photonic systems critical for current and next generation telecommunication, computer, sensor, transportation, and defense technologies. Improvement in molecular first hyperpolarizability (β), and in turn electro-optic activity, is crucial to optimizing device performance. Common hybrid density functional theory (DFT) methods, while attractive due to their computational scaling, often perform poorly for optical properties in systems with substantial intramolecular charge-transfer character, such as OEO chromophores. This study evaluates the utility of the long-range corrected (LC) DFT methods for computation of the molecular second-order nonlinear optical response. We compare calculated results for a 14-molecule benchmark set of OEO chromophores with the corresponding experimentally measured β and one-photon absorption energy, λmax. We analyze the distance dependence of the fraction of exact exchange in LC-DFT methods for accurately computing these properties for OEO chromophores. We also examine systematic tuning of the range-separation parameter to enforce Koopmans'/ionization potential theorem. This tuning method improves prediction of excitation energies but is not reliable for predicting the hyperpolarizabilities of larger chromophores since the tuning parameter value can be too small, leading to instabilities in the computation of βHRS. Additionally, we find that the size dependence of the optimal tuning parameter for the ionization potential has the opposite size dependence of optimal tuning parameter for best agreement with the experimental λmax, suggesting the tuning for the ionization potential is unreliable for extended conjugated systems.

Benzocyclobutene barrier layer for suppressing conductance in nonlinear optical devices during electric field poling

We measured the electro-optic (EO) coefficients (r33) of thin-film devices made from several monolithic, high number density organic EO chromophores with and without additional charge barrier layers. We found that a cross-linkable benzocyclobutene layer was very effective in suppressing unwanted, leakage current, keeping the effective poling voltage nearly identical to the applied voltage. This barrier layer proved to be superior to a titanium dioxide (TiO2) barrier layer. The suppression of the leakage current in combination with a new chromophore enabled the construction of EO devices that had r33 values in the range of 400–500 pm V−1 with poling fields ≥ 85 V μm−1.

Comparison of nonlinear optical chromophores containing different conjugated electron-bridges: the relationship between molecular structure-properties and macroscopic electro-optic activities of materials

In electro-optic (EO) materials, realization of large EO coefficients for organic EO materials requires the simultaneous optimization of chromophore first hyperpolarizability, acentric order, molecular shape etc.

Hydrogen-bonded network: An effective approach to improve the thermal stability of organic/polymer electro-optic materials

Binary polymer blends that can form hydrogen-bond networks were prepared and used to fabricate the guest-host organic electro-optic (EO) materials. The hydrogen-bonds network in the solid films of novel guest-host organic EO materials was verified by FT-IR spectra. After corona poling, two kinds of EO films revealed the maximum EO coefficient value (r33) of 19.5 and 30 pm/V at the wavelength of 1310 nm. The EO activity and the order parameter of the chromophores in the poled films both indicated that the hydrogen-bond network has little impact on the motion of the chromophores while poling. These new EO films exhibited better temporal stability after 250 h at 55 and 85 °C compared with the conventional guest-host systems that have the similar or higher Tgs. Moreover, the results also indicated that polymer blends as the host of EO materials can change the dielectric strength of the materials, which directly influence the poling efficiency.