Organic Electro-optic Materials Research Articles

(Keynote) Silicon-Organic Hybrid Photonics: Integration of Electro-Optical Polymers in a Photonic Integrated Circuit Technology

Photonic integrated circuits (PIC) based on silicon-on-insulator technologies play a crucial role for the development of large-scale on-chip optical devices [1]. They address a variety of applications in optical sensing [2] and signal processing [3]. Many research efforts have been undertaken to develop a wafer-scale PIC platform by making use of well-established semiconductor fabrication processes. Active photonic devices based on standard silicon-on-insulator based PICs rely typically on the plasma dispersion effect in silicon to induce a refractive index change as function of an applied voltage. In this case, the silicon waveguide is doped in such a way that it forms a pn-junction. This is, however, accompanied by optical losses due to absorption and fundamental speed restrictions are related to carrier injection and removal. To overcome these limitations, it is desirable to integrate materials with large off-resonant electro-optical (EO) effect, which allows a change of refractive index by applying a dc or modulated electric field. In this way, no carrier transport is necessary giving rise to ultra-high speed data communication. Here, the silicon waveguide is structured like a capacitor. It consists of two silicon rails in which the active EO material is located in between. Functionalized organic materials are from great interest since they exhibit extraordinary high EO effects. In particular, they can exhibit both, a linear and quadratic EO effect [4]. The advantage of organic EO materials is twofold. First, they exhibit strong EO effects and are highly transparent in the telecommunication wavelength range. Second, they exhibit off-resonant EO effects avoiding unwanted non-parametric processes like two photon absorption and free carrier absorption. The combination of organic materials with silicon-on-insulator based waveguides is known as silicon-organic hybrid (SOH) technology. Recent research has demonstrated a high-speed SOH phase shifter having bandwidths above 100 GHz [5]. Besides that, EO modulators with energy consumption in the atto-Joule per-bit regime have been realized, and also higher modulation formats have been demonstrated [6]. These results show clearly the potential of SOH devices for a next generation of signal processing. However, the integration of organic materials into a well-established silicon-on-insulator technology is still challenging because typical semiconductor fabrication processes are used with relatively high process-temperatures destroying the organic materials. To tackle this issue, an integration approach was proposed in which the organic material is deposited in a post-process. In this work, we review recent results on the hybrid integration of organic EO materials in a silicon-on-insulator technology. We outline some of the identified challenges regarding process compatibility and present preliminary results on the integration of organic materials in a photonic integrated circuit (PIC) technology. Here, we are focusing on EO applications for high speed data transfer employing the linear and quadratic EO effect. As an example, we demonstrate an intensity modulator fabricated in a 0.25 µm SiGe BiCMOS pilot line using 200 mm silicon-on-insulator wafers. This approach gives perspective for monolithically hybrid-integrated photonic devices in an electronic PIC (EPIC)-technology.

Unraveling Excitonic Effects for the First Hyperpolarizabilities of Chromophore Aggregates

Excitonic interactions often significantly affect the optoelectronic properties of molecular materials. However, their role in determining the nonlinear optical response of organic electro-optic materials remains poorly understood. In this paper, we explore the effects of excitonic interactions on the first hyperpolarizability for aggregates of donor–acceptor chromophores. We show that calculations of the first hyperpolarizabilty of chromophore aggregates based on a two-state model agree well with the more rigorous coupled perturbed Hartree–Fock method. We then use both time-dependent density functional theory calculations and the molecular exciton approximation to parametrize the two-state model. Use of the molecular exciton approximation to the two-state model (i) is appropriate for disordered aggregates (unlike band theory), (ii) is computationally efficient enough for calculating the first hyperpolarizability of materials that consist of thousands of interacting chromophores, and (iii) allows the unra...

Ultranarrow-band metagrating absorbers for sensing and modulation.

Nanostructured plasmonic metamaterials are an excellent platform for narrowband optical absorption, which has wide applications in sensing, filtering, modulation, and emission tailoring. However, achieving a subnanometer absorption bandwidth for optical sensing and dynamical control of light is still challenging. Here, we propose an asymmetric metagrating structure and make use of the propagating surface plasmonic mode that has a small dissipation rate, to achieve perfect optical absorption with a bandwidth of 0.28 nm near the wavelength of 1.55 μm. Our proposed structure can be used in solution environments as a chemical or biological sensor in the visible spectral range just by changing the structural parameters. The sensor possesses a sensitivity of 440 nm/RIU and figure of merit of 1333.33 RIU-1. In addition, by combining an organic electro-optic material with this metagrating, our device can be reconfigurable with a dynamic range of 15.52 dB. Therefore, our proposed metagrating platform not only works as an ultranarrow-band absorber, but also can be employed for optical sensing and dynamic control of light.

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.

Nonlinearities of organic electro-optic materials in nanoscale slots and implications for the optimum modulator design.

The performance of highly nonlinear organic electro-optic (EO) materials incorporated into nanoscale slots is examined. It is shown that EO coefficients as large as 190pm/V can be obtained in 150 nm wide plasmonic slot waveguides but that the coefficients decrease for narrower slots. Possible mechanism that lead to such a decrease are discussed. Monte-Carlo computer simulations are performed, confirming that chromophore-surface interactions are one important factor influencing the EO coefficient in narrow plasmonic slots. These highly nonlinear materials are of particular interest for applications in optical modulators. However, in modulators the key parameters are the voltage-length product UπL and the insertion loss rather than the linear EO coefficients. We show record-low voltage-length products of 70 Vµm and 50 Vµm for slot widths in the order of 50 nm for the materials JRD1 and DLD164, respectively. This is because the nonlinear interaction is enhanced in narrow slot and thereby compensates for the reduced EO coefficient. Likewise, it is found that lowest insertion losses are observed for slot widths in the range 60 to 100 nm.

Plasmonic modulator with >170 GHz bandwidth demonstrated at 100 GBd NRZ.

We demonstrate a plasmonic Mach-Zehnder (MZ) modulator with a flat frequency response exceeding 170 GHz. The modulator comprises two phase modulators exploiting the Pockels effect of an organic electro-optic material in plasmonic slot waveguides. We further show modulation at 100 GBd NRZ and 60 GBd PAM-4. The electrical drive signals were generated using a 100 GSa/s digital to analog converter (DAC). The high-speed and small-scale devices are relevant for next-generation optical interconnects.

An electro-optic polymer-cladded TiO2 waveguide modulator

Organic electro-optic (EO) materials and their hybrid systems have received considerable attention for high-performance modulators. In this work, we demonstrate the design and fabrication of an EO modulator with a low half-wave voltage (Vπ) and a RF modulation response up to at least 10 GHz. The polymer used in this work has a glass transition temperature of 172 °C, which makes it stable even under high temperature RF sputtering. As a result, an EO polymer-cladded titanium dioxide (TiO2) waveguide structure can be utilized to enlarge the confinement factor, and thus, enable a low Vπ. The response of the modulator from 4 to 10 GHz has been measured, which shows a flat frequency response up to 10 GHz and a potential for the application in several tens GHz.

Silicon-Organic Hybrid (SOH) and Plasmonic-Organic Hybrid (POH) Integration

Silicon photonics offers tremendous potential for inexpensive high-yield photonic-electronic integration. Besides conventional dielectric waveguides, plasmonic structures can also be efficiently realized on the silicon photonic platform, reducing device footprint by more than an order of magnitude. However, neither silicon nor metals exhibit appreciable second-order optical nonlinearities, thereby making efficient electro-optic modulators challenging to realize. These deficiencies can be overcome by the concepts of silicon-organic hybrid (SOH) and plasmonic-organic hybrid integration, which combine SOI waveguides and plasmonic nanostructures with organic electro-optic cladding materials.

Dielectric and phase behavior of dipolar spheroids.

The Stockmayer fluid, composed of dipolar spheres, has a well-known isotropic-ferroelectric phase transition at high dipole densities. However, there has been little investigation of the ferroelectric transition in nearly spherical fluids at dipole densities corresponding to those found in many polar solvents and in guest-host organic electro-optic materials. In this work, we examine the transition to ordered phases of low-aspect-ratio spheroids under both unperturbed and poled conditions, characterizing both the static dielectric response and thermodynamic properties of spheroidal systems. Spontaneous ferroelectric ordering was confined to a small region of aspect ratios about unity, indicating that subtle changes in sterics can have substantial influence on the behavior of coarse-grained liquid models. Our results demonstrate the importance of molecular shape in obtaining even qualitatively correct dielectric responses and provide an explanation for the success of the Onsager model as a phenomenological representation for the dielectric behavior of polar organic liquids.

Femtojoule electro-optic modulation using a silicon–organic hybrid device

Energy-efficient electro-optic modulators are at the heart of short-reach optical interconnects, and silicon photonics is considered the leading technology for realizing such devices. However, the performance of all-silicon devices is limited by intrinsic material properties. In particular, the absence of linear electro-optic effects in silicon renders the integration of energy-efficient photonic–electronic interfaces challenging. Silicon–organic hybrid (SOH) integration can overcome these limitations by combining nanophotonic silicon waveguides with organic cladding materials, thereby offering the prospect of designing optical properties by molecular engineering. In this paper, we demonstrate an SOH Mach–Zehnder modulator with unprecedented efficiency: the 1-mm-long device consumes only 0.7 fJ bit−1 to generate a 12.5 Gbit s−1 data stream with a bit-error ratio below the threshold for hard-decision forward-error correction. This power consumption represents the lowest value demonstrated for a non-resonant Mach–Zehnder modulator in any material system. It is enabled by a novel class of organic electro-optic materials that are designed for high chromophore density and enhanced molecular orientation. The device features an electro-optic coefficient of r33≈180 pm V−1 and can be operated at data rates of up to 40 Gbit s−1.

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.

Poling efficiency enhancement of tethered binary nonlinear optical chromophores for achieving an ultrahigh n3r33 figure-of-merit of 2601 pm V−1

The poling efficiency enhancement of tethered binary dipolar chromophores significantly improves the performance of organic electro-optic materials for optical modulation.

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.

High-Speed, Low Drive-Voltage Silicon-Organic Hybrid Modulator Based on a Binary-Chromophore Electro-Optic Material

We report on the hybrid integration of silicon-on-insulator slot waveguides with organic electro-optic materials. We investigate and compare a polymer composite, a dendron-based material, and a binary-chromophore organic glass (BCOG). A record-high in-device electro-optic coefficient of 230 pm/V is found for the BCOG approach resulting in silicon-organic hybrid Mach-Zehnder modulators that feature low $U_{\pi }L$ -products of down to 0.52 Vmm and support data rates of up to 40 Gbit/s.

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.

Active unidirectional propagation of surface plasmons at subwavelength slits

Highly efficient, active and compact, unidirectional surface plasmon (SP) propagator composed of double subwavelength slits; filled with organic electro-optic (EO) material is proposed and investigated. By selecting appropriate structure parameters, obtained by solving phase relations between slits, the relative phase of SP generated at the slit exit aperture can be tailored. Simulation results show under normal illumination and external voltage of 8.7 V, SP launching efficiency of 55% and unidirectional SP extinction ratio about 47dB at wavelength of 632.8 nm is achieved. The power consumption of the structure is on the order of 9 fJ/bit which meet the power consumption limitation for optical devices. Moreover, the structure is very compact with effective total length of 1.2 µm and thickness of 0.6 µm.

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.

Molecular engineering of nanoscale order in organic electro-optic glasses

The rational design of bulk nanoscale order in organic electro-optic materials, where the strong dipole–dipole interactions tend to dominate over the weaker forces exploited for self-assembly processes, remains an attractive yet elusive goal. Towards this end, a series of pseudo-discotic dipolar nonlinear optical chromophores have been synthesized and fully characterized. Theoretical guidance and an iterative molecular design process have succeeded in engineering long-range nanoscale order in organic electro-optic glasses. Small-angle thin-film X-ray diffraction experiments demonstrate a self-assembled lamellar morphology in a majority of these materials. Cryogenic crystallography, using a synchrotron X-ray source, afforded the structure of a representative system. This structure, in concert with thin-film X-ray diffraction, atomic force microscopy, UV-vis-NIR absorption spectroscopy, and refractive index experiments elucidated the nanoscale order in the films. Application of these materials in electro-optics is discussed.

Theory-Guided Design of Organic Electro-Optic Materials and Devices

Integrated (multi-scale) quantum and statistical mechanical theoretical methods have guided the nano-engineering of controlled intermolecular electrostatic interactions for the dramatic improvement of acentric order and thus electro-optic activity of melt-processable organic polymer and dendrimer electro-optic materials. New measurement techniques have permitted quantitative determination of the molecular order parameters, lattice dimensionality, and nanoscale viscoelasticity properties of these new soft matter materials and have facilitated comparison of theoretically-predicted structures and thermodynamic properties with experimentally-defined structures and properties. New processing protocols have permitted further enhancement of material properties and have facilitated the fabrication of complex device structures. The integration of organic electro-optic materials into silicon photonic, plasmonic, and metamaterial device architectures has led to impressive new performance metrics for a variety of technological applications.