High Electro-optic Coefficient Research Articles

Fabrication and Characterization of Optical Devices on Lithium Niobate on Insulator Chips

The high electro-optic coefficients and nonlinear coefficients of lithium niobate make it a highly promising material for optical modulator design, experiments in cavity quantum electrodynamics, and microwave photonics. The two most common techniques for photonic device fabrication on lithium niobate are titanium diffusion and proton exchange. However, due to very low refractive index contrast (usually less than 0.1), the waveguides formed through these processes have low mode confinement, high bending losses, and often, are not polarization independent, making it difficult to fabricate compact photonic structures on the material. Here, we report our new design: rib waveguide on lithium niobate on insulator chips. Lithium niobate on insulator chips contains a thin film of lithium niobate (thickness: ∼700 nm) adhering on an insulator substrate like SiO2. Such devices provide good vertical and lateral index contrast and mode confinement. Also, because of the smaller waveguide widths attainable through etch based techniques, a greater control of the critical dimensions of devices is possible. We fabricated and characterized optical devices - waveguides, power splitters, and microring resonators on lithium niobate on insulator chips, employing robust fabrication techniques, namely EBL and ion milling. The structures were characterized in terms of propagation loss and quality factor. The fabricated structures showed good performance, and the fabrication techniques have potential for use in the mass manufacture of lithium niobate based optical devices.

Dendrimer Dipole Excitation: A New Mechanism for Terahertz Generation

An electro-optic dendrimer (EO dendrimer) material has been designed for high power terahertz generation. An ordinary poly (amidoamine organosilicon) (PAMAMOS) dendrimer was modified via doping and poling to generate a high electro-optic coefficient nanomaterial. Measured EO coefficient varied from ~130 pm/V at 633 nm to ~90 pm/V at 1553 nm. An emitter designed from this EO dendrimer generates milliwatts of continuous wave (CW) terahertz radiation (T-ray) when pumped by a CW laser of suitable wavelength. The mechanism termed as the dendrimer dipole excitation (DDE) works via excitation of the dipole population generated by the doping process. The doping protocol also generates a distribution of dipole moments, as opposed to fixed dipoles in the lattice of a crystalline material; thus, when excited by a suitable pump laser, these dipoles radiate a broadband frequency that range from 0.1 THz to ~ 30 THz. A terahertz time-domain spectrometer (TeraSpectra) was designed with this DDE terahertz source. As a test of the spectrometer functionality, a standard polyethylene calibration was conducted. It was found that TeraSpectra reproduces several known absorbance peaks of polyethylene. It also produces additional absorbance peaks not observed before. It is surmised that the ultra-high sensitivity of T-ray enables observation and discovery of additional absorbance peaks that are not visible via other spectroscopy such as visible, UV, FTIR or Raman.

Introduction of fluorine to change the dielectric environment of nonlinear optical chromophores for improved electro-optic activities

Fluorine was non-conjugatedly linked to the donor of nonlinear optical chromophore to investigate the effect of highly electronegative fluorine on intramolecular charge-transfer and electro-optic activities. 1HNMR spectra and UV–vis absorption were used to investigate that introduction of fluorine change the dielectric environment of intramolecular charge-transfer. Fluorinated chromophore showed more intensified absorption and its absorption properties was more stable to dielectric change in solution. In solid state of guest-host electro-optic polymer films, this dielectric environment stabilization was revealed to attenuate the dipole–dipole interactions of chromophores. Higher electro-optic coefficients (148pm/V) and higher chromophore loading density (30wt%) of fluorinated material chr-2/APC than benchmark material chr-1/APC (116pm/V at 25wt%) was achieved.

High Performance Ridge Type PLZT Optical Switch With Offset Upper Electrode

(Pb,La)(Zr,Ti)O3 (PLZT) has a high electro-optical coefficient and is very attractive for optical waveguide switches with high speed, low driving voltage, and compact size. The $1\times 2$ PLZT optical switch using ridge type Mach–Zehnder interference structure has been designed and fabricated. By introducing offset upper electrodes instead of the conventional top electrodes, the insertion loss of the fabricated $1\times 2$ PLZT optical switch was decreased 9.8 dB significantly with a crosstalk of −25.3 dB and the driving voltage <10 V.

Tuning the defect mode in ternary photonic crystal with external voltage for designing a controllable optical filter

In this work, behavior of defect mode in one-dimensional ternary photonic crystal (1DTPC) structure with arrangement of (MgF2/Ag/TiO2)5LiNbO3(TiO2/Ag/MgF2)5 was investigated under the applied external electric dc voltage. The defect layer is lithium niobate (LiNbO3), an electro-optical (EO) material whose refractive index is voltage-dependent with high EO coefficient. In comparison, magnesium fluoride (MgF2) and titanium dioxide (TiO2) layers have very low EO coefficients. A narrow localized defect mode with perfect transmittance was appeared inside the photonic band gap. Under applying the positive or negative biases, red shift and blue shift was observed in the defect mode, respectively. More than 120 nm tunability was obtained under externally applied voltage in the range of −200 V to 200 V. The physical interpretation is very simple. Change in optical path-length displaces the localized wavelength of the defect mode due to Bragg interface condition. The externally tunable localized mode can be employed in designing a controllable optical filter, one of the essential devices for new-generation all-optical integrated circuits.

Polymer enabled 100 Gbaud connectivity for datacom applications

Polymers hold the promise for ultra-fast modulation of optical signals due to their potential for ultra-fast electro-optic (EO) response and high EO coefficient. In this work, we present the basic structure and properties of an efficient EO material system, and we summarize the efforts made within the project ICT-POLYSYS for the development of high-speed transmitters based on this system. More specifically, we describe successful efforts for the monolithic integration of multi-mode interference (MMI) couplers and Bragg-gratings (BGs) along with Mach–Zehnder modulators (MZMs) on this platform, and for the hybrid integration of InP active elements in the form of laser diodes (LDs) and gain chips (GCs). Using these integration techniques and the combination of the hybrid optical chips with ultra-fast indium phosphide double heterojunction bipolar transistor (InP-DHBT) electronics, we develop and fully package a single 100Gb/s transmitter and a 2×100Gb/s transmitter that can support serial operation at this rate with conventional non-return-to-zero on-off-keying (NRZ-OOK) modulation format. We also present the experimental evaluation of the devices, validating the efficiency of the monolithic and hybrid integration concepts and confirming the potential of this technology for single-lane 100Gb/s optical connectivity in data-center network environments. Results from transmission experiments to this end include the achievement of BER close to 6·10−9 in B2B configuration, the achievement of BER lower than 10−7 for propagation over standard single-mode fiber (SSMF) with total length up to 1000m, and the achievement of BER at the level of 10−5 after 1625m of SSMF. Finally, plans for the use of the EO polymer system in a more complex hybrid integration platform for high-flexibility/high-capacity transmitters are also outlined.

Performance enhancement of strip-loaded electro-optic modulator using photobleaching-assisted method

A low-cost hybrid electro-optic (EO) material with a high EO coefficient and excellent poling stability is used as the basis for a conventional Mach–Zehnder interferometer modulator with a strip-loaded waveguide structure, and thorough modeling, design and fabrication processes are proposed. To improve the performance of this strip-loaded EO modulator, a simple photobleaching technique is introduced into the fabrication process. Using this design, the confinement is increased by a factor of 1.71, and coupling loss reduction of 2.3 dB per facet is obtained at the same time. The device exhibits an insertion loss of 8.9 dB and an EO coefficient of 20.9 pm/V at 1550 nm with stable operation. This waveguide structure and the fabrication process are shown to be valuable for EO modulators and switches application.

Short-conjugated zwitterionic cyanopyridinium chromophores: Synthesis, crystal structure, and linear/nonlinear optical properties

A new class of short-conjugated zwitterionic cyanopyridinium chromophores with the absorption maxima less than 550 nm were designed and synthesized. These chromophores display a good solubility in organic solvents and exist in a charge-separated ground state, as confirmed by IR, X-ray crystallography, negative solvatochromism and fluorescence studies. They can be doped up to 10% by weight into poly(ether sulfone) and attached onto a methacrylate polymer by grafting. Without optimization of the poling conditions, a high electro-optic coefficient can be readily achieved using the chromophore-grafted nonlinear optical polymers.

3D simulation of low loss photonic crystal-based splitters on lithium niobate in mach-zehnder interferometer

We present 3D simulation of low loss splitters made on Lithium Niobate used in Mach-Zehnder interferometers. The interferometer consists of the splitters designed with photonic crystal structures at splitting point and bend positions for reduced size and loss, and straight sections as channel waveguides with high electro-optical coefficient r33 of Lithium Niobate for reduced interactive length and modulating voltage π. The simulation shows a reduced coupling loss to 1.37 dB with the channel waveguides made suspended and Vπ?is only 0.42V for interactive length of 200 um.

Precursor adsorption efficiency of titanium tetra isopropoxide in the presence of a barium β-diketonate precursor

Barium titanate is a promising candidate for integrated photonic application due to its high electro-optic coefficient; therefore the need arises for a suitable deposition technique which provides high film quality at low temperatures (below 500°C) and reasonable growth rates. We report on combinatorial high vacuum chemical vapor depositions of titanium(IV) tetra isopropoxide and barium hexafluoroacetylacetonate (hfa)–tetraglyme on silicon substrates aiming at the deposition of barium titanate. The barium precursor was synthesized and characterized regarding its evaporation kinetics. Co-depositions of both precursors were carried out in the absence and presence of reactive gas partners such as H2, H2O, O2 and O3. Plain substrates deposited in the absence of ozone, however, do not show any titanium incorporation. We call this phenomenon obviating precursor interaction. Elongated masking structures were designed and manufactured on silicon substrates to modify locally the precursor impinging rate of one of both precursors to study the nature of this obviating behavior. It was found, that the blocking results from a reduced sticking coefficient, i.e. a fast desorption of the precursor prior to decomposition. When adding ozone, distinct, sharply separated regions were identified on the substrate, which we separate, according to their chemical composition and morphology, into TiO2, Ba-containing and barium and titanium mixed deposit. We found that the transition between TiO2 and Ba-deposit is dependent on the titanium to barium precursor impinging ratio. Depending on the barium precursor flux, the third region of mixed deposition of barium and titanium forms in the Ba deposit. The mixed deposit indicates a promising starting point for a combinatorial high vacuum chemical vapor deposition optimization to evaluate the feasibility of a high quality barium titanate film by this technique.

Simulations for dual-rail driven electro-optic modulators of BaTiO3 crystal thin-film waveguides

In this work, a dual-rail EO modulator structure is presented with the advantages of extremely high electro-optic (EO) coefficient r51 of BaTiO3 crystal, the quadratic modulation effect and the thin-film waveguide structure where electric fields are applied at both transverse and horizontal directions of optical beam propagation to create a dual-rail driven modulation effect to the refractive index of optical channel. A new theoretical model of the quadratic modulation effect to the dual-rail driven regime is proposed. The dependences of Vπ2L on the physical parameters of devices such as the two electrode gaps, the r51 value and the birefringence of BaTiO3 crystal thin-film are simulated, then the intrinsic relationships among the half-wave voltage Vπ, interaction length L, EO coefficient r51, birefringence Beo and electrode gap G are investigated. As an illustration, for an extremely small L value of 1mm, <6.0V Vπ values correspond to 600pm/V r51, −0.007 Beo and 7.5μm G. Finally, the challenges and possibilities to the new regime of dual-rail driven EO modulators are discussed.

Photorefractive Damage in congruent LiNbO3. Part I. Zinc doped Lithium Niobate Crystals

The experimental measurements of the photorefractive damage in a series of zinc doped congruent lithium niobate are reported and discussed. The first experimental technique used is based on the measurement with time of the photoinduced distortion of an HeNe beam spot at 632.8 nm. The second method is based on the direct measurement of the photoinduced birefringence variation with time, using a powerful laser beam at 514.5 nm as gating radiation, and a laser weak beam at 632.8 nm as probe. The dependences of the photorefractivity and of the photosensitivity on the power and dopant concentration have been investigated. The photorefractive damage is highlight by the electro-optic behaviour with the zinc concentration and the defect structure of the doped crystals. We conclude that doping above a second threshold concentration with divalent Zn ions leads to a significant decrease of the photorefraction with respect to pure congruent crystals. In this concentration range, LiNbO3:Zn are strongly photorefractive damage resistant and thus present an interesting alternative for modulating devices to the stoichiometric lithium niobate. Link to the high electro-optic coefficients, these crystals are very suitable for Q-switching applications.

Mid-Range Structure of Niobium–Sodium–Phosphate Electro-Optic Glasses

The mid-range structure of glasses of the glass-forming system xNb(2)O(5)-(60 - x)P(2)O(5)-40Na(2)O, with x varying from 0 to 40, has been investigated by Raman spectroscopy and electro-optical Kerr measurements. It was found that the mid-range inhomogeneities in the glasses under study vary with x and their compositions and structures dramatically differ from the gross glass composition. It is shown that inhomogeneities with the 8NaNbO(3) + (Nb(2)O(5)·P(2)O(5)) composition are responsible for the electro-optical sensitivity of the glasses. The electro-optical structural elements of the inhomogeneities of this type are assumed to be fragments of the quasicubic lattice of crystalline NaNbO(3) demonstrating a rather high electro-optic Kerr coefficient. The obtained results are discussed in terms of the concepts of constant stoichiometric groupings and crystal motifs.

Synthesis and electro-optical features of a high T g polymer system with excellent electro-optic activity and thermal stability

Three chromophores with tricyanofuran and tricyanopyrroline electron acceptors were synthesized and doped in high glass transition temperature (T g) polymer poly(N-(4-acetoxylphenyl)maleimide-co-styrene, NAPMI-co-ST). The electro-optic (EO), optical, and thermal properties of the doped poly(NAPMI-co-ST) were characterized and discussed. After being corona poled under 12 kV, this high T g polymer material showed excellent EO activity and thermal stability. The highest EO coefficient (r 33) reached 48.2 pm V−1 (1,310 nm) and could remain 90 % of the original value for 100 h at 85 °C. The EO coefficient was relatively higher compared with other high T g EO polymers. The thermal stability was also very good and the manufacture process was convenient and applicable for device fabrication.

Analysis and Implementation of Reflection-Type Electro-Optic Phase Diffraction Grating

A reflection-type electro-optic phase diffraction grating using the novel transparent lead magnesium niobate-lead titanite (PMNT) electro-optic ceramic is analyzed theoretically and implemented experimentally. The Au/Ti interdigital electrodes are sputtered on one surface of ceramic wafer which is used to change the material refractive index by the quadratic electro-optic effect. The phase modulation characteristic and the optical beam diffraction patterns induced by a controllable electric field are analyzed theoretically and simulated numerically. Using a high electro-optic coefficient transparent PMNT ceramic wafer, some experimental results are given to verify the theoretical analysis and they are accordant very well. Finally, some potential applications for the reflection-type electro-optic phase diffraction grating in the ultrafast laser application areas are also discussed in this paper.

Synthesis and nonlinear optical properties of novel y-type polyurethanes containing different concentrations of chromophore

A new series of Y-type polyurethanes containing different concentrations of nonlinear optical (NLO) chromophore with aniline donor and tricyanofurane (TCF) acceptor have been successfully prepared, and characterized by FTIR, UV-Vis, and 1H-NMR spectra. New polyurethanes were synthesized with different chromophore contents by introducing diol N, N-dihydroxyethylaniline or 4-[N, N-(dihydroxyethyl)amino]benzaldehyde. These NLO polyurethanes exhibit good film forming property and high thermal stability up to 281°C. The highest electro-optic coefficient (r33) of polymers is up to 39 pm V−1 measured by simple reflection technique at 1310 nm, and the temporal stability of the poling-induced order at elevated temperature of 80°C was much improved through the introducing of hydrogen bonding interaction in this system. © 2012 Wiley Periodicals, Inc. J. Appl. Polym. Sci., 2013

On the origin of the high Kerr coefficient measured in thallium–zinc–tellurite glasses

Measurements of Kerr electrooptical sensitivity of several zinc–thallium–tellurite glasses are presented, and composition dependence of Kerr sensitivity is compared with the dependence of the second harmonic generation efficiency collected for optically poled TeO2–TlO0.5–ZnO glasses. These data being analyzed jointly with Raman measurements data allowed us to conclude that the high electrooptical Kerr coefficient and nonlinearity of Tl2O–ZnO–TeO2 glasses, and their sharp increase with augmenting concentration of thallium oxide TlO0.5 above 15% should be attributed to the presence of Tl+ cations having very high non-linear polarizability most likely related to their electronic lone pairs.

Short hybrid polymer/sol-gel silica waveguide switches with high in-device electro-optic coefficient based on photostable chromophore

The highest electro-optic (EO) coefficient to date is achieved in short polymeric directional coupler switches based on hybrid EO polymer/sol-gel silica waveguides. Optimized poling conditions in such waveguides give a highest in-device EO coefficient of 160 pm/V at 1550 nm using highly efficient and photostable guest–host EO polymer SEO100. Adiabatic waveguide transitions from the passive sol-gel core to active EO polymer cores surrounding the sol-gel core are shown using EO polymer cores with a coplanar tapered structure. Switching voltages of 8.4 and 10.5 V are achieved for electrodes that are 2.1 and 1.5 mm long, respectively, which are half those of EO switches containing the chromophore AJLS102.

High Electro-Optic Coefficient in Organic NLO Polymers: Poling and Study of the Relaxation Process

We report a high electo-optic coefficient (r33) of 175 pm/V in guest-host polymer films made by mixing amorphous polycarbonate with the chromophore, PYR-3 that was synthesized by our team. We have previously shown that PYR-3 has a large 2nd order nonlinear optical figure of merit. The measured r33 is five times larger than that of conventional inorganic materials such as lithium niobate. The decay of r33 with time has been studied for more than 1000 hours at room temperature and ambient atmosphere, and we find that the r33 value gradually reduced to a stable value of 34 pm/V after 40 days.

A NOVEL LEAD-FREE TRANSPARENT CERAMIC WITH HIGH ELECTRO-OPTIC COEFFICIENT

A novel lead-free transparent electro-optic ceramic (K0.5Na0.5)0.95 Li0.05Nb0.95Bi0.05O3+ 2 mol % excess Bi2O3 has been fabricated by hot-press sintering. The ceramic possesses a dense, fine-grained and pure-perovskite structure. Owing to the decreases in optical anisotropy of the crystal structure and optical scattering by the fine grains, the ceramic becomes visually clear. It exhibits a high transmittance of 60% in the near-IR region as well as a high effective linear electro-optic coefficient of 102 pm V-1. Although a relaxor behavior is induced, the ceramic exhibits reasonably good ferroelectric and piezoelectric properties (d33 = 64 pC N-1).