High Electro-optic Coefficient Research Articles

Enhanced electrooptic modulation efficiency utilizing slow-wave optical propagation

The use of slow-wave optical propagation to promote highly efficient electrooptic modulation of light is investigated theoretically. The proposed modulators utilize a traveling wave (TW) design in which a grating integrated with a single-mode waveguide induces coupling between forward- and reverse-propagating waves. This contradirectional coupling leads to a reduction in the average optical propagation speed in the forward direction. The "slow" waveguide structures provide two features which facilitate improved modulator performance over conventional "fast" TW designs: (1) optical/microwave velocity matching in substrates with high electrooptic coefficients and dielectric constants and (2) enhancement of electrooptic phase shift due to the "dwell time" of the light in the modulation region. For the ideal case of perfect velocity matching, these two factors lead to a potential improvement of nearly an order of magnitude in electrical power dissipation over velocity-matched designs in the conventional lithium niobate (LN) substrate material. Additional orders-of-magnitude improvement in the required electrical power could result from the use of tungsten bronze substrates such as strontium barium niobate (SBN), which have such higher electrooptic coefficients than LN. The prediction of a large reduction in electrical power dissipation is confirmed by calculations for specific slow-wave designs utilizing multireflector etalons in SBN, although response speed limitations result from the fact that perfect velocity matching is not achieved.

Fabrication and optical characterization of Pb(Mg1/3Nb2/3)O3-PbTiO3 planar thin film optical waveguides

Highly (110)-oriented and (100) nearly epitaxial 0.7PMN-0.3PT thin films were deposited on (1012) sapphire and (100) LaAlO3 substrates, respectively, using a dip-coating method. Optical waveguide characterization and electro-optic effect measurements of the film on sapphire substrate were demonstrated. Low propagation loss of 4.1 dB/cm and high quadratic electro-optic coefficient of 0.75×10−16 (m/V)2 were obtained at wavelength of 632.8 nm. Epitaxial PMN-PT thin films will be suitable for integrated optic devices.

Nonlinear optical properties of push–pull polyenes for electro-optics

Improved nonlinear organic chromophores of varying conjugation length with either thiobarbituric acid or 3-dicyanomethylene-2,3-dihydrobenzothiophene-1,1-dioxide (FORON® Blue) acceptors have been synthesized and investigated for their nonlinear optical properties. Very large quadratic hyperpolarizabilities β(-2ω;ω,ω) have been found, up to 25,700×10-48 esu at λ=1.91µm. In a guest–host polymer very high electro-optic (EO) coefficients, of up to 55 pm/V, have been determined at λ=1.31µm with 20-wt % chromophore loading. We find good agreement between molecular parameters evaluated by electric-field-induced second-harmonic generation (EFISH) and the measurements of guest–host solid–solid solutions. The latter method is well suited to the determination of the product of dipole moment μ and hyperpolarizability β quickly and reliably at the wavelength of interest for EO applications without the complications associated with EFISH measurements.

Hybrid integrated optical devices utilizing thin films of lithium niobate

Abstract A variety of thin film LiNbO3 processes and devices are reviewed in this paper. Integrated optical, electronic, electro-optical, and ferroelectric devices based on LiNbO3 thin films have been built by different research groups and their properties have been characterized. The lowest propagation loss at 633 nm was 0.9dB/cm for LiNbO3 on Al2O3 and 1.9 dB/cm for LiNbO3 on SiO2/Si waveguides. The highest electro-optic coefficient, r 33, published so far is 10.7 × 10−12 m/V for a LiNbO3 thin film deposited on (110) Al2O3. The optical properties of an MFSFET with LiNbO3 as the gate insulator have also been investigated. It has been shown that it is possible to shift the parameters of this transistor with exposure to incident light.

Epitaxial Multilayers of (Sr,Ba)Nb2O6 and Conducting Films on (001) Mgo Substrates

Abstract(Sr,Ba)Nb2O6 (SBN) is a very promising material for nonlinear optical applications because of its high electro-optic and nonlinear optical coefficients. For these applications, SBN requires ferroelectric poling along the optical axis. Conducting layers such as Pt or YBCO must therefore be deposited to provide electrodes above and below the SBN film.We have investigated the epitaxial growth of multilayers of Sr0.61Ba0.39Nb2O6/Pt and Sr0.61Ba0.39Nb2O6/YBCO thin films on (001) MgO substrates by pulsed laser deposition. X-ray diffraction 2θ scans indicate epitaxial growth of SBN/Pt/MgO and SBN/YBCO/MgO heterostructures with their c-axes perpendicular to the substrate plane. X-ray phi scans indicate single crystal Pt and YBCO growth with one in plane orientation. atomic Force Microscopy shows surface roughness of 2.19 nm, and no evidence of particles is observed.

Highly Active and Thermally Stable Ctlromophores and Polymers for Electro-Optic Applications

AbstractWe have developed two new classes of highly active and thermally stable nonlinear optical (NLO) chromophores based on the use of efficient thiophene conjugating units, a N,N-diphenylamino electron-donating group, and a 1,1′-dicyanovinyl substituted electron-accepting group. We have also developed a facile and generally applicable method to functionalize NLO chromophores onto high temperature polymers to demonstrate both high electro-optic (E-O) coefficients and long-term alignment stability at 100 °C.

Thermally stable cross-linked polymers for electro-optic applications

The electro-optic properties of a new variety of cross-linked polymer referred to as Red-acid Magly are being reported. Its linear and nonlinear optical properties are characterized in comparison with the classical side-chain polymer DR1-MMA. The combined curing and poling processes are optimized so as to improve the thermal stability of the nonlinear optical properties. Polymer films are shown to sustain a high electro-optic coefficient of 12 pm/V at 1.32$~\mu$m during several weeks at 85 °C. An electro-optic phase modulator based on this polymer has been fabricated and demonstrated.

Thermally stable poled polymers: Highly efficient heteroaromatic chromophores in high temperature polymides

In this paper, we report our recent developments in achieving thermally stable polyimides that possess large second order nonlinear optical activity. We have developed several classes of novel chromophores based on the combination of efficient thiophene conjugating units and novel electron-donating and electron-accepting functional groups. Through these developments, we have synthesized chromophores that possess nonresonant βμ values as high as 9100 × 10-48 esu measured at 1.9 µm. These chromophores also possess improved thermal and chemical stabilities. The incorporation of these chromophores in high temperature polyimides produces E-0 materials with high linear electro-optic coefficients (in excess of 15 μm/V at 1.3 μm) and long-term temporal stability at an elevated temperature of 150°C for more than 750 h.

Thermally Stable Poled Polymers: Highly Efficient Heteroaromatic Chromophores In High Temperature Polyimides

ABSTRACTIn this paper, we report our recent developments in achieving thermally stable polyimides that possess large second order nonlinear optical activity. We have developed several classes of novel chromophores based on the combination of efficient thiophene conjugating units and novel electron-donating and electron-accepting functional groups. Through these developments, we have synthesized chromophores that possess non-resonant βμ values as high as 9,100 × 10−48 esu measured at 1.9 μm. These chromophores also possess improved thermal and chemical stabilities. The incorporation of these chromophores in high temperature polyimides produces E-O Materials with high linear electro-optic coefficients (in excess of 15 pm/V at 1.3 μm) and long-term temporal stability at an elevated temperature of 150°C for more than 800 hours.

Observation of photorefractivity in a fullerene-doped polymer composite.

A photorefractive effect was observed in a fullerene (${\mathrm{C}}_{60}$) -doped organic composite containing polyvinylcarbazole and a second-order molecule, diethyl-amino-nitrostyrene (DEANST). Electro-optic modulation and degenerate four-wave mixing experiments were conducted. The material shows relatively high electro-optic coefficient, photocharge generation quantum efficiency, and photorefractive diffraction efficiency.

Epitaxially induced secondary grain growth in Ti:LiNbO3 optical waveguides

Lithium niobate (LiNbO3) is one of the most promising materials for use in hybrid optical waveguide devices because of its high electro-optic coefficient and its availability as large single crystals. Optical waveguides in LiNbO3 are most commonly made by Ti indiffusion in which strips of Ti metal (between 10 and 100 nm thick) are deposited on a single crystal LiNbO3 substrate. The device is then heated to temperatures around 1000°C typically for 6 hours. During this time, the Ti diffuses into the LiNbO3 to form a Ti-rich LiNbO3 solid solution. This solid solution has a higher refractive index than the substrate and forms the waveguide region. Factors controlling the indiffusion process, however, are not very well understood and very little is known about the microstructural changes which occur during Ti indiffusion. In this study, the microstructure of Ti:LiNbO3 optical waveguides was examined as a function of time and temperature using transmission electron microscopy (TEM).

Waveguides in Lithium Niobate

Lithium niobate (LiNbO3 is considered to be the leading electrooptical material for fabrication of active waveguides, modulators, and switches for application in integrated optical circuits. LiNbO3 is a ferroelectric material with a high Curie temperature of 1210°C. This is essential to permit rapid diffusion at high temperature, e.g., >1000°C, without domain reversal. Its high electrooptic coefficient allows modulation at relatively low voltages. Waveguides are fabricated in LiNbO3 by Ti indiffusion around 1000°C, by Li2O outdiffusion, and, more recently, by Li+/H+ ion exchange. We report results of a study on diluted Li+/H+ ion exchange. This process is attractive because it allows control of the index change between 0.003 and 0.1 Furthermore, the fabricated waveguides are polarization selective, have high optical damage resistance, and do not exhibit index instability as is the case with complete Li+/H+ exchange. The variation in both the index of refraction and the diffusion coefficient with composition is very nonlinear. Mechanisms contributing to these nonlinear properties are discussed.

Nonlinear Optical Polymers: Challenges and Opportunities in Photonics

ABSTRACTIn polymer structures, highly correlated virtual excitations of the π-electrons are responsible for the exceptionally large nonresonant nonlinear optical responses observed. Extremely large resonant nonlinear optical responses are also achievable in certain π-electron systems, which can be treated as optical Bloch systems. In addition to their obvious scientific importance, these large optical nonlinearities potentially make possible the implementation of powerful, new nonlinear optical devices and systems. After a description of nonlinear optical processes in polymers, two examples are presented. First, saturable absorption and optical bistability in ultrathin organic polymer films are described, illustrative of resonant third order processes. Saturable absorption studies of glassy polymer films consisting of quasi two-dimensional conjugated disc-like structures of silicon naphthalocyanine demonstrate that on-resonance the system behaves as an optical Bloch system with a linear absorptivity coefficient α0 of 1 × 105 cm−1 and an intensity dependent refractive index n2 of 1 × 104 cm2/kW in the wavelength range of standard laser diodes. A resonant nonlinear optical response of π-electron excitations provides the nonlinear interaction essential to the onset of bistability. Electronic absorptive optical bistability is observed on a nanosecond time scale in a nonlinear Fabry-Perot interferometer employing the saturably absorbing naphthalocyanine film as the nonlinear optical medium. As a second example, the nonresonant second order process of linear electrooptic effects in poled polymer films, is discussed. For such a second order nonlinear optical process, the broken global centrosymmetry is achieved by electric field poling of a thin polymer film. With high electrooptic coefficients of 10–50 pm/V and low dielectric constants of 3–4, poled polymers have potentially great advantages over inorganic crystals as electrooptic materials. As one device illustration, the application of poled polymers in electrooptic waveguide operations is presented.

Crystallization of Silicon on Electro-Optic Plzt by a Laser Beam Modulated in Shape and Intensity Profile

AbstractPLZT, a ferroelectric material with a high quadratic electro-optic coefficient, is combined with silicon technology via chemical vapor deposition to enable the realization of Si/PLZT spatial light modulators. Laser crystallization is required to produce device quality silicon deposited on PLZT. The critical issues in the laser crystallization are (i) achieving crystallization in the absence of a good seed, and (ii) preventing damage to the PLZT. To prevent PLZT damage during the laser heating, a 3.5 μm silicon dioxide layer is used as a thermal buffer between the silicon and the PLZT and the effective scanning time is shortened to 100 μs. A double humped cw argon ion laser with a shaped beam is used to achieve successful crystallization with good surface smoothness and large grain size. This technology and devices fabricated on these samples will be discussed.

Electro-optic effects of (Pb, La)(Zr, Ti)O3 thin films prepared by rf planar magnetron sputtering

Epitaxial (Pb, La)(Zr, Ti)O3 thin films were grown successfully on (0001) sapphire substrates at below 600 °C. High quadratic electro-optic coefficient of 0.8×10−16 (m/V)2 was also obtained at 6328 Å. The rf planar magnetron sputtering process improved the quality of the film, which will be suitable for thin-film integrated optic devices.