Linear Electro-optic Research Articles

K0.5Na0.5NbO3‐Based Lead‐Free Transparent Electro‐Optic Ceramics Prepared by Pressureless Sintering

Transparent lead‐free electro‐optic (EO) ceramics (K0.5Na0.5) 0.9Li0.1Nb0.9Bi0.1O3 have been fabricated by pressureless sintering. The ceramics have a fine‐grained structure and cubic‐like symmetry. The comodification with Li and Bi induces a diffuse phase transition, causing the ceramics become relaxor‐like and contain polar nanoregions. Our results reveal that excess Bi2O3 can further enhance the changes in the crystal structure and dielectric behavior of the ceramics, causing them become more cubic‐like and more relaxor‐like, respectively. These can reduce the light scattering arisen from birefringence and domain walls, and thus improving the optical properties. For the ceramics added with 4 and 6 mol% excess Bi2O3, the optical transmittance reaches a high value of 60%–70% in the near‐infrared region. The ceramics also exhibits a good linear EO response, giving an effective EO coefficient of 30–40 pm/V.

Optically induced anisotropy and electrooptics in ferroic organic nanocomposites

The occurrence of linear electrooptics and optical anisotropy is observed in the ferroic crystals \([\hbox {NH}_{2}(\hbox {C}_{2}\hbox {H}_{5})_{2}]_{2} \hbox {CuCl}_{4 }\) embedded into polymethylmethacrylate polymer matrix. The studies were performed for the optically treated bulk crystals and the large-size nanocrystals (with sizes higher than 50 nm) embedded into the polymer matrix. The specimens were both in a form of bulk as well as in the form of the composite films. To understand the mechanisms giving the principal role to the effect we have performed polarized absorption measurements for the samples treated in the external dc-electric field and bicolour 532 nm/1,064 nm laser treatments. The behaviour of the birefringence and of the linear electrooptics coefficients were studied within a temperature range covering the incommensurate phase existence.

Ab initio study of elasticity, piezoelectricity, and nonlinear optical performance in monoclinic NaAsSe2

Monoclinic NaAsSe2 (γ-NaAsSe2) has been reported to be a potential IR nonlinear optical material due to the very strong second harmonic generation (SHG). Here, we present a systematic density functional theory (DFT) study on the elasticity, SHG response, linear electro-optical (EO) effect, and piezoelectricity of this compound and analyze the microscopic origins of these properties. We find that γ-NaAsSe2 has smaller values of elastic moduli than those of AgGaSe2, and behaves in a ductile manner. The strong SHG response observed experimentally is well predicted by the 2n+1 theorem andthe “sum over states” expressions based on the DFT. As with the strong SHG response, γ-NaAsSe2 also presents large linear EO coefficient (r11=32.7pm/V, as large as that of LiNbO3), and piezoelectric coefficient (e11=1.27C/m2, about seven times larger than that of α-quartz). The highly distorted structure and the strong hybridizations between As and Se sp orbitals are identified as the chief sources of these unique properties. Our results demonstrate the potential applications of γ-NaAsSe2 in piezoelectric, electro-optical, and nonlinear optical devices.

Enhanced electrooptical active materials based on n-hexyl group flexible isolation in NLO chromophores

Two principally novel organic nonlinear optical chromophores (1 and 2) with flexible (n-hexyl group) or rigid isolated (benzyl group) group are designed and successfully synthesized. The prepared chromophores were characterized by MS, 1H-NMR and UV–Vis spectra. Their thermal stability was studied by thermal gravimetric analyzer and differential scanning calorimetry. Poled films of the chromophores doped in amorphous polycarbonate afford the maximum electro-optic tensor coefficient (r33) equal to 39 pm/V, 63 pm/V for chromophore 1 and chromophore 2, respectively at the wavelength 1,064 nm. The reason of so large differences between these two chromophores’ linear electrooptics coefficients were explained within a framework of performed quantum chemical calculations and it is crucially dependent on distances between the chromophore molecules and the polymer chains.

Photoinduced Pockels effect in the Nd-doped ZnO oriented nanofilms

Photoinduced linear electrooptical (EO) effect was studied for Zinc oxide nanofilms doped by Nd. We have chosen Nd rare earth impurity with different contents to explore its influence on photoinduced EO. The kinetics of the photoinduced EO changes was studied. We have chosen the bicolor coherent 10 ns Nd:YAG laser pulses with power densities varying up to about 1 GW/cm2 as the photoinducing beam. The enhancement of the photoinduced liner EO and its relaxation was explored. Additionally, the study of the photoinduced trapping levels within the energy gap was performed and the explanation of the obtained results is given.

Electrooptic Properties of Submonolayer Polydiacetylene Nanoparticle Film Probed by Surface Plasmon Resonance Spectroscopy

Electrooptic (EO) properties were studied for a polydiacetylene (PDA) nanoparticle (NP) submonolayer film on a thin gold film at a surface coverage of σ∼0.084, using the attenuated total reflection geometry at the surface plasmon resonance condition. Both linear and second-order EO effects (Pockels and Kerr effects) were observed from the PDA NP submonolayer, although the Pockels effect will be absent in the PDA NP film under the electric dipole approximation. The second- and third-order nonlinear susceptibilities of the PDA NPs are determined to be χ(2) ∼102 pm/V and χ(3) ∼10-9 esu, respectively. The susceptibilities are almost compatible with those in general nonlinear optical materials. The origin of the Pockels effect in the PDA NP film is also discussed.

Measurement of the linear electro-optic tensor of cubic boron nitride single crystals

The transverse electro-optic (EO) modulation system is built based on cubic boron nitride (cBN) single crystals unintentionally doped and synthesized at a high pressure and high temperature. The photoelectric output of the system includes two parts that can be measured respectively and the value of elements in the linear EO tensor of the cBN crystal can be obtained. This method does not need to measure the absolute light intensity. All of the surfaces of the tiny cBN crystals whose hardness is next to the hardest diamonds are {111} planes. The rectangular parallelepiped cBN samples are obtained by cleaving along {110} planes and subsequently grinding and polishing f112g planes of the tiny octahedral cBN flakes. Three identical non-zero elements of the EO tensor of the cBN crystal are measured via two sample configurations, and the measured results are very close, about 3.68 and 3.95 pm/V, respectively, which are larger than the linear EO coefficients of the general III-V compounds.

The nonlinear optical susceptibility and electro-optic tensor of ferroelectrics: first-principle study

Abstract The nonlinear optical properties of some ABO3 materials (BaTiO3, KNbO3, LiTaO3 and LiNbO3) are studied by density functional theory (DFT) in the local density approximation (LDA) expressions based on first-principle calculations. Our goals are to give the details of the calculations for linear and nonlinear optical properties, including the linear electro-optic (EO) tensor for some ABO3 structures with oxygen octahedral structures using first-principles methods. These results can then be used in the study of the physics of ferroelectrics, specifically, we present calculations of the second harmonic generation response coefficient X ijk(2) (−2ω, ω, ω) over a large frequency range for ABO3 crystals. The electronic linear EO susceptibility X ijk(2) (−ω, ω,0) is also evaluated below the band gap. These results are based on a series of the LDA calculations using DFT. Results for X ijk(2) (−ω, ω,0) are in agreement with experiments below the band gap. The results are compared with the theoretical calculations and the available experimental data.

Wave-coupling theory of linear electro-optic effect for ultrashort laser pulses

A wave-coupling theory of linear electro-optic (EO) effect for ultrashort laser pulses has been developed. As one of its applications, the linear EO effect of ultrashort Gaussian pulses propagating in LiNbO3 crystal is demonstrated, with the intensity lower than the damage threshold. It is found that the EO effect is sensitive to both the azimuth angle and the polar angle of wave vector of light. And the larger the applied electric field, the shorter the crystal used for optimization of EO coupling is. The numerical results show that, with the duration of input pulse changing from 150 fs to 5 fs, the durations of output pulses are broadened due to the group-velocity dispersion, as well as the first- and second-order refractive dispersion. In addition, when input pulse duration is larger than 20 fs, in contrast with zero chirp, the output durations can be slightly compressed if a positive chirp input pulse is used. The validities of slowly varying amplitude approximation and continuous wave approximation are also discussed. The influence of self-phase modulation and cross-phase modulation on the EO effect is also discussed briefly.

Simultaneous characterization of the electro-optic, converse-piezoelectric, and electroabsorptive effects in epitaxial (Sr,Ba)Nb2O6 thin films

Implementation of the linear electro-optic (EO) effect in thin film waveguides is expected to allow drastic reductions in the drive voltage, power, and dimensions of devices devoted to light modulation. It should also enable the realization of electrically tunable photonic crystal devices. In this paper we introduce a method which eliminates systematically the sources of the unreliability which strongly affects thin film EO characterization. Based on a Fabry–Perot reflective configuration, the method enables characterizing simultaneously the EO, converse-piezoelectric, and electroabsorptive effects in a film. It provides the magnitude and sign of each of the involved coefficients, and allows accounting for the whole of experimental data versus angle of incidence for both transverse-electric and transverse-magnetic polarizations. At λ=633 nm and room temperature, the results obtained with an epitaxial strontium barium niobate (SrxBa1−xNb2O6, x=0.60) ferroelectric thin film, are: r13=+8.5±1.3 pm/V, r33=+38.9±0.5 pm/V, d33=Δe/ΔV=+21±4 pm/V, and Δko/ΔV=(+9.8±0.6)×10−6, where r13 and r33 are two linear EO coefficients, d33 is a converse-piezoelectric coefficient, and e, ko, V represent, respectively, the film thickness, film ordinary extinction coefficient, and applied voltage. Converse-piezoelectric and electroabsorptive effects are found significant in the film response at a frequency below piezoelectric resonance. Diagonal and effective EO coefficients of the (Sr,Ba)Nb2O6 (SBN) film explored in the present work are larger than those of a crystal of lithium niobate (LN) at the same wavelength λ=633 nm. Taking into account the significant difference in dielectric permittivity between the two materials, advances and potential of LN and SBN thin film paths are compared.

Second-order nonlinear optical response of zigzag BN single-walled nanotubes

A theory based on the two-band tight-binding approximation for $\ensuremath{\pi}$ electrons is developed to describe the second-order nonlinear optical (NLO) properties of arrays of uniformly sized and well-aligned boron-nitride single-walled nanotubes (BN-SWNTs) with a zigzag achiral structure. It is assumed that the coherent light beam at frequency $\ensuremath{\omega}$, incident upon the nanotube sample, is linearly polarized along the symmetry axis of the nanotubes. The long-axis NLO susceptibility ${\ensuremath{\chi}}^{(2)}(\ensuremath{\omega})$ of those nanotubes is calculated within the independent nanotube approximation and in neglecting local-field effects. Using the perturbation-theory formalism in the crystal-momentum representation, we derive an explicit analytic expression for the ${\ensuremath{\chi}}^{(2)}(\ensuremath{\omega})$ and apply it to study three distinct second-order NLO effects possible in the BN-SWNTs due to their noncentrosymmetric structure---namely, second-harmonic generation (SHG), linear electro-optical (LEO) effect, and nonlinear optical rectification (NOR). The theory is illustrated by numerical model calculations of the SHG, LEO, and NOR susceptibility spectra for several representative BN-SWNT ensembles consisting of large-diameter nanotubes. The calculated SHG spectra are found to be dominated by the highly peaked $2\ensuremath{\omega}$ resonance at half the band-gap energy of the BN-SWNTs, where the absorption of light is negligible. Distinct features are also found in the LEO and NOR susceptibility spectra, e.g., a sudden switching of the susceptibility from a positive peak value to a negative peak one in the near vicinity of the fundamental absorption edge. A fairly large magnitude of those susceptibilities, reaching the order of ${10}^{\ensuremath{-}7}\text{ }\text{esu}$ under off-resonant conditions and up to ${10}^{\ensuremath{-}6}\text{ }\text{esu}$ in the resonant case, suggests that BN-SWNTs are a promising material for various electro-optical device applications.

Influence of crystal phase and transparent substrates on electro-optic properties of lead zirconate titanate films

Pb ( Zr x Ti 1 − x ) O 3 [x=0.52, PZT(52) near morphotropic phase boundary], tetragonal PZT(65), and rhombohedral PZT(20) thin films have been fabricated on different transparent substrates by radio frequency sputtering. The optical studies show that the band gap energies and refractive indices of the PZT thin films are crystal phase dependent. The largest electro-optic (EO) coefficient of 219.6 pm/V has been achieved by controlling the crystal phase of the PZT thin films. The linear EO coefficients of PZT(52) films on the (Pb0.86La0.14)TiO3-coated glass, indium tin oxides, and MgO substrates are also studied. Such study could contribute to the crystal phase and substrate dependent PZT films for electro-optic devices and multifunctional integrated circuits.

Electro-optic effect and optical rectification in gold nanoparticles immobilized above a gold surface

Second-order nonlinear optical effects, a linear electro-optic (EO) effect and optical rectification (OR), were observed in surface immobilized gold nanospheres (SIGNs) above a gold surface with a gap of about 1.4 nm. The EO and OR measurements were performed in an attenuated total reflection geometry under a surface plasmon resonance condition. While it has been reported that the SIGN structure shows considerable second-harmonic generation activity, there has been no report on a linear EO effect and OR in the SIGN structure. The effective second-order susceptibilities of a single SIGN structure were evaluated to be 13 pm V−1 (EO) and 40 pm V−1 (OR). Although the theory predicts that the susceptibilities of EO and OR will be equal, the observed susceptibilities were different. The reasons for this discrepancy are discussed.

Optical and electro-optic properties of micrometric thick lead zirconate titanate films on (Pb0.86,La0.14)TiO3/glass

Highly (100)-oriented Pb(Zr0.52,Ti0.48)O3 (PZT) films of up to ∼2.4 μm are deposited on the (Pb0.86,La0.14)TiO3 (PLT)-coated amorphous substrate by one cycle of sputtering and annealing. The films possess fibrous columnar nanostructures and vertically grow on the PLT/glass substrate. Microstructure analysis and property characterization indicate that the films possess very promising optical, waveguiding, and electro-optic (EO) properties, as well as low propagation loss. The transparency of >80% in the range of λ=400–1200 nm and the linear EO coefficient of 341.4 pm/V have been measured in the films. Due to the large EO coefficient and the micrometric thickness, a giant phase retardation of 2.93 rad has been measured under an electric field of 50 kV/mm.

Optical modulator using metal-oxide-semiconductor type Si ring resonator

Electric-field drive optical modulators using a Si ring resonator were fabricated on silicon-on-insulator (SOI) wafers. The fabricated resonators consisted of Si waveguides with width and thickness of 1.0 and 0.3 μm, respectively. In order to induce the linear electro-optic (EO) effect in the Si core layer, the strain was applied by covering the layer with Si3N4 film (0.26 μm thick) deposited by low pressure chemical vapor deposition (LPCVD) at 750 °C. The vertical electric-field was applied to the Si waveguide through the top and bottom cladding layers, and the optical output from the drop port at the resonance wavelength was measured. At a wavelength of 1501.6 nm, the optical modulation of 33% was obtained at 200V (electric-field at the silicon surface ∼3 × 105 V/cm, nearly the breakdown field). The resonance wavelength was shifted toward the short wavelength side by applying both positive and negative voltages, this shift was explained by carrier concentration modulation. The linear EO effect in the Si core layer was not observed, presumably because the strain in the Si core layer was too small.

Spatial anisotropy of linear electro-optic effect in crystal materials: II. Indicative surfaces as efficient tool for electro-optic coupling optimization in LiNbO 3

The electro-optic (EO) properties of pure and MgO-doped LiNbO 3 crystals are characterized by indicative surfaces (IS) describing the spatial anisotropy of the longitudinal and transverse linear EO effects. By analyzing the IS and their stereographic projections, we have determined the geometries with maximum EO coupling. The maximum magnitudes of the linear EO effect are revealed in the longitudinal and transverse geometries along the directions that substantially deviate from the principal crystallographic axes of both crystals. Spatial analysis of the IS thus yields a set of the optimized sample geometries recommended for designers who develop EO modulators or deflectors. Just by switching from the standard cell geometry to the optimized cell geometry, as determined in the present work, one may improve almost three times the modulation efficiency of EO devices based on lithium niobate crystals. This results in a corresponding reduction of their driving voltages being evidently of great practical importance for many applications.

Electro-optic Anisotropy of Epitaxially Grown (Pb,La)(Zr,Ti)O3 Films Fabricated by Modified Sol–Gel Process

High-quality epitaxially grown (Pb,La)(Zr,Ti)O3 (PLZT) films were successfully fabricated on single-crystalline sapphire (012) substrates using a modified sol–gel process. The crystallographic relationship between the PLZT film and the sapphire substrate, and the detailed electro-optic (EO) properties of the films were investigated. Large linear EO coefficients, as large as 510 pm/V, were obtained when an electric field was applied at a 45° angle to the PLZT {110}. In turn, smaller EO coefficients were obtained when an electric field was applied at 0° and 90° angles to the PLZT {110}. A strong EO anisotropy was observed according to the configuration of the input light polarization and the applied electric field.

Electron–phonon coupling and vibrational modes contributing to linear electro‐optic effect of the efficient NLO chromophore 4‐(N,N‐dimethylamino)‐N‐methyl‐4′‐toluene sulfonate (DAST) from their vibrational spectra

AbstractThe optimized geometry and structural features of the most prospective electro‐optic crystal 4‐(N,N‐dimethylamino)‐N‐methyl‐4′‐toluene sulfonate (DAST), and the vibrational spectral investigations have been comprehensively described with the near infrared Fourier transform (NIR FT) Raman and Fourier transform infrared (FT‐IR) spectra supported by the density functional theoretical (DFT) computations to elucidate the contribution of vibrational modes to the linear electro‐optic (LEO) effect. Mulliken population analysis and natural bond orbital (NBO) analysis have also been carried out to analyze the effects of intramolecular charge transfer (ICT), intramolecular hydrogen bonding and hyperconjugative interactions on the geometries. The influence of CT interaction between the phenyl ring and the dimethylamino group of the nonlinear optical (NLO) chromophore on the endocyclic and exocyclic angles, and the electronic effects such as hyperconjugation and back‐donation on the methyl hydrogen atoms have been examined. The concurrent intense activation of Raman and IR activities of the effective conjugation vibrational coordinate, which significantly contributes to the LEO effect resulting from the strong electron–phonon (e/ph) coupling, has been analyzed in detail. The effects of frontier orbitals, highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO), transition of electron density (ED) transfer and the influence of planarity in the stilbazolium ring on the first hyperpolarizability are also discussed. Copyright © 2008 John Wiley & Sons, Ltd.

Field-stimulated electrooptics in quinoline and stilbene chromophore derivatives incorporated into photopolymer thin films

Optically and electric-field induced linear electrooptics effect (PILEOE) was discovered during investigations of the 3-(4-Nitrophenyl)-6-N,N-diphenylamine-1-phenyl-1H-pyrazolo[3,4-b]quinoline and stilbene chromophores embedded into polymer PMMA matrices. The PILEOE was discovered in the quinoline and stilbene derivatives incorporated into the polymer PMMA matrices. one can see that the PILEOE coefficients achieve their maxima after 130–140 s of the bicolor photo-treatment with simultaneous applied electric field. For the both polarization geometries corresponding to two different electrode-laser polarizations the maximal value of the PILEOE coefficient is achieved for the two pyrazoloquinoline molecule and lower Pockels effect is achieved for the stilbene molecule. The behavior is changed drastically for cross-polarized (electrode-pump polarizations). In this case the effect is very low for pyrazoloquinoline chromophore (A and B) and is substantially larger for the C chromophore. Moreover, the absolute value of the achieved PILEOE is almost the same for co- and cross-polarized experimental geometry. The performed molecular geometry optimization and quantum chemical simulation have shown that the observed features are different for the two class of the chromophore and by different influence of external bicolor light on the photo-structural changes. For the quinoline crucial role belongs to co-planar nitrogen atoms, which are very sensitive to the geometry of photoinduced electrooptics effect. For the case of the stilbene role of the cis–trans isomerization is not so crucial and the corresponding anisotropy is almost absent contrary to the generally adopted model. For all the investigated chromophore maximal values of the PILEOE effect is smaller in the direction perpendicular the pump polarization (cross-polarized geometry) compared to the parallel (co-polarized) ones. The anisotropy is substantially larger for the quinoline chromophore compared to the stilbene ones. Analysis of photoinduced optical absorption and IR spectra of quinoline show principal role of the nitrogen-induced co-planarity inside the aromatic groups in the observed PILEOE anisotropy.

Temperature Dependences of Piezoelectric and Linear Electrooptic Properties of Rhombohedral 0.67Pb(Mg1/3Nb2/3)O3–0.33PbTiO3 Single Crystal Oriented in Direction

We characterized the temperature dependences of the linear electrooptical (E-O) and piezoelectric coefficients of the rhombohedral 0.67Pb(Mg1/3Nb2/3)O3–0.33PbTiO3 single crystals oriented along the <111> direction (the spontaneous polarization direction). The E-O coefficients of the 0.67PMN–0.33PT single crystals were r333=107 pm/V at room temperature and 950 pm/V at 80 °C. The piezoelectric coefficient d311 of the 0.67PMN–0.33PT single crystals increased from 45 pC/N at room temperature to 1015 pC/N at 80 °C. Large E-O and piezoelectric coefficients at 80 °C were from the engineering domain configuration induced by a transition from the rhombohedral phase othorhombic phase.