Thermal Poling Research Articles

Optical creation and erasure of the linear electrooptical effect in silica fiber.

We study the creation and erasure of the linear electrooptical effect in silicate fibers by optical poling. Carriers are released by exposure to green light and displaced with simultaneous application of an internal dc field. The second order nonlinear coefficient induced grows with poling bias. The field recorded (~10⁸ V/m) is comparable to that obtained through classical thermal poling of fibers. In the regime studied here, the second-order nonlinearity induced (~0.06 pm/V) is limited by the field applied during poling (1.2 × 10⁸ V/m). Optical erasure with high-power green light alone is very efficient. The dynamics of the writing and erasing process is discussed, and the two dimensional (2D) field distribution across the fiber is simulated.

Hindering effect of the core-cladding interface in thermally poled optical fibers.

In this paper, the hindering effect of the core-cladding interface (CCI) in thermally poled optical fiber is investigated based on the two-dimensional charge dynamics model. The influence of the mobility of charge carriers at the CCI, and the mobility and concentration of charge carriers in the fiber core on the thermal poling process is presented. It is found that lower mobility of charge carriers at the CCI is responsible for the hindering effect of the CCI. The hindering effect of the CCI could either be enhanced by increasing the mobility and concentration of charge carriers in the fiber core or be overcome by decreasing the mobility and concentration of sodium ions in the fiber core, which could increase the χ(2) in propagation mode. The results provide theoretical insight into the underlying mechanism of the hindering effect and may find applications in thermal poling optical waveguides and multilayered structures.

Frequency Tunable Continuous THz Wave Generation in a Periodically Poled Fiber

An all-fiber approach to terahertz generation using a periodically poled optical fiber is proposed and experimentally demonstrated. In the proposed approach, a continuous-wave THz wave is generated at a periodically poled fiber by beating two optical wavelengths from two laser sources with the wavelength spacing corresponding to the frequency of the THz wave. The key component in the system is the periodically poled fiber, which is made by a twin-hole fiber with the fiber core residing between two holes. The twin-hole fiber is then thermally poled at a temperature of ~ 260 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">°</sup> C with a voltage of 3.3 kV applied to the silver electrodes inside the two holes to introduce second-order nonlinearity. The quasi phase matching (QPM) condition is achieved by periodically erasing the thermal poling induced second-order nonlinearity with an ultraviolet laser, which enhances the energy conversion efficiency. The proposed approach is validated by an experiment. The emission of a THz wave centered at 3.8 THz with an output power of 0.5 μW is observed. The frequency tunability between 2.2 and 3.8 THz is also experimentally demonstrated.

Design of Thermally Poled Twin-Core Fiber Coupler With Electrical Tunability

Tunable coupler based on thermally poled twin-core fiber (TCF) with a hole-core-core-hole configuration was proposed in this letter. The thermal poling process therein was investigated based on the 2-D charge dynamics model. Effective χ <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">(2)</sup> in the twin-cores pointing toward opposite directions were introduced. By subjecting the poled TCF to modulating voltage, the tunability of coupling ratio between the twin-cores was investigated. The results indicate that the coupling characteristics of thermally poled twin-cores could be controlled flexibly by modulating voltage, and consequently achieving a stable and highly compacted electrically tunable TCF coupler.

Nanoprofiling of alkali-silicate glasses by thermal poling

Formation of nanoscale surface relief on the anodic surface of an alkali-silicate glass in the course of thermal poling in open anode configuration and related structural/compositional changes in subanodic region of the glass are studied. Surface of poled glass beneath anodic electrode goes down relatively to the unpoled glass area. The depth of the relief increases linearly with the electric charge transferred through the glass in poling till ~200mC/cm2 charge density, when the depth saturates at ~190nm. While the depth-charge dependence is linear, the glass surface keeps initial quality, and the saturation region corresponds to arising voids and channels directed in the depth of the poled glass. Our evaluations show that the volume difference resulting from the replacement of alkaline and alkaline earth ions by hydrogen is insufficient to provide observed relief. Raman scattering measurements evidence that the poling induces rearrangement of the structure of the glass via losing non-bridging oxygen atoms and forming molecular oxygen. This makes the glass very similar to silica one. These processes are mainly responsible for the surface relief formation.

Optical fiber poling by induction.

Conventional thermal poling methods require direct physical contact to internal fiber electrodes. Here, we report an indirect electrostatic induction technique using electrically floating wires inside the fiber combined with external electric fields that can allow for facile poling of complex microstructured fibers (MOFs) of arbitrarily long lengths. In combination with our unique ability to use liquid gallium electrodes, inducing second-order nonlinearities inside otherwise difficult to access multi-core or multi-hole MOFs now becomes entirely feasible and practical. The formation of a permanent second-order nonlinearity is unequivocally demonstrated by realizing quasi-phase-matched frequency doublers using periodic UV erasure methods in the induction-poled fibers. The second-order susceptibility created inside the fiber is driven by the potential difference established between the floating electrodes, which we calculate via numerical simulations.

Plasmonic molecules via glass annealing in hydrogen.

Growth of self-assembled metal nanoislands on the surface of silver ion-exchanged glasses via their thermal processing in hydrogen followed by out-diffusion of neutral silver is studied. The combination of thermal poling of the ion-exchanged glass with structured electrode and silver out-diffusion was used for simple formation of separated groups of several metal nanoislands presenting plasmonic molecules. The kinetics of nanoisland formation and temporal evolution of their size distribution on the surface of poled and unpoled glass are modeled.PACS78.67.Sc (nanoaggregates; nanocomposites); 81.16.Dn (self-assembly); 68.35.bj (surface structure of glasses); 64.60.Qb (Nucleation); 81.16.Nd (micro- and nanolithography)

Study of thermally poled fibers with a two-dimensional model.

A two-dimensional (2D) numerical model is implemented to describe the movement of ions under thermal poling for the specific case of optical fibers. Three types of cations are considered (representing Na(+), Li(+) and H3O(+)) of different mobility values. A cross-sectional map of the carrier concentration is obtained as a function of time. The role of the various cations is investigated. The assumptions of the model are validated by comparing the predictions to experimental data of the time evolution of the nonlinearity induced. A variational analysis of poling parameters including temperature, poling voltage, sign of the bias potential and initial ionic concentrations is performed for a particular fiber geometry. The analysis allows identifying the impact of these parameters on the induced second-order nonlinearity in poled fibers.

Silver plasmon resonance effects in AgPO3/silica photonic bandgap fiber.

We report on the application of an external electric field and its tuning effect in the guiding properties of a composite AgPO3/silica photonic bandgap fiber. The application of an electric field leads to the poling of the soft glass inclusion, resulting in the formation of a silver-induced plasmonic band, predominantly affecting the short wavelength guiding regimes while inducing polarization dependent losses. These spectral effects are attributed to the formation of silver nanoparticles within the AgPO3 glass matrix, driven by thermal poling.

Structural and compositional modification of a barium boroaluminosilicate glass surface by thermal poling

In addition to inducing second-order nonlinear properties, significant structural and compositional alteration can be imparted to glass surfaces during the process of thermal poling. In this work, we focus on how thermal poling affects a structurally complex, nominally alkali-free boroaluminosilicate display glass composition. We provide evidence for electrolysis of the glass network, characterized by the migration of both cations (Ba2+, Na+) and anions (O−, F−) towards opposing electrode interfaces. This process results in oxidation of the positively biased electrode and forms a network-former rich, modifier-depleted glass surface layer adjacent to the anodic interface. The modified glass layer thickness is qualitatively correlated to the oxidation resistance of the electrode material, while extrinsic ions such as H+/H3O+ at not found in the depletion layer to compensate for the migration of modifier cations out of the region. Rather, FTIR spectroscopy suggests a local restructuring of the B2O3–Al2O3–SiO2 network species to accommodate the charge imbalance created by the exodus of network-modifying cations, specifically the conversion of tetrahedral B(4) to trigonal B(3) as Ba or Na ions are removed from B-related sites in the parent network. The resultant poling-induced depletion layer exhibits enhanced hydrolytic resistance under acidic conditions, and the IR spectra are substantially unlike those produced by acid leaching the same glass.

Trapped Molecular and Ionic Species in Poled Borosilicate Glasses: Toward a Rationalized Description of Thermal Poling in Glasses

Molecular and ionic species of nitrogen oxides have been injected, formed, and then trapped within a borosilicate glass during a thermal poling process. This original observation denotes a new aspect of poling mechanisms for ionic glasses. For each poled borosilicate glasses studied, compositional, structural, and second harmonic generation profiles of their subanodic polarized layers have been characterized. A description of the space charge implementation process, involving interactions between charged and chemically active species formed both within plasma discharges at the glass/electrode interface and within the polarized glass matrix, is proposed.

Creating large second-order optical nonlinearity in optical waveguides written by femtosecond laser pulses in boro-aluminosilicate glass

The thermal poling technique was applied to optical waveguides embedded in a commercial boro-aluminosilicate glass, resulting in high levels of induced second-order optical nonlinearity. The waveguides were fabricated using the femtosecond laser direct-write technique, and thermally poled samples were characterized with second harmonic optical microscopy to reveal the distribution profile of the induced nonlinearity. It was found that, in contrast to fused silica, the presence of waveguides in boro-aluminosilicate glass led to an enhancement of the creation of the second-order nonlinearity, which is larger in the laser written waveguiding regions when compared to the un-modified substrate. The magnitude of the nonlinear coefficient d33 achieved in the core of the laser-written waveguides, up to 0.2 pm/V, was comparable to that in thermally poled fused silica, enabling the realization of compact integrated electro-optic devices in boro-aluminosilicate glasses.

Formation and 2D-patterning of silver nanoisland film using thermal poling and out-diffusion from glass

We fabricated silver nanoisland films and patterned silver nanoisland films using out-diffusion of silver from glass in the course of the ion-exchanged glass substrate annealing in reducing hydrogen atmosphere. The choice of the annealing conditions allows to provide prevailing of silver nanoisland formation over the formation of silver nanoparticles in the bulk of the glass. The procedure of the patterned film formation includes (i) silver-sodium ion exchange in the glass, (ii) thermal poling of the ion-exchanged glass with a profiled anodic electrode, and (iii) annealing the glass in hydrogen. The formation of silver nanoislands in unpoled regions on the glass surface allowed us to avoid any post-processing of very fragile silver island film in formation of 2D-patterned nanoisland structures. Poling of the glass with properly profiled electrode was used for the formation of random chains and ordered arrays of separate silver nanoislands. Depending on processing parameters, a typical island size in the films and chains varied from several to tens of nanometers, and was down to 200 nm in the arrays.

Quasi-phase-matched second-harmonic generation in thermally poled twin-hole silica-glass optical fiber by mercury-lamp exposure

Second-harmonic generation (SHG) from a twin-hole fiber by thermal poling was studied. Quasi-phase-matching (QPM) was performed with a low-pressure mercury lamp. Erasure of the second-order nonlinearity was studied by uniform exposure, and after that QPM was performed using a photomask erasing the nonlinearity periodically. SHG was measured with a Q-switched Nd3+:YAG laser. The second-harmonic power increased by a factor of 7 after QPM. A ray-tracing analysis on the light distribution of the lamp after the photomask was performed, and the effect on QPM is discussed. A model for the second-order nonlinearity by charge layers is also discussed.

Real-Time Investigation of Thermal Poling Process in Optic Fiber

Thermal poling could make centrosymmetric fused silica optical fibers generate second-order nonlinearity effect and linear electooptic effect. In order to investigate the influence of thermal poling parameters on linear electooptic effect, a real-time test system, which mainly consists of an all polarization maintaining fiber Mach-Zehnder interferometer, has been utilized to monitor the whole thermal poling process in fibers. The processing parameters in thermal poling, such as applied poling voltage, poling duration and temperature, have been measured in real time. Based on those measurements, their influence on the linear electrooptic effect has been discussed. Experiment results show that the linear electrooptic coefficient would increase when a stronger electric field is applied on fibers. Considering the anti-high-voltage breakdown capability of fibers, a DC voltage from 3KV to 4KV is suitable for polarization in thermal poling. When using 3KV, the optimum poling duration is about 16 minutes and the best temperature for thermal poling is around 190°C. Keywords: electro-optic effect, poled fiber, thermal poling, real time test system, fiber optic interferometer

Photothermal poling of glass complexes Ag2S–Ga2S3–P2S5

The studies of the dc-electric field induced second harmonic generation (SHG) for fundamental wavelength of the Ag2S–Ga2S3–P2S5 using nanosecond Er:glass laser at wavelength 1540nm and elevated temperatures (thermal poling) were performed. We have found that the highest output SHG efficiency is obtained for the maximal Ga2S3 content and with simultaneous presence of Ag2S. We have shown that the SHG effect exists only during simultaneous dc- and thermal poling at about 120°C. After switching off of the dc-electric field the effect is completely reversible. The maximally achieved value of the effective second-order optical susceptibility is equal to about 0.75pm/V at 1540nm wavelength, which allows to propose to apply these materials for the laser operation.

Thermal poling behavior and SHG stability in arsenic-germanium sulfide glasses

Second-order optical properties of thermally poled arsenic-germanium sulfide glasses have been investigated. Parallel studies of glass structure changes upon poling and/or visible cw-laser irradiation and complete SHG quantitative analysis have been performed. Key parameters and poling mechanisms influencing largely SHG stability and efficiency have been pointed out.

Direct Laser-Writing in silver-zinc doped phosphate glasses: correlated linear and nonlinear optical properties

*yannick.petit@u-bordeaux1.fr For the last decade, direct laser writing (DLW) has kept on providing impressive achievements in material processing, with a very broad range of both fundamental phenomena and subsequent applications. Among the numerous advantages of femtoseconde DLW, one can list its large versatility with respect to the irradiated material, its ability for both bulk and surface structuring, and direct experimental access to three-dimension laser-induced micro-scale modifications. For the last years, we have been improving photosensitive glasses as tailored silver containing phosphate glasses [1], and their interaction with femtosecond infrared lasers [2, 3]. We designed laser/glass interactions to engineer laser-induced modifications of both linear and nonlinear optical responses. These modifications include absorption and fluorescence excitation and emission properties [4], but also localized nonlinear chi 3 enhancement leading to efficient third harmonic generation [5]. Recently, we reported DLW of localized second-order nonlinear response, leading to second harmonic generation in our initially centro-symmetric phosphate glass [6]. Such observation resulted from laserinduced space charge separation, at the root of a strong remnant buried static electric field whose amplitude up to ~10 9 V/m is similar to what reported elsewhere from thermal poling of glasses [7]. In this invited talk, we will provide an overview about results obtained with DLW in our tailored silver-containing phosphate glasses. We will specifically show recent progress about electric field induced second harmonic generation (EFISHG), by describing both original spatial distributions as well as detailed polarization analysis to prove the EFISHG behavior [8]. Moreover, we will present correlative microscope measurements, including both the nonlinear EFISHG signal and the linear fluorescence response, each property originating from space charge separation and from silver cluster production, respectively. Such correlative study provides new interpretation of the fluorescent silver cluster formation and stabilization since we observed that such fluorescent species appear to be a subsequent consequence of the laser-induced static electric field [8]. Silver clusters are shown to appear where DLW leads such species to be stable in adequate reduction/oxidation conditions, as seen from the correlated spatial distributions of EFISHG and fluorescence. We will thus conclude with our new scheme of interpretation, for the understanding of the formation, stabilization and localization of the fluorescent silver clusters. Finally, DLW with our tailored glasses makes these materials promising for many applications as perennial high density 3D data storage or as integrated elementary bricks for future nonlinear photonics devices.

Laser assisted modification of poled silver-doped nanocomposite soda-lime glass

Thermal poling assisted homogenization of polydisperse Ag nanoparticles embedded in the soda-lime glass is demonstrated. The homogenization leads to the narrowing of the localized surface plasmon resonance. The subsequent irradiation with linearly polarized ultrashort laser pulses induces spectrally defined and four times larger dichroism than in non-poled sample.

Dissolution of metal nanoparticles in glass under a dc electric field

The theory of electric-field-assisted dissolution of metal nanoparticles embedded in glass is presented. We demonstrate that the major mechanism of dissolution is associated with the ejection of ions and electrons caused by the ultrahigh electric field, which is built in the vicinity of a nanoparticle when the external electric field is applied and thermal poling of glass takes place. Our estimations show that for a silver nanoparticle embedded in soda-lime glass, the local electric field strength can be as high as 0.5 V nm−1 and the nanoparticle dissolves in a few seconds. The developed theory reproduces well the experimental results on electric-field-assisted dissolution of silver nanoparticles in glass.