Ferroelectrics are dielectric materials possessing a switchable spontaneous polarization, which have attracted a growing interest for a broad variety of applications such as ferroelectric lithography, artificial photosynthesis, random and dynamic access memories (FeRAMs and DRAM), but also for the fabrication of devices for nonlinear optics, etc. All the aforementioned applications rely on the control of the ferroelectric domains arrangement, or the charge distribution and transport. In this regard, the main prerequisite is the engineering of the spontaneous polarization, obtained by reversing its orientation or locally inhibiting it. In the latter case, the interface created by the spatial discontinuity of the spontaneous polarization generates local charge accumulation, which can be used to extend the capabilities of ferroelectric materials. This thesis shows how engineering the spontaneous polarization in lithium niobate (LN) by means of proton exchange (PE), a temperature-activated ion exchange process, can be used to develop novel approaches for ferroelectric domain structuring, as well as fabrication of self-assembled nanostructures and control of ionic/electronic transport in this crystal. In particular, it is shown how the electrostatic charge at PE:LN junctions lying below the crystal surface can effectively counteract lateral domain broadening, which in standard electric field poling hampers the fabrication of ferroelectric gratings for Quasi-Phase Matching with periods shorter than 10 μm. By using such an approach, ferroelectric gratings with periods as small as ~ 8 μm are fabricated and characterized for efficient nonlinear optical applications. The viability of the approach for the fabrication of denser gratings is also investigated.  The charge distribution at PE:LN junctions lying on the crystal surface is modelled and used to drive the deposition of self-assembled nanowires by means of silver photoreduction. Such a novel approach for PE lithography is characterized for different experimental conditions. The results highlight a marked influence of the orientation of the spontaneous polarization, the deposition times, as well as the reactants concentrations and the doping of the substrate with MgO. Based on the fact that proton exchange locally reduces the spontaneous polarization, a quick and non-destructive method for imaging PE regions in lithium niobate with nanoscale resolution is also developed by using Piezoresponse Force Microscopy. Moreover the relative reduction of the piezoelectric d33 coefficient associated to PE is estimated in lithium niobate substrates with and without MgO-doping. Finally, by using advanced Scanning Probe Microscopy techniques, the features of charge transport in PE regions are further investigated with nanoscale resolution. A strong unipolar response is found and interpreted in light of ionic-electronic motion coupling due to the interplay of interstitial protons in the PE regions, nanoscale electrochemical reactions at the tip-surface interface, and rectifying metal-PE junctions. / <p>QC 20150325</p>

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Integration of acoustics and photonics devices on the same chip will enable various applications including: building miniaturized sensors, on-chip filtering and optical signal processing, high speed modulation, as well as non-linear optical devices. As an example of the capabilities enabled by such integration, we target the development of a rotation sensor gyroscope based on the acousto-optic effect. The gyroscope components are integrated on a Lithium Niobate Over Insulator (LNOI) substrate because it is a unique platform that exhibits exceptional acoustic as well as photonic properties. However, acoustics and photonics have never been integrated on such substrate, which required the development of a new fabrication process and the design of novel components.. The main challenges we had to overcome and resulted in innovative demonstrations of fabrication processes and devices are:  Developing a robust fabrication process for etching lithium Niobate (LN) waveguides and integrating them with acoustic transducers: A robust fabrication process was developed on the LNOI platform, which can integrate patterning sub-micron features together with microscale ones on the same 3’’ substrate. Furthermore, the developed fabrication process enabled integrating metallic Al electrodes together with etched LN waveguides, which is required for building various components like electro-optic modulators and acousto-optic modulators.  Coupling light in and out of chip: Gratings couplers were designed for optimum coupling of the TE polarized light. The optimization was based on FDTD simulation on LUMERICAL. The grating couplers realization enabled estimation of the light coupling loss in and out of the chip. The measured coupling loss was about 9 dB per coupler in the best case which is much more than the estimated from simulation. That difference is attributed to the alignment accuracy of the photonic chip.  Integrating photonic waveguides/resonators and coupling light between them: LNOI waveguides and photonic resonators were designed and built. The photonic resonators enabled extraction of the losses of waveguides by monitoring the photonic resonator Quality factor, Q, or Finesse (F). Directional couplers (DCs) are commonly used as coupling elements to photonic resonators. However, etching narrow gaps in LN is a challenge that we avoided by using multi-mode interference (MMI) couplers, where butterfly MMI couplers were designed as coupling element to photonic racetrack (RT) resonators aiming for critical coupling condition. Additionally 3-dB MMI couplers were designed to be used as beam combiners in the Mach-Zehnder interferometer (MZI). The built RT resonators enabled extraction of the propagation losses in the etched LNOI photonic waveguides, which were found to be equal to2.5 dB/cm.  Building high efficiency electro-optic modulators (EOMs): The EOM is used in the AOG to compensate for temperature variations and other environmental variation affecting the rotation measurement. The EOM realization enabled extraction of the electro-optic (EO) coefficient for the LN thin film, which permits to evaluate the magnitude of the control voltages required to stabilize the system. EOMs of two different types were demonstrated, one is based on a photonic RT while the other is based on an Asymmetric MZI (AMZI). The RT EOM represents the first demonstration for such device with etched waveguides in Y cut LNOI platform. Modulation bandwidth of 4 GHz, wavelength tuning rate of 0.32 pm/V and an ER of more than 10 dB were experimentally measured for the RT EOM. For the AMZI, a half wave voltage length product of 16.8 Vcm was experimentally measured. Although, it is not the best we can get from this LNOI platform because of our wide waveguides, feeding that EO coefficient to the AOG system model ensures that the temperature variation from -54 oC to 25 oC can be compensated by applying a maximum voltage of 64.5 V.  Building efficient acousto-optic modulators (AOMs): The AOM enabled the extraction of the acousto-optic (AO) coefficient, which directly impacts the AOG scale factor (SF). Additionally, two different types of AOMs were demonstrated, one is based on an MZI embedded inside a SAW cavity while the other is based on a photonic RT whose coupling condition is under EO control. For the MZI AOM, the SAW resonator enhances the modulation efficiency due to the resonator Q such that the phase shift per square root of power extracted from the measurements is a factor of 3x higher than what previously reported on a GaAs platform, which makes it, to the author’s knowledge, effectively the highest AO modulation ever attained on chip. On the other hand, the EO tuned RT AOM showcases integration of various functionalities on same platform to build efficient AOM that can be operated at the desired wavelength. The EO tuning not only changes the operating optical wavelength but also ensures the critical coupling condition needed for efficient modulation. This design takes advantage of the unique AO and EO properties of LN, hence showcasing important building blocks for RF-photonic applications. By addressing all the previous challenges through the demonstration of high performance components, we were able to prototype the first acousto-optic gyroscope. That prototype represents the first demonstration of a novel rotation sensing technique, which combines the following advantages: (i) large mass (there is no suspended mass in the sensing mechanism and hence no limits on increasing the mass and no concerns about stiction issues during fabrication), and (ii) high shock resistance (since the sensing mechanism is strain based, the AOG has no moving parts that would not survive high G accelerations). The AOG SF is estimated comparing three photonic phase sensing techniques which are MZI, RT as well as RT coupled to MZI (MZI/RT). The phase sensitivity is estimated in terms of the cavity F for each technique. That theoretical analysis is verified by experimental measurement for the SF for both the MZI and the RT AOGs. The measured SF for the MZI is 48 nv/(o/sec) while it is about 9 nv/(o/sec) for the RT AOG. The SF is lower for the RT AOG because the Finesse (F~6) of the RT is not as high as expected. Nevertheless, these prototypes represent a proof of concept for our novel method for sensing rotation. Future work could prove that this AOG concept could be disruptive. Reducing the losses in the LNOI waveguide is a key challenge that can be overcome and has been already demonstrated by other groups showcasing 100x lower propagation loss. The estimated F from our model in that case would increase by approximately 50x, hence improving the gyroscope SF by the same factor. Further improvement of 100x is possible by increasing the SAW wavelength and Q. A separate challenge that needs to be addressed is the laser and photodetector integration on chip, which will reduce the coupling loss and the sensitivity to optical alignment.

A computer-controlled ellipsometer was used to obtain data on the birefringence changes induced in lithium niobate crystals by irradiation with an argon-ion laser. The instrument was set to take readings on a rectangular grid of points before and after irradiation. Irradiation was performed with circular and with narrow rectangular light beams. Results are compared with the prediction of the model of Chen in which he postulated the presence of an internal field Eo in the direction of the c-axis of the crystal. According to his theory, electrons, photoexcited from traps by the laser light, drift under the influence of Eo along the c-axis before being retrapped. The resulting space-charge field set between positive ionised centres and trapped electrons gives rise to the observed birefringence changes via the linear electro-optic effect of the crystal. Approximate mathematical models based on Chen's theory are used to solve the problem with narrow rectangular light beams. Results are compared with experiments. / Applied Science, Faculty of / Electrical and Computer Engineering, Department of / Graduate

Le niobate de lithium (LN), en raison de ses propriétés, électro-optiques, optiques non linéaires et photoréfractives (PR) , s'impose comme un matériau de choix pour des applications en modulation, filtrage, stockage holographique ou conversion de fréquence optique. L'effet PR est gouverné à la fois par les centres profonds extrinsèques (Fe2+ et Fe3+ le plus souvent) et par les antisites niobium en site (Nbli5+), qui constituent des pièges préférentiels pour les électrons arrachés aux donneurs profonds et forment ainsi des polarons liés Nbli4+, caractérisables par une large bande d'absorption photo-induite (API) s"étendant dans le rouge et le proche infrarouge, ainsi que par une bande de photoluminescence (PL) légèrement décalée en longueur d'onde par rapport à la précédente. Nous présentons ici une étude expérimentale de la PL polaron dans le LN congruent dopé fer, fonction de la température et de l'intensité lumineuse excitatrice, en régime continu et en régime pulsé, ainsi qu'un modèle phénoménologique à trois centres permettent d'interpréter toutes les caractéristiques observées. Nous montrons que la PL permet en principe de doser les donneurs profonds dans le LN congruent en qualité infime, que soit leur nature. Pour les ions Fe2+, la détectivité est typiquement de 0,25 ppm à l'ambiante, ce qui est bien meilleur que la spectroscopie d'absorption. La PL résolue spatialement permet en outre, contrairement aux autres techniques, de cartographier la concentration de donneurs profonds à l'échelle micrométrique, d'où son potentiel pour la caractérisation de guides d'onde, de composants optiques intégrés ou autres microstructures / Because of their electro-optical, non-linear optical and photorefractive effet, lithium niobate crystals (LN) are used in many applications such as modulation, filtering, holographic storage or frequency conversion. Its known that the photorefractive effect is influenced by both extrincic deep centers (Fe2+ and Fe3+ in most cases) and by niobium antisites (Nbli5+) which constitute preferential sites to trap an electron hence giving a small bound polaron (Nbli4+). This defect is characterized by a photo_indiced obsorption (API) broad band in the NIR range, as well as slightly Stokes shifted photoluminescence band (PL). We presnt nerein an experime,tal study of the polaron related PL in iron doped congruent lithium niobate as a function of temperature and incident intensity in CW and pulsed regime. We also propose a phenomenological threecenter model as an interpretation of all observed results. We show that the PL permits one to determine the concentration of deep centres in congruent LN in trace amounts, whatever their nature. For Fe 2+ ions, the sensitivity is typically around 0,25 ppm at RT, which is better than absorption spectroscopy. In addition, unlike other techniques, the PL is spatially resolved (micrometer scale) which allows to maps the concentration of deep donors, hence its potential for characterizating waveguides, integrated optical components and other microstructures

In iron-doped lithium niobate and other similar crystals, exposure to light of appropriate wavelength induces small changes in the refractive index. This phenomenon is called the photorefractive effect. It allows phase holograms to be stored in these crystals. The work to be described was undertaken to obtain a better understanding of the mechanisms of the photorefractive effect and to investigate possible engineering applications. The photorefractive effect is believed to involve the spatial redistribution of photoexcited electrons among traps. This causes a space charge field to develop-which modulates the refractive index via the linear electro-optic effect. A new bulk photovoltaic effect special to ferroelectric crystals, first recognized by Glass et al., is important in the photorefractive effect in these crystals. It is shown that the finite electron transport length in this effect makes the photovoltaic current distribution spatially shifted from the light intensity pattern that causes it. Moreover, it is shown that the spatially varying photovoltaic current component which is responsible for the hologram formation decreases as the spatial frequency of the light interference pattern increases. Hologram writing by the photorefractive effect is modelled for arbitrary electron transport length. The treatment allows for the feedback effect of the space charge field and for the dark conductivity of the crystal. The model applies to uniform illumination and constant applied voltage conditions. It is shown that except in crystals where diffusion dominates the .hologram is spatially shifted from the light intensity pattern that caused it because of the finite electron transport length associated with the bulk photovoltaic effect. Experimental results which bear upon the bulk photovoltaic effect and the associated electron transport length are reported. Hologram writing with an arbitrary one-dimensional light intensity distribution is modelled allowing for the feedback effect of the space charge field at all writing times, A large scale space charge field associated with the envelope of the light is shown to affect the writing process. It is found that for any type of intensity distribution an increase in the fraction of the crystal which is illuminated improves the efficiency of the hologram writing process. Also for partially illuminated crystals the storage efficiency improves as the photoconductivity approaches the dark conductivity value from above. For a fully illuminated crystal the storage efficiency improves as the ratio of the photoconductivity to dark conductivity increases. Experimental observations of the effect of the large scale field on hologram storage are reported. Beam distortion and optically induced scattering are two problems encountered while storing holograms in lithium hipbate. We report experimental observations and theoretical models for these phenomena. It is shown that beam distortion is due to the defocusing action of the large scale refractive index change due to the envelope of light. Light scattering is suggested to be due to the lens action of the index variations due to laser speckles inside the crystal. A theoretical treatment of the spatial filtering properties of volume holograms is presented. Practical applications of volume holograms in the fields of interferometric testing and optical. communications are also discussed. / Applied Science, Faculty of / Electrical and Computer Engineering, Department of / Unknown

Integrated optical devices use light to perform circuit functions. One class of devices using Ti indiffused waveguides in LiNb0₃ are investigated in this thesis. The applications studied include series and multibranch interferometers, a comparatorless A/D converter, and a high voltage sensor. An Impedance transformation equation for an exponential line is derived and used to obtain relations for the propagation constant of an optical waveguide with linearly graded index. The results are used to study the effect of a Ta₂O₅ film on the propagation constant of a Ti diffused waveguide. Heating in O₂ of the TA₂0₅ film, loading one arm of the Mach-Zehnder modulator, is shown to tune the modulator. The fabrication of devices in Y-cut LiNb0₃ by Ti diffusion is described. Microscope objectives are used to couple 0.6328 μm light into 4 μm wide waveguides through polished edges. An application of the Mach-Zehnder modulator and a two mode BOA (Bifurcation optique active) modulator to high voltage measurement is discussed. The output Intensity of the series interferometric filter is calculated. Measured results on a two section filter are presented. Also, the output intensity of a multibranch interferometric filter is determined. Experimental results on a three branch filter are included. / Applied Science, Faculty of / Electrical and Computer Engineering, Department of / Graduate

Exposure of some insulating crystals such as lithium niobate to light of appropriate wavelength induces small changes in the refractive index. This effect has been named the photorefractive effect. It allows phase holograms to be stored in these crystals. The work to be described was undertaken in order to obtain a better understanding of the hologram storage process which is believed to involve the spatial redistribution of photoexcited electrons among traps. This causes a space charge field to develop which modulates the refractive index via the linear electro-optic effect. A new reliable criterion for deciding whether the Raman-Nath or the Bragg regime of diffraction will be observed with a given hologram was proposed. It is shown that the distinction between "thin" and "thick" holograms is invalid as a criterion for which regime operates. The new bulk photovoltaic effect proposed by Glass et al. is an important mechanism in the photorefractive effect in ferroelectric crystals. It is shown that as formulated by Glass et al. it is formally equivalent to a fictional "virtual field" acting on the photo-liberated electrons provided that their migration length is short compared to the grating spacing. Hologram writing by the photorefractive effect was modelled in progressive stages of complexity. All the models were based on the assumption that the transport length of the free electrons is short compared to the grating spacing. This appeared to be a generally accepted assumption. The first treatment allowed for the feedback effect of the space charge field and for the dark conductivity. It was for uniform illumination and constant applied voltage. The effects of the modulation ratio and the applied field were investigated. This treatment was then modified to allow for the effect of the absorption constant in reducing the intensity of the light as it propagates through the crystal. It was shown that the hologram becomes nonuniform through the crystal thickness as a result of this effect. Hologram writing with one-dimensional Gaussian beams was modelled allowing for the feedback effect of the space charge field. A large scale space charge field associated with the envelope of the light pattern was shown to affect the writing process. It was found that an increase in the fractional illumination of the crystal improves the writing process. The dark conductivity is shown to have an important effect on the process. The final model was again for uniform illumination and allowed not only for the feedback effect of the photoinduced field and the effect of the dark conductivity and absorption but also for the interaction between the hologram being written and the light pattern which is writing it. This causes energy transfer between the two writing beams, thus modifying the light pattern. Optical erasure of holograms with the light incident either on and off the Bragg angle was modelled. The treatment allows for the feedback effect of the space charge fields and for the effect of the absorption in reducing the light intensity. The model allowed for the interaction between the diffracted and the reading beams for the case of incidence at the Bragg angle. The resulting interference pattern writes a new hologram which may add to or subtract from the hologram to be erased. An experimental method is described in which a normally incident ancillary light beam of different wavelength than that used to write the hologram allows the diffraction efficiency to be determined without errors due to multiple internal reflections. A limited experimental investigation was made of hologram storage in LiNbO₃. Photocurrent and optical measurement were carried out on the same crystal. Almost 100% relative diffraction efficiency was observed. The value of the virtual field obtained from holographic measurement was found to agree within 10% with the value obtained from photocurrent measurements. During hologram writing, energy transfer of up to 70% between the two writing beams was observed. However, since the "virtual" field in these experiments was much larger than the diffusion equivalent field, the model predicted only about 5% energy transfer. It is, therefore, suggested that the transport length of the photoexcited electrons in the crystal used, was not short compared to the grating spacing. It is also shown that light induced scattering can cause serious error in measuring the diffraction efficiency expecially during optical erasure. / Applied Science, Faculty of / Electrical and Computer Engineering, Department of / Graduate

Annealed proton-exchanged (APE) MgO:PPLN ridge waveguides have been successfully developed, and its optical properties have been researched. Nonlinear wavelength conversion experiments have been successfully conducted using the developed waveguide. The APE MgO:PPLN ridge waveguide has been proved suitable for high efficiency nonlinear optical conversions across its transparency spectrum from visible to mid-IR light. With distinctive features including flexibility, high efficiency and low cost, this new type of waveguide can find potential applications in various application fields. / Thesis / Candidate in Philosophy