LiNbO3 Planar Waveguide Research Articles

Investigation of optical absorption enhancement of plasmonic configuration by graphene on LiNbO3-SiO2 structure

A novel plasmonic structure is demonstrated by combining graphene with a planar LiNbO3 thin layer, which is simple and easy to fabricate compared to the complex design of general graphene surface plasmons devices. Graphene from the chemical vapor deposition is investigated and characterized to be a continuous and uniform monolayer or fewlayer. LiNbO3 capped by graphene layer show an extraordinary absorption enhancement in an attenuated total reflection (ATR) measurement at a wide bandwidth of 500–4000 cm−1, which can be explained by resonance absorption resulting from the coupling of graphene surface plasmons with optical modes of LiNbO3-SiO2 Fabry–Perot cavity and LiNbO3 planar waveguide. The simulation results are generally consistent with the ATR experimental results. The absorption spectra versus temperature of this plasmonic configuration is also investigated, which show that increasing the testing temperature not only highlights the atomic vibrational peaks of graphene, but also enhances the absorption at several characteristic absorption frequencies due to the enhanced coupling between the surface plamons excitations and the optical modes.

Experimental evidence for preservation of electro-optic property in titanium-diffused lithium niobate waveguide

Abstract Electro-optic (EO) property of titanium-diffused LiNbO3 optical waveguide was studied using prism coupling technique. The EO-effect-induced increment ΔN of effective refractive index (at the wavelength 1311 nm) was measured as a function of externally applied electric field E for TM0 and TM1 modes guided in the titanium-diffused LiNbO3 planar waveguide. By using the linearity between ΔN and E, the EO coefficient r33 is determined. For reference, the measurement was also performed on a bulk material of congruent LiNbO3, yielding r33 = 24.9 ± 0.8 p.m./V, which is in qualitative agreement with that obtained by extrapolating previously reported result to the 1311 nm wavelength, ~24 p.m./V, confirming the feasibility of the measurement method. Measurements on the titanium-diffused planar waveguide give r33 = 25.7 ± 0.6 p.m./V for the TM0 mode and 25.7 ± 0.4 p.m./V for the TM1 mode. Both values can be considered as identical within the error, and are the same as that of the bulk material, indicating that the EO property is well preserved in the titanium-diffused LiNbO3 waveguide. The main reason is that the LiNbO3 crystalline phase is well preserved in the titanium-diffused waveguide layer.

Off-Normal Incidence Coupling for Perfectly Phase-Matched Second Harmonic Generation in a Sub-Micron LiNbO3 Planar Waveguide

An angular coupling configuration is shown to facilitate phase-matched second harmonic generation between the TE <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1</sub> fundamental and TE <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">4</sub> second harmonic modes of a LiNbO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> planar waveguide. For an incident coupling angle of 10°, 2.6% coupling efficiency is achieved for the TE <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1</sub> waveguide mode, thus permitting visible light generation at the wavelength of 402.4 nm through phase-matching to the TE <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">4</sub> second harmonic mode. For a 2.8 mm-long LiNbO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> planar waveguide excited at an energy of 1.8 nJ, the second harmonic generation conversion efficiency is 4.6 × 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-5</sup> . The ability of this angular coupling scheme to facilitate phase-matched frequency-conversion makes it ideal for use in optical computing, optical communications, and quantum computing.

Effect of titanium in LiNbO3 on domain growth during e-beam writing

We report the features of domain growth during e-beam recording on non-polar Y-surfaces of LiNbO3 crystals doped with titanium at various concentrations (CTi). The nominally pure LiNbO3 of the congruent composition (CLN), LiNbO3 doped with 0.5 mol% TiO2 (CLN-0.5Ti), and the Ti:LiNbO3 planar waveguide formed as a result of high-temperature diffusion of titanium were comparatively studied. The titanium concentration at the surface of Ti: LiNbO3 was ∼7.6 at%, then CTi gradually decreased to 0.5–0.8 at% аt a depth of ∼4–5 μm. The combination of the etching technique with non-destructive methods of observation of the created domain structures of a planar type (low-voltage SEM and SHG microscopy) made it possible to detect the effect of titanium concentration on the features of the domain growth. It has revealed that the domain sizes and the average rate of their frontal growth (Vf) in CLN and CLN-0,5Ti are different. The observed differences are discussed in the framework of the current model of the intrinsic defect structure of LiNbO3. A peculiar three-dimensional structure of domain gratings in the Ti:LiNbO3 over the waveguide depth was found. The differences in the formation of the upper and lower parts of periodic gratings in Ti:LiNbO3 are explained by a significant increase in conductivity near the surface of the waveguide in comparison with deeper layers. The obtained results are useful to match the position and design of planar domain gratings in Ti:LiNbO3 optical waveguides on non-polar surfaces for better conversion efficiency of integrated nonlinear devices based on quasi-phase-matching.

Nonlinear Cherenkov radiations modulated by mode dispersion in a Ti in-diffused LiNbO3 planar waveguide.

We report nonlinear Cherenkov radiations (NCRs) in a Ti in-diffused LiNbO3 planar waveguide. The radiations were modulated exploiting different polarizations and orders of the guided modes, the fundamental wavelengths and the working temperatures. Some characteristics related to NCRs, such as radiation angles and relative intensities were investigated in detail. The experimental results matched well with theoretical calculations.

Waveguiding Er3+/Yb3+:LiNbO3 films prepared by a sol–gel method using polyvinylpyrrolidone

LiNbO3 waveguiding structures are mainly prepared by the chemical or structural modification of LiNbO3 single crystals. An alternative method is the preparation of epitaxially grown LiNbO3 films with waveguiding ability using sol–gel procedures. Our aim was to test the use of 2-methoxyethanol solutions containing polyvinylpyrrolidone (PVP) in the preparation of Er3+/Yb3+doped LiNbO3 thin films. The solutions were spin-coated on sapphire (0001) substrates and then heat-treated under different regimes on the basis of thermal analysis results. The microstructure of the films was compared using scanning electron microscopy and atomic force microscopy. Optical properties in the IR range were validated by the measurement of luminescence spectra and waveguiding properties (m-line spectroscopy). The influence of the thermal treatment and of PVP molar mass used (29,000 or 360,000g/mol) was established. Also the influence of additional crystallization on the films properties was tested. A suitable combination of PVP molecular mass and thermal treatment led to the successful preparation of LiNbO3 planar waveguides in the IR range. Thus, the sol–gel deposition from solutions containing PVP is a feasible non-hydrolytic method applicable for the preparation of planar waveguides.

Control of scandium diffusion in LiNbO3 single crystal by co-diffusion of titanium

The mutual influence of co-diffused Ti4+ and Sc3+ in LiNbO3 single-crystal was studied. Sc3+/Ti4+ co-doped LN plates were prepared by annealing (at 1060 °C in wet O2) Z-cut congruent LiNbO3 substrates coated with stacked Sc2O3 and Ti films, which have different thicknesses and coating sequences. After the annealing, the refractive index and composition at the undoped and Sc3+-only doped surface parts were characterized by prism coupling technique. The Sc3+ and Ti4+ profiles were analyzed by secondary ion mass spectrometry. The results show that the Sc3+-doping has little contribution to the LiNbO3 refractive index. Li2O out-diffusion is not measurable. In the only diffusion case, the Sc3+ diffuses five times slower than the Ti4+. In the Sc3+/Ti4+ co-diffusion case, the Ti4+ assists and controls the Sc3+ diffusion. The Sc3+ diffusivity increases linearly with the initial Ti-metal thickness. In contrast, the Sc3+ affects the Ti4+ diffusion little. Neither the Sc2O3 film thickness nor the coating sequence influences the diffusivity of the two ions. The co-diffusion features are explained. A model for the co-diffusion system is suggested and verified experimentally. The Ti:Sc:LiNbO3 planar and strip waveguide are optically characterized. The results show that the Sc3+ presence does not affect the waveguide characteristics and the waveguide is optical-damage-resistant.

Relationship between refractive index increase and Ti4+ concentration in Ti:LiNbO3 waveguide fabricated by Ti4+ diffusion in near-stoichiometric LiNbO3 substrate

Multi-mode Ti:LiNbO3 planar waveguide was fabricated by in-diffusion of 100-nm-thick Ti-metal film coated onto a near-stoichiometric LiNbO3 substrate prepared by vapor transport equilibration. The crystalline phase and composition in the waveguide were characterized. The guided modes in the planar waveguide were characterized by prism coupling technique. The refractive index profile is constructed from the measured mode indices using the inverse Wentzel–Krames–Brillouin method, and correlated with the Ti4+- concentration profile, which was obtained from secondary-ion-mass-spectroscopy analysis. The results show that the waveguide still retains the LiNbO3 phase, its composition is near-stoichiometric. The index change and Ti4+-concentration follow an exponential relationship with a power index 0.4 for an ordinary ray and 1.07 for an extraordinary ray. A comparison of various congruent and near-stoichiometric bulk materials or waveguides allows to conclude that the relationship is different from congruent to near-stoichiometric composition, from one fabrication method to another, and from bulk material to waveguide configuration. The difference is explained qualitatively.

Green up-conversion of swift C5+ ion irradiated planar waveguide in Er3+, MgO codoped nearly stoichiometric LiNbO3 crystal

We report on the green up-conversion emission from Er3+, MgO codoped nearly stoichiometric LiNbO3 planar waveguides fabricated by the swift C5+ ion irradiation. The characteristics of the waveguides are investigated by using a continuous wave laser at wavelength of 800nm. The reconstructed distribution of the refractive index of the C ion irradiated waveguides shows a “barrier and well” profile. The up-conversion emissions of green light, corresponding to the transition of 4H11/2→4I15/2 and 4S3/2→4I15/2 bands, have been observed in the waveguides with intensity of 40% of that from the bulk.

Relation of Refractive Index to Ti-Concentration in Near-Stoichiometric ${\rm Ti{:}LiNbO}_{3}$ Waveguide

Multi-mode near-stoichiometric (NS) Ti:LiNbO3 planar waveguide was fabricated by co-work of Li-rich vapor transport equilibration and in-diffusion of Ti-film on an initially congruent LiNbO3 substrate. The Ti-concentration is profiled by secondary ion mass spectrometry. The refractive index profile is constructed from measured mode indices and correlated with the Ti-concentration profile. The results show that the index change and Ti-concentration follow an exponential relationship with a power index 0.75/0.49 for the ordinary/extraordinary ray. The relationship is different from that of either conventional congruent waveguide or homogeneously Ti-doped NS bulk material or NS waveguide fabricated by direct Ti-diffusion in an NS substrate.

Efficient high-power frequency doubling of distributed Bragg reflector tapered laser radiation in a periodically poled MgO-doped lithium niobate planar waveguide

We report on efficient single-pass, high-power second-harmonic generation in a periodically poled MgO-doped LiNbO3 planar waveguide using a distributed Bragg reflector tapered diode laser as a pump source. A coupling efficiency into the planar waveguide of 73% was realized, and 1.07 W of visible laser light at 532 nm was generated. Corresponding optical and electro-optical conversion efficiencies of 26% and 8.4%, respectively, were achieved. Good agreement between the experimental data and the theoretical predictions was observed.

Mode-coupling Cerenkov sum-frequency-generation in a multimode planar waveguide

We present experimental and theoretical studies of the mode-coupling Cerenkov sum-frequency radiations in a multimode LiNbO3 planar waveguide. The radiations result from the coupling of different guided modes of the fundamental wave, which have the same optical frequencies but different propagation constants. At the same time, scattering-involved Cerenkov sum-frequency-generation was also observed and discussed. Our theoretical predictions are in well accordance with the experimental results.

Periodic domain structures formed under electron-beam irradiation in LiNbO3 plates and Ti:LiNbO3 planar waveguides of the Y cut

Regular domain structures on the Y cuts of the LiNbO3 substrates and Ti:LiNbO3 waveguide structures based on these substrates have been fabricated under electron-beam irradiation. It has been revealed that the domains in undoped and titanium-doped LiNbO3 crystals are formed as a result of different processes. It has been demonstrated using chemical etching and waveguide second-harmonic generation that regular domain structures in the Ti:LiNbO3 waveguides are formed at a depth of approximately 8 μm from the surface, where the titanium concentration does not exceed 2 mol %. The quasi-synchronous waveguide optical second harmonic generation with an efficiency of 8.8% has been obtained using the fabricated structures.

Optical waveguides in LiNbO3 and stoichiometric LiNbO3 crystals by proton exchange

The formation of optical planar waveguides in LiNbO3 and stoichiometric LiNbO3 crystals by proton exchange was reported. The prism-coupling method was used to characterize the dark-line spectroscopy at the wavelength of 633 and 1539 nm, respectively. The mode optical near-field outputs from proton-exchanged LiNbO3 and SLN waveguides at 633 nm were presented. The mode field from stoichiometric LiNbO3 (SLN) waveguide is lighter and more uniform than that from LiNbO3 waveguide, which means the quality of the waveguide in SLN crystal is better than that of the LiNbO3 waveguide. For proton-exchanged LiNbO3 waveguides, the evolution of the refractive index profile with annealing was presented. The disorder profiles of Nb atoms in proton-exchanged LiNbO3 waveguides were obtained by Rutherford backscattering/channeling technique. It is shown that the longer the exchange time, the larger the displacement of Nb atoms.

Proposals for Fabrication of Long-Period Grating in LiNbO3 Strip Waveguides

The structure of a strip LiNbO3 waveguide clad with a planar LiNbO3 waveguide is proposed to fabricate a long-period waveguide grating (LPWG). It is shown theoretically that an LPWG device based on such a special LiNbO3 waveguide structure is of the merit of thermal stability The methods for fabrication of planar waveguide, channel waveguide and grating are suggested and discussed. Detailed design consideration is exemplified based on a soft-proton-exchange planar waveguide clad Ti-diffused LiNbO3 (Z-cut) strip waveguide.

MgO:LiNbO3 planar waveguide formed by MeV O2+ implantation and its annealing characteristics

A z-cut magnesium-doped LiNbO3 is implanted by O2+ at 1.5–4.5 MeV and with doses of 4–12 × 1014 ions/cm2 at room temperature. Planar optical waveguides were obtained and characterized by the prismcoupling technique at wavelength of 633 and 1539 nm. Implanted samples were annealed at different temperatures and time durations. After different annealing treatments, mode profiles and optical losses were measured and analyzed.

Green light generation in a periodically poled Zn-doped LiNbO3 planar waveguide fabricated by He+ implantation

An optical planar waveguide is investigated in ZnO-doped periodically poled lithium niobate formed by He+ implantation. Optical losses were found to be 2 dB/cm. Second-harmonic generation is used to produce a green laser beam by quasi-phase matching in the obtained waveguide. The conversion efficiency was found to be 10-2%/W. Photorefractive resistance properties are reported and discussed relating to un-doped periodically poled lithium niobate.

Tailoring of refractive index profiles in LiNbO3 optical waveguides by low-fluence swift-ion irradiation

Proton-exchange LiNbO3 planar optical waveguides have been irradiated with swift ions (Cl 30 MeV) at very low fluences in the range 5 × 1010−5 × 1012 cm−2. Large modifications in the refractive index profiles, and therefore in the optical performance, have been obtained due to the generation of amorphous nano-tracks by the individual ion impacts. Moreover, a fine tuning of the refractive index can be achieved by a suitable control of the fluence (δn/δϕ ∼ 10−14 cm2 or δn ≈ 10−5 for δϕ = 109 cm−2). An effective medium approach has been used to account for those changes and determine the amorphous fraction of material. The results have been compared with information extracted from complementary RBS channelling experiments. From the calculated amorphous fractions a radius of ∼2 nm for the amorphous tracks have been estimated.

Electric field periodical poling of lithium niobate crystals after soft-proton-exchanged waveguide fabrication

We report on the periodic polarization of α-phase proton- exchanged LiNbO3 planar waveguides down to 5 μm period lengths using structured electrodes and electrical fields. Unlike a majority of previous work, room-temperature electric field poling has been carried out after waveguide fabrication. Chemical etching of the large guiding surface has revealed homogeneous and high-quality domains with duty cycles close to 0.5. Moreover, it was verified by diffraction techniques and etching that the domain structure is preserved more than 2 μm along the thickness of the waveguide. The excellent optical performances of single-domain α-phase waveguides were not affected by the periodic poling.

Optical damage in x-cut proton exchanged LiNbO3 planar waveguides

The optical damage of different proton exchanged LiNbO3 planar waveguides has been experimentally studied by measuring the intensity output of a single beam as a function of the intensity input. Parallel measurements of photovoltaic currents (sometimes referred to as photocurrents) have been carried out with the same setup as a function of the input intensity and they have been correlated to the optical damage data. The following proton exchanged phases have been studied and compared with the substrate: α, β1, β2, and reverse proton exchanged (RPE). The greatest intensity thresholds for optical damage, about 2×103 times greater than that of the substrate, have been obtained in RPE guides supporting ordinary polarization and in β1,2 guides which support extraordinary polarization. On the other hand, the lowest photovoltaic currents have been measured in β1,2 phases. As a function of the light intensity, the strong superlinear behavior exhibited by the photovoltaic current in the α phase is almost absent in the others. In addition, it has been experimentally ascertained that a scattering increase produces a decrease of the threshold intensity for optical damage. The role of those different effects is discussed extensively together with the electro-optic properties of the different guides.