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A theoretical study has been performed on effect of polarization state of incident plane wave on performance of photonic nanojet (PNJ) generated by a sphere or an ellipsoid or an elliptical cylinder scatterer, among which the sphere has a higher while the ellipsoid and elliptical cylinder have a relatively low geometric symmetry. The study shows that the polarization state of incident wave affects mainly full width at half maximum of the PNJ in case of the sphere scatterer, whilst influences mainly the focal length in case that the scatterer is an ellipsoid or elliptical cylinder. Further study focuses on the effect on the focal length of the PNJ generated by either an ellipsoid or an elliptical cylinder scatterer, and the influences of type, geometric size and orientation of scatterer on the effect. The results show that the effect prefers to take place for a scatterer with a lower symmetry, and the type, geometric size and orientation of scatterer all can influence the effect. There exists a maximum effect at a certain symmetry and the maximum effect depends on the type, geometric size and orientation of scatterer. In the case of the elliptical cylinder, the focal length of PNJ generated can increase by as much as two times as the polarization state is changed. [ABSTRACT FROM AUTHOR]

The preparation of substituted phenols via a direct carbonyl desaturation is an intrinsic challenging task in modern synthetic chemistry. Herein, we develop a modular catalytic protocol for the synthesis of substituted phenols via the direct dehydrogenative aromatization of simple cyclohexanones in a metal‐free manner. A broad variety of substrates bearing an assortment of functional substituent groups are compatible with this protocol. Preliminary mechanistic investigations revealed that the reaction underwent a hydride leaving mechanism in the presence of a catalytic amount of tetrabutylammonium iodide. The current approach features simple operation, low reagent cost and green conditions. Insight gained from our studies is expected to advance general efforts towards the green synthesis of structurally diverse phenols via readily available raw materials, offering an alternative strategy for the carbonyl desaturation in organic synthesis. [ABSTRACT FROM AUTHOR]

Growth of Er3+‐doped 100‐nm thick LiNbO3(LN) films on Z‐cut LiTaO3(LT) single‐crystal by e‐beam evaporation and in‐situ anneal at 600–800°C was studied. Crystalline phase, crystallographic orientation, Er3+ spectroscopic properties, constituent element valence states, defects, Er3+ concentration and stoichiometry of the films were characterized. The results show that the films are in LN single‐crystal state and orient along with c crystallographic axis. Constituent elements Li, O and Er are present in their respective single valence states Li+, O2−, Er3+, while the Nb is in two valence states Nb5+ and Nb4+. The films have Li2O‐contents around congruent point. Antisite(NbLi4+) is the major defect. Its concentration decreases, while Li2O‐content, crystallinity degree, and fluorescence intensity increase with increased growth temperature. The presence of Er3+ in the grown films is in the form of LN crystalline phase. Its concentration is considerably lower than that in initial target material because of lower e‐beam evaporation efficiency of heavy Er3+. [ABSTRACT FROM AUTHOR]

Mode size converter enables to match two waveguides with different mode sizes. It has already become an indispensable component in modern integrated photonics. Here, we propose a strategy for implementing an on-chip mode size converter consisted of a gradient geometry metasurface and two cascaded waveguide tapers on the lithium-niobate-on-insulator platform. The metasurface, which is formed by a series of length-graded silica nanostrips embedded into LN waveguide, functions as focusing of incident wave and exaltation of conversion efficiency. A mode size converter that enables to match a fundamental TE 00 mode in a 10-μm-wide waveguide with that in a 0.8-μm-wide one was simulated to verify the feasibility of the strategy. We show that the device is not only compact with a length of 22.9 μm, but also shows an ultra-broad operation band as much as 1150 nm (from 1100 to 2250 nm), over which the conversion efficiency keeps >90 %. In addition, we show that the device has a good robustness to fabrication deviations. The strategy proposed here can be flexibly extended to other waveguide cross-section sizes and other guided modes. • A strategy is proposed for realizing an on-chip ultra-broadband mode size converter. • It uses gradient geometry metasurface and cascaded tapers on LNOI platform. • It is proved by simulating a device connecting a wide waveguide with a narrow one. • The device is valid for on-chip use and has an ultra-broad band as much as 1150 nm. • The strategy has a good scalability, the device has an excellent fabrication tolerance. [ABSTRACT FROM AUTHOR]

Er3+/Yb3+ doped LiNbO 3 (LN) single crystal films with varying Yb3+ doping level were grown on z -cut LiTaO 3 bulk single crystals via using e-beam evaporation method. The crystallographic orientation, elemental distribution, fluorescence properties, ionic valence states, Er3+ concentration, Yb3+ concentration and Li 2 O content of films were carefully studied. Results from XRD, SEM and Raman scattering conclude that the predominant phase of these films is LN and they are oriented along (006). The results of energy dispersive spectroscopy indicate that constituent elements are uniformly distributed over the entirety of the films. The visible and near-infrared luminescence intensities rise with increasing Yb3+-doping concentration. The Li 2 O content of films remains constant at 48.8 mol% with growing Yb3+-doping level based on the optical birefringence method, which is consistent with the Raman results. XPS results suggest that the Er3+ and Yb3+ concentrations in the films are lower than that in the polycrystalline powder since e-beam evaporation has a low efficiency of heavy Er3+ and Yb3+ ions. • The Er3+, Yb3+ doped LiNbO 3 single crystal thin films were grown by the e-beam evaporation method. • The fluorescence properties enhance clearly with a rise in Yb3+-doping concentration. • The accurate Li 2 O content have been evaluated by the birefringence method. • The Er3+ and Yb3+ concentration in the films have been calculated by the XPS. [ABSTRACT FROM AUTHOR]

Diffusivity of Er3+ into LiNbO3 crystal has been studied as a function of Li2O content and crystal orientation. The Li2O content in single-crystal LiNbO3 plates was adjusted by Li-rich vapor transport equilibration (VTE) technique. Five X-cut and five Z-cut congruent substrates were VTE treated at first at 1100 °C for different durations ranging from 6 to 74 h, and then were subjected to a special post-VTE anneal procedure at the same temperature for 60 h to obtain homogeneous Li2O content. The Li2O content was determined from the measured fundamental optical absorption edge. Its homogeneity is verified by measuring the optical absorption edge as a function of the crystal thickness. After the composition characterization, the substrates were coated with 45 nm thick Er film and annealed at 1130 °C for 40 h in argon atmosphere. The Er3+ profile was analyzed by secondary ion mass spectrometry. The results show that the Er3+ diffusivity decreases monoexponentially with the increase in the Li2O content in both cases of X- and Z-cut. As the Li2O content is increased from the congruent point (48.4 mol %) to the near-stoichiometric composition (∼49.9 mol %), the diffusivity decreases from 0.107 (0.072) to 0.017 (0.018) μm2/h for the Z (X)-cut substrate. The diffusion shows considerable anisotropy for the lower Li2O content and the diffusivity in a Z-cut substrate is larger. The anisotropy weakens gradually with the increase in the Li2O content, and the diffusivities in the X- and Z-cut substrates ultimately tend to a same value (∼0.02 μm2/h) at the Li-rich phase boundary. After the Er diffusion procedure, endface coupling experiment was carried out to check if a planar waveguide layer was formed near the Er-diffused surface due to the Li outdiffusion. The results show that the Li outdiffusion did not accompany the Er diffusion procedure. [ABSTRACT FROM AUTHOR]

The thermal diffusion of Er3+ into X- and Z-cut congruent LiNbO3 crystal in Li-enriched atmosphere [i.e., vapor transport equilibration (VTE)], created by Li3NbO4–LiNbO3 two-phase powder at the temperature around 1130 °C, was attempted. Single-crystal x-ray diffraction, micro-Raman, photoluminescence spectroscopy, and secondary ion mass spectrometry (SIMS) were used to study the crystalline phase with respect to Er3+ ion and the Er3+ diffusivity. The results show that the thickness of the Er film coated should not be thicker than 10 nm for an X-cut plate and 15 nm for a Z-cut plate. In this case, the diffusion is complete if the duration is long enough (>150 h) and the Er3+ ions in the diffused layer still retain the LiNbO3 phase. On the other hand, if the initial thickness of the Er metal film is thicker than 10 nm for the X-cut plate and 15 nm for the Z-cut plate, the diffusion will be incomplete no matter how long the duration is. This is because the residual Er3+ ions form irremovable ErNbO4 grains on the surface of the crystal. SIMS analysis on an X-cut VTE (1130 °C/192 h) and a Z-cut VTE (1129 °C/158 h) crystal coated, respectively, with 10 and 15 nm thick Er film reveals that the Er diffusion shows obvious anisotropy with the mean diffusion coefficients of 0.0155 and 0.0957 μm2/h, respectively. The surface concentrations are 1.5×1020 and 1.0×1020 at./cm3, respectively. The diffused Er3+ ions follow the stretched-exponential decay profile with a stretching factor of 1.85 and 3.5, respectively. The Li/Nb ratio in the Er-diffused layer is similar to 99.4% for the X-cut sample coated with 10 nm thick Er film and 99.3% for the Z-cut crystal coated with 15 nm thick Er film. The rms roughness of the diffused surface is better than 6 and 4 nm for the X-cut and Z-cut samples, respectively. [ABSTRACT FROM AUTHOR]

Standard thermal diffusion of Er3+ into X-cut congruent LiNbO3 crystal with the MgO doping level close to the antiphotorefractive threshold concentration, 4.5 mol%, was attempted at the temperature close to Curie point of the crystal. Single-crystal x-ray diffraction, photoluminescence spectroscopy and secondary ion mass spectrometry (SIMS) were used to study the crystalline phase with respect to Er3+ ions and the depth profile of the diffused Er3+ ions. The results show that the thickness of the Er metal film coating should not be thicker than 4.6 nm. In this case, the diffusion is complete, the diffused surface is clean, and the Er3+ ions in the diffused layer still retain the LiNbO3 phase. On the other hand, the diffusion will be incomplete if the initial thickness of the Er metal film is thicker than 4.6 nm. The residual Er3+ ions form ErNbO4 grains on the surface of the crystal. SIMS analysis shows that the diffused Er3+ ions follow a monoexponential decay profile. The experimental results also show that the diffusivity of Er3+ in the X-cut congruent Mg(4.5 mol %):LiNbO3 is similar to 4.69×10-3 μm2/h at 1130 °C. Further vapor transport equilibration treatment following the standard diffusion procedure results in transformation of the diffused Er3+ ions from LiNbO3 into the ErNbO4 phase and, hence, substantial spectral change of the 1.5 μm emission. [ABSTRACT FROM AUTHOR]

Congruent LiNbO3 crystals doped with 5.5/1, 6/0.15, and 7/0.8 mol %/mol % of Zn/Er were thermally treated at 1120 °C over 101 h using vapor transport equilibration (VTE) technique. Unpolarized visible and infrared absorption spectra of the VTE-treated and corresponding as-grown crystals were recorded at room temperature. It is shown that VTE treatment has brought these crystals closer to stoichiometric composition. X-ray powder diffraction and optical absorption results show that the VTE treatment induces the crystalline phase transformation with respect to Er3+ from the original Er3+:LiNbO3 to the ErNbO4 in all crystals studied. The formation of the ErNbO4 phase results in substantial absorption changes that include the drop of the transparency in the visible region, the reduction of Er3+ absorption, and the Zn-doping-level-dependent changes in spectral shape and peaking position at 0.98 and 1.5 μm regions. Based upon the measured absorption spectra, the influences of Zn-doping level and VTE treatment on the Er3+ spectroscopic properties were studied by using the Judd-Ofelt (JO) theory. The results show that the VTE treatment has a little effect on the fluorescence branch ratio, while a large effect on the JO parameters, oscillator strength, and electron transition probability. The Zn doping level affects the JO parameters, oscillator strength, and transition probabilities of some individual manifolds. In addition, the VTE treatment leads to the increases of the theoretical radiative lifetimes of almost all manifolds considered. For the emission at 1.5 μm, the lifetime increase is as much as 30% and the theoretical results are in well agreement with the experimental data. The VTE-induced lifetime increase is qualitatively explained. [ABSTRACT FROM AUTHOR]

A model is proposed for describing two-dimensional diffusion of Ti into an initially congruent LiNbO3 crystal under a Li-enriched atmosphere created by a mixed two-phase (Li3NbO4 and LiNbO3) powder at elevated temperature [vapor transport equilibration (VTE)]. The influence of VTE treatment on Ti diffusivity is considered in the model. To solve the model, four key input parameters including Li-concentration-dependent Li and Ti diffusivities and two switching times t1 and t2 were determined. Prior to solve the Ti-diffusion model, a one-dimensional VTE model is solved at first to obtain the dynamic Li2O concentration depth profile. Both the Li-diffusion and Ti-diffusion models were solved by using finite difference method. Based on secondary-ion-mass spectrometry analysis, the validity of the VTE and Ti-diffusion models as well as the numerical method employed are confirmed. After that, diffusion at 1100 °C of an 8-μm-wide, 100-nm-thick Ti strip defined on the surface of a Z-cut or an X-cut substrate was simulated for the VTE duration up to 130 h. Based on the numerical results, the Ti-(Li-)diffusion characteristics are discussed in the aspects of (1) the relation of depth and width profile function of Ti concentration to the VTE duration, (2) the substrate cut effect on both the Ti and Li diffusions, (3) the relation of the 1/e Ti-concentration depth and half-width to the VTE duration, and (4) the VTE duration dependence of the mean [Li]/[Nb] ratio within the Ti-diffused layer. [ABSTRACT FROM AUTHOR]

• A thin-film lithium niobate TM-pass waveguide polarizer is proposed. • Subwavelength grating metamaterial for polarization generation. • Simulation shows that the device can operate with a bandwidth ∼415 nm. • The device shows reasonable extinction ratio and insertion loss. An ultra-broadband TM-pass waveguide polarizer based on thin-film LiNbO 3 -on-insulator waveguide and subwavelength grating (SWG) metamaterials is proposed and modeled. The device consists of an SWG in the central part and two coupling regions in the input and output sides. The SWG, formed by a series of bending nano-stripes, functions as a filter of polarization, and the two coupling regions serve to degrade mode mismatch loss. By tailoring the duty cycle of the SWG metamaterial, the SWG structure supports only the TM guided mode, while the TE guided mode is cutoff and thus filtered out. Simulation shows that, with a SWG meta-material formed by a series of 90°-bending nano-stripes, the polarizer can achieve an ultra-large bandwidth of ∼415 nm (1.26–1.675 μm) with an insertion loss < 1.1 dB and an extinction ratio > 29.4 dB. Moreover, the proposed polarizer tolerates a 5 nm deviation in both width and thickness of the nano-stripes. Such an ultra-broadband device may find its use in high-capacity optical communication systems, and is proposed firstly on the basis of the lithium-niobate-on-insulator platform, to the best of our knowledge. [ABSTRACT FROM AUTHOR]

Two optimized methods for accurately measuring the 1.5 µm lifetime of Er[sup 3+] in LiNbO[sub 3] crystals and waveguides are proposed. The first method, suitable for measurement of bulk material, was suggested by forming an annealed proton exchanged sandwich structure and fluorescence was collected through an adjustable aperture. This excitation collection scheme was used to investigate bulk Er-doped LiNbO[sub 3] crystals with eight different Er[sup 3+] doping levels, three different crystal thicknesses, and three different cut orientations. The 1.5 µm lifetime was investigated in three configurations: with both a sandwich and an aperture, with a sandwich but without an aperture, and without not only a sandwich but also without an aperture. In addition, the effect of the aperture was also studied. The second method, suitable for a waveguide structure, was proposed on the basis of transversely gathering the fluorescence aided by an outcoupling fiber. It was optimized by comparing three different fluorescence collection schemes. Using this method, the lifetimes of some Ti: Er:LiNbO[sub 3] strip waveguides were measured. In addition, the effect of radiation trapping on the polarized fluorescence spectrum was also investigated. The experimental results are discussed and summarized. [ABSTRACT FROM AUTHOR]

(The origin of the formation of nanoprecipitates of ErNbO[SUB4] induced by vapor transport equilibration (VTE) in Er:LiNbO[SUB3] crystals was investigated by comparing x-ray powder diffraction patterns of VTE-treated, annealed and as-grown Er:LiNbO[SUB3] crystals. The results indicated that annealing an Er:LiNbO[SUB3] crystal in ambient atmosphere, whether the crystal is x or z cut, and whether it is highly or lower doped, does not influence its crystalline structure, and hence cannot cause the formation of the precipitate. The elevated temperature is not a sufficient requirement for the formation of this additional phase, but is only an essential prerequisite, together with the high-doping level of erbium. The incorporation of a certain amount of additional Li[SUP+] ions accompanying the VTE procedure is considered as the direct reason for the formation of the precipitate. The mechanism of the formation of the precipitates of ErNbO[SUB4] was tentatively suggested from both microscopic and macroscopic points of view. In addition, the origin of VTE-induced cracking, which occurs only in higher-doped x-cut crystals, was also considered based on the experimental results of annealed crystals. The incorporation of a certain amount of additional Li[SUP+] ions by the VTE procedure is again correlated with the cracking. [ABSTRACT FROM AUTHOR]

At room temperature, polarized visible and near infrared emission characteristics of 488 nm pumped vapor transport equilibration (VTE) treated Er:LiNbO[sub 3] crystals were investigated in comparison with the corresponding as-grown ones. The observed characteristics of the VTE crystal, whether it precipitates or not, include the retained polarization dependence, slight shift (less than 5 Å) of peak position, narrowing of some individual emission peaks, the appearance of additional emission peak near 1536.5 nm and the about 10% lengthening of 1.5 µm lifetime. In addition, in the case of lower Er doping level, the spectral shape are well reserved for the VTE crystals and no obvious changes in both visible and IR emission intensity. On the other hand, for those highly doped crystals, the VTE treatment not only results in a definite change of relative intensity between some emission peaks in IR region, but also causes the drop of both visible and IR emission intensity. Combining with the related absorption characteristic allow to conclude that the drop of the emission intensity results from the scattering of the nanocrystalline grains inside these crystals formed by VTE treatment. [ABSTRACT FROM AUTHOR]

The visible and infrared transmission spectra of vapor transport equilibration (VTE) treated Er:LiNbO[sub 3] crystals, which have different doping levels (0.2%, 0.4%, and 2.0% Er per cation site), different cut orientation (X and Z cut) and different VTE duration (80, 120, 150, and 180 h), were recorded at room temperature in the wavelength range of 250-3700 nm. All of 2.0 mol % doped VTE crystals have precipitated whether X cut or Z cut, while the others have not. Their absorption characteristics were summarized and discussed in contrast to those of corresponding as-grown crystals. The OH[sup -] absorption feature of VTE treated Er:LiNbO[sub 3] is found to be different from that of pure VTE LiNbO[sub 3] crystal. The significant reduction of OH[sup -] absorption band implies that the hydrogen content in the VTE crystals has been reduced substantially whether the crystal precipitates or not. The electron transition absorption characteristics of the lower-doped, not precipitated crystals mainly include the higher transmittance, slight shift of peak or band position, slight absorption intensity change, the appearance of some additional peaks or bands, the narrowing of the peak width (full width at half maximum), and the definite blueshift of the optical absorption edge. The spectral changes are associated with the redistribution of Er[sup 3+] spectroscopic sites induced by the VTE procedure. In comparison with those lower-doped VTE crystals, the highly doped VTE crystals display more significant absorption characteristics: the significant enhancement of 1480 nm pumping band and the obvious weakening of 1531 nm peak, the appearance of many additional peaks in the infrared region, and the interesting evolution of the transmittance with the wavelength. These substantial spectral changes are unambiguously conducted with the formation of a precipitate ErNbO[sub 4] induced by the VTE treatment in these crystals. The mechanism for the formation of the precipitate has... [ABSTRACT FROM AUTHOR]

• Polarization beam splitter based on LNOI triple-waveguide coupler is proposed. • α-Si nanostrip is used to manipulate the coupling of TM polarization mode. • Higher polarization extinction ratio and lower insertion loss are achieved. • The device is compact and suitable for on-chip applications. • A larger feature size (∼200 nm) that facilitates fabrication is involved. Polarization beam splitter (PBS) is a fundamental component of integrated photonics to manipulate wave polarization state, and most PBS devices were designed on the popular SOI. As an alternative, a PBS may be implemented on the basis of thin-film LiNbO 3 -on-insulator (LNOI), a promising platform for developing various compact or ultra-compact photonic active and passive devices. However, smaller modal birefringence of the LNOI waveguide makes it not easy to implement a PBS of high-performance. Here, we propose a compact, high-performance PBS on the basis of a triple-waveguide coupler consisted of two LNOI ridge waveguides and an amorphous silicon (α-Si) nanostrip assisted waveguide. By making use of the α-Si nanostrip, the PBS works in the case that the TM polarization mode meets the phase matching condition while the TE polarization mode does not. Numerical results show that the PBS is compact with a length of only 31 μm. The device can achieve an ultra-high polarization extinction ratio (>29.4 dB) and an ultra-low insertion loss (<0.066 dB) in the operating wavelength range of 1500–1600 nm for the TE polarization mode, and a polarization extinction ratio of > 20 dB and an insertion loss of < 0.16 dB over a ∼ 60 nm bandwidth from 1527 nm to 1587 nm for the TM polarization mode. Moreover, the device has a larger feature size that facilitates fabrication. Overall, this device has the advantages of compact size, higher polarization extinction ratio, lower insertion loss, and larger feature size, which is suitable for on-chip applications. [ABSTRACT FROM AUTHOR]

A post-growth Li-rich vapor transport equilibration (VTE) technique was used to prepare near-stoichiometric Er3+ doped LiNbO 3 thin films. Er3+ fluorescence property, defect feature and Li 2 O content of the films were characterized. The results show that the visible and near-infrared fluorescence intensity increases remarkably by ∼10 times the VTE time is increased from 0 to 5 h. The enhancement can be explained from viewpoint of Nb4+ defects concentration. X-ray photoelectron spectrometer study confirms presence of antisite Nb Li 4+ in the films and Nb Li 4+ is major defect. Its concentration decreases with increased VTE time. The Li 2 O content in the films is evaluated to be 48.8–49.7 mol% from the birefringence results. It increases with a rise in VTE time, matching with the characteristic of antisite defects. As defects concentration decreases, the non-radiative transition weakens , which is beneficial to improve fluorescence properties. • A Li-rich vapor transport equilibration technique was used to prepare near-stoichiometric Er3+ doped LiNbO 3 films. • Visible and near-infrared fluorescence intensity increases by ∼10 times as VTE time is increased to 5 h. • Li 2 O contents in the grown films were evaluated from measured birefringence to be from 48.8 to near-stoichimetric 49.7 mol%. • The enhancement can be explained from viewpoint of Nb4+ defects concentration measured by XPS. [ABSTRACT FROM AUTHOR]

• Photonic hooks (PHs) array generated by a multi–dielectric structure is studied. • PHs formation is ascribed to optical inhomogeneity of multi-dielectric structure. • PHs performance can be effectively tailored by geometric and optical parameters. • Both 1D and 2D PHs arrays bent outward or inward can be generated. • Both 1D and 2D PHs arrays show application potential in field of 3D manipulation. Generation of array of photonic hooks (PHs) by a multi–dielectric structure that consists of periodically arrayed scatterer units has been systematically studied. The study focuses on the effects of geometric and optical parameters of the multi–dielectric structure on the characteristics of generated PHs, especially their bending angles. These geometric and optical parameters include size, shape, refractive index and number of scatterer units, and their adjacent spacing. The results show that the spacing, size and refractive index play a predominant role in tailoring the performance of the PHs array, and the spacing affects mainly the characteristics of a pair of symmetrical PHs while the size and refractive index differences between scatterer units affect mainly those of asymmetrical PHs as a result of the introduction of optical inhomogeneity. In the aspect of scatterer shape, for a multi–dielectric structure that consists of some cylinders, as the light wave is incident parallel (perpendicular) to the cylinder axis, its scattering characteristics are similar to those of the multi–dielectric structure that consists of some cuboids (spheres). In particular, for a structure consisted of cylinders or cuboids with long length in the z direction, an intensive photonic nanojet is additionally generated and located between the paired PHs provided that the wave is incident perpendicular to the cylinder axis or the longest side of cuboid. In the aspect of scatterer unit number, the multi–dielectric structures of odd and even scatterer units generate PHs with different characteristics. The paired PHs generated by multi–dielectric structure with equal spacings always bend outward. However, the paired PHs may bend inward provided that the multi–dielectric structure is designed properly. We have also studied the features of 2D PHs array. The results show that the 2D PHs array displays similar features to the 1D case. The application potential of both 1D and 2D PHs arrays is evaluated. [ABSTRACT FROM AUTHOR]

• A TM-pass polarizer based on an air-slot embedded TFLNOI waveguide is proposed. • Designed structure supports only a TM mode while TE mode being in cut-off state. • A symmetric tetragonal dipyramid air-slot is designed to degrade insertion loss. • Both slot geometry and silica thickness take effects on polarization performance. • Optimal device shows good performance and ultra-compactness valid for on-chip use. An air-slot assisted TM-pass waveguide polarizer based on thin-film lithium niobate on insulator (TFLNOI) is proposed. It consists of a segment of ridge-type TFLNOI single-mode strip waveguide and an embedded air-slot. The air-slot has a symmetric tetragonal dipyramid structure consisted of a cuboid in the central part and two identical tetragonal pyramids in two sides, which function as polarization generation and adiabatic approximation to degrade insertion loss, respectively. Waveguide properties and transmission characteristics of the device were simulated using finite difference method. The results show that the waveguide structure with a properly designed geometry supports only a TM mode, while does not the TE mode, which is cut-off in the cuboid air-slot region. Slot length, width and silica substrate thickness all take considerable effects on the insertion loss and extinction ratio. Qualitative and comprehensible explanations are given for the effects. An optimized polarizer has been suggested that exhibits excellent polarization performance, including a larger extinction ratio of 28.4 dB, a lower insertion loss of 0.29 dB, and a broader bandwidth of ∼110 nm (1490–1600 nm) with an extinction ratio >20 dB and an insertion loss <0.5 dB. In addition, the proposed device has a length of only 13 µm satisfying on-chip use. [ABSTRACT FROM AUTHOR]

A thermodynamic study was performed on diffusion growth of Ga 3+ -doped LiTaO 3 (LT) thin film for integrated optics. Some Ga 3+ -doped LT thin films were grown on LT surface by in-diffusion of homogeneously coated Ga 2 O 3 film at the temperature range of (1273 to 1473) K. After growth, the refractive indices at Ga 3+ -doped and un-doped surface parts were measured by prism coupling technique and Li composition there was evaluated from the measured refractive indices. The results show that Ga 3+ dopant has small effect on the LT index. Li 2 O out-diffusion is not measurable. The Ga 3+ profile in the grown thin film was analysed by secondary ion mass spectrometry. It is found that the grown Ga 3+ ions follow a complementary error function profile. A thermodynamic model for Ga 3+ diffusion growth is suggested and verified experimentally. From the measured Ga 3+ profiles, some thermodynamic parameters were obtained. These include diffusivity, diffusion constant, chemical activation energy, solubility, solubility constant and enthalpy of solution. These parameters are crucial to design and growth of a desired Ga 3+ -doped LT thin film for integrated optics. Comparison shows that Ga 3+ grows in LT two orders faster than Ti 4+ because of its smaller radius. Ga 3+ diffusivity/solubility in LT is one order/1.5-fold lower than in LiNbO 3 because Ta has a larger atomic mass and hence a lower mobility than Nb. [ABSTRACT FROM AUTHOR]

Some Er 3+ -doped LiTaO 3 plates were prepared by in-diffusion of Er-metal film locally coated onto congruent Z -cut substrate in air at a wide temperature range from 1000 to 1500 °C. After diffusion, Er 3+ -doping effect on LiTaO 3 refractive index and Li 2 O out-diffusion arising from Er 3+ in-diffusion were studied at first. Refractive indices at the doped and undoped surface parts were measured by prism coupling technique and the surface composition was estimated. The results show that Er 3+ dopant has small contribution to the LiTaO 3 index. Li 2 O out-diffusion is slight (Li 2 O content loss <0.3 mol%) for the temperature below 1300 °C while is moderate (Li 2 O content loss <0.6 mol%) for the temperature above 1400 °C. The Er 3+ profile was studied by secondary ion mass spectrometry. The study shows that the diffused Er 3+ ions follow either a complementary error function or a Gaussian profile. Characteristic parameters including diffusivity, diffusion constant, activation energy, solubility, solubility constant and heat of solution were obtained and compared with the LiNbO 3 case. The comparison shows that the diffusivity and solubility in LiTaO 3 are considerably smaller than in LiNbO 3 because of the difference of Ta and Nb in atomic weight. [ABSTRACT FROM AUTHOR]

Er 3+ /Ti 4+ -co-doped LiNbO 3 plates were prepared by annealing congruent substrates coated with stacked Er- and Ti-metal films having different thicknesses and coating sequences. The crystalline phase, OH − absorption, Er 3+ -doping effect on LiNbO 3 refractive index and Li + -composition in the Er 3+ /Ti 4+ -co-doped layer were studied. The results show that the LiNbO 3 phase dominates in the co-doped layer, the Er 3+ -doping affects little the LiNbO 3 refractive index, slight Li 2 O out-diffusion occurred in the diffusion procedure, and the annealing procedure does not affect the OH − absorption band noticeably. The Er 3+ /Ti 4+ -co-diffusion characteristics were studied by secondary ion mass spectrometry. The study shows that Ti 4+ diffuses much faster than Er 3+ . The Ti 4+ co-diffusion assists and controls the Er 3+ -diffusion and Er 3+ diffusivity increases linearly with initial Ti-metal thickness. In contrast, neither Er-metal thickness nor coating sequence of Ti- and Er-metal films influences the Ti 4+ or Er 3+ diffusivity. The co-diffusion features are explained. A co-diffusion model is proposed. [ABSTRACT FROM AUTHOR]

The mutual influence of co-diffused Ti and Sc in LiNbO single-crystal was studied. Sc/Ti co-doped LN plates were prepared by annealing (at 1060 °C in wet O) Z-cut congruent LiNbO substrates coated with stacked ScO and Ti films, which have different thicknesses and coating sequences. After the annealing, the refractive index and composition at the undoped and Sc-only doped surface parts were characterized by prism coupling technique. The Sc and Ti profiles were analyzed by secondary ion mass spectrometry. The results show that the Sc-doping has little contribution to the LiNbO refractive index. LiO out-diffusion is not measurable. In the only diffusion case, the Sc diffuses five times slower than the Ti. In the Sc/Ti co-diffusion case, the Ti assists and controls the Sc diffusion. The Sc diffusivity increases linearly with the initial Ti-metal thickness. In contrast, the Sc affects the Ti diffusion little. Neither the ScO 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:LiNbO planar and strip waveguide are optically characterized. The results show that the Sc presence does not affect the waveguide characteristics and the waveguide is optical-damage-resistant. [ABSTRACT FROM AUTHOR]

In this paper, we have tried to prepare MgAl2O4 spinel by molten salt synthesis using hydroxide as solvent. By controlling the amount of KCl in KOH, pure flower-like MgAl2O4 nanostructures could be synthesized at 1150 °C. Powder X-ray diffraction, field emission scanning electron microscopy, and transmission electron microscopy were used to investigate the structures and morphologies of the obtained products. The results showed that the 3D flower-like nanostructures consisted of numerous 2D nanoflakes with a thickness of about several tens of nanometers. Further investigations on the possible formation mechanism revealed that a self-sacrificing template process dominated the formation of MgAl2O4 and the growth of the flower-like products was attributed to the competition between the growth rates along the directions perpendicular and parallel to the Al2O3 particle surfaces. Finally, an impedance-type humidity sensor was fabricated based on the flower-like MgAl2O4 nanostructures and tested for its humidity performance showing that the as-prepared MgAl2O4 nanostructures are suitable for high-performance humidity sensors. [ABSTRACT FROM AUTHOR]

Large amounts of MgAl 2 O 4 micro-rods were successfully synthesized using the molten-salt technology. The effect of KCl contents on the formation of MgAl 2 O 4 micro-rods was investigated. The structure and morphology of MgAl 2 O 4 were investigated by means of powder X-ray diffraction, field emission scanning electron microscopy, and transmission electron microscopy, respectively. The experimental results showed that the contents of KCl significantly influenced the formation of MgAl 2 O 4 micro-rods. MgAl 2 O 4 micro-rods could be prepared at 1150 °C with a weight ratio of 100:1 between the salt and the starting materials. The formation of MgAl 2 O 4 micro-rods could be suggested to be due to the inhomogeneous nucleation and orientated growth perpendicularly to the surfaces of Al 2 O 3 grains. An impedance-type humidity sensor was finally fabricated based on the as-prepared MgAl 2 O 4 micro-rods. According to tests of the humidity performance, MgAl 2 O 4 micro-rods might be suitable for high-performance humidity sensors. [ABSTRACT FROM AUTHOR]

Zr4+/Ti4+-codoped LiNbO3 plates were prepared by local codiffusion of stacked ZrO2 and Ti metal films coated onto Z-cut congruent LiNbO3 substrates in wet O2 at 1060°C. The metal and oxide films have different thicknesses and coating sequences. After diffusion, the Zr4+ doping effect on the refractive index of LiNbO3 and the Li2O out-diffusion issue were studied by the prism coupling technique. The codiffusion characteristics of Zr4+ and Ti4+ were studied by secondary ion mass spectrometry. The results show that the Zr4+ doping has little contribution to the refractive index of the crystal. Li2O out-diffusion is not measurable. In the Zr4+-only diffusion case, the diffusivity of Zr4+ is four times smaller than that of Ti4+. In the Zr4+/Ti4+ codiffusion case, the Ti4+ codiffusion assists the Zr4+ diffusion. The Zr4+ diffusivity increases linearly by two more times with the increase in initial Ti film thickness from 0 to 200 nm. On the other hand, the Zr4+ affects the Ti4+ diffusion little. Neither the ZrO2 film thickness nor the coating sequence of Ti metal and ZrO2 oxide films influences the diffusivity of the two ions. All the codiffusion characteristics are explained. A Zr4+/Ti4+ codiffusion model is suggested that consists of two independent diffusion equations with a Zr4+ diffusivity dependent of Ti4+ concentration and a constant Ti4+ diffusivity. In addition, the existence of a waveguide in the Zr4+/Ti4+-codoped layer is verified experimentally, and the optical-damage-resistant feature of the waveguide is verified by two-beam hologram recording experimental results. [ABSTRACT FROM AUTHOR]

Multi-mode Ti:LiNbO 3 planar waveguide was fabricated by in-diffusion of 100-nm-thick Ti-metal film coated onto a near-stoichiometric LiNbO 3 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 Ti 4+ - concentration profile, which was obtained from secondary-ion-mass-spectroscopy analysis. The results show that the waveguide still retains the LiNbO 3 phase, its composition is near-stoichiometric. The index change and Ti 4+ -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. [ABSTRACT FROM AUTHOR]

Diffusion properties of Sc3+ in LiNbO3 crystal were studied, together with other two related issues: Sc3+-doping effect on LiNbO3 refractive index and Li2 O out-diffusion arising from Sc3+ in-diffusion. To reach the goal, some Sc3+-doped LiNbO3 crystal plates were prepared by in-diffusion of Sc2 O3 film coated onto Z-cut congruent substrates in air at different temperatures ranging from 1030°C to 1130°C. After diffusion, the refractive indices at the doped and undoped parts of surface were measured by prism coupling technique and the surface composition was evaluated from the measured index. The results show that the Sc3+ doping has little contribution to the substrate index and the Li2 O out-diffusion is not measurable. The Sc3+ profile was analyzed by secondary ion mass spectrometry. Some characteristic parameters such as temperature-dependent diffusivity, diffusion constant, activation energy, and surface concentration are obtained. The Sc3+/ Ti4+ co-diffusion characteristics were also studied. Comparison shows that in the single-diffusion case, the Sc3+ diffusivity is considerably smaller than the Ti4+ diffusivity. In the Sc3+/ Ti4+ co-diffusion case, the Ti4+ co-diffusion results in substantial increase in Sc3+ diffusivity while the Ti4+ diffusivity changing little. The diffusion characteristics are qualitatively explained. The Ti4+-assisted Sc3+ diffusion would be utilizable in shortening the fabrication period of an optical-damage-resistant Ti-diffused LiNbO3 waveguide doped with Sc3+, and some considerations for the fabrication are given. [ABSTRACT FROM AUTHOR]

A numerical study has been performed on electric field profile features of photonic nanojet generated by a SiO 2 microcylinder with a varied diameter. The study shows that a strong (44.2I 0) nanojet with 0.7λ lateral width and 7.8λ length can be obtained, which, as a line light source, has a potential for application in fields of nanopatterning and nanolithography. Along with the study on a single-layer microcylinder, we further studied the characteristics of the nanojet generated by a two-layer microsphere consisted of an interior LiNbO 3 microsphere and an exterior semiconductor layer, which have refractive indices of 2.34 and ~3.3 @ 500 nm, respectively. The study focuses on nanojet generation condition and the effects of refractive indices of microsphere materials and background, and working wavelength on electric field distribution characteristics of the nanojet. By immersing the two-layer microsphere into a proper liquid such as the de-ionized water, an ultra-long (>14λ), ultra-strong (>50I 0) and narrow enough (0.56λ) nanojet could be created. All of the characteristics and effects are qualitatively interpreted in terms of ray optics in combination with Mie scattering theory. [ABSTRACT FROM AUTHOR]

Abstract: We report that potassium meta-phosphate (KPO3) is an adequate solvent for high-temperature fluxing salt of LiNbO3 crystal. As the KPO3 is used as the solvent, the solubility of LiNbO3 is as high as 3 g g−1 at 1050 °C. The dissolving is fast. Neither solute nor solvent evaporates from the melt during the dissolving procedure. A clear solution is obtained and verified valid for crystal composition analysis using chemical method of inductively coupled plasma atomic emission spectroscopy. To help for understanding the dissolving mechanism, the obtained fluxing melt product was further characterized using X-ray diffraction, nuclear magnetic resonance and Raman scattering spectroscopy. The results show that the melt is amorphous. The P ion in the melt is in the form of low condensation and the Nb ion is likely in the form of Lindqvist ion. Finally, the dissolving mechanism is discussed. [Copyright &y& Elsevier]

The influence of factors on the growthof the Er3+/Tm3+-codoped LiNbO3(LN)crystal by codiffusion ofstacked Er and Tm metal thin films was studied. These factors includethe thicknesses and the coating sequence of the Er and Tm metal films,and the growth is characterized by the diffusivity and solubility.To achieve the goal, Er3+/Tm3+-codoped LN crystalswere grown by codiffusion in air at 1130 °C of stacked Er andTm metal thin films coated onto the surface of Z-cut congruent LNplates. The metal films have different thicknesses and coating sequences.After the growth, the Er3+and Tm3+profileswere analyzed by secondary ion mass spectrometry. The Er3+/Tm3+diffusivity and surface concentration were obtainedfrom the measured profiles. The diffusivity is influenced by neitherthe thickness nor the coating sequence of metal films. The solubilityconsists of Er3+and Tm3+components, and thetwo components equal the respective concentrations in the diffusionreservoir and change with the initial metal film thickness. Nevertheless,their sum, i.e., the solubility, remains a constant at a given temperatureand shows a weaker effect of either the crystal cut or the coatingsequence of Er and Tm films, together with the two components. Inaddition, the emission characteristics of the Er3+/Tm3+diffusion-codoped LN were investigated. The results showthat the codoping allows the combination of the wavelength emissionsof both ions, and the resultant emission band in the telecommunicationwindow around 1.5 μm is as wide as 150 nm, providing the possibilityof S+C+L broadband amplification by employing the commercial 980 and795 nm laser diodes as the pump sources. [ABSTRACT FROM AUTHOR]

Co-diffusion characteristics of Er3+ and Tm3+ ions in LiNbO3 crystal have been studied, together with two other properties: doping effect on refractive index of substrate, and Li out-diffusion. Er3+/ Tm3+-codoped LiNbO3 single-crystals were prepared by co-diffusion of stacked Er- and Tm-metal thin films, coated onto parts of the surfaces of X- and Z-cut congruent LiNbO3 crystal plates, at different temperatures and different durations in air. After diffusion, the surface refractive indices of the doped and undoped parts were measured by prism coupling technique, and the surface composition was evaluated from the measured indices. The results show that the dopings have negligible effect on the substrate index and the Li out-diffusion depends mainly on the diffusion temperature. The Er3+ and Tm3+ profiles were studied using secondary ion mass spectrometry, and a co-diffusion dynamical model is proposed and verified experimentally. From the measured profiles, the temperature-dependent Er3+/ Tm3+ diffusivity and solubility were obtained together with other relevant parameters, such as diffusion constant, activation energy, solubility constant and heat of solution. The Er3+ and Tm3+ co-diffusion exhibits similar temperature dependent diffusivity, and the dependence is similar to that of single-diffusion. In the co-diffusion case, the total solubility consists of two parts: Er3+ and Tm3+ parts, there is no anisotropy, and the value is equal to that of single-diffusion case, in which both ions have the similar solubility. The Er3+ and Tm3+ parts of solubility change relatively with the initial metal film thicknesses and hence have different temperature dependences. Nevertheless, their sum, i.e., the co-diffusion solubility, remains a constant at a given temperature. [ABSTRACT FROM AUTHOR]

Abstract: A thermodynamic study on Li-poor vapor transport equilibration (VTE) in MgO-doped LiNbO3 crystal was carried out. A thorough thermodynamic VTE model is established that considers all of possible factors including the depth relative to crystal surface, VTE temperature and time. To solve the model, the composition on crystal surface was studied as a function of VTE temperature and time. To achieve that, a number of Li-deficient MgO(5 mol% in melt):LiNbO3 crystals were produced by the VTE under different temperatures from 1010 to 1130 °C for different durations up to 395 h, the crystalline phase contained was determined by X-ray powder diffraction and their surface compositions were determined from the measured birefringence. The results show that the Li-deficient crystal keeps the LiNbO3 phase. The surface Li2O-content has an Arrhenius relationship to the VTE temperature and a square-root dependence on the VTE duration. Based upon the VTE temperature and time dependences of surface Li2O-content, the solution to the thermodynamic VTE is obtained and verified experimentally. The solution is crucial to design and produce a Li-deficient MgO:LiNbO3 crystal with desired Li2O-content profile. By using the solution, one can predict the Li2O-content depth profile after a certain VTE time for a given VTE temperature. [Copyright &y& Elsevier]

Diffusion properties of Tm3+ in congruent LiNbO3 crystal have been investigated, together with other two related issues, i.e., Tm3+-doping contribution to refractive index of LiNbO3 substrate and Li out-diffusion. Four X-cut and four Z-cut congruent LiNbO3 substrates locally coated with 15-31 nm-thick Tm-metal films were annealed in surrounding air under different temperatures of 1030°C-1130°C for different durations of 20-70 h. After anneal, refractive index at Tm3+-doped and Tm3+-free parts of crystal surface was measured at the wavelengths of 1311 and 1553 nm and surface Li2 O contents were evaluated from measured refractive index. The results show that Tm3+ doping has a weak effect on substrate index and a small contribution to index increment in waveguide layer in comparison with Ti4+- or Zn2+ doping. The Li2 O content at the Tm3+-doped surface equals that at the Tm3+-free surface. The Li out-diffusion depends mainly on the diffusion temperature. Below 1100°C, the Li out-diffusion is not measurable. At 1130°C, a 30-h diffusion procedure may cause 0.2-0.3 mol% slight loss of Li2 O content. Secondary ion mass spectrometry was used to study the Tm3+ diffusion properties. The results show that the diffused Tm3+ ions in all samples follow a complementary error function profile. From measured Tm3+ profiles, characteristic diffusion parameters such as diffusivity, diffusion constant, activation energy, solubility, solubility constant, and heat of solution were obtained and discussed in comparison with the case of Er3+ diffusion. In comparison with Er3+ diffusion, the Tm3+ diffusion shows similar anisotropy and temperature dependence of solubility. In the aspect of diffusivity, under lower temperature the Tm3+ has a lower diffusivity than the Er3+, and their diffusivity difference reduces with the increased temperature and becomes null at 1130°C. [ABSTRACT FROM AUTHOR]

Influence of factors on growth ofoff-congruent LiNbO3single-crystal by Li-rich/Li-poorchemical vapor transport equilibration(VTE) was studied. These factors include the molar ratio of raw materialsLi2CO3and Nb2O5for preparingtwo-phase crucible, the hollow space volume for VTE growth, and theperiod of use of two-phase crucible. To achieve the goal, a numberof off-congruent LiNbO3plates were grown by Li-rich/Li-poorVTE technique using four two-phase crucibles having different molarratios of raw materials, hollow space volumes, and periods of use.The VTE-induced crystal composition alteration was characterized bymeasurement of birefringence. A comparison of results of differentcrucibles shows that in the Li-rich VTE case the first factor hasless effect, while the latter two may cause a difference of Li2O content as much as 0.4 mol %. Larger hollow space volumeand older crucible tend to degrade the VTE growth efficiency. Instead,the growth by Li-poor VTE is less affected by the three factors becauseof much lower Li2O gas pressure and hence slow growth ratein comparison with that by Li-rich VTE. [ABSTRACT FROM AUTHOR]

We have measured the unpolarized and polarized emission spectra of 4 I9/2→4 I15/2 (810 nm) and 4 S3/2→4 I13/2 (860 nm) electronic transitions of Er3+ in LiNbO3 crystal under different incident directions and polarization states of excitation beam. From the measured emission spectra, the emission and absorption cross-section spectra were calculated based upon McCumber theory. It is found that Er3+ electronic transition shows interesting excitation beam direction effect in polarization dependence, spectral shape, and cross-section value. Both transitions are highly π-polarized as the excitation beam was aligned perpendicular to the optical axis of crystal while being highly σ-polarized as the excitation beam was oriented parallel to the optical axis of crystal. The spectral shape in the case of the perpendicular excitation is very different from that in the case of parallel excitation. The cross-section value in the perpendicular excitation case is at least 1.5 times larger than that in the parallel excitation case. These excitation direction effects are independent of the polarization state of excitation light, and are attributed to the selective Er3+ site excitation. In addition, the Er3+ 860 nm emission lifetime was measured to be 27 ± 5 μs and the quantum efficiency of the emission is 2.5%. [ABSTRACT FROM AUTHOR]

Abstract: Secondary ion mass spectrometry (SIMS) was used to study the diffusion properties of Mg2+ and Ti4+ ions in a peculiar optical-damage-resistant near-stoichiometric (NS) Ti:Mg:LiNbO3 waveguide structure that a number of strip waveguides are embedded in a planar waveguide. The structure was fabricated on an initially congruent, undoped Z-cut substrate with a technological process in sequence of planar waveguide, strip waveguide, MgO diffusion and post Li-rich vapor transport equilibration (VTE) treatment. The results show that the Mg2+-profiles in both strip and planar waveguide layers are desirably homogeneous with a concentration 1.7±0.3mol% above the optical damage concentration threshold. The Ti4+-profile in the strip waveguide follows a sum of two error functions along the lateral direction with a width 13±1.0μm and a sum of two Gaussian functions in the depth direction with a 1/e depth of ∼11μm all. The Ti4+ ions in the planar waveguide obey a Gaussian profile. From the measured profiles, characteristic diffusion parameters such as mean Mg2+ and Ti4+ diffusivities and surface Ti4+-concentration are evaluated. The diffusivity is discussed in comparison with the previously reported results on single Mg2+, Ti4+ diffusion and Mg2+/Ti4+ co-diffusion in pure (or bulk MgO-doped), congruent (or NS) LiNbO3 crystal. Finally, the keys to the succeeding in fabrication are highlighted. [Copyright &y& Elsevier]

Abstract: Rare-earth-doping and various post-growth heat treatments effects on OH− absorption in singly Er- and Yb-doped, doubly Er/Mg- and Er/Yb-doped LiNbO3 crystals have been investigated. The OH− absorption in the rare-earth-doped crystals shows definite anisotropy and is stronger in a Z-cut plate. Rare-earth-doping causes considerable OH− absorption reduction. MgO-codoping induces additional reduction. Slight/heavy rare-earth-doping causes slight/considerable spectral shape change. Thermal reduction treatment causes OH− absorption vanishing whether the crystal is originally congruent or near-stoichiometric. Further oxidation treatment renders the absorption reemerged. Li-rich vapor transport equilibration (VTE) treatment causes significant absorption decrease. For a slightly rare-earth-doped Li-rich VTE-treated crystal, its spectral feature is similar to that of the undoped near-stoichiometric crystal; for a heavily rare-earth-doped VTE-treated crystal, its spectral shape preserves the feature of the heavily rare-earth-doped as-grown crystal. For the Er/Mg-codoped crystal studied, the Li-rich VTE also induces an abrupt conversion of peaking position from 3483 cm−1 to 3535 cm−1. The absorption in the Li-rich VTE-treated crystals also reveals aging effects in spectral shape, peaking position and absorption intensity. Li-poor VTE induces significant absorption increase. In addition, the effect of OH− presence on lifetime of Er3+ emission at 1.5 μm was also studied. The results show that the effect is small for the as-grown congruent crystals and slight for Li-rich VTE-treated ones. [Copyright &y& Elsevier]

Abstract: We have measured OH absorption spectra of a 0.47-mm-thick Z-cut MgO (5mol% in melt):LiNbO3 crystal subjected to post-growth Li-poor vapor transport equilibration (VTE) treatments at 1100°C for different durations ranging from zero to 395h. These spectra allow the evolution of OH absorption characteristics with prolonged VTE to be followed. After 2h of VTE process an additional absorption appears at 3483cm−1. A transition regime that the original 3536cm−1 and new 3483cm−1 absorptions simultaneously appear exists within the VTE duration range of 2–16h. In this regime, the 3536cm−1 absorption becomes weak gradually and eventually disappears around 16h while the 3483cm−1 absorption increases remarkably with the prolonged VTE. The presence of transition regime gives a hint that the 3483cm−1 absorption is due to the VTE-induced formation of a new center. Based upon the spectral features, we have suggested a simple three-layer (two on surface and one in the center of crystal plate) model to describe the depth profile of the photorefractive damage MgO concentration threshold in the crystal in the transition regime. A Li out-diffusion theory is suggested and correlated with the model. To support the Li out-diffusion theory, some additional experiments have been done. These include the depth profile characterization of VTE-induced Li2O content reduction and the measurement of the surface Li2O content as a function of the VTE duration. A quantitative analysis and discussion shows that the model is well supported by the experimental results. [Copyright &y& Elsevier]

Under Li-poor atmosphere, local Er3+ diffusion into highly MgO-doped LiNbO3 ( LN) crystal has been investigated. Three Z-cut 5 mol% (in growth melt) MgO-doped LN substrates locally coated with 44-60 nm thick Er-metal films underwent the respective Li-poor vapor transport equilibration ( VTE) treatment at 1060°C, 1100°C, and 1130°C for the same 90 h duration. After the VTE, the reductions of Li2O content at the Er-free and Er3+-doped parts of the surface of the three samples, determined from the measured birefringence, have a similar value of 1.0 ± 0.1, 2.0 ± 0.1, and 2.3 ± 0.1 mol%, respectively. Secondary ion mass spectrometry was used to study the Er3+ diffusion properties and the VTE effect on Mg2+ profile. From the measured Er3+ profiles, characteristic diffusion parameters such as 1/e diffusion depth, diffusivity, diffusion constant, activation energy, surface concentration, solubility, solubility constant, and heat of solution are determined. A comparison with the related results previously reported allows to conclude that heavy MgO-doping results in substantial decrease of both the diffusivity and solubility, and under the same diffusion temperature the diffusivity and solubility increase by as much as one order of magnitude and two times, respectively, as the diffusion atmosphere goes from the traditional argon to the Li-poor atmosphere. The Mg2+ profiles reveal that the VTE may induce MgO diffusion and hence an inhomogeneous profile in the Er3+-diffused layer. This undesired effect is strong for the 1130°C VTE temperature whereas is relatively much weak for 1060°C. It is tentatively explained in terms of the Er3+ diffusion and Li-poor VTE mechanisms. [ABSTRACT FROM AUTHOR]

We have measured the polarized visible and near-infrared, and unpolarized mid-infrared (2.7 \mum) emission spectra of photorefractive-damage-resistant locally Er-Mg-doped near-stoichiometric (NS) Ti:Mg:Er:LiNbO3 strip waveguide, fabricated on an X-cut initially congruent LiNbO3 substrate in sequence by local Er doping in air, Mg-Ti pre-diffusion in wet O2 and post Li-rich vapor transport equilibration treatment. From the measured emission spectra, the emission and absorption cross section spectra were calculated based upon McCumber theory. The spectroscopic features are discussed in comparison with the spectra recorded from the area outside the waveguide, and with the previously reported results of bulk-doped NS Er:Mg:LiNbO3 crystals and congruent Er:LiNbO3 bulk material or Ti:Er:LiNbO3 waveguide structure. The results show that the spectra of the NS waveguide are traditional and reveal small differences from those spectra outside the waveguide in spectral shape, polarization dependence, as well as cross section values. In contrast, the cross section values of the NS waveguide show considerable differences from those of bulk-doped NS material and congruent bulk material or waveguide structure. The 552, 673, 996, and 1531 nm emission lifetimes of the Er^3+ ions outside the waveguide were also measured, and found to be comparable to the results of the bulk-doped NS crystal and the congruent bulk material or waveguide structure. [ABSTRACT FROM PUBLISHER]

Abstract: An X-cut and a Z-cut Li-deficient, non-congruent LiNbO3 crystal were prepared by carrying out Li-poor vapor transport equilibration (VTE) treatment on one mm thick congruent plates at 1100°C for 170h. From the measured optical absorption edges (OAE), the Li2O content is evaluated to be 47.74±0.13mol% in the X-cut plate and 47.61±0.13mol% in the Z-cut VTE crystal. The composition homogeneity is verified by measuring the OAE as a function of the crystal thickness. The visible optical absorption and powder X-ray diffraction studies allow to conclude that both VTE crystals retain still the LiNbO3 phase. OH− absorption characteristics of the two VTE crystals were investigated in comparison with those of the as-grown crystals. The results show that the Li-poor VTE treatment only slightly modifies the area, spectral width, spectral shape and background absorption of the OH− band. Comparison shows that the Li-poor VTE effects on the OH− absorption have large differences from the Li-rich VTE effects in bandwidth and areas of three component peaks at 3466, 3481 and 3488cm−1. These spectral differences are explained on the basis of the H+ site occupation and crystal defect models taking into account the VTE effect on crystal defect concentration. The Li-poor VTE effects are attributed to the VTE inducing the redistribution of OH− ions in different sites. [Copyright &y& Elsevier]

Abstract: Polarized downconversion, 980-nm-upconversion and near-infrared emission characteristics of vapor-transport-equilibrated (VTEed) bulk Er (0.4mol%)/MgO (6mol%)-codoped LiNbO3 crystals were investigated. The downconversion and upconversion visible emissions display similar VTE effects including the drop of emission intensity and the weakening of polarization dependence. At 0.98 and 1.5μm regions, the VTE has a weak effect on the emission intensity, but a strong effect on the spectral shape. The crystalline phases in these bulk Er/Mg-codoped VTE crystals are determined by comparing their infrared emission characteristics with those of pure ErNbO4 powder and locally Er-doped MgO (4.5mol%):LiNbO3 crystal. The results show that the Er3+ ions present in these bulk Er/Mg-codoped VTE crystals as a mixture of Er:LiNbO3 and ErNbO4 phases. The percentages of the ErNbO4 phase contained in these VTE crystals were evaluated from the 1531 and 1536nm characteristic absorption areas. The contents of constituent elements were determined by chemical analysis. [Copyright &y& Elsevier]

Abstract: Congruent LiNbO3 crystals with Zn/Er-doping levels of 5.5/1, 6/0.15 and 7/0.8 mol%/mol% were thermally treated at 1100 °C over 101 h using vapor transport equilibration (VTE) technique. It is shown that the VTE treatment has brought these crystals closer to stoichiometric composition. X-ray powder diffraction and Er3+ spectroscopic characterizations show that the VTE treatment induced formation of micron-sized ErNbO4 precipitates on the surface and in the bulk of all crystals studied. It was observed using an optical microscope that the precipitates grow preferably along X, Y and Z axes of the host matrix, and show different morphologies, densities and dimensions, depending on the Er3+ and Zn3+ doping level. The formation origin of the precipitate is qualitatively explained. [Copyright &y& Elsevier]

Abstract: Polarized visible and infrared emission characteristics of Er3+ ions in vapor-transport-equilibration (VTE)-treated LiNbO3 crystals codoped with different concentrations of Zn and Er were investigated in comparison with corresponding as-grown crystals. The results show that the VTE treatment leads to substantial spectral changes of Er3+ emissions at 0.65, 0.98 and 1.5μm regions, and the spectral changes in the 0.98 and 1.5μm regions appear to be Zn-concentration-dependent. It is concluded in combination with X-ray powder diffraction results and optical absorption characteristics reported previously that the VTE treatment resulted in crystalline phase transformation with respect to Er3+ ions from original LiNbO3 to ErNbO4 phase in all crystals studied. The formation of the ErNbO4 phase and the Zn2+ codopants are responsible for the VTE-induced substantial spectral changes. The emission characteristics of the ErNbO4 precipitates in the Zn/Er-codoped crystals are found to be very different from those of the ErNbO4 precipitates in the only Er-doped crystal in the infrared region, and the difference is attributed to the influence of the Zn2+ codopant on the Er3+ ion environment. The mechanism of the crystalline phase transformation is qualitatively explained from the viewpoint of the declined solubility of Er3+ ion in a Li-rich LiNbO3 crystal and from the phase diagram of Li2O–Nb2O5 system. [Copyright &y& Elsevier]

• Er3+/Yb3+:NaLa(WO 4) 2 phosphor was synthesized by solid-state reaction method. • The phosphor has a mean grain size 5 μm and a cutoff phonon energy 927 cm−1. • There exist optimal Er doping level of 5 mol% and Yb codoping level of 20 mol%. • The phosphor shows strong fluorescence, higher thermal stability and sensitivity. • The phosphor is promising for optical thermometry in a broad range of 293–800 K. Er3+/Yb3+-codoped NaLa(WO 4) 2 phosphor samples with different Er3+ and Yb3+ doping levels were synthesized by solid-state chemical reaction for optical thermometry and fully characterized. The results show that each phosphor sample mainly consists of NaLa(WO 4) 2 crystalline phase and has a nominally stoichiometric chemical composition. The phosphor grains have massive polyhedron morphology, good size homogeneity with a mean value of ∼5 μm and a cutoff phonon energy of 927 cm−1. The 530 and 550 nm upconversion fluorescence of Er3+ in the phosphor samples and their temperature characteristics were investigated under 980 nm excitation. The results show that the phosphor exhibits strong upconversion fluorescence and the maximum fluorescence intensity is observed at the optimal Er3+ and Yb3+ doping levels of 5 and 20 mol%, respectively. The two-photon process is responsible for the upconversion fluorescence. The spectral structure of either 530 or 550 nm fluorescence band is thermally stable. The peak absolute thermal sensitivity is observed at ∼490 K and has a peak value of ∼9 × 10-3 K-1. As expected, Er3+ or Yb3+ doping level has little effect on the thermal sensitivity. The phosphor displays a wide dynamical response range of thermal sensitivity from 293 to 800 K, where the sensitivity varies within (6.5–9.0) × 10-3 K-1. A comparison study shows that the temperature characteristics obtained from the cooling procedure are almost the same as those obtained from the heating procedure. Besides, the effects of Er3+/Yb3+ doping level and heating/cooling on fluorescence color chromaticity were also studied. Based on the results, the Er3+/Yb3+-codoped NaLa(WO 4) 2 phosphor shows strong fluorescence, good thermal stability, high thermal sensitivity and broad response range of thermal sensitivity. Thus, it is a promising material for thermal detection over a wide temperature range of 293−800 K. [ABSTRACT FROM AUTHOR]

Abstract: Er3NbO7 phosphor was synthesized by sintering a mixture of Er2O3 and Nb2O5 powder in a molar ratio of 3:1 at 1600°C over 55h. Optical absorption and emission characteristics of Er3+ ions in the calcined Er3NbO7 powder were investigated and discussed compared with ErNbO4 phosphor and a Z-cut congruent Er (2mol%):LiNbO3 single crystal. The absorption and emission studies show that, due to different crystal structures, the spectroscopic properties of these niobates have some differences in spectral shape, peak position, and relative intensity, especially at 1.5μm. The most obvious spectral feature of the Er3NbO7 is that the spectral structure of band instead of peak is observed in its absorption or emission spectrum due to the existence of local structural disorder and multiple Er3+ sites. The Er3NbO4 shows stronger upconversion emission than the single crystal but weaker than the ErNbO4. Experimental results show that energy transfer upconversion and/or excited state absorption play a dominant role in the upconversion emissions, and, at higher pump level (>200mW), the thermal effect becomes significant and results in drop of the upconversion intensity. The 1.5μm lifetimes of Er3+ ion in the Er3NbO7, ErNbO4 phosphor, and in the Er:LiNbO3 crystal are measured to be ∼5.3, 2.0, and 2.4ms, respectively. In combination with the measured Raman spectra, the quantum efficiency, multiphonon nonradiative decay rate, and theoretical radiative lifetime of the 1.5μm emission of the two powder materials are expected. The differences in upconversion intensity and measured 1.5μm lifetime between the three materials are explained qualitatively. [Copyright &y& Elsevier]

Abstract: Starting with evolution equation of signal optical power in an Er-doped channel waveguide, rigorous theoretical expressions used for correct determination of net gain from signal enhancement measured by using pump-on/off method are derived. These expressions allow to clarify the argument on relationships between net gain, internal gain and signal enhancement in some earlier literatures. Physical implications of net gain, internal gain and signal enhancement expressions are analyzed. Standardized definitions for the three physical quantities are proposed. The definition for net gain is proposed on the basis of practical application of an EDWA in optical fiber communication and the definitions for the latter two are suggested based on the physical implication analysis of their expressions. [Copyright &y& Elsevier]

Abstract: A comprehensive, complete theory suitable for a long period grating (LPG) on/in a planar or channel waveguide is described on the basis of coupled mode theory. The theory aims at key characteristic parameters with respect to the effect of the LPG, including resonance wavelength, transmission power of the guided mode in the waveguide, full-width at half-maximum (FWHM) of the power attenuation band in transmission spectrum of the guided mode, and coupling coefficient between a guided and a coupled mode. The type of the grating including relief and index modulation, different polarization state combinations of the guided mode and the resonant mode, and slanted grating structure are considered in this theory. The influences of the coupling between two guided modes and between a guided mode and a substrate mode are discussed. The role of cladding layer in a long period waveguide grating is demonstrated in particular. Finally, the expressions for the sensitivity of resonance wavelength as a function of external temperature, pressure and surrounding refractive index are derived. [Copyright &y& Elsevier]

Abstract: Influence of thermal reduction on intrinsic (bipolarons), extrinsic (Er3+) defects and OH− groups in Er:LiNbO3 crystals, which were as-grown and VTE-treated (VTE: vapor transport equilibration) before being reduced, was studied by measuring the polarised or unpolarised optical absorption in visible and near infrared regions. A wide and strong band extending from the optical absorption edge up to the infrared region and peaking around 500nm (∼2.5eV), resulting from the absorption of reduction-induced bipolarons, is observed. Meanwhile, the thermal reduction also induces an additional, relatively much narrow absorption band around 370nm in a crystal whether it is Er-doped or undoped and whether it is congruent or originally VTE-treated. Both the 500nm and the 370nm bands show similar polarisation dependence. The thermal reduction treatment hardly influences Er3+ spectroscopic properties such as absorption amplitude, linewidth, peaking position and polarisation dependence. The original VTE effects on the spectroscopic properties of Er:LiNbO3 crystal are essentially retained still. The thermal reduction has a similar effect on the OH− absorption to a strong VTE treatment: the removal of the OH− groups contained in the crystal. [Copyright &y& Elsevier]