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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]

• 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]

• 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]

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]

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]

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]

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]

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]

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]

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]

Abstract: The emission and absorption spectra of Er3+ ions in Er2O3 powder are investigated at room temperature. The crystal field splitting of 4I15/2, 4I13/2, 4I9/2 and 4S3/2 multiplets of Er3+ was calculated using a parameterized Hamiltonian including crystal field terms. A final standard deviation of 5.3cm−1 is obtained between 22 calculated and experimental Stark. The strength crystal field parameters of Er2O3 powder are compared to those of erbium doped Y2O3 single crystal. In addition, the location of the ground and the first excited states of Er3+ in Er2O3 powder is determined. The 4I15/2 ground state of Er3+ is located at 12750cm−1 below the valence band edge. The 4I13/2 and 4I11/2 multiplets are set in resonance with the valence band. A configuration coordinate diagram is proposed for Er3+ in Er2O3 powder. [Copyright &y& Elsevier]

Abstract: X-, Y- and Z-cut congruently grown bulk Er(1.6, 2.0mol%):LiNbO3 crystals have been thermally treated using the vapor transport equilibration (VTE) technique. Optical absorption and X-ray powder diffraction measurements have been carried out to verify the existence of ErNbO4 precipitates induced by the VTE procedure in these crystals. Scanning electron microscope (SEM) has been used for the direct observation of crystallographic morphology of the ErNbO4 precipitates. The results show that the precipitates lie mainly in the (0001) crystallographic plane of the substrate and preferably grow with a crystallographic morphology of flat polyhedrons with rough dimensions of submicron×micron×micron. The precipitates on the surfaces of Z-cut substrates have only three orientations mutually aligned at approximately 60° or 120° angles being nearly parallel to the crystallographic axes of the host matrix. The SEM results show that VTE conditions and the type of crystallographic cut of the substrate remarkably affect the size and density of the precipitates. [Copyright &y& Elsevier]

The formation of ErNbO4 powder, prepared by calcining an Er2O3 (50 mol%) and Nb2O5 (50 mol%) powder mixture at 1100 and 1600 °C for different durations, was investigated by using X-ray diffraction. The experimental results have displayed that although the solid-state reaction had started to some extent when the mixture was pre-calcined at 1100 °C for a duration of 13 h, the two original phases Er2O3 and Nb2O5 still dominated the mixture. When the duration of the calcination reaction was increased to 120 h at the same temperature, the resultant mixture experienced a nearly complete phase transformation. Accordingly, the ErNbO4 phase was dominant phase in the mixture. Nevertheless, a small portion of the raw powder still existed in the mixture. When the calcining temperature was elevated to 1600 °C, ErNbO4 powder with higher purity could be obtained for a relatively much shorter duration (only up to several tens of hours). A simple formation mechanism of ErNbO4, an elevated-temperature-assisted solid-state chemical reaction: Er2O3+Nb2O5⇌2ErNbO4, is suggested. In addition, the present experimental results offer important evidence for the formation of the additional phase ErNbO4 induced in Er:LiNbO3 crystals by vapour transport equilibration (VTE) treatment. [Copyright &y& Elsevier]

Visible and near infrared absorption and emission (488 nm excitation) characteristics of ErNbO4 powder, which were prepared by calcining the Er2O3 (50 mol%) and Nb2O5 (50 mol%) powder mixture at 1100 and 1600 °C for different durations, have been investigated at room temperature. The absorption and emission characteristics of these calcined ErNbO4 powder were summarized and discussed in comparison with those of Er2O3. Weak emission of Er2O3 relative to the calcined ErNbO4 is mainly conducted with absorption difference at the excitation wavelength 488 nm. The obvious spectral changes from Er2O3 to calcined ErNbO4 samples are related to an elevated-temperature-assisted phase transformation according to the solid-state chemical reaction equation: Er2O3 + Nb2O5 ⇌ 2ErNbO4, which results in the changes of the ion environment of Er3+ and hence changes of the Stark levels of Er3+. The further spectral change as the strengthened calcination results from the improvement of ErNbO4 purity in the calcined mixture. The borders between two green transitions and between two near infrared transitions in the emission spectra of both calcined samples and Er2O3 were tentatively identified by referencing earlier reported emission spectra of the precipitated Z-cut VTE Er(2.0 mol%):LiNbO3 crystal and the match relation between absorption and emission spectra of the ErNbO4 powder. A comparison was performed on the spectra of calcined ErNbO4 powder and those of VTE Er(2.0 mol%):LiNbO3 crystals. The results allow to preliminarily deem the contribution of ErNbO4 precipitates, generated inside these crystals by the VTE procedure, to the spectra of these crystals. [Copyright &y& Elsevier]

Starting with two dimensional, scalar wave equation, a variational equation was established for the fundamental TE and TM modes guided in Ti : LiNbO3 waveguides on the basis of assuming a symmetric Gaussian mode field function in the width direction and two-half Gaussian trial functions in the depth direction. The controllable waveguide fabrication parameters, including channel width, diffusion temperature, initial Ti-strip thickness and diffusion time, dependent of fundamental mode size, effective pump area, coupling efficiency between pump and laser modes, and the coupling loss between a Ti : LiNbO3 waveguide and a fiber were numerically calculated for Z-cut Er : Ti : LiNbO3 channel waveguide lasers at three possible emission wavelengths 1532,1563 and 1576 nm and two possible pump wavelengths 1480 and 980 nm. The calculated results were compared with those of Gaussian/Hermite–Gaussian mode field distribution in detail. [Copyright &y& Elsevier]

Photonic jet (PJ) is generated by illuminating a dielectric scatterer with symmetric four Airy beams (SFAiB). The PJ properties have been systematically studied as a function of characteristic parameters of both the SFAiB and the scatterer, including the geometric parameter D that controls lateral position of each Airy beam component, decay factor, focal point position of the SFAiB, as well as the refractive index of the scatterer. The results show that the PJ performance can be optimized by all of these parameters as a result that the diffraction and self-focusing effects of the SFAiB itself, and the scatterer's focusing function simultaneously play roles in the PJ generation in case of SFAiB illumination. The illumination by SFAiB has advantages over that by plane wave or Gaussian beam in flexibility of tailoring the PJ performance and in possibility of generating a photonic hook with asymmetric illumination caused by transverse offset of focal point of incident SFAiB. In addition, the SFAiB illumination makes it possible to generate a PJ outside a scatterer in case that the scatterer has a higher refractive index of n s > 2.0, increasing the flexibility in both material choice and structural design of scatterer for generation of a PJ for some potential applications. • Photonic jet (PJ) is generated by symmetric four Airy beams (SFAiB) illumination. • SFAiB's diffraction and self-focusing, scatter's focusing effects enable to tailor PJ performance. • SFAiB illumination has advantages over that by plane wave or Gaussian beam. • The merits include flexibility in tailoring PJ performance and possibility of generating a photonic hook with asymmetric illumination. • SFAiB illumination enables to generate a PJ outside a scatterer with a higher refractive index of n s > 2.0, increasing flexibility in material choice and structural design of scatterer. [ABSTRACT FROM AUTHOR]

• Human awareness detection dynamically reacts to high-altitude environment. • Early and late components co-mediate the reaction with exposure duration. • Opposite change of two components as a response to the reduced cognitive resources. • Increased late component and fast reaction resist high-altitude exposure. The psychological effects of long-term exposure to high-altitude environments have attracted great attention. These effects are usually attributed to the diminished cognitive resources due to high-altitude exposure. This study employed electroencephalography (EEG) to investigate the effects of exposure duration on awareness detection tasks. Neither reaction time nor accuracy showed the direct effects of the exposure duration, so did the model indexes obtained from drift diffusion model analysis. However, event-related potentials (ERP) analysis revealed that exposure duration was associated with changes in the visual awareness negativity (VAN) and the late positivity (LP) components, which in turn affected reaction time. Specifically, longer exposure durations were associated with lower VAN and higher LP, resulting in shorter reaction times and greater drift rate. In contrast to previous studies, the reverse relationship between VAN and LP may reflect a compensatory response to the reduced cognitive resources caused by high-altitude exposure. Additionally, increased LP and shorter reaction times with exposure duration may reflect a resistance to the high-altitude environment. We also conducted time–frequency analysis and found that theta power did not vary with exposure duration, suggesting that the reduction in cognitive resources remains stable in these individuals over time. Overall, our study provides new insights into the dynamic effects of high-altitude environments on awareness detection in the presence of reduced cognitive resources. [ABSTRACT FROM AUTHOR]

• The Y-junction and T-junction 1 × 2 power splitters based on LNOI platform are designed by an intelligent algorithm. • The combination of GA and FEM based on "Phc-like" structure is proposed. • Symmetric operations and two-dimensional models are used to improve optimization efficiency. • The optimized devices offer good performance and ultra-compact footprints suitable for on-chip use. Optical power splitters are critical components of photonic integrated circuits based on the lithium-niobate-on-insulator (LNOI) platform. However, the lithium niobate power splitters designed by traditional strategies are usually large in size, which limits their practical applications for on-chip high dense integration. Here, we focus on the efficient design of ultra-compact power splitters based on the LNOI platform. The Y-junction and T-junction 1 × 2 power splitters based on photonic-crystal-like (PhC-like) structures are designed by a genetic algorithm. The footprint of the functional region for Y-junction and T-junction power splitters are only 2.6 μm × 2.6 μm and 2.85 μm × 4.94 μm, respectively. Simulation results show that the insertion losses are about 0.1 dB and 0.7 dB at 1550 nm for these two kinds of devices. This work can as a generic approach for designing various ultra-compact functional devices based on LNOI platform. [ABSTRACT FROM AUTHOR]

• Polarization beam splitter(PBS) based on LNOI triple-waveguide coupler is proposed. • A plasmonic waveguide is used to exalt modal birefringence and control TM mode coupling. • Good performance of high polarization extinction ratio and low insertion loss is expected. • The device has compact size for on-chip application and definite fabrication tolerance. • Present work offers a promising strategy for compact, efficient PBS based on LNOI. A compact polarization beam splitter based on a triple-waveguide plasmonic directional coupler is proposed and modeled. The directional coupler consists of two usual lithium-niobate-on-insulator (LNOI) ridge waveguides and a metal-capped plasmonic LNOI waveguide located between them. It utilizes stronger polarization dependence of modal birefringence of a plasmonic waveguide. By appropriately designing the plasmonic waveguide geometry, the TM component wave meets the resonant coupling condition and outputs at cross port. Instead, the TE component wave does not meet the resonant coupling condition, cannot be resonant in the triple-waveguide directional coupler and outputs at through port directly. Excellent performance is expected for an optimal device. First, the device shows an ultra-high polarization extinction ratio(PER) of 28.3 dB and an ultra-low insertion loss(IL) of 0.06 dB for the TE-polarized wave, and of 31.7 dB and of 0.31 dB in order for the TM-polarized wave. Second, the working bandwidth with PER > 25 dB and IL < 0.08 dB is 100 nm (1500–1600 nm) for the TE-polarized wave, and that with PER > 20 dB and IL < 0.4 dB is 30 nm (1534–1564 nm) for the TM-polarized wave. Third, the device has a compact structure of 33 μm in length and exhibits definite fabrication tolerance. Overall, the proposed device exhibits ultra-high PER and ultra-low IL, and has a compact size suitable for on-chip use. A comparison shows that present device exhibits advantage in IL and/or size over that based on other LNOI structures or the SOI platform. [ABSTRACT FROM AUTHOR]

A new aluminum ion fluorescent probe (4-(diethylamino)-2-hydroxybenzylidene)isoquinoline-1-carbohydrazide (HL 1 ) has been conveniently synthesized and characterized. HL 1 exhibited a highly selective and pronounced enhancement for Al 3+ in the fluorescence emission over other common cations by forming a 2:1 complex, with a recognition mechanism based on excited-state intramolecular proton transfer (ESIPT) and intramolecular charge transfer (ICT). The strong fluorescent emission can be observed even at ppm level concentration of the probe in the presence of Al 3+ with 41 fold intensity enhancement at 545 nm. HL 1 displays good linear relationship with Al 3+ in the low concentration and the limit of detection is 8.08 × 10 −8  mol/L. Similar molecules with different substituents on salicylaldehyde phenyl ring were synthesized for studying the structure-activity relationship. Density-functional theory (DFT) calculations are in agreement with the proposed mechanism. It is confirmed that HL 1 could be used to detect Al 3+ ions in real sample by fluorescence spectrometry and Al 3+ ions in cells by bioimaging. [ABSTRACT FROM AUTHOR]

• The study found notable distinctions in the visual, somatomotor, dorsal network, as well as in the DMN, frontoparietal network, and cerebellum when comparing EC and EO states. • The graph theory analysis revealed that the weakened degree and functional connectivity in the SFG and the MCC among StD individuals. The SFG and the MCC connections were correlated with perceived stress. • The functional connectivity between the SFG and the SFGmed was discovered to modulate the relationship between perceived stress and StD. Perceived stress is an acknowledged risk factor for subthreshold depression (StD), and fluctuations in perceived stress are thought to disrupt the harmony of brain networks essential for emotional and cognitive functioning. This study aimed to elucidate the relationship between eye-open (EO) and eye-closed (EC) states, perceived stress, and StD. We recruited 27 individuals with StD and 33 healthy controls, collecting resting state fMRI data under both EC and EO conditions. We combined intrinsic connectivity and seed-based functional connectivity analyses to construct the functional network and explore differences between EC and EO conditions. Graph theory analysis revealed weakened connectivity strength in the right superior frontal gyrus (SFG) and right median cingulate and paracingulate gyrus (MCC) among participants with StD, suggesting an important role for these regions in the stress-related emotions dysregulation. Notably, altered SFG connectivity was observed to significantly relate to perceived stress levels in StD, and the SFG connection emerges as a neural mediator potentially influencing the relationship between perceived stress and StD. These findings highlight the role of SFG and MCC in perceived stress and suggest that understanding EC and EO states in relation to these regions is important in the neurobiological framework of StD. This may offer valuable perspectives for early prevention and intervention strategies in mental health disorders. [ABSTRACT FROM AUTHOR]

• Mid-IR DFG based on mode phase match in a dual-layer LiNbO 3 waveguide is modeled. • The device shows excellent performance of ultra-high efficiency and ultra-broad band. • It is due to two reversed ferroelectric domain orientations in the dual-layer waveguide. • The reversed domain orientations enable to enhance nonlinear interaction and efficiency. • Present work offers a universal strategy towards more efficient on-chip nonlinear devices. Although there is a great breakthrough in second harmonic generation based on the birefringence and quasi phase matching (QPM) concepts, the development of an efficient difference frequency generation (DFG) based on modal phase matching (MPM) is still in its infancy at present. In this paper, we propose an ultra-broadband and ultra-high conversion efficiency mid-infrared DFG on the basis of MPM in a dual-layer waveguide consisting of two reversed poled LiNbO 3 thin-films on sapphire. The ultra-high conversion efficiency is ascribed to significantly enhanced nonlinear interaction realized by making use of spatial profile of optical nonlinearity of LN crystal produced by two reversed ferroelectric domain orientations in the dual-layer waveguide. The proposed DFG device has been modeled on the basis of a 0.98 μm pump wave and a 1.34–1.44 μm signal wave, the frequency difference of which generates a mid-infrared idler wave centered at 3.32 μm wavelength. The results show that the device exhibits excellent DFG performance, including an ultra-high nonlinear conversion efficiency 434.6 % W−1 cm−2 and an ultra-broad tuning band 340 nm meanwhile, which far exceed the performance of conventional MPM devices, even the QPM ones. [ABSTRACT FROM AUTHOR]

Generation of multiple photonic hooks (PHs) at THz region in reflection mode by a multi-dielectric structure that consists of periodical or aperiodical scatterer units on top surface of a reflection screen has been systematically studied. The study focuses on effects of geometric and optical parameters of the multi-dielectric structure on characteristics of the PHs generated, especially the bending angle. These geometric and optical parameters include spacing between adjacent scatterer units, length of reflection screen, as well as the size, refractive index and number of scatterer units. The results show that these geometric and optical parameters are key factors in tailoring the performance of the multiple PHs generated. The spacing determines bending direction of PHs and there exists a critical spacing value. Smaller than this critical value, the PHs generated bend outwards. Larger than this critical value, the PHs bend inwards. In addition, the spacing can be also used to effectively adjust the intensity and bending angle of the PHs. Screen length affects mainly PH's bending. The size and refractive index differences between scatterer units introduce additional optical inhomogeneity that leads to the generation of asymmetrical multiple PHs. Finally, the possibility of generation of an arrayed PHs in the reflection mode is demonstrated. • Generation of photonic hooks (PHs) array in reflection mode is studied. • PHs formation is ascribed to optical inhomogeneity of multi-dielectric structure. • PHs performance can be effectively tailored by geometric and optical parameters. • PHs generated at smaller or larger spacing bend in opposite directions. • Optical-axis asymmetric structure generates asymmetrical PHs array. [ABSTRACT FROM AUTHOR]

A theoretical study has been performed on 1.76 μm on-chip amplification characteristics of Tm3+ in a rib waveguide of LiNbO 3 on insulator (Tm:LNOI) under 795 nm wavelength pumping. Steady-state rate equations and propagation equations of signal and pump waves are established on the basis of a three-level model of Tm3+ system with the cross relaxation process 3H 4 →3F 4 : 3H 6 →3F 4 included. The validity of the three-level model is demonstrated by referencing previously reported experiment result of Er3+-doped LNOI waveguide amplifier and by examining the effect of Tm3+ blue upconversion emissions on the 1.76 μm amplification. Besides the relations of signal gain to propagation length, launched pump power and input signal power, the effects of waveguide cross-section geometry and Tm3+ concentration on the gain characteristics are also studied. The results show that the waveguide cross-section geometry affects definitely the gain performance via its effect on mode field. The Tm3+ concentration affects the signal gain also via its effect on the 3H 4 →3F 4 : 3H 6 →3F 4 cross relaxation coefficient, besides the conventional effect that higher active ion concentration yields larger signal gain. A comparison with conventional Ti4+(Zn2+)-diffused Tm3+-doped LN [Ti(Zn):Tm:LN] waveguide shows that the Tm:LNOI waveguide has much better gain performance than the Ti(Zn):Tm:LN waveguide, including much stronger pump power and propagation distance dependences of signal gain, much higher signal gain and much lower threshold pump power. These features are associated with significantly increased Tm3+ population inversion extent due to ultra-compact mode field and larger overlapping factor of mode field and Tm3+ population profiles in the Tm:LNOI waveguide. • 1.8 μm amplification in Tm:LNOI waveguide is simulated based on three-level model. • Quantitative relations of signal gain to controllable parameters are established. • Tm:LNOI amplifier is more efficient than Ti(Zn):Tm:LN due to ultra-compact mode. • Waveguide cross-section geometry affects definitely gain via its effect on mode field. • Tm3+ concentration affects gain via its effect on cross relaxation process. [ABSTRACT FROM AUTHOR]

Low-loss gallium(Ga 3+ )-diffused erbium(Er 3+ )-doped LiTaO 3 (Ga:Er:LiTaO 3 ) channel waveguide was fabricated using erbium metal film diffusion followed by photolithographically patterned gallium oxide(Ga 2 O 3 ) strip diffusion. Secondary ion mass spectrometry analysis shows that Er 3+ concentration follows a Gaussian profile, Ga 3+ concentration follows a sum of two error functions at the waveguide surface and a Gaussian profile in the depth direction. Optical studies show that the waveguide supports single-mode propagation at 1.5 μm wavelength with a coupling loss <1 dB and a waveguide loss 0.2(0.3) dB/cm for the TE(TM) mode, and with a small polarization dependence in mode field distribution. Stable optical gain of ∼1.0 dB/cm was measured at 1547 nm wavelength with 220 mW pump power of 980 nm wavelength, indicating good photorefractive damage resistance of the waveguide. [ABSTRACT FROM AUTHOR]

• Er3+/Yb3+:GdNbO 4 micron-sized phosphor was synthesized by solid-state method. • Er3+ and Yb3+ concentrations have apparent effects on grain morphology. • The phosphor shows strong UCL, good thermal stability and adequate sensitivity. • The phosphor shows good chemical stability in high salinity environment. • The phosphor is promising for both thermometry and maritime protection. Micron-sized GdNbO 4 :Er3+/Yb3+ phosphors were prepared by employing a high-temperature solid-state reaction. The crystalline phase, morphology, composition, spectroscopic properties, and impact of rare-earth ion concentration, temperature and high salinity environment were systematically characterized. The extracted results show that phosphor is dominated by the existence of the GdNbO 4 crystalline phase. Their morphology and agglomeration phenomenon are strongly dependent on the rare-earth concentration. The phosphor exhibits also excellent up-conversion luminescence (UCL) properties under 980 nm excitation, while a two-photon process accounts for UCL. The following optimal concentrations were selected for enhanced UCL intensity: 10 mol% for Yb3+ and 3 mol% for Er3+. The phosphor shows excellent thermometric performances including strong UCL, good thermal stability, appropriate sensitivity and a wide sensitivity response range. To evaluate the accuracy of the present optical thermometer, the temperature of a heated bench has been comparatively measured by using the proposed optical thermometer and a commercial thermocouple with an accuracy of 1 K. By comparing the temperature values measured by the two methods, it can be concluded that the present optical thermometer has an accuracy better than 1.6 K. In addition, the phosphor exhibits good chemical stability in high salinity environment. Interestingly, its crystalline phase, structure, and UCL spectral structure, intensity and color are all preserved after one-month immersion into a high salinity solution. [Display omitted] GdNbO 4 :Er3+/Yb3+ phosphor was prepared by high-temperature solid-state method. Crystalline phase, morphology, composition, spectroscopic properties, and effects of rare-earth concentration, temperature and high salinity environment were fully characterized. The results show that the phosphor is promising for dual-application in thermometry and safety marks for maritime protection. [ABSTRACT FROM AUTHOR]

Without any templates, hollow structured La 2 Mo 2 O 9 particles are successfully prepared at room temperature by an ageing-assisted salt solvent process. Powder X-ray diffraction, field emission scanning electron microscopy, and transmission electron microscopy are used to investigate structure and morphologies of the obtained products. It is found that the ageing process has a significant influence on the particle morphology evolution when the mixed precursors are disposed at room temperature. With the ageing time changed from one week to 12 weeks, the particles morphologies evolve gradually from ellipsoid to sphere. The as-prepared hollow structured La 2 Mo 2 O 9 spheres have an average diameter of about 800 nm and shell thickness of about 100 nm. For elucidating the possible formation mechanism, the influences of the single salt on the particles morphologies evolution are also investigated. The final results indicate KCl is a key factor for formation of La 2 Mo 2 O 9 spheres and the formation of the hollow structure should have close relations with the surface tension in the ageing assisted process. [ABSTRACT FROM AUTHOR]

Previous research has established a strong link between attention and visual mental imagery, but it's remained uncertain whether attention networks influence individual differences in the vividness of visual mental imagery. In our study, we examined 140 participants, assessing the vividness of imagery using the Vividness of Visual Imagery Questionnaire in both eyes-open and eyes-closed conditions. We employed the Attention Network Test, coupled with EEG recording, to characterize three attention sub-networks: alerting, orienting, and executive control. To pinpoint the specific attentional networks associated with the vividness of visual mental imagery, we utilized latent profile analysis to categorize participants into distinct subgroups. Additionally, we constructed a regression mixture model to explore how attention networks predict different latent categories of visual imagery vividness. Our findings revealed that the efficiency of the alerting network, as indicated by the N1 component, demonstrated a positive correlation with the vividness of visual imagery. This electrophysiological evidence underscores the role of the alerting network in shaping individual differences in the vividness of visual mental imagery. [ABSTRACT FROM AUTHOR]

• On-chip 1.53 μm amplification in Er:LNOI micro-photonic amplifier was simulated. • The simulation was performed in comparison with Ti:Er:LiNbO 3 waveguide amplifier. • Numerical model is confirmed by consistency of numerical and experimental results. • The Er:LNOI amplifier is more efficient than the Ti:Er:LiNbO 3 waveguide. • Due to higher Er3+ population inversion extent and ultra-compact mode of Er:LNOI. On-chip 1.53 μm amplification in 980-nm-pumped Er3+-doped lithium niobate on insulator (Er:LNOI) photonic wire (PW) has been studied theoretically in comparison with that in Ti:Er:LiNbO 3 waveguide on the basis of three- and six-level models of Er3+. We show that the six-level model gives more accurate analysis and its validity is confirmed by comparing the simulated results with the previously reported experiment data. The study shows that the Er:LNOI PW has a larger Er3+ population inversion extent than the Ti:Er:LiNbO 3 because of larger overlapping factor of mode field and Er3+ population profiles, and ultra-compact mode field. As a result, the Er:LNOI PW displays better gain performance than the Ti:Er:LiNbO 3 as described below. (1) Maximum/saturation gain of Er:LNOI PW can be as much as two/three times higher than that of the Ti:Er:LiNbO 3. (2) Gain of the Er:LNOI PW increases with propagation length more strongly in the initial stage and decreases more slowly beyond optimal length. It also shows stronger dependences on both pump power (in the initial stage) and signal power. (3) Gain in the Er:LNOI PW saturates more easily, and the Er:LNOI PW has a threshold pump power one order of magnitude lower than the Ti:Er:LiNbO 3. [ABSTRACT FROM AUTHOR]

• The shorter access time was more explained by the lower threshold. • The heart rate predicted the access time and threshold at high altitudes. • The cardiac deceleration-acceleration pattern was slightly various at high-altitudes. Many studies have indicated a strong relationship between cardiac and brain activities, both of which are sensitive to high-altitude exposure. This study combined a consciousness access task and electrocardiograms (ECG) to uncover conscious awareness in response to high-altitude exposure and its relation to cardiac activity. When compared with the low-altitude groups, the behavioral results showed that the high-altitude participants shortened the time of access to visual awareness of grating orientation, which was accompanied by a faster heart rate, excluding the influence of pre-stimulus heart rate, extent of cardiac deceleration after presenting the stimulus, and task difficulty. Although there were post-stimulation cardiac deceleration and post-response acceleration at both high and low altitudes, a slight increase in heart rate after stimulation at high altitudes may indicate that participants at high altitudes could quickly readjust their attention to the target stimulus. More importantly, the drift diffusion model (DDM) was used to fit the access time distribution of all participants. These results suggest that shorter time at high altitudes might be due to the lower threshold, suggesting that less evidence in high-altitude participants was required to access visual consciousness. The participants' heart rates also negatively predicted the threshold through a hierarchical drift diffusion modeling (HDDM) regression. These findings imply that individuals with higher heart rates at high altitudes have a greater cognitive burden. [ABSTRACT FROM AUTHOR]

Abstract Visible and infrared absorption and emission spectra of Er3+-doped strontium gadolinium gallate (SrGdGa 3 O 7) single-crystal were measured at room temperature. From the measured fluorescence spectra, the Er3+ emission cross section spectra were calculated at first. Then, by utilizing McCumber relation, the absorption cross section spectra were further calculated. In parallel, the absorption cross section spectra were also calculated from the measured absorption spectra of the crystal, and compared with those spectra calculated from the McCumber relation. The comparison shows that the absorption cross section spectra obtained using the McCumber relation are in reasonable agreement with those deduced from the measured absorption spectrum. Graphical abstract Visible, near-infrared, and mid-infrared emission and absorption cross section spectra of Er3+ in SrGdGa 3 O 7 single-crystal have been calibrated from measured fluorescence spectra using the McCumber theory. The absorption cross-section spectrum was also calibrated from the measured absorption spectrum. Comparison shows that the absorption cross section spectra calibrated from the McCumber theory are reasonably consistent with those from the absorption spectrum. Present work presents complete cross section knowledge of the studied crystal. fx1 [ABSTRACT FROM AUTHOR]

Abstract Optical and spectroscopic properties of Er3+/Pr3+-codoped Gd 3 Ga 5 O 12 single-crystal grown by Czochralski method have been studied. Absorption spectrum of the crystal was measured at room temperature and Er3+ absorption cross-section spectral distributions were calibrated from it. The Er3+ spectroscopic properties were analyzed on the basis of the Judd-Ofelt theory. Fundamental spectroscopic parameters were obtained that include absorption coefficient, cross-section spectral distribution, electronic transition oscillator strength, Judd-Ofelt parameters, fluorescence branch ratio, transition probability, radiative and fluorescence lifetimes, and quantum efficiency as well. Study shows that Pr3+-codoping results in substantial change of Er3+ spectroscopic properties, in qualitative consistent with the previous fluorescence studying result. In addition, refractive index of the crystal was measured at different wavelengths using prism coupling method, and the Sellmeier equation reported previously for Er3+-only doped Gd 3 Ga 5 O 12 crystal is corrected. Highlights • Juud-Ofelt spectroscopic properties of Er3+/Pr3+:Gd 3 Ga 5 O 12 crystal were studied. • Pr codoping results in substantial increase of the Judd-Ofelt parameters. • Pr codoping induces shortening of Er3+ 2.7 and 1.5 μm lifetimes from ms to sub-ms. • The effect is related to two resonant energy transfers from Er3+ to Pr3+. • Different efficiencies of the two transfers enable to realize 2.7 μm lasing. [ABSTRACT FROM AUTHOR]

Abstract Gallium-diffused lithium niobate (LiNbO 3) channel optical thin-film was fabricated by diffusion of photolithographically patterned gallium-oxide stripes on a Z-cut congruent LiNbO 3 substrate. Composition, optical properties of the channel thin-film and gallium diffusion profiles were characterized. The results show that the thin-film is in congruent composition environment, confines single-mode ordinary and extraordinary waves with a loss <0.5 dB/cm at 1.5 μm wavelength. The confined mode field follows Gaussian/Hermite-Gaussian profile in the width/depth direction of the channel thin-film, and shows definite anisotropy. The diffused gallium ions follow a sum of two error functions in the width and a Gaussian function in the depth direction of the channel optical thin-film. Two-dimensional gallium concentration and refractive index profiles in the channel thin-film are modeled. [ABSTRACT FROM AUTHOR]