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The development of nanoribbon-like structures is an effective strategy to harness the potential benefits of graphenic materials due to their excellent electrical properties, advantageous edge sites, rapid electron transport, and large specific area. Herein, parallel and connected magnetic nanostructured nanoribbons are obtained through the synthesis of reduced graphene oxide (rGO) using NiCl2 as a precursor with potential applications in nascent electronic and magnetic devices. Several analytical techniques have been used for the thorough characterization of the modified surfaces. Atomic force microscopy (AFM) shows the characteristic topographical features of the nanoribbons. While X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD), and Raman spectroscopy provided information on the chemical state of Ni and graphene-like structures, magnetic force microscopy (MFM) and scanning Kelvin probe microscopy (SKPFM) confirmed the preferential concentration of Ni onto rGO nanoribbons. These results indicate that the synthesized material shows 1D ordering of nickel nanoparticles (NiNPs)-decorating tiny rGO flakes into thin threads and the subsequent 2D arrangement of the latter into parallel ribbons following the topography of the HOPG basal plane. [ABSTRACT FROM AUTHOR]

This study experimentally and numerically validates the commonly employed technique of laser-induced heating of a material in optical temperature sensing studies. Furthermore, the Er3+-doped glass microspheres studied in this work can be employed as remote optical temperature sensors. Laser-induced self-heating is a useful technique commonly employed in optical temperature sensing research when two temperature-dependent parameters can be correlated, such as in fluorescence intensity ratio vs. interferometric calibration, allowing straightforward sensor characterization. A frequent assumption in such experiments is that thermal homogeneity within the sensor volume, that is, a sound hypothesis when dealing with small volume to surface area ratio devices such as microresonators, but has never been validated. In order to address this issue, we performed a series of experiments and simulations on a microsphere supporting whispering gallery mode resonances, laser heating it at ambient pressure and medium vacuum while tracking the resonance wavelength shift and comparing it to the shift rate observed in a thermal bath. The simulations were done starting only from the material properties of the bulk glass to simulate the physical phenomena of laser heating and resonance of the microsphere glass. Despite the simplicity of the model, both measurements and simulations are in good agreement with a highly homogeneous temperature within the resonator, thus validating the laser heating technique. [ABSTRACT FROM AUTHOR]

The laser performance and crystalline micro-structural properties of near-infrared step-index channel waveguides fabricated inside Neodymium doped YAG laser ceramics by means of three-dimensional sub-picosecond pulse laser direct writing are reported. Fluorescence micro-mapping of the waveguide cross-sections reveals that an essential crystal lattice re-distribution has been induced after short pulse irradiation. Such lattice re-distribution is evidenced at the waveguide core corresponding to the laser written refractive index increased volume. The waveguides core surroundings also present diverse changes including slight lattice disorder and bi-axial strain fields. The step-index waveguide laser performance is compared with previous laser fabricated waveguides with a stress-optic guiding mechanism in absence of laser induced lattice re-distribution. [ABSTRACT FROM AUTHOR]

Featured Application: The investigated YAS glass microspheres doped with Nd3+ ions have great potential as remote optical temperature sensors. Moreover, they are suitable for the development of laser devices with low power thresholds. Yttria–alumina–silica (YAS) glass microspheres doped with Nd3+ ions were excited with a 532 nm continuous laser in order to study the dependency of the wavelength of the whispering gallery mode (WGM) peaks on the temperature of the sample. This was possible due to a previous calibration of the 808 nm and 890 nm emission bands of the Nd:YAS glass sample for different temperatures using the fluorescence intensity ratio (FIR) technique. A maximum sensitivity of 15 × 10−6 K−1 and a temperature resolution limit of 0.2 K were obtained for the microsphere sensor. Moreover, laser emission at 1064 nm was observed by continuous pumping at 532 nm, and a power threshold of 100 mW was determined. Upconversion emissions of Nd3+ were also studied by exciting the sample at 808 nm. [ABSTRACT FROM AUTHOR]

We report on the direct femtosecond laser ablation of sub-λ/4 (80–250 nm) holes in LiNbO3 crystals and on its local near-field imaging. We show that the near-field transmission of holes can feature an attenuation of ∼75% at hole central position, and a ∼20% transmission enhancement at its sides. This high-contrast ring-shaped near-field distribution is found to be in agreement with simulations, suggesting the surface relief as the main contrast mechanism. [ABSTRACT FROM AUTHOR]

We report on the direct laser writing of a three-dimensional photonic structure in a neodymium doped yttrium aluminum garnet laser ceramic. The fabricated structure consists of a voxel-based face centered cubic lattice showing a stop band centered at 2.4 μm. Confocal luminescence imaging was used to study the submicron low refractive-index voxels constituting the photonic structure, as well as to elucidate how the luminescence properties of the neodymium laser ions are modified by the writing process. [ABSTRACT FROM AUTHOR]

This paper provides a generic way to fabricate a high-index contrast tapered waveguide platform based on dielectric crystal bonded on glass for sensing applications. As a specific example, KLu(WO4)2 crystal on a glass platform is made by means of a three-technique combination. The methodology used is on-chip bonding, taper cutting with an ultra-precise dicing saw machine and inductively coupled plasma-reactive ion etching (ICP-RIE) as a post-processing step. The high quality tapered waveguides obtained show low surface roughness (25 nm at the top of the taper region), exhibiting propagation losses estimated to be about 3 dB/cm at 3.5 μm wavelength. A proof-of-concept with crystal-on-glass tapered waveguides was realized and used for chemical sensing. [ABSTRACT FROM AUTHOR]

Abstract: In this work, we have examined three new bistable systems based on the luminescence of three different crystals activated with trivalent rare earth ions. We have focussed our attention on Yb3+ ions activators, for which the most relevant results are obtained. The first crystal, Sr0.6Ba0.4Nb2O6, is a ferroelectric material with a relatively low phase transition temperature (∼370K), which provides bistability in the luminescence of Yb3+ ions due to the thermal hysteresis associated with phase transition. The second crystal, LiNbO3, provides an intrinsic bistability in the luminescence of Yb3+ ions, which is driven by changes in the excitation intensity. In the third crystal, NdPO4, a new mechanism of excitation intensity driven bistability is obtained when activated with Yb3+ ions, due to a interplay between the Nd3+↔Yb3+ energy transfer and back transfer processes. [Copyright &y& Elsevier]

This paper provides a generic way to fabricate a high-index contrast tapered waveguide platform based on dielectric crystal bonded on glass for sensing applications. As a specific example, KLu(WO 4 ) 2 crystal on a glass platform is made by means of a three-technique combination. The methodology used is on-chip bonding, taper cutting with an ultra-precise dicing saw machine and inductively coupled plasma-reactive ion etching (ICP-RIE) as a post-processing step. The high quality tapered waveguides obtained show low surface roughness (25 nm at the top of the taper region), exhibiting propagation losses estimated to be about 3 dB/cm at 3.5 m wavelength. A proof-of-concept with crystal-on-glass tapered waveguides was realized and used for chemical sensing., Competing Interests: The authors declare no conflict of interest.

We report on efficient laser operation of the first holmium monoclinic double tungstate waveguide laser fabricated by femtosecond direct laser writing. A depressed-index buried channel waveguide with a 60 μm diameter circular cladding was inscribed in 5 at.% Ho 3+ :KGd(WO 4 ) 2 . It was characterized by confocal microscopy and μ-Raman and μ-luminescence spectroscopy, indicating well-preserved crystallinity of its core. Pumped by a thulium bulk laser, the holmium waveguide laser generated 212 mW at 2055 nm with a slope efficiency of 67.2%. The waveguide propagation losses were 0.94±0.2  dB/cm.

We report on the first erbium (Er 3+ ) doped double tungstate waveguide laser. As a gain material, we studied a monoclinic Er 3+ :KLu(WO 4 ) 2 crystal. A depressed-index buried channel waveguide formed by a 60 µm-diameter circular cladding was fabricated by 3D femtosecond direct laser writing. The waveguide was characterized by confocal laser microscopy, µ-Raman and µ-luminescence mapping, confirming that the crystallinity of the core is preserved. The waveguide laser, diode pumped at 981 nm, generated 8.9 mW at 1533.6 nm with a slope efficiency of 20.9% in the continuous-wave regime. The laser polarization was linear (E || N m ). The laser threshold was at 93 mW of absorbed pump power.

We report mid-infrared LiNbO 3 depressed-index microstructured cladding waveguides fabricated by three-dimensional laser writing showing low propagation losses (~1.5 dB/cm) at 3.68 µm wavelength for both the transverse electric and magnetic polarized modes, a feature previously unachieved due to the strong anisotropic properties of this type of laser microstructured waveguides and which is of fundamental importance for many photonic applications. Using a heuristic modeling-testing iteration design approach which takes into account cladding induced stress-optic index changes, the fabricated cladding microstructure provides low-loss single mode operation for the mid-IR for both orthogonal polarizations. The dependence of the localized refractive index changes within the cladding microstructure with post-fabrication thermal annealing processes was also investigated, revealing its complex dependence of the laser induced refractive index changes on laser fabrication conditions and thermal post-processing steps. The waveguide modes properties and their dependence on thermal post-processing were numerically modeled and fitted to the experimental values by systematically varying three fundamental parameters of this type of waveguides: depressed refractive index values at sub-micron laser-written tracks, track size changes, and piezo-optic induced refractive index changes.

A novel fiber-coupled indium tin oxide (ITO) coated glass slide sensor for performing surface plasmon polariton chemical monitoring in the ∼3.5  μm mid-infrared (IR) range is reported. Efficient mid-IR fiber coupling is achieved with 3D laser written waveguides, and the coupling of glass waveguide modes to ITO surface plasmon polaritons (SPPs) is driven by the varying phase matching conditions of different aqueous analytes across the anomalous dispersion range determined by their molecular fingerprints. By means of using both a mid-IR fiber supercontinuum source and a diode laser, the excitation of SPPs is demonstrated. The sensor sensitivity is tested by discriminating CH from OH features of ethanol in water solutions, demonstrating an instrumental ethanol limit of detection of 0.02% in a wide concentration range of at least 0%-50%. The efficient optical monitoring of mid-IR SPPs in smart glass could have a broad range of applications in biological and chemical sensing.

Mid-infrared lithium niobate cladding waveguides have great potential in low-loss on-chip non-linear optical instruments such as mid-infrared spectrometers and frequency converters, but their three-dimensional femtosecond-laser fabrication is currently not well understood due to the complex interplay between achievable depressed index values and the stress-optic refractive index changes arising as a function of both laser fabrication parameters, and cladding arrangement. Moreover, both the stress-field anisotropy and the asymmetric shape of low-index tracks yield highly birefringent waveguides not useful for most applications where controlling and manipulating the polarization state of a light beam is crucial. To achieve true high performance devices a fundamental understanding on how these waveguides behave and how they can be ultimately optimized is required. In this work we employ a heuristic modelling approach based on the use of standard optical characterization data along with standard computational numerical methods to obtain a satisfactory approximate solution to the problem of designing realistic laser-written circuit building-blocks, such as straight waveguides, bends and evanescent splitters. We infer basic waveguide design parameters such as the complex index of refraction of laser-written tracks at 3.68 µm mid-infrared wavelengths, as well as the cross-sectional stress-optic index maps, obtaining an overall waveguide simulation that closely matches the measured mid-infrared waveguide properties in terms of anisotropy, mode field distributions and propagation losses. We then explore experimentally feasible waveguide designs in the search of a single-mode low-loss behaviour for both ordinary and extraordinary polarizations. We evaluate the overall losses of s-bend components unveiling the expected radiation bend losses of this type of waveguides, and finally showcase a prototype design of a low-loss evanescent splitter. Developing a realistic waveguide model with which robust waveguide designs can be developed will be key for exploiting the potential of the technology.

We report on the direct low-repetition rate femtosecond pulse laser microfabrication of optical waveguides in KTP crystals and the characterization of refractive index changes after the thermal annealing of the sample, with the focus on studying the potential for direct laser fabricating Mach-Zehnder optical modulators. We have fabricated square cladding waveguides by means of stacking damage tracks, and found that the refractive index decrease is large for vertically polarized light (c-axis; TM polarized) but rather weak for horizontally polarized light (a-axis; TE polarized), this leading to good near-infrared light confinement for TM modes but poor for TE modes. However, after performing a sample thermal annealing we have found that the thermal process enables a refractive index increment of around 1.5x10(-3) for TE polarized light, while maintaining the negative index change of around -1x10(-2) for TM polarized light. In order to evaluate the local refractive index changes we have followed a multistep procedure: We have first characterized the waveguide cross-sections by means of Raman micro-mapping to access the lattice micro-modifications and their spatial extent. Secondly we have modeled the waveguides following the modified region sizes obtained by micro-Raman with finite element method software to obtain a best match between the experimental propagation modes and the simulated ones. Furthermore we also report the fabrication of Mach-Zehnder structures and the evaluation of propagation losses.

We report on the experimental observation of "focus splitting" when light is tightly focused into a uniaxial lithium niobate crystal along its optical axis. This effect consists in the focal spot being split into two major sub-peaks along the axial direction. For the microfabrication applications such as three-dimensional photonic crystal fabrication and waveguide writing, this effect is highly undesired since it can lead to the generation of multiple distinct voxels in the vicinity of the focus. The splitting is caused by different birefringence induced aberrations for the ordinary and extraordinary polarization eigenmodes. We present numerical simulations which support our observations and suggest methods to avoid this effect.

We report on the fabrication of active optical channel waveguides in Er(3+)/MgO co-doped near stoichiometric lithium niobate crystals by means of selective low-dose oxygen ion implantation through a specially designed photoresist stripe mask. After post-implantation treatment at 260 degrees C for 1 h, the channel waveguides possess a propagation loss of approximately 1.7 dB/cm. The micro-luminescence investigation reveals that fluorescence emissions at approximately 1.5 microm in the waveguides are well preserved with respect to the bulk, exhibiting possible applications for integrated active photonic devices.

We report on the confocal Raman characterization of the micro-structural lattice changes induced during the high-repetition rate ultrafast laser writing of buried optical waveguides in lithium niobate (LiNbO(3)) crystals. While the laser beam focal volume is characterized by a significant lattice expansion together with a high defect concentration, the adjacent waveguide zone is largely free of defects, undergoing only slight rearrangement of the oxygen octahedron in the LiNbO(3) lattice. The close proximity of these two zones has been found responsible for the propagation losses of the guided light. Subjacent laser-induced periodic micro-structures have been also observed inside the laser focal volume, and identified with a strong periodic distribution of lattice defects.