Low-loss 3D-laser-written mid-infrared LiNbO3 depressed-index cladding waveguides for both TE and TM polarizations.

We report mid-infrared LiNbO3 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.

Heuristic modelling of laser written mid-infrared LiNbO3 stressed-cladding waveguides.

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.

Low-repetition rate femtosecond laser writing of optical waveguides in KTP crystals: analysis of anisotropic refractive index changes.

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.

Effect of Pt nanoparticles on the optical gas sensing properties of WO3 thin films.

Thin films of tungsten trioxide were deposited on quartz substrates by RF magnetron sputtering. Different annealing temperatures in the range from 423 to 973 K were used under ambient atmosphere. The influence of the annealing temperature on the structure and optical properties of the resulting WO3 thin films were studied. The surface morphology of the films is composed of grains with an average size near 70 nm for the films annealed between 773 and 973 K. Some of the WO3 thin films were also coated with Pt nanoparticles of about 45 nm in size. Spectrometric measurements of transmittance were carried out for both types of WO3 samples in the wavelength range from 200-900 nm, to determine the effect of the exposure to two different gases namely H2 and CO. Films showed fast response and recovery times, in the range of few seconds. The addition of Pt nanoparticles enables reducing the operation temperature to room temperature.

Enhancing thermometric efficiency: a wavelength excitation analysis in LiSrGdW3O12:Tb3+ for superior single band ratiometric (SBR) thermometry.

This study delves into the Single Band Ratiometric (SBR) method for luminescence thermometry, specifically employing Tb3+-doped LiSrGdW3O12 (LSGW) as a novel phosphor. The prepared samples crystallize with the tetragonal scheelite structure, with the optimal Tb3+ concentration pinpointed at 0.3Tb3+ ions. When stimulated at diverse wavelengths exhibit luminescence characterized by varying heat dependencies. By utilizing two Fluorescence Intensity Ratio (FIR) parameters for the 544 nm green emission, firstly excited at 405 nm and 488 nm, and then at 405 nm and 379 nm, the Sr relative thermal sensitivity of the luminescent thermometer peaks at 3.56% K-1 and 4.21% K-1, respectively, within the temperature range of 290-440 K. The temperature resolution (δT) of the luminescent thermometer is calculated to be δT1 = 0.68 K and δT2 = 0.75 K for T = 290 K, respectively. These outcomes underscore the applicability of Tb3+ ions for SBR thermometry, emphasizing the impact of the excitation wavelength on the thermal sensitivity. The study lays the groundwork for developing highly sensitive temperature probes by elucidating the interplay of material properties and physical processes.

Fabrication of Quasi-Sinusoidal Surface Relief Optical Transmission Gratings in Pyrex and IOG Glasses by Implantation with Oxygen and Nitrogen Ion Microbeams of the 5-6 MeV Energy Range.

A method for the fabrication of high diffraction efficiency optical transmission gratings with quasi-sinusoidal profile in glasses by microbeams of medium-mass ions of 5-6 MeV energy was devised and demonstrated. Gratings with a 30 µm grating constant have been manufactured and characterized by interference microscopy and microprofilometry. The obtained surface profiles of the gratings were found to be quasi-sinusoidal with up to 265 nm amplitude. Measured highest first-order diffraction efficiencies were around 26% in both Pyrex and IOG glasses. Such gratings could serve as coupling elements in integrated optics and photonic integrated circuits.

Passive Q-switching of the diode pumped Tm3+:KLu(WO4)2 laser near 2-µm with Cr2+:ZnS saturable absorbers.

Single pulse energies as high as 145 µJ were generated with a passively Q-switched diode-pumped Tm:KLu(WO4)2 laser using poly-crystalline Cr2+:ZnS as a saturable absorber. The maximum average power reached 0.39 W at a pulse repetition rate of 2.7 kHz with pulse durations in the 25 - 30 ns range. The maximum peak power amounted to 6 kW. The obtained results agree well with theoretical analysis.

Efficient thin-disk Tm-laser operation based on Tm:KLu(WO4)2/KLu(WO4)2 epitaxies.

A diode-pumped thin-disk laser based on Tm:KLu(WO4)2/KLu(WO4)2 epitaxies is realized. The emission is in the 1850-1945 nm spectral range for Tm-doping between 5 and 15 at. %. The maximum slope efficiency of 47% with respect to the absorbed power obtained with 5 at. % Tm:KLu(WO4)2/KLu(WO4)2 corresponds to a maximum output power of ~6 W in cw operation.

Diode-pumped 2 µm vibronic (Tm3+, Yb3+):KLu(WO4)2 laser.

We report on laser operation in a (6 at. % Tm, 5 at. % Yb):KLu(WO4)2 codoped crystal. The vibrational frequencies of KLu(WO4)2 are coupled to the electronic transitions of Tm3+ at 1946 nm, creating virtual final laser levels at higher energy than the ground level 3H6 of Tm3+. The longest recorded laser wavelength was 2039 nm, which is longer than permitted by a pure electronic transition in Tm3+ ions in KLu(WO4)2. We show that every laser wavelength can be explained with the electron-phonon coupling effect, where the vibration frequencies were determined through Raman spectroscopy.

Continuous-wave laser generation at ~2.1 µm in Ho:KRE(WO4)2 (RE = Y, Gd, Lu) crystals: a comparative study.

The laser performance of the monoclinic Ho:RE(WO4)2 (RE = Y, Gd, Lu) crystals is compared under identical experimental conditions. The comparison deals with the laser transition of Ho3+ at ~2.1µm by using two different pump sources, a diode laser operating at 1941 nm and a diode-pumped Tm:KLu(WO4)2 laser operating at 1946 nm. The results show internal slope efficiencies of ~60% and output powers exceeding 400 mW. The laser performance of Ho:KY(WO4)2 and Ho:KLu(WO4)2 is quite similar and superior to Ho:KGd(WO4)2.

Emission properties of hydrothermal Yb(3+), Er(3+) and Yb(3+), Tm(3+)-codoped Lu2O3 nanorods: upconversion, cathodoluminescence and assessment of waveguide behavior.

Yb(3+) and Ln(3+) (Ln(3+) = Er(3+) or Tm(3+)) codoped Lu(2)O(3) nanorods with cubic Ia3 symmetry have been prepared by low temperature hydrothermal procedures, and their luminescence properties and waveguide behavior analyzed by means of scanning near-field optical microscopy (SNOM). Room temperature upconversion (UC) under excitation at 980 nm and cathodoluminescence (CL) spectra were studied as a function of the Yb(+) concentration in the prepared nanorods. UC spectra revealed the strong development of Er(3+) (4)F(9/2) → (4)I(15/2) (red) and Tm(3+) (1)G(4) → (3)H(6) (blue) bands, which became the pre-eminent and even unique emissions for corresponding nanorods with the higher Yb(3+) concentration. Favored by the presence of large phonons in current nanorods, UC mechanisms that privilege the population of (4)F(9/2) and (1)G(4) emitting levels through phonon-assisted energy transfer and non-radiative relaxations account for these observed UC luminescence features. CL spectra show much more moderate development of the intensity ratio between the Er(3+) (4)F(9/2) → (4)I(15/2) (red) and (2)H(11/2), (4)S(3/2) → (4)I(15/2) (green) emissions with the increase in the Yb(3+) content, while for Yb(3+), Tm(3+)-codoped Lu(2)O(3) nanorods the dominant CL emission is Tm(3+) (1)D(2) → (3)F(4) (deep-blue). Uniform light emission along Yb(3+), Er(3+)-codoped Lu(2)O(3) rods has been observed by using SNOM photoluminescence images; however, the rods seem to be too thin for propagation of light.

Lanthanide doped luminescence nanothermometers in the biological windows: strategies and applications.

The development of lanthanide-doped non-contact luminescent nanothermometers with accuracy, efficiency and fast diagnostic tools attributed to their versatility, stability and narrow emission band profiles has spurred the replacement of conventional contact thermal probes. The application of lanthanide-doped materials as temperature nanosensors, excited by ultraviolet, visible or near infrared light, and the generation of emissions lying in the biological window regions, I-BW (650 nm-950 nm), II-BW (1000 nm-1350 nm), III-BW (1400 nm-2000 nm) and IV-BW (centered at 2200 nm), are notably growing due to the advantages they present, including reduced phototoxicity and photobleaching, better image contrast and deeper penetration depths into biological tissues. Here, the different mechanisms used in lanthanide ion-doped nanomaterials to sense temperature in these biological windows for biomedical and other applications are summarized, focusing on factors that affect their thermal sensitivity, and consequently their temperature resolution. Comparing the thermometric performance of these nanomaterials in each biological window, we identified the strategies that allow boosting of their sensing properties.

Effect of the Size and Shape of Ho, Tm:KLu(WO4)2 Nanoparticles on Their Self-Assessed Photothermal Properties.

The incorporation of oleic acid and oleylamine, acting as organic surfactant coatings for a novel solvothermal synthesis procedure, resulted in the formation of monoclinic KLu(WO4)2 nanocrystals. The formation of this crystalline phase was confirmed structurally from X-ray powder diffraction patterns and Raman vibrational modes, and thermally by differential thermal analysis. The transmission electron microscopy images confirm the nanodimensional size (~12 nm and ~16 nm for microwave-assisted and conventional autoclave solvothermal synthesis) of the particles and no agglomeration, contrary to the traditional modified sol-gel Pechini methodology. Upon doping with holmium (III) and thulium (III) lanthanide ions, these nanocrystals can generate simultaneously photoluminescence and heat, acting as nanothermometers and as photothermal agents in the third biological window, i.e., self-assessed photothermal agents, upon excitation with 808 nm near infrared, lying in the first biological window. The emissions of these nanocrystals, regardless of the solvothermal synthetic methodology applied to synthesize them, are located at 1.45 µm, 1.8 µm and 1.96 µm, attributed to the 3H4 → 3F4 and 3F4 → 3H6 electronic transition of Tm3+ and 5I7 → 5I8 electronic transition of Ho3+, respectively. The self-assessing properties of these nanocrystals are studied as a function of their size and shape and compared to the ones prepared by the modified sol-gel Pechini methodology, revealing that the small nanocrystals obtained by the hydrothermal methods have the ability to generate heat more efficiently, but their capacity to sense temperature is not as good as that of the nanoparticles prepared by the modified sol-gel Pechnini method, revealing that the synthesis method influences the performance of these self-assessed photothermal agents. The self-assessing ability of these nanocrystals in the third biological window is proven via an ex-vivo experiment, achieving thermal knowledge and heat generation at a maximum penetration depth of 2 mm.

CW lasing of Ho in KLu(WO4)2 in-band pumped by a diode-pumped Tm:KLu(WO4)2 laser.

We demonstrate continuous wave (CW) room temperature laser operation of the monoclinic Ho(3+)-doped KLu(WO(4))(2) crystal using a diode-pumped Tm(3+):KLu(WO(4))(2) laser for in-band pumping. The slope efficiency achieved amounts to ~55% with respect to the absorbed power and the maximum output power of 648 mW is generated at 2078 nm.

Single crystal growth, optical absorption and luminescence properties under VUV-UV synchrotron excitation of type III Pr3+:KGd(PO3)4.

Scintillator materials are widely used for a variety of applications such as high energy physics, astrophysics and medical imaging. Since the ideal scintillator does not exist, the search for scintillators with suitable properties for each application is of great interest. Here, Pr3+-doped KGd(PO3)4 bulk single crystals with monoclinic structure (space group: P21) are grown from high temperature solutions and their structural, thermal and optical properties are studied as possible candidates for scintillation material. The change in the unit cell parameters as a function of the Pr3+ level of doping and temperature is studied. Differential thermal analysis reveals that KGd0.942Pr0.058(PO3)4 is stable until 1140 K. The 5d3, 5d2 and 5d1 levels of Pr3+ with respect to the 3H4 ground state are centred at 166, 196 and 218 nm, respectively, in this host. The luminescence of KGd0.990Pr0.010(PO3)4, by exciting these 5d levels, shows intense emissions centred at 256 and 265 nm from the 5d1 to 3F3,4 and 1G4 levels of Pr3+ with a short decay time of 6 ns. The 6P3/2,5/2,7/2 → 8S7/2 transitions of Gd3+ appear after exciting the 5d levels of Pr3+ and the 4 f levels of Gd3+, showing an energy transfer between Pr3+ and Gd3+.

KLu(WO4)2/SiO2 Tapered Waveguide Platform for Sensing Applications.

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 m wavelength. A proof-of-concept with crystal-on-glass tapered waveguides was realized and used for chemical sensing.

Thermal properties of monoclinic KLu(WO4)2 as a promising solid state laser host.

Thermal analysis of the monoclinic solid state laser host KLu(WO4)2 is presented. The specific heat was measured by the relaxation method in the temperature range from 1.9 to 385 K: its value at room temperature is 0.324 J/gK. The Debye temperature and the sound velocity amount to 303+/-3 K and 3734 m/s. The linear thermal expansion tensor was measured by X-ray powder diffraction from room temperature up to 773 K. The eigenvalues of this tensor are alpha'(11)=8.98 x 10(-6) K(-1), alpha'(22)=3.35 x 10(-6) K(-1), and alpha'(33)=16.72 x 10(-6) K(-1), with the maximum value in the a-b crystallographic plane, at 31.94 degrees from the N(g) principal optical axis. The thermal diffusivity and its anisotropy in the temperature range between 300 and 500 K were measured by the pyroelectric method to determine the thermal conductivity tensor. The eigenvalues of the thermal conductivity are kappa'(11)=2.95 Wm(-1)K(-1), kappa'(22)=2.36 Wm-1K-1, and kappa'(33)=4.06 Wm(-1)K(-1), with the maximum value along a direction again in the a-b crystallographic plane, at 40.75 degrees from the N(g) principal optical axis. Simulation of the temperature distribution in a bulk sample of KLu(WO4)2 with dimensions 3 x 3 x 3 mm(3) shows that pump and laser beam directions along the N(p) principal optical axis in terms of thermal effects are preferable because the propagation is along a quasi-isothermal path.

Optimization of the Synthesis and Physical Characterization of Praseodymium-Doped Type III KGd(PO3)4 Nanocrystals.

Scintillator materials are used as detectors in the ray imaging techniques for medical diagnosis. Because the ideal medical scintillator material does not exist, many efforts are being made to find new materials that satisfy a greater number of properties. Here, the synthesis conditions of Pr:KGd(PO3)4 nanocrystals by the modified Pechini method are optimized to obtain a single crystalline phase of those that form the polymorphism of KGd(PO3)4. The interest lies in the type III phase because less quenching by Pr3+ concentration is expected. By performing transmittance measurements and because of the wide transparency window of the type III KGd(PO3)4 host, the 3H4 → 5d1 absorption transition of Pr3+ has been observed in the vacuum ultraviolet spectral range. After creating electron-hole pairs in the host due to the excitation of the material by X-ray radiation, the bands corresponding to the 5d1 → 3H4, 3H5, 3H6 and 5d1 → 3F3, 3F4, 1G4 transitions of Pr3+ have been observed in the near-visible spectral range, being these 5d → 4f transitions interesting for scintillation applications. Therefore, the type III Pr:KGd(PO3)4 nanocrystals allow the conversion from high-energy radiation to visible or near-visible light.

Single crystal growth, optical absorption and luminescence properties under VUV-UV synchrotron excitation of type III Ce3+:KGd(PO3)4, a promising scintillator material.

Scintillator materials have gained great interest for many applications, among which the medical applications stand out. Nowadays, the research is focused on finding new scintillator materials with properties that suit the needs of each application. In particular, for medical diagnosis a fast and intense response under high-energy radiation excitation is of great importance. Here, type III Ce3+-doped KGd(PO3)4 single crystals with high crystalline quality are grown and optically characterized as a new promising scintillator material. The 4f → 5d electronic transitions of Ce3+ are identified by optical absorption. The optical absorption cross section of Ce3+ for the electronic transition from the 2F5/2 to the 5d1 level is 370 × 10-20 cm2. The luminescence of KGd0.996Ce0.004(PO3)4 crystal by exciting the 5d levels of Ce3+ with VUV-UV synchrotron radiation shows down-shifting properties with strong emissions at 322 and 342 nm from the 5d1 to 2F5/2 and 2F7/2 levels of Ce3+ with a short decay time of ~16 ns, which is very suitable for scintillator applications. Moreover, these intense emissions are also observed when Gd3+ is excited since an energy transfer from Gd3+ to Ce3+ exists.

New microarchitectures of (Er,Yb):Lu2O3 nanocrystals embedded in PMMA: synthesis, structural characterization, and luminescent properties.

We report the formation of two-dimensional disordered arrays of poly(methyl)methacrylate (PMMA) microcolumns with embedded single size distribution of Lu0.990Er0.520Yb0.490 nanocrystals, (Er,Yb):Lu2O3, using a disordered porous silicon template. The cubic (Er,Yb):Lu2O3 nanocrystals, which crystallize into the cubic system with Ia3¯ space group, were synthesized using the modified Pechini method. Electronic microscopic techniques were used to study the distribution of the nanocrystals in the PMMA columns. Cathodoluminescence was used to observe the visible luminescence of the particles. Red emission attributed to 4 F9/2 → 4I15/2 erbium transition is predominant in these new composites.