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We report on the continuous-wave and passively Q-switched operation of a compositionally “mixed” heavily doped 16.6 at.% Yb:(Y,Lu)3Al5O12 garnet crystal at cryogenic temperatures (100–200 K), pumped by a volume Bragg grating stabilized diode laser emitting at 969 nm. At 140 K, in the continuous-wave regime, a maximum output power of 10.65 W was achieved at ~ 1029 nm with a slope efficiency of 56% (versus the incident pump power), a laser threshold of 1.05 W and excellent beam quality. Using Cr4+:YAG as a saturable absorber, the passively Q-switched laser generated pulses with an energy/duration of 0.15 mJ/201 ns, respectively, at a repetition rate of 39.7 kHz, corresponding to a peak power of 0.39 kW.

We present the laser characteristics of a 5 at.% Tm:YLF crystal using a modular setup at cryogenic temperatures emitting around 2 µm. Continuous-wave laser operation was achieved by pumping the laser crystal using a Volume Bragg Grating-stabilized laser diode emitting at 793 nm. A maximum output power of 6.5 W was achieved at 80 K corresponding to a slope efficiency of 66.0% with respect to the absorbed power with excellent beam quality.

This work produces a unique microfluidic device that acts as a “lab on a chip,” conducting separation, mixing, and concentration of microparticles similar to that required by cell sorting flow cytometry applications. The passive two-dimensional device shows to be successful at separating polystyrene (PS) beads between 5 and 20 µm in diameter, mixing them with an external media, and concentrating them by 250% continuously with minimal sample preparation, while still being inexpensive, and effective. By implementing the microfluidic device, the processing steps are done within seconds due to its high throughput of 2 mL min−1, wherein different hydrodynamic phenomena such as Dean's forces, inertial lift forces, and enhanced diffusion are taken advantage of.

Single-crystalline layers of 12 at.% Tb3+, 5 at.% Gd3+:LiYF4 were grown by the liquid phase epitaxy method on (001) oriented bulk undoped LiYF4 substrates using LiF as a solvent. The growth temperature was 737-740 & DEG;C, the growth duration was 15 -25 min, and the layer thickness was 40-90 & mu;m. The structural, morphological, vibronic and spectroscopic properties of the layers were studied. Tb3+ ions were uniformly distributed in the layers with no diffusion into the substrate. Polarized Raman spectroscopy confirmed the orientation of the layers (growth along the [001] direction). Under excitation in the blue, the layers exhibited intense green emission. For the 5D4-, 7F5 Tb3+ transition, the peak stimulated-emission cross-section is 1.28 x 10-21 cm2 at 542.0 nm for at-polarization. The luminescence lifetime of the 5D4 Tb3+ state is 5.05 ms at room temperature. The crystal-field splitting of Tb3+ multiplets was determined at low temperature. The developed epitaxies are promising for green and yellow waveguide lasers.& COPY; 2023 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement

We study a cascade laser scheme involving the 3H4 → 3H5 and 3F4 → 3H6 consecutive transitions in Tm3+-doped materials as a promising technique to favor laser emission at 2.3 µm. We examine the conditions in terms of the Tm3+ doping levels for which the cascade laser is beneficial or not. For this, Tm:LiYF4 lasers based on crystals with several doping levels in the range of 2.5 - 6 at.% with and without cascade laser are studied. For low doping of 2.5 at.% Tm3+, adding the laser emission at 1.9 µm allows to double the output power at 2.3 µm, whereas for high doping of 6 at.%, allowing the laser to operate at 1.9 µm totally suppresses the laser emission at 2.3 µm. An analytical model is developed and confronted with experimental results to predict this doping-dependent phenomenon and forecast the potential benefits. This study of cascade laser emission on the 3H4→ 3H5 and 3F4→ 3H6 transitions versus the Tm3+ doping level is finally extended to other well-known Tm3+-doped laser materials.

We report on the optical spectroscopy and continuous-wave (CW) laser operation of ytterbium-doped yttrium gallium garnet (Yb:YGG) crystal at cryogenic temperatures. A maximum absorption cross-section of 1.7 × 10-20 cm2 with spectral bandwidth of 2.4 nm centered at 970.5 nm and a maximum emission cross-section of 6.5 × 10-20 cm2 with spectral bandwidth of 4.6 nm centered at 1024 nm were determined at 120 K. Cryogenic continuous-wave laser operation was achieved using a VBG stabilized laser diode emitting around 969 nm as pump source. At 120 K, a maximum output power of 17.50 W was achieved around 1024 nm corresponding to a slope efficiency of 87% with respect to the absorbed power.

“Mixed” calcium rare-earth aluminate laser host crystals Ca(Gd,Lu)AlO4 (CALGLO) with up to 10.8 at.% Lu codoped with Tm3+ and Ho3+ ions are grown by the Czochralski method along the [001] direction. The segregation of rare-earth ions is studied. The crystal structure is refined by the Rietveld method. Tm,Ho:Ca(Gd,Lu)AlO4 crystallizes in the tetragonal system (sp. gr. I4/mmm) exhibiting a K2NiF4 type structure. The lattice constants are a = 3.6585(6) Å and c = 11.9660(9) Å for a crystal with a composition of CaGd0.8947Lu0.0551Tm0.0448Ho0.0054AlO4. The stability of Ca(Gd,Lu)AlO4 solid-solutions is discussed. The polarized Raman spectra are measured, revealing a most intense mode at 311 cm−1 and a maximum phonon frequency of ∼650 cm−1. The polarized absorption spectra are measured. The transition intensities for the Ho3+ ion are analyzed using the modified Judd-Ofelt theory accounting for configuration interaction.

A series of Dy3+-doped sodium lanthanum orthosilicate oxyapatites, NaLa9?xDyx(SiO4)6O2 (NLSO:xDy, x = 0–0.3), were synthesized by the solid-state reaction method at 1100 °C, with the aim of developing yellow emitting phosphors for applications in w-LEDs. Their crystal structure, morphology and particle size distribution, electronic structure, Raman spectra, concentration- and temperature-dependent luminescent properties were investigated for the first time. The crystal structure was refined by the Rietveld method. Undoped NLSO is hexagonal (space group P63/m – C26h) with lattice constants a = b = 9.6917(3) Å and c = 7.1836(4) Å. It is shown that Dy3+ ions substitute for the La3+ ones in two types of sites with C3 and Cs point symmetries. The first-principle calculations for undoped NLSO revealed an indirect bandgap of 5.06 eV. The NLSO:0.2Dy phosphor exhibited intense yellow emission with CIE 1931 chromaticity coordinates of (0.469, 0.495) and a correlated color temperature of 3150 K owing to the hypersensitive electric-dipole transition 4F9/2?6H13/2 (571 nm) dominating in the luminescence spectrum. Excellent thermal stability was found for this apatite phosphor (the activation energy is 0.23 ± 0.02 eV). © 2021 The Authors

We present a comparative study of two Lu-based oxide ceramics doped with Yb3+ ions, namely Yb:Lu3Al5O12 (garnet) and Yb:Lu2O3 (sesquioxide), promising for thin-disk lasers. The ceramics are fabricated using nanopowders of 3.6 at.% Yb:Lu2O3 and Al2O3 produced by laser ablation: Yb:Lu3Al5O12 – by vacuum sintering at 1800 °C for 5 h with the addition of 1 wt% TEOS as a sintering aid, and Yb:Lu2O3 – by vacuum pre-sintering at 1250 °C for 2 h followed by Hot Isostatic Pressing at 1400 °C for 2 h under Ar gas pressure of 207 MPa. The comparison includes the structure, Raman spectra, transmission, optical spectroscopy and laser operation. The crystal-field splitting of Yb3+ multiplets is revealed for Lu3Al5O12. A continuous-wave (CW) Yb:Lu3Al5O12 ceramic microchip laser generates 5.65 W at 1031.1 nm with a slope efficiency of 67.2%. In the quasi-CW regime, the peak power is scaled up to 8.83 W. The power scaling for the Yb:Lu2O3 ceramic laser is limited by losses originating from residual coloration and inferior thermal behavior. © 2020 Elsevier Ltd and Techna Group S.r.l.

Polarization anisotropy of luminescent properties of europium (III) ions in low-symmetry C2 sites is studied using monoclinic (sp. gr. C2/c) tungstate crystal KY(WO4)2. The 5D0 → 7FJ (where J = 0...6) transitions are characterized for the principal light polarizations. Polarization selection rules for the magnetic dipole 5D0 → 7F1 transition are presented. The stimulated-emission cross-sections for Eu3+ ions relevant for laser operation are determined.

© 2020, The Author(s). 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+.

© 2020 Optical Society of America under the terms of the OSA Open Access Publishing Agreement We report on the crystal growth, spectroscopy and first laser operation of a novel double molybdate compound - Tm:KY(MoO4)2. This orthorhombic (sp. gr. Pbna) crystal exhibits strong anisotropy of the spectroscopic properties due to its layered structure. The maximum stimulated emission cross-section for the 3F4 → 3H6 transition is 2.70×10−20 cm2 at 1856nm with a bandwidth of >110 nm (for E || b). The lifetime of the 3F4 state is 2.29 ms. Crystalline films and plates (thickness down to 70 µm) of high optical quality are obtained by mechanical cleavage along the (100) plane. Continuous-wave diode-pumped laser operation is achieved in such thin films and plates yielding a maximum output power of 0.88 W at ∼1.9 µm with a slope efficiency of 65.8% and a linearly polarized laser output. Vibronic lasing is demonstrated at ∼2.06 µm. Tm:KY(MoO4)2 is promising for microchip and thin-disk lasers.

© 2020 Optical Society of America under the terms of the OSA Open Access Publishing Agreement Depressed-index buried and surface channel waveguides (type III) are produced in a bulk 3.5 at.% Tm3+:CALGO crystal by femtosecond direct-laser-writing at kHz repetition rate. The waveguides are characterized by confocal microscopy and µ-Raman spectroscopy. Under in-band-pumping at 1679 nm (3H6 → 3F4 transition) by a Raman fiber laser, the buried channel waveguide laser with a circular cladding (diameter: 60 µm) generated a continuous-wave output power of 0.81 W at 1866-1947 nm with a slope efficiency of 71.2% (versus the absorbed pump power) and showed a laser threshold of 200 mW. The waveguide propagation losses were as low as 0.3 ± 0.2 dB/cm. The laser performance under in-band pumping was superior compared pumping at ∼800 nm (3H6 → 3H4 transition), i.e., the convetional pump wavelength. Vibronic laser emission from the WG laser above 2 µm is also achieved. The low-loss behavior, the broadband emission properties and good power scaling capabilities indicate the suitability of Tm3+:CALGO waveguides for mode-locked laser operation at ∼2 µm.

© 2020 Optical Society of America. We report on the diverse pulsed operation regimes of a femtosecond-laser-written Yb:KLuW channel waveguide laser emitting near 1040 nm. By the precise position tuning of a carbon-nanotube-coated saturable absorber (SA) mirror, the transition of the pulsed operation from Q-switching, Q-switched mode-locking and finally sub-GHz continuous-wave mode-locking are obtained based on the interplay of dispersion and mode area control. The Q-switched pulses exhibit typical fast SA Q-switched pulse characteristics depending on absorbed pump powers. In the Q-switched mode-locking, amplitude modulations of the mode-locked pulses on the Q-switched envelope are observed. The radio-frequency spectrum represents the coexistence of Q-switching and mode-locking signals. In the purely mode-locked operation, the waveguide laser generates 2.05-ps pulses at 0.5 GHz.

© 2020, The Author(s). We present the spectroscopic and laser characteristics of a 3 at.% Tm:Y2O3 transparent ceramic at cryogenic temperatures. An absorption cross section of 4.7 × 10−21 cm2 with a bandwidth of 0.7 nm centered at 793.3 nm and an emission cross section of 29.0 × 10−21 cm2 centered at 1930.9 nm were estimated at 80 K. Continuous-wave laser operation was achieved using broadband and VBG stabilized laser diodes emitting around 793 nm as pump sources. With the VBG pump diode, a maximum output power of 6.4 W was achieved at 80 K corresponding to a slope efficiency of 52.0% with respect to absorbed power.

© 2019 Optical Society of America. A channel waveguide (WG) buried immediately below the surface of a Yb:KLuW crystal is used as a laser gain medium for passive Q-switching by both evanescent- and direct-field interactions with single-walled carbon nanotubes (SWCNTs) near 1040 nm. The SWCNTs used as saturable absorbers (SAs) are deposited on top of the half-ring-type channel WG fabricated via femtosecond direct laser writing. The Q-switched WG laser delivers 88.5 ns pulses at a 1.16 MHz repetition rate with a maximum average output power of 680 mW. For the two different interaction schemes with SWCNT-SAs, the pulse characteristics, depending on the output coupling ratio and absorbed pump power, are experimentally investigated and compared to the results of theoretical analyses of the SA Q-switched operation.

© 2020 Elsevier B.V. The research for new scintillator materials with improved properties is gaining growing interest due to their wide variety of applications, including medical diagnosis. Here, radioluminescence studies under synchrotron X-ray excitation of bulk single crystals of type III Ce:KGd(PO3)4 and Pr:KGd(PO3)4 (Ce:KGdP and Pr:KGdP) with high crystalline quality have been carried out. The X-ray excited radioluminescence spectra show emission bands corresponding to 5d1 → 4f electronic transitions of Ce3+ and of Pr3+ in type III KGdP, being of great interest for scintillation applications. In the case of Ce:KGdP crystals, the absence of the 6PJ → 8S7/2 transitions of Gd3+ (~312 nm) can be ascribed to an efficient energy transfer from Gd3+ to Ce3+. In undoped and Pr3+-doped KGdP crystals, a change in coloration in the irradiated regions was observed after prolonged X-ray irradiation, which could be related to the presence of color centers.

© 2019 by the authors Inertial focusing conditions of fluorescent polystyrene spherical particles are studied at the pointwise level along their pathlines. This is accomplished by an algorithm that calculates a degree of spreading function of the particles’ trajectories taking streaklines images as raw data. Different confinement ratios of the particles and flow rates are studied and the results are presented in state diagrams showing the focusing degree of the particles in terms of their position within a curve of an asymmetric serpentine and the applied flow rate. In addition, together with numerical simulation results, we present empirical evidence that the preferred trajectories of inertially focused spheres are contained within Dean vortices’ centerlines. We speculate about the existence of a new force, never postulated before, to explain this fact.

© 2019 Optical Society of America. Both direct- and evanescent-field interactions with carbon nanotubes (CNTs) are applied to achieve stable Q-switched operation of Yb:KYW planar waveguide lasers. The performance characteristics were investigated in a same cavity configuration and analyzed in detail in the following three cases, CNTs deposited onto end mirror (M-coating), output coupler (OC-coating) and top surface of the planar waveguide (WG-coating). Maximum output powers, repetition rates, and minimum pulse durations are 61 mW, 1103 kHz and 215 ns for OC-coating, 39 mW, 1052 kHz and 275 ns for WG-coating, and 26 mW, 1119 kHz and 217 ns for M-coating, respectively. From the calculation of the configuration-dependent stability range, the beam size and the electric field distribution in the Yb:KYW planar waveguide, it is confirmed that the evanescent-field interaction scheme makes stable Q-switching possible with much lower intensities at saturable absorber compared to the direct-field interaction scheme in the presented waveguide laser operation.

© 2019 Optical Society of America. We report the generation of mid-infrared (~2 µm) high repetition rate (MHz) sub-100 ns pulses in buried thulium-doped monoclinic double tungstate crystalline waveguide lasers using two-dimensional saturable absorber materials, graphene and MoS 2 . The waveguide (propagation losses of ~1 dB/cm) was micro-fabricated by means of ultrafast femtosecond laser writing. In the continuous-wave regime, the waveguide laser generated 247 mW at 1849.6 nm with a slope efficiency of 48.7%. The laser operated at the fundamental transverse mode with a linearly polarized output. With graphene as a saturable absorber, the pulse characteristics were 88 ns / 18 nJ (duration / energy) at a repetition rate of 1.39 MHz. Even shorter pulses of 66 ns were achieved with MoS 2 . Graphene and MoS 2 are therefore promising for high repetition rate nanosecond Q-switched infrared waveguide lasers.

© 2019, The Author(s). We optimized the deposition conditions of polycrystalline nanoporousGaN coatings produced by Chemical Vapor Deposition on Si substrates, by exploring the effect produced by the Ga holder shape, the initial amount of Ga, the reaction deposition time and the metallic catalyst used. Such polycrystalline films probed to act as antireflective coatings by reducing the reflectance of Si substrates by 50% or more, and that of flat GaN samples by 40% in the UV and 83% in the visible, at the same time that they exhibit an almost constant reflectance from 400 to 800 nm, important to develop UV sensors with enhanced sensitivity. Furthermore, the polycrystalline nanoporous coatings we developed exhibit hydrophobic behaviour, with a static contact angle of 119°, and a contact angle hysteresis of 4.5°, which might contribute to enlarge the durability of such functional films, by the self cleaning effect induced.

© 2018 The spectroscopic properties of Tb 3+ ions in monoclinic KLu(WO 4 ) 2 double tungstate crystal are studied with polarized light. The absorption spectra in the visible, near- and mid-IR including the transitions to all lower-lying 7 F J (J = 0 … 5) excited states are measured. The maximum absorption cross-section for the 7 F 6 ? 5 D 4 transition is 3.42 × 10 ?21 cm 2 at 486.7 nm for light polarization E || N m . The transition probabilities for Tb 3+ ions are calculated within the Judd-Ofelt theory modified for the case of an intermediate configuration interaction (ICI). The radiative lifetime of the 5 D 4 state is 450 ?s and the luminescence quantum yield is > 90%. The polarized stimulated-emission cross-section spectra for all 5 D 4 ? 7 F J (J = 0 … 6) emission channels are evaluated. The maximum ? SE is 11.4 × 10 ?21 cm 2 at 549.4 nm (for E || N m ). Tb 3+ :KLu(WO 4 ) 2 features high transition cross-sections for polarized light being promising for color-tunable visible lasers and imaging.

© 2018 Optical Society of America. We report on the crystal growth, structure, Raman and optical spectroscopy of novel mixed tetragonal vanadates, Yb:Lu 1-x-y Y x La y VO 4 . Optical absorption, stimulatedemission, and gain cross-section spectra of Yb 3+ are determined for p and s polarizations. For a Yb:Lu 0.74 Y 0.23 La 0.01 VO 4 crystal, the absorption bandwidth is > 10 nm, the ? SE is 1.1 × 10 -20 cm 2 at 1013 nm, the gain bandwidth is > 40 nm (for p-polarization), and the radiative lifetime of the 2 F 5/2 state is ~305 ?s. The Stark splitting of the Yb 3+ multiplets is determined using low-temperature (6 K) spectroscopy. A diode-pumped a-cut 2 at.% Yb:Lu 0.74 Y 0.23 La 0.01 VO 4 laser generated 5.0 W at 1044 nm with a slope efficiency of 43%. The developed materials are promising for sub-100 fs mode-locked lasers at ~1 ?m.

© 2018 Optical Society of America under the terms of the OSA Open Access Publishing Agreement. : We report on the first erbium (Er3+) doped double tungstate waveguide laser. As a gain material, we studied a monoclinic Er3+:KLu(WO4)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 || Nm). The laser threshold was at 93 mW of absorbed pump power.

© 2018 Optical Society of America. A Tm 3+ monoclinic double tungstate planar waveguide laser is passively Q-switched (PQS) by a saturable absorber (SA) based on single-walled carbon nanotubes (SWCNTs) randomly oriented in a polymer film. The laser is based on a 18 µm-thick 5 at.% Tm:KY 1-x-y Gd x Lu y (WO 4 ) 2 active layer grown on an undoped (010)-oriented KY(WO 4 ) 2 substrate by liquid phase epitaxy with determined propagation losses 0.7 ± 0.2 dB/cm. The PQS laser generated a maximum average output power of 45.6 mW at 1.8354 µm with a slope efficiency of 22.5%. Stable 83-ns-long laser pulses with an energy of 33 nJ were achieved at a repetition rate of 1.39 MHz. The use of SWCNTs as SA is promising for generation of sub-100 ns pulses in such waveguide lasers at ~2 µm.

© 2018 Optical Society of America under the terms of the OSA Open Access Publishing Agreement. A Holmium thin-disk laser based on a 3 at.% Ho:KY(WO 4 ) 2 /KY(WO4)2 epitaxy and single-bounce pumping by a 1960 nm Tm-fiber laser is passively Q-switched with a GaSb-based quantum-well semiconductor saturable absorber mirror. It generates an average output power of 551 mW at 2056 nm with a slope efficiency of 44% (with respect to the absorbed pump power). The best pulse characteristics (energy and duration) are 4.1 ?J /201 ns at a repetition rate of 135 kHz and the conversion efficiency with respect to the continuous-wave regime is as high as 93%.

© 2018 American Chemical Society. 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)4nanocrystals 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)4host, the3H4? 5d1absorption 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?3H4,3H5,3H6and 5d1?3F3,3F4,1G4transitions 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)4nanocrystals allow the conversion from high-energy radiation to visible or near-visible light.

© 2018, The Author(s). 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)4single 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 the2F5/2to the 5d1level is 370 × 10−20cm2. The luminescence of KGd0.996Ce0.004(PO3)4crystal 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 5d1to2F5/2and2F7/2levels 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.

We report on the first erbium (Er3+) doped double tungstate waveguide laser. As a gain material, we studied a monoclinic Er3+:KLu(WO4)2 crystal. A depressed-index buried channel waveguide formed by a 60 ¿m-diameter circular cladding was fabricated by 3D femtosecond direct laser writing. The waveguide was characterized by confocal laser microscopy, ¿-Raman and ¿-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 || Nm). The laser threshold was at 93 mW of absorbed pump power

Open Access, A Tm3+ monoclinic double tungstate planar waveguide laser is passively Qswitched (PQS) by a saturable absorber (SA) based on single-walled carbon nanotubes (SWCNTs) randomly oriented in a polymer film. The laser is based on a 18 ¿m-thick 5 at.% Tm:KY1-x-yGdxLuy(WO4)2 active layer grown on an undoped (010)-oriented KY(WO4)2 substrate by liquid phase epitaxy with determined propagation losses 0.7 ± 0.2 dB/cm. The PQS laser generated a maximum average output power of 45.6 mW at 1.8354 ¿m with a slope efficiency of 22.5%. Stable 83-ns-long laser pulses with an energy of 33 nJ were achieved at a repetition rate of 1.39 MHz. The use of SWCNTs as SA is promising for generation of sub-100 ns pulses in such waveguide lasers at ~2 ¿m.

© 2018 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/). The dynamics of a spherical particle in an asymmetric serpentine is studied by finite element method (FEM) simulations in a physically unconstrained system. The two-way coupled time dependent solutions illustrate the path of the particle along a curve where a secondary flow (Dean flow) has developed. The simulated conditions were adjusted to match those of an experiment for which particles were focused under inertial focusing conditions in a microfluidic device. The obtained rotational modes inferred the influence of the local flow around the particle. We propose a new approach to find the decoupled secondary flow contribution employing a quasi-Stokes flow.

We report on inkjet-printing of graphene saturable absorbers (SAs) suitable for passive Qswitching of ~2-μm bulk and waveguide lasers. Using graphene-SA in a microchip Tm:KLu(WO4)2 laser, 1.2 μJ/136 ns pulses are generated at 1917 nm., QC 20171030

© 2017 Optical Society of America. Monoclinic thulium-doped magnesium monotungstate, Tm 3+ :MgWO 4 , is promising for efficient power-scalable continuous-wave (CW) and passively Q-switched lasers at > 2 ?m. Under diode pumping at 802 nm, a compact CW laser based on Z-cut Tm:MgWO 4 generated 3.09 W at 2022-2034 nm with a slope efficiency of 50% which represents the highest output power ever achieved with this type of laser host. Stable passive Q-switching of the Tm:MgWO 4 laser is demonstrated for the first time, to the best of our knowledge, using single-walled carbon nanotubes, graphene, and Cr 2+ :ZnS saturable absorbers. Using the latter, the best performance is achieved with 16.1 ?J/13.6 ns pulses at 2017.8 nm with a maximum average output power of 0.87 W and a peak power of 1.18 kW.

© 2017 Optical Society of America. Multi-watt continuous-wave (CW) operation of tetragonal rare-earth calcium aluminate Yb:CaLnAlO 4 Ln Gd;Y) crystals in plano–plano microchip lasers was demonstrated with an almost quantum-defect-limited slope efficiency. Pumped at 978 nm by an InGaAs laser diode, a 3.4 mm long 8 at. % Yb:CaGdAlO 4 laser generated 7.79 W at 1057–1065 nm with a slope efficiency of ? 84% (with respect to the absorbed pump power). An even higher ? 91% was achieved with a 2.5 mm long 3 at. % Yb:CaYAlO 4 laser, from which 5.06 W were extracted at 1048–1056 nm. Both lasers produced linearly polarized output (?-polarization) with an almost circular diffraction-limited beam (M 2 x,y < 1.1). The output performance of the developed lasers was modeled, yielding an internal loss coefficient as low as 0.004?0.007 cm ?1 . In addition, their spectroscopic properties were revisited.

Increasing Yb3+ absorption efficiency is currently desired in a number of applications including bio-imaging, photovoltaics, near infrared driven photocatalysis or ultra-short pulsed solid-state lasers. In this work, silver nanoparticles, which are connected forming disordered networks, have been self-assembled on Yb3+ doped RbTiOPO4 crystals to produce a remarkable enhancement of Yb3+ absorption, and hence in the photoluminescence of this ion. The results are interpreted taking into account the near-field response of the plasmonic networks, which display strong amplification of the electric field at the maximum of Yb3+ excitation at around 900 nm, together with the anisotropic character of the Yb3+ transitions in RbTiOPO4. We show that in the near field regime, the scattering of the plasmonic networks produces additional polarization field components to those of the incident field, which allows access to the largest transition dipolar moment of Yb3+ ions in RbTiOPO4. As a result, a much more efficient route for Yb3+ excitation takes place at the immediacy of the plasmonic networks. This work provides fundamental insights for improving the optical properties of rare earth ions by the suitable design of metallic nanoparticle arrangements, and constitutes a promising step towards the development of new multifunctional solid-state lasers.

We study cryogenic laser operation of an Yb-doped KLu(WO4)2 crystal pumped with a volume Bragg grating (VBG) stabilized diode laser at 981 nm. In the continuous wave laser regime, a maximum output power of 4.31 W is achieved at 80 K with a slope efficiency of 44.0% with respect to the incident pump power. Using a 85% initial transmission Cr:YAG crystal for passive Q-switching, an average output power of 2.11 W is achieved at 100 K for a repetition rate of 19 kHz. The pulse energy, pulse duration and peak power amount to 111 ¿J, 231 ns and 0.48 kW, respectively.

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

We report on the crystal growth, spectroscopic investigation and laser performance of Tm3+-doped monoclinic magnesium tungstate (Tm:MgWO4), for the first time, to the best of our knowledge. A high-quality crystal has been grown by the top seeded solution growth method. The relevant spectroscopic properties are characterized in terms of absorption, luminescence and Raman spectroscopy. Judd-Ofelt (J-O) analysis is performed to evaluate the spontaneous emission probabilities and the radiative lifetimes. The absorption, stimulatedemission and gain cross-section spectra are determined for the principal light polarizations. The first laser action in the 2 ¿m spectral range is demonstrated in the regime of continuous-wave operation with a maximum output power of 775 mW and a slope efficiency of 39%.

DOI: 10.1088/1555-6611/aa5c15 URL: http://iopscience.iop.org/article/10.1088/1555-6611/aa5c15/meta;jsessionid=400897093FD1F1097EB0E6468F81AD1D.c4.iopscience.cld.iop.org Filiació URV: SI, We report on the first Tm3+-doped double tungstate waveguide laser passively Q-switched by a graphene saturable absorber using a 12.4 ¿m-thick 3 at.% Tm:KY0.58Gd0.22Lu0.17(WO4)2 epitaxial layer grown on a (0 1 0)-oriented pure KY(WO4)2 substrate. This laser generated 5.8 nJ/195 ns pulses at 1831.8 nm corresponding to a pulse repetition frequency of 1.13 MHz. These are the shortest pulses achieved in passively Q-switched Tm waveguide lasers. The laser slope efficiency was 9% and the Q-switching conversion efficiency reached 45%. Graphene is promising for the generation of ns pulses at ¿2 ¿m in Tm3+-doped double tungstate waveguide lasers operating in the MHz-range.

In this paper we study the optical properties of nanoporous gallium nitride (GaN) epitaxial layers grown by chemical vapour deposition on non-porous GaN substrates, using photoluminescence, cathodoluminescence, and resonant Raman scattering, and correlate them with the structural characteristic of these films. We pay special attention to the analysis of the residual strain of the layers and the influence of the porosity in the light extraction. The nanoporous GaN epitaxial layers are under tensile strain, although the strain is progressively reduced as the deposition time and the thickness of the porous layer increases, becoming nearly strain free for a thickness of 1.7 ¿m. The analysis of the experimental data point to the existence of vacancy complexes as the main source of the tensile strain.

DOI: 10.1364/OE.25.003682 URL: https://www.osapublishing.org/oe/abstract.cfm?uri=oe-25-4-3682 Filiació URV: SI Memòria, We report on the crystal growth, spectroscopic investigation and laser performance of Tm3+-doped monoclinic magnesium tungstate (Tm:MgWO4), for the first time, to the best of our knowledge. A high-quality crystal has been grown by the top seeded solution growth method. The relevant spectroscopic properties are characterized in terms of absorption, luminescence and Raman spectroscopy. Judd-Ofelt (J-O) analysis is performed to evaluate the spontaneous emission probabilities and the radiative lifetimes. The absorption, stimulated-emission and gain cross-section spectra are determined for the principal light polarizations. The first laser action in the 2 ¿m spectral range is demonstrated in the regime of continuous-wave operation with a maximum output power of 775 mW and a slope efficiency of 39%.

Porous GaN layers are grown on silicon from gold or platinum catalyst seed layers, and self-catalyzed on epitaxial GaN films on sapphire. Using a Mg-based precursor, we demonstrate p-type doping of the porous GaN. Electrical measurements for p-type GaN on Si show Ohmic and Schottky behavior from gold and platinum seeded GaN, respectively. Ohmicity is attributed to the formation of a Ga2Au intermetallic. Porous p-type GaN was also achieved on epitaxial n-GaN on sapphire, and transport measurements confirm a p-n junction commensurate with a doping density of 1018 cm 3. Photoluminescence and cathodoluminescence confirm emission from Mg-acceptors in porous p-type GaN.

Europium-doped La2O3 nanocrystalline powders with sizes ranging from 4 nm to 300 nm have been obtained by the modified Pechini, hydrothermal with conventional furnace, hydrothermal with microwave furnace, and precipitation with ultrasonic bath methods. X-ray diffraction techniques were used to study the evolution of the prepared gels towards the desired crystalline phase. We determined the size and the morphology of the nanoparticles by electronic microscopy. Finally, we studied and analyzed the luminescence properties of the trivalent europium in the hexagonal La2O3 nanocrystals by photoluminescence and cathodoluminescence.

Europium-doped La2O3 nanocrystalline powders with sizes ranging from 4 nm to 300 nm have been obtained by the modified Pechini, hydrothermal with conventional furnace, hydrothermal with microwave furnace, and precipitation with ultrasonic bath methods. X-ray diffraction techniques were used to study the evolution of the prepared gels towards the desired crystalline phase. We determined the size and the morphology of the nanoparticles by electronic microscopy. Finally, we studied and analyzed the luminescence properties of the trivalent europium in the hexagonal La2O3 nanocrystals by photoluminescence and cathodoluminescence.

Porous GaN layers are grown on silicon from gold or platinum catalyst seed layers, and self-catalyzed on epitaxial GaN films on sapphire. Using a Mg-based precursor, we demonstrate p-type doping of the porous GaN. Electrical measurements for p-type GaN on Si show Ohmic and Schottky behavior from gold and platinum seeded GaN, respectively. Ohmicity is attributed to the formation of a Ga2Au intermetallic. Porous p-type GaN was also achieved on epitaxial n-GaN on sapphire, and transport measurements confirm a p-n junction commensurate with a doping density of 1018 cm 3. Photoluminescence and cathodoluminescence confirm emission from Mg-acceptors in porous p-type GaN.

DOI: 10.1364/OL.42.003490 Filiació URV: SI, We report on the first Holmium (Ho3+) monoclinic double tungstate thin-disk laser. It is based on a 250 µm-thick 3 at. % Ho:KY(WO4)2 active layer grown on a (010)-oriented KY(WO4)2 substrate. When pumped by a Tm-fiber laser at 1960 nm with a single-bounce (single double-pass) pump geometry, the continuous-wave Ho:KY(WO4)2 thin-disk laser generated 1.01 W at 2057 nm corresponding to a slope efficiency ¿ of 60% and a laser threshold of only 0.15 W. Implementing a double-bounce (second double-pass) for the pump, the output of this laser was scaled to 1.57 W with ¿ = 55%. The maximum stimulated-emission cross-section ¿SE of the Ho3+ ions in the epitaxial layer reaches 2.5×10-20 cm2 at 2056.5 nm for E || Nm. The Ho:KY(WO4)2 epitaxial structures are promising for multi-watt mode-locked thin-disk lasers at ~2.06 µm.© 2017 Optical Society of America

© 2017 Optical Society of America. Multi-watt continuous-wave (CW) operation of tetragonal rare-earth calcium aluminate Yb:CaLnAlO 4 Ln Gd;Y) crystals in plano–plano microchip lasers was demonstrated with an almost quantum-defect-limited slope efficiency. Pumped at 978 nm by an InGaAs laser diode, a 3.4 mm long 8 at. % Yb:CaGdAlO 4 laser generated 7.79 W at 1057–1065 nm with a slope efficiency of ? 84% (with respect to the absorbed pump power). An even higher ? 91% was achieved with a 2.5 mm long 3 at. % Yb:CaYAlO 4 laser, from which 5.06 W were extracted at 1048–1056 nm. Both lasers produced linearly polarized output (?-polarization) with an almost circular diffraction-limited beam (M 2 x,y < 1.1). The output performance of the developed lasers was modeled, yielding an internal loss coefficient as low as 0.004?0.007 cm ?1 . In addition, their spectroscopic properties were revisited.

Depressed-index channel waveguides with a circular and photonic crystal cladding structures are prepared in a bulk monoclinic Tm:KLu(WO4)2 crystal by 3D direct femtosecond laser writing. The channel waveguide structures are characterized and laser operation is achieved using external mirrors. In the continuous-wave mode, the maximum output power of 46 mW is achieved at 1912 nm corresponding to a slope efficiency of 15.2% and a laser threshold of only 21 mW. Passive Q-switching of a waveguide with a circular cladding is realized using single-walled carbon nanotubes. Stable 7 nJ/50 ns pulses are achieved at a repetition rate of 1.48 MHz. This first demonstration of ?2???m fs-laser-written waveguide lasers based on monoclinic double tungstates is promising for further lasers of this type doped with Tm3+ and Ho3+??ions.

DOI: 10.1364/OE.25.003682 URL: https://www.osapublishing.org/oe/abstract.cfm?uri=oe-25-4-3682 Filiació URV: SI Memòria, We report on the crystal growth, spectroscopic investigation and laser performance of Tm3+-doped monoclinic magnesium tungstate (Tm:MgWO4), for the first time, to the best of our knowledge. A high-quality crystal has been grown by the top seeded solution growth method. The relevant spectroscopic properties are characterized in terms of absorption, luminescence and Raman spectroscopy. Judd-Ofelt (J-O) analysis is performed to evaluate the spontaneous emission probabilities and the radiative lifetimes. The absorption, stimulated-emission and gain cross-section spectra are determined for the principal light polarizations. The first laser action in the 2 ¿m spectral range is demonstrated in the regime of continuous-wave operation with a maximum output power of 775 mW and a slope efficiency of 39%.

We report on the crystal growth, spectroscopic investigation and laser performance of Tm3+-doped monoclinic magnesium tungstate (Tm:MgWO4), for the first time, to the best of our knowledge. A high-quality crystal has been grown by the top seeded solution growth method. The relevant spectroscopic properties are characterized in terms of absorption, luminescence and Raman spectroscopy. Judd-Ofelt (J-O) analysis is performed to evaluate the spontaneous emission probabilities and the radiative lifetimes. The absorption, stimulatedemission and gain cross-section spectra are determined for the principal light polarizations. The first laser action in the 2 ¿m spectral range is demonstrated in the regime of continuous-wave operation with a maximum output power of 775 mW and a slope efficiency of 39%.