Emission cross sections and spectroscopy of Ho3+ laser channels in KGd(WO4)2 single crystal

The spectroscopic properties of Ho3+ laser channels in KGd(WO4)2 crystals have been investigated using optical absorption, photoluminescence, and lifetime measurements. The radiative lifetimes of Ho3+ have been calculated through a Judd-Ofelt (JO) formalism using 300-K optical absorption results. The JO parameters obtained were Ω2=15.35×10-20 cm2, Ω 4=3.79×10-20 cm2, Ω6 =1.69×10-20 cm2. The 7-300-K lifetimes obtained in diluted (8·1018 cm-3) KGW:0.1% Ho samples are: τ(5F3)=0.9 μs, τ( 5S2)=19-3.6 μs, and τ(5F5 )=1.1 μs. For Ho concentrations below 1.5×1020 cm-3, multiphonon emission is the main source of non radiative losses, and the temperature independent multiphonon probability in KGW is found to follow the energy gap law τph -1(0)=βexp(-αΔE), where β=1.4×10-7 s-1, and α=1.4×103 cm. Above this holmium concentration, energy transfer between Ho impurities also contributes to the losses. The spectral distributions of the Ho3+ emission cross section σEM for several laser channels are calculated in σ- and π-polarized configurations. The peak a σEM values achieved for transitions to the 5I8 level are ≈2×10-20 cm2 in the σ-polarized configuration, and three main lasing peaks at 2.02, 2.05, and 2.07 μm are envisaged inside the 5I7→5I8 channel.This work was supported by Comisión Interministerial de Ciencia y Tecnología under Projects MAT99-1077 and 2FD97-0912.Peer reviewe

Femtosecond laser-written double-cladding waveguides in Nd:GdVO4crystal: Raman analysis, guidance, and lasing

We report on waveguide lasers at 1064.5 nm in femtosecond laser-written double-cladding waveguides in Nd∶GdVO4 crystals. The cladding waveguides guide both transverse electric (TE)- and transverse magnetic (TM)-polarized modes with considerably symmetric single-modal profiles and show good transmission properties (propagation loss as low as 1.0 dB∕cm). The detailed structure of the single and double claddings has been imaged by means of μ-Raman analysis, and the observed slight fabrication asymmetries with respect to an ideal circular cladding are in well agreement with the observed differences in TE/TM propagation losses. Importantly, the Raman imaging shows the complete absence of lattice defect at the laser active volume. Under the optical pumping at 808 nm, a maximum output power up to 0.43Wof the continuous wave waveguide laser with a slope efficiency of 52.3% has been achieved in the double-cladding waveguide, which is 21.6% and 23% higher than that from a single-inner cladding waveguide. Furthermore, the maximum output power of the waveguide laser is 72% higher than that of the double-line waveguide due to the double-cladding designThe work is supported by the National Natural Science Foundation of China (No. 11274203), the Specialized Research Fund for the Doctoral Program of Higher Education of China (No. 20130131130001), and Junta de Castilla y León under project SA086A12-2. Support from the Centro de Láseres Pulsados (CLPU) is also acknowledged. This work was supported by the Spanish Government under project MAT2011- 29255-C02-02 and by the Generalitat de Catalunya under project 2014SGR1358

Efficient tunable laser operation of Tm:KGd(WO4)2 in the continuous-wave regime at room temperature

Tm:KGd(WO/sub 4/)/sub 2/ is studied as a three-level laser on the /sup 3/F/sub 4/ /spl rarr/ /sup 3/H/sub 6/ transition and a tunable source in the 2-/spl mu/m spectral range, operating at room temperature. An overall tunability extending from 1790 to 2042 nm is achieved with maximum output powers of 400 mW for an absorbed pump power of 1 W. Various doping levels, pump wavelengths and polarization configurations are compared and the advantages of the monoclinic double tungstates over other Tm-hosts are outlined

Optical and structural characterisation of epitaxial nanoporous GaN grown by CVD

Producción CientíficaIn 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 μm. The analysis of the experimental data point to the existence of vacancy complexes as the main source of the tensile strain.Ministerio de Economía, Industria y Competitividad (Projects No. TEC2014-55948-R and MAT2016-75716-C2-1-R (AEI/FEDER, UE)Junta de Castilla y León (programa de apoyo a proyectos de investigación – Ref. Projects VA293U13 and VA081U16)Comisión Interministerial de Ciencia y Tecnología (Proyect CICYT MAT2010-20441-C02)Ministerio de Economía, Industria y Competitividad (Projects No.ENE2014-56069-C4-4-R

Transition of pulsed operation from Q-switching to continuous-wave mode-locking in a Yb:KLuW waveguide laser

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.National Research Foundation of Korea (2018H1A2AA1061480, 2019R1A2C3003504, 2020R1A4A2002828); Spanish Government (FIS2017-87970-R, MAT2016-75716-C2-1-R (AEI/FEDER,UE)); Junta de Castilla y León (SA287P18); Generalitat de Catalunya (2017SGR755)

Oriented zinc oxide nanorods: A novel saturable absorber for lasers in the near-infrared

Zinc oxide (ZnO) nanorods (NRs) oriented along the crystallographic [001] axis are grown by the hydrothermal method on glass substrates. The ZnO NRs exhibit a broadband (1–2 µm) near-IR absorption ascribed to the singly charged zinc vacancy VZn−1. The saturable absorption of the ZnO NRs is studied at ≈1 µm under picosecond excitation, revealing a low saturation intensity, ≈10 kW/cm2, and high fraction of the saturable losses. The ZnO NRs are applied as saturable absorbers in diode-pumped Yb (≈1.03 µm) and Tm (≈1.94 µm) lasers generating nanosecond pulses. The ZnO NRs grown on various optical surfaces are promising broadband saturable absorbers for nanosecond near-IR lasers in bulk and waveguide geometries

Heuristic modelling of laser written mid-infrared LiNbO_3 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.This work was supported by the Spanish MINECO under project MAT2013-47395-C4-4-R, TEC2014-55948-R and FIS2013-44174-P. Catalan Government 2014SGR1358. European Commission (ACP2-GA-2013-314335-JEDI ACE). F. D. acknowledges additional support 2010-ICREA-02 for excellence in research

Plasmonic enhancement of second harmonic generation from nonlinear RbTiOPO4 crystals by aggregates of silver nanostructures

We demonstrate a 60–fold enhancement of the second harmonic generation (SHG) response at the nanoscale in a hybrid metal-dielectric system. By using complex silver nanostructures photochemically deposited on the polar surface of a ferroelectric crystal, we tune the plasmonic resonances from the visible to the near-infrared (NIR) spectral region, matching either the SH or the fundamental frequency. In both cases the SHG signal at the metal-dielectric interface is enhanced, although with substantially different enhancement values: around 5 times when the plasmonic resonance is at the SH frequency or up to 60 times when it matches the fundamental NIR radiation. The results are consistent with the more spatially-extended near-field response of complex metallic nanostructures and can be well explained by taking into account the quadratic character of the SHG process. The work points out the potential of aggregates of silver nanostructures for enhancing optical nonlinearities at the nanoscale and provides an alternative approach for the development of nanometric nonlinear photonic devices in a scalable way.This work has been supported by the Spanish Ministry of Economy and Competitiveness (MINECO) under projects MAT2013-43301-R, FIS2013-41184-P, MAT2013-47395-C4-4-R and TEC2014-55948-R. The authors also acknowledge Comunidad de Madrid under grant S2013/MIT-2740, Dept. of Industry of the Basque Country under project nanoGUNE’14, and Catalan Authority under project 2014SGF1358. F.D. acknowledges additional support through the ICREA Academia awards 2010ICREA-02 for excellence in research. LSG acknowledges FPU13/02476 grant from the Spanish Ministry of Education.Peer reviewe