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The fabrication of diffraction gratings by ultrashort direct laser writing in poly-hydroxyethyl-methacrylate (PHEMA) polymers used as soft contact lenses is reported. Diffraction gratings were inscribed by focusing laser radiation 100 µm underneath the surface of the samples. Low- and high-repetition rate Ti:sapphire lasers with 120 fs pulsewidth working at 1 kHz and 80 MHz respectively were used to assess the role of thermal accumulation on microstructural and optical characteristics. Periodic patterns were produced for different values of repetition rate, pulse energy, laser wavelength, distance between tracks, and scanning speed. Compositional and structural modifications of the processed areas were studied by micro-Raman spectroscopy showing that under certain parameters, thermal accumulation may result in local densification. Far-field diffraction patterns were recorded for the produced gratings to assess the refractive index change induced in the processed areas ; This research was funded by the PIT2 program of the University of Murcia's own research plan. Fundación Seneca grant No 20647/JLI/18, Junta de Castilla y León (project SA287P18), MINECO (project FIS2017-87970-R) and European Union's Horizon 2020 research and innovation programme under the Marie Skłodowska-Curie IF No 795630 are also acknowledged.

We report novel results on the fabrication of near-infrared waveguides inside lithium niobate (LiNbO3) crystals with different three-dimensional beam-splitting architectures, comparing the effects that each type of architecture has on the propagation losses and mode evolutions. Optimized waveguides are then studied in detail to obtain the refractive index profiles within the femtosecond-laser-written claddings with sub-micron resolution. This knowledge is currently impossible to obtain with experimental techniques and allows for the proper understanding of the laser-writing process, as well as to design novel waveguides and photonic circuits with optimized properties. ; Junta de Castilla y León (SA046U16); MINECO (FIS2013-44174-P, FIS2015-71933-REDT, FIS2017-87970-R); National Natural Science Foundation (grant No. 61775120); European Union's Horizon 2020 research and innovation program under the Marie Skłodowska-Curie Individual Fellowship (grant No. 747055).

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 design ; The 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.

The evolution of the isolated pyrrole molecule has been followed after excitation in the 265–217 nm range by using femtosecond time delayed ionization. The transients collected in the whole excitation range show the vanishing of the ionization signal in the femtosecond time scale, caused by the relaxation along a πσ* type state (3s a1←π 1a2), which is the lowest excited electronic state of the molecule. This surface is dissociative along the NH bond, yielding a 15 ± 3 fs lifetime that reflects the loss of the ionization cross-section induced by the ultrafast wavepacket motion. Although a weak πσ* absorption is detected, the state is mainly reached through internal conversion of the higher bright ππ* transitions, which occurs with a 19 ± 3 fs lifetime. In addition to its resonant excitation, the intense ππ* absorption extending in the 220–190 nm interval is also out-of-resonance populated at energies far to the red from its absorption onset. This coherent adiabatic excitation of the ππ* transition should follow the excitation pulse (coherent population return effect), but instead the system relaxes toward the lower πσ* surface through a conical intersection during the interaction time, leading to the population of πσ* state at wavelengths as long as 265 nm. According to the observed behavior, the time evolution of the system in the full excitation range studied is modeled by a coherent treatment that provides key insights on the photophysical properties of the molecule. ; This study was funded by Spanish MICINN (Grant No. CTQ2010-17749) and Consolider Program "Science and Applications of Ultrafast Ultraintense Lasers" (Grant No. CSD2007-00013), and by the Basque Government through the "Ayudas para apoyar las actividades de grupos de investigación del sistema universitario vasco" program. The experiments and theoretical calculations were carried out at the SGIker laser facility, and IZO-SGI of the UPV/EHU, respectively.

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).

We report on the fabrication, µ-Raman characterization, and continuous-wave laser operation of a channel waveguide with a hexagonal optical-lattice-like cladding fabricated in monoclinic Tm:KLu(WO4)2 crystal by femtosecond direct laser writing. µ-Raman spectroscopy indicates preservation of the crystalline quality in the core region and an anisotropic residual stress field. When pumped by a Ti:Sapphire laser at 802 nm, the Tm:KLu(WO4)2 buried channel waveguide laser generated 136 mW at 1843.7 nm with a slope efficiency of 34.2% and a threshold as low as 21 mW, which are the record characteristics for femtosecond-laser-written Tm crystalline waveguide lasers. The variation of the output coupling resulted in discrete wavelength tuning of the laser emission from 1785 to 1862 nm. The propagation losses in the waveguide are ~1.2 ± 0.3 dB/cm. ; E. K. acknowledges financial support from the Generalitat de Catalunya under grants 2016FI_B00844 and 2017FI_B100158. F.D. acknowledges additional support through the ICREA academia award 2010ICREA-02 for excellence in research. X. M. acknowledges support from the European Union's Horizon 2020 research and innovation programme under the Marie Skłodowska-Curie grant agreement No 657630. A. R. acknowledges funding from the European Union's Horizon 2020 research and innovation programme under the Marie Skłodowska-Curie Individual Fellowship Grant Agreement No. 747055. P. L. acknowledges financial support from the Government of the Russian Federation (Grant 074-U01) through ITMO Post-Doctoral Fellowship scheme.

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. ; Spanish MINECO and FEDER under project MAT2016-75716-C2-1-R, TEC2014-55948-R and FIS2013-44174-P; Catalan Government 2014SGR1358 and Junta de Castilla y León (Projects UIC016, SA046U16); European Commission (ACP2-GA-2013-314335-JEDI ACE); F. D. acknowledges additional support 2010-ICREA-02 for excellence in research; G. Martin acknowledges support from ASHRA for mid-IR astrophotonic devices development in collaboration with international groups.

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.

In this work the fabrication of buried optical waveguides by femtosecond laser inscription in the 0.8CaSiO3–0.2Ca3(PO4)2 eutectic glass doped with Nd3+ ions is reported. The glass samples were prepared by melting the eutectic powder mixture in a Pt–10 wt.% Rh crucible at 1600 °C and pouring it in a preheated brass mould. Afterwards, the glass was annealed to release the inner stresses. Buried waveguides were fabricated by focusing beneath the surface a pulsed Ti:sapphire laser with a pulsewidth of 120 fs working at 1 kHz. Two adjacent parallel tracks were written to define a region where an increase in the refractive index occurs. The effects produced by the variation of the laser pulse energy as well as the lateral separation between tracks, scanning speed and focusing distance were studied. After the laser processing, the near-field intensity distribution at 633 nm of the waveguide's modes was studied demonstrating the confinement of both, the TE as the TM polarizations. In order to diminish the losses induced by colour centres absorption, heat treatments were carried out in the samples. The waveguide's modes were compared with respect to the samples without heat treatments. The spectroscopic properties of the neodymium ions have been characterized to evaluate in what extent their optical properties could be modified by the waveguide fabrication process and to elucidate the potential application of such waveguides as integrated laser sources. ; This work was supported by the Spanish Government MEC under Projects No. MAT2009-14282-C02-02, MAT2010-17753, FIS2011-27968, FIS2009-09522, Consolider SAUUL CSD2007-00013, Basque Country Government (IT-331-07) and TEC2010-21574-C02-01. Daniel Sola thanks the JAE-DOC program and the Science and Technology Inter-Ministry commission of Spain and FEDER funds of the EC under project MAT2009-13979-C03-03 for the financial support of his contract.

We report on the first watt-level ultrafast laser inscribed Thulium waveguide (WG) lasers. Depressed-index buried channel WGs with a circular cladding (type III) are produced in monoclinic Tm3+:KLu(WO4)2 crystals. Laser operation is achieved under conventional (3H6 → 3H4) and in-band (3H6 → 3F4) pumping. In the former case, employing a Raman fiber laser emitting at 1679 nm as pump, the continuous-wave Tm channel WG laser generated 1.37 W at 1915–1923 nm with a record-high slope efficiency of 82.7% (with respect to the absorbed pump power), a threshold of only 17 mW and a spatially single-mode output with linear polarization. The WG propagation losses were 0.2 ± 0.3 dB/cm. Passive Q-switching of Tm channel WG lasers is achieved using Cr2+:ZnS and Cr2+:ZnSe saturable absorbers. With Cr2+:ZnS, record-short pulses of 2.6 ns/6.9 μJ at a repetition rate of 8.0 kHz were generated. The developed WGs are promising for compact GHz mode-locked lasers at ~2 μm. ; Spanish Government, MINECO (MAT2016-75716-C2-1-R (AEI/FEDER,UE), FIS2017-87970-R); Junta de Castilla y León (SA287P18); Generalitat de Catalunya (2017SGR755).

We report on the direct laser fabrication of step-index waveguides in fused silica substrates for operation in the 3.5 μm mid-infrared wavelength range. We demonstrate core-cladding index contrasts of 0.7% at 3.39 μm and propagation losses of 1.3 (6.5) dB/cm at 3.39 (3.68) μm, close to the intrinsic losses of the glass. We also report on the existence of three different laser modified SiO2 glass volumes, their different micro-Raman spectra, and their different temperature-dependent populations of color centers, tentatively clarifying the SiO2 lattice changes that are related to the large index changes. ; Spanish Government (MAT2013-47395-C4-4-R, TEC2014-55948-R, TEC2014-52642-C2-1-R, FIS2013-44174-P); Catalan Government (2009SGR235, 2014FI_B 00274); European Commission (EC) (ACP2-GA-2013-314335-JEDI ACE).

In this work we present the formation of laser induced periodic surface structures (LIPSS) on spin-coated thin films of several model aromatic polymers including poly(ethylene terephthalate), poly(trimethylene terephthalate) and poly carbonate bis-phenol A upon irradiation with femtosecond pulses of 795 and 265 nm at fluences well below the ablation threshold. LIPSS are formed with period lengths similar to the laser wavelength and parallel to the direction of the laser polarization vector. Formation of LIPSS upon IR irradiation at 795 nm, a wavelength at which the polymers absorb weakly, contrasts with the absence of LIPSS in this spectral range upon irradiation with nanosecond pulses. Real and reciprocal space characterization of LIPSS obtained by Atomic Force Microscopy (AFM) and Grazing Incidence Small Angle X-ray Scattering (GISAXS), respectively, yields well correlated morphological information. Comparison of experimental and simulated GISAXS patterns suggests that LIPSS can be suitably described considering a quasi-one-dimensional paracrystalline lattice and that irradiation parameters have an influence on the order of such a lattice. Fluorescence measurements, after laser irradiation, provide indirect information about dynamics and structure of the polymer at the molecular level. Our results indicate that the LIPSS are formed by interference of the incident and surface scattered waves. As a result of this process, heating of the polymer surface above its glass transition temperature takes place enabling LIPSS formation. © 2013 the Owner Societies. ; Funding from MICINN, Spain (Projects CTQ2010-15680,FIS2009-09522, MAT2012-33517 and CSD2007-00013) and Junta de Castilla y León (Project SA086A12-2). The experiments performed at BW4 in HASYLAB (DESY,Germany) were supported by the European Union (Contract RII3-CT-2004-506008 (IA-SFS).MINECO (previous MICINN), Spain, Ramón y Cajal contract (RYC-2011-08069), FPI fellowship and PTA contract ; Peer Reviewed

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. ; National Research Foundation of Korea (2017R1A4A1015426, 2018H1A2A1061480); Spanish Government (FIS2017-87970-R, MAT2016-75716-C2-1-R (AEI/FEDER, UE)); Junta de Castilla y León (SA287P18); Generalitat de Catalunya (2017SGR755).

Granite has been widely used as a structural and ornamental element in public works and buildings. In damp climates it is almost permanently humid and its exterior surfaces are consequently biologically colonized and blackened We describe a comparative analysis of the performance of two different laser sources in removing biological crusts from granite surfaces: nanosecond Nd:YVO4 laser (355 nm) and femtosecond Ti:Sapphire laser at its fundamental wavelength (790 nm) and second harmonic (395 nm). The granite surface was analyzed using scanning electron microscopy, attenuated total reflection – Fourier transform infrared spectroscopy and profilometry, in order to assess the degree of cleaning and to characterize possible morphological and chemical changes caused by the laser sources. ; This work is supported by the CTM2010-19584, FIS2009-09522 and CSD2007-00013 research projects (Ministerio de Ciencia y Tecnología, Spanish Government) and by SA086A12-2 project (Junta de Castilla y León, Spain). FTIR, profilometry and SEM analyses were conducted at the University of Vigo's research centre (CACTI).

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 μm fs-laser-written waveguide lasers based on monoclinic double tungstates is promising for further lasers of this type doped with Tm3+and Ho3+ ions . ; Ministerio de Economía y Competitividad (MINECO) (FIS2013-44174-P, FIS2015-71933-REDT, MAT2013-47395-C4-4-R, MAT2016-75716-C2-1-R, TEC 2014-55948-R); Departament d'Innovació, Universitats i Empresa, Generalitat de Catalunya (DIUE) (2014SGR1358); Junta de Castilla y León (UIC016, SA046U16); Institució Catalana de Recerca i Estudis Avançats (ICREA) Academia (2010ICREA-02); H2020 Marie Skłodowska-Curie Actions (MSCA) (657630); Government of the Russian Federation (074-U01); National Research Foundation of Korea (NRF) Korea of MSIP (2016R1A2A1A05005381).