Reflectivity loss due to dust deposition over primary mirrors is a key issue for ground-based astronomical observatories. Suspended atmospheric dust falls over the optical surface affecting its performance and optical quality. Several in-situ and off-site techniques have been developed for eliminating dust particulates from optical surfaces, but all of them present important drawbacks like being time-consuming, expensive, contaminant or even involving dangerous operations in off-site approaches. Alternatively, a method based on laser-matter interaction and SENER’s experience in ground telescopes, has been tested and developed. Laser cleaning is a discipline that covers a large range of applications, from semiconductor physical decontamination to the removal of oxides from metallic surfaces. The wide catalogue of laser systems currently available, allows us to investigate and design the best laser source for astronomical mirror maintenance. This has allowed us to develop a system that performs a proper cleaning for astronomical mirrors, which is currently bridging the gap between the laboratory demonstrator and the telescope application. In this communication we present the development of a laser cleaning system for the removal of dust particulates and contaminants attached to telescope mirror surfaces. Several configurations have been tested in the laboratory, under different conditions, and cleaning was performed over aluminized Zerodur® flat samples, that were naturally contaminated in Atacama’s desert and Canary Islands, aiming at the definition, manufacturing, and testing of a demonstrator prototype. Results show a large improvement of reflectivity after cleaning and contribute to the design of a custom efficient laser cleaning system adapted to different observatories.
Pulsed Laser Deposition (PLD) is used to produce Er-doped lead-niobium germanate (PbO–Nb2O5–GeO2) and fluorotellurite (TeO2–ZnO–ZnF2) thin film glasses. Films having high refractive index, low absorption and large transmission are obtained in a narrow processing window that depends on the actual PLD configuration (O2 pressure ∼a few Pa, Laser energy density ∼2-3 J cm-2 for the results presented in this work). However, Er-doped thin film glasses synthetized at room temperature using these experimental parameters show poor photoluminescence (PL) performance due to non-radiative decay channels, such as a large OH- concentration. Thermal annealing allows improving PL intensity and lifetime (τPL), the latter becoming close to that of the parent Er-doped bulk glass. In addition, the use of alternate PLD from host glass and rare-earth targets allows the synthesis of nanostructured thin film glasses with a controlled rare-earth concentration and in-depth distribution, as it is illustrated for Er-doped PbO–Nb2O5–GeO2 film glasses. In this case, PL intensity at 1.53 μm increases with the spacing between Er-doped layers to reach a maximum for a separation between Er-doped layers ≥ 5 nm, while τPL is close to the bulk value independently of the spacing. Finally, the comparison of these results with those obtained for films grown by standard PLD from Er-doped glass targets suggests that nanostructuration allows reducing rare-earth clustering and concentration quenching effects.
In this work, we report the near infrared and upconversion emissions of Er3+-doped transparent fluorotellurite glassceramics obtained by heat treatment of the precursor Er-doped TeO2-ZnO-ZnF2 glass. Structural analysis shows that ErF3 nanocrystals nucleated in the glass-ceramic sample are homogeneously distributed in the glass matrix with a typical size of 45±10 nm. The comparison of the fluorescence properties of Er3+-doped precursor glass and glass-ceramic confirms the successful incorporation of the rare-earth into the nanocrystals. An enhancement of the red upconversion emission due to 4F9/2→4I15/2 transition together with weak emission bands due to transitions from 2H9/2, 4F3/2,5/2, and 4F7/2 levels to the ground state are observed under excitation at 801 nm in the glass-ceramic sample. The temporal evolution of the red emission together with the excitation upconversion spectrum suggest that energy transfer processes are responsible for the enhancement of the red emission.
Integrated optics demand waveguides on Si platforms with different funtionalities and thus thin film technologies become essential tools for its development. Incorporating rare-earth ions or metal nanoparticales in dielectric hosts are respectively of interest for producing active media for gain devices or non-linear optical media with a high potential for all-optical switching. In this work, pulsed laser deposition is used to produce these materials with "dopant" distributions controlled within the nanometre scale embedded in a deilectric host. Examples will be given in which the control of this distribution is essential for achieving optimised optical transmission, propagation losses or photoluminescence lifetime. These examples are additionally used to show that controlling the separation between "dopants" becomes a useful tool to provide new insights in the understanding of interaction mechanisms.
In this work, pulsed laser deposition has been used to produce Er3+ doped lead-niobium-germanate thin films from Er3+ doped 50Geo2-25PbO-25Nb2O5 transparent glasses with an Er2O3 content in the 0.5-3 weight % range. In all cases, by pumping the 4I11/2 level of Er3+, the as deposited films are optically active showing at room temperature the 4I13/2 →4I15/2 emission centered around 1530 nm. The luminescence intensity increases as the Er3+ content in the targets increases, whereas the opposite behavior is observed for the luminescence lifetime. The emission intensity and lifetimes of the 1.5 μm emission increase upon annealing.
Nanocomposite thin films formed by metal or semiconductor nanocrystal (NCs) embedded in a host exhibit interesting nonlinear optical properties relate to the small size of the NCs. These properties make these materials potential candidates for the development of all-optical switching devices. The challenge is to produce nanocomposite materials with controlled and suitable characteristics. The present work aims to show that nanocomposite materials produced by pulsed laser deposition (PLD) might have superior structural and non-linear optical properties than those obtained by other techniques. This result will be illustrated in systems formed by metallic NCs embedded in an Al2O3 host. Fundamental aspects related to the nucleation and growth mechanisms or the reactivity of the NCs with the host will be discussed. Finally, the excellent nonlinear properties of the PLD synthesized composites will be illustrated in the case of Cu:Al2O3 films, in which the dependence of the nonlinear third order optical susceptibility has been investigated as a function of the NCs size and (chi) (3) values as large as 10-7 esu have been achieved.
Thin films of 4-cyano-4'-pentylbiphenyl (5CB) and E7 liquid crystals have been fabricated by pulsed laser deposition. The suitability of different lasers (ArF, KrF, XeCl and CO2) has been investigated over a range of fluence using visible-UV and infrared absorption and optical polarizing microscopy to characterize the films. High performance liquid chromatography and matrix assisted laser desorption ionization mass spectroscopy were used to assess the extent of decomposition of the films. The high photon energy of ArF and KrF excimer lasers produce severe and partial decomposition of the deposited films respectively, whilst films deposited using the CO2 laser also present partial degradation, most likely related to thermal processes during the laser-target interaction. Films with near identical structure to that of the starting LC target and good textures were obtained by XeCl laser deposition up to fluences of 130 mJ/cm2.
Switching characteristics of ferroelectric thin films (103 to 104 angstrom) at the crossover field from subsonic (via domain wall motion) to supersonic (via random reversal of individual dipoles) switching is investigated. The switching behavior of TGS (singlecrystal) and PZT (ceramic) films is discussed.
A first attempt to growth thin films of Nb doped PZT with 96% of Zr by pulsed laser deposition (PLD) has been done on Pt-sputtered Si (100) single crystal substrates. The target [1.5% Nb doped Pb(Zr0.965Ti0.035)O3] has been ablated in oxygen atmosphere with a focused KrF excimer laser beam. By changing the laser repetition rate and the deposition time, the thickness of the films was selected in the 150-400 nm range. The lead deficiency of the film was reduced by using an oxygen pressure of 75 mTorr as well as by holding the substrates at temperatures not higher than 500 degree(s)C. The results of the characterization show that a first (150-180 nm) layer is the main responsible of the lead deficiency, the low resistivity and the lack of ferroelectric response of the films grown.
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