Picosecond laser ablation of thin copper films

The ablation process of thin copper films on fused silica by picosecond laser pulses is investigated. The ablation area is characterized using optical and scanning electron microscopy. The single-shot ablation threshold fluence for 40 ps laser pulses at 1053 nm has been determinated toF _thres = 172 mJ/cm². The ablation rate per pulse is measured as a function of intensity in the range of 5 × 10⁹ to 2 × 10¹¹ W/cm² and changes from 80 to 250 nm with increasing intensity. The experimental ablation rate per pulse is compared to heat-flow calculations based on the two-temperature model for ultrafast laser heating. Possible applications of picosecond laser radiation for microstructuring of different materials are discussed.     

Enhanced blue-green-red up-conversion and 1.3-μm emission of Pr/Yb-codoped oxyhalide tellurite glasses with PbCl2 doping

We report on the blue-green-red up-conversion spectroscopic properties of Pr³⁺/Yb³⁺-codoped oxyhalide tellurite glasses upon excitation of a conventional 980 nm laser diode (LD). Significant enhancement of the blue-green-red up-conversion emission intensity has been observed with increasing PbCl₂ doping. The up-conversion intensity has a quadratic dependence on incident pump laser power, indicating a two-photon process. The population of the Pr³⁺ upper ³P₀ emitting level was accomplished through a combination of a ground state absorption, energy transfer and excitated state absorption. 1.3-μm emission in the second telecom window originated from Pr³⁺:¹G₄→³H₅ transition has also been investigated upon excitation at 980 nm LD. The measured peak wavelength and full width at half-maximum of the fluorescent are 1335 nm and ∼100 nm, respectively. An enhanced 1.3μm emission with increasing PbCl₂ doping has also been observed. Codoping of Yb³⁺ significantly enhance both the blue-green-red up-conversion emission and 1.3-μm emission intensity by way of a nonradiative Yb³⁺:²F₅→Pr³⁺:¹G₄ energy transfer.     

铯原子调制转移光谱在激光稳频中的应用

利用铯原子D₂线的调制转移光谱的鉴频特性,实现了光栅外腔式主振荡器-功率放大型(MOPA)半导体激光器相对于铯原子6²S_1/2F=4→6²P_3/2 F’=5超精细跃迁的偏频锁定,在100 ms内激光频率的相对起伏小于±280 kHz.相对于一般的饱和吸收锁频方法而言,调制转移光谱在锁频中的应用不仅彻底消除了Doppler背景对锁频的影响,而且还避免了对激光器直接进行频率扰动而带来的附加频率噪音.     

Evaluation of stoichiometric rare-earth molybdate and tungstate compounds as laser materials

Photoluminescence spectra, photoluminescence decay curves and Raman scattering spectra have been investigated for stoichiometric rare-earth molybdate and tungstate compounds. NaNd(MoO₄)₂ and NaNd(WO₄)₂ show emissions due to the transition ⁴F_3/2→⁴I_9/2 in Nd³⁺. A possibility of laser oscillation in NaNd(MoO₄)₂ is pointed from comparisons of the emission intensity and the decay time constant with NaNd(WO₄)₂ where laser oscillations have been reported. In NaLa(MoO₄)₂ and NaLa(WO₄)₂, observed emissions which are not related to La³⁺ are probably due to the transitions in MoO₄^2- and WO₄^2- molecular ions, respectively, in scheelite crystal. Raman spectra of these compounds are similar, probably related to the same crystal structure. LiEr(MoO₄)₂ shows the emissions due to transitions ²H_11/2→⁴I_15/2, ⁴S_3/2→⁴I_15/2 and ⁴F_9/2→⁴I_15/2 in Er³⁺, respectively, which are believed to be observed for the first time.     

Development of a 2D dosimetry system based on the optically stimulated luminescence of Al2O3

A laser-scanning 2D dosimetry system based on the Optically Stimulated Luminescence (OSL) signal from Al₂O₃ films was built and demonstrated. The main challenge of using the OSL from Al₂O₃ for 2D dosimetry by laser scanning is the long lifetime (∼35 ms) of the main luminescence centers in this material (F-centers). In this work, we demonstrated the possibility of performing 2D dosimetry by laser scanning using a combination of the fast UV emission of F⁺-centers (lifetime <7 ns) and the slow F-center emission of Al₂O₃:C, and an algorithm to correct for the slow F-center luminescence lifetime. We also investigated the possibility of using Al₂O₃:C,Mg, to take advantage of its greater F⁺-center emission compared to Al₂O₃:C. Results from 6 MV photon beam irradiations from a clinical linear accelerator were compared to radiographic and radiochromic film profiles showing a good qualitative agreement.     

Synthesis and laser patterning of Bi-doped Y3Fe5O12 crystals in germanosilicate glasses

New germanosilicate glasses giving the crystallization of yttrium iron garnet Y₃Fe₅O₁₂ (YIG) and Bi-doped YIG, 23Na₂O-xBi₂O₃-(12−x)Y₂O₃-25Fe₂O₃-20SiO₂-20GeO₂ (mol%), are developed, and the laser-induced crystallization technique is applied to the glasses to pattern YIG and Bi-doped YIG crystals on the glass surface. It is clarified from the Mössbauer effect measurements that iron ions in the glasses are present mainly as Fe³⁺. It is suggested from the X-ray diffraction analyses and magnetization measurements that Si⁴⁺ ions are incorporated into YIG crystals formed in the crystallization of glasses. The irradiations (laser power: 32-60 mW and laser scanning speed: 7 μm/s) of continuous wave Yb:YVO₄ fiber laser (wavelength: 1080 nm) are found to induce YIG and Bi-doped YIG crystals, indicating that Fe³⁺ ions in the glasses act as suitable transition metal ions for the laser-induced crystallization. It is suggested that YIG and Bi-doped YIG crystals in the laser irradiated part might orient. The present study will be a first step for the patterning of magnetic crystals containing iron ions in glasses.     

Type I FA (Rb, Cs) and II FA (Li, Na) tunable laser activities and donor-acceptor properties of O and O at KCl (001) surface: first principle calculations

Type I F_A (Rb⁺, Cs⁺) and II F_A (Li⁺, Na⁺) tunable laser activities, adsorptivity and donor-acceptor properties of O and O⁻ adsorbates at the flat surface of KCl crystal were investigated using an embedded cluster model and ab initio methods of molecular electronic structure calculations. Ion clusters were embedded in a simulated Coulomb field that closely approximates the Madelung field of the host surface, and the nearest neighbor ions to the defect site were allowed to relax to equilibrium. Based on the calculated Stokes shifted optical transition bands, F_A tunable laser activities were found to be inversely proportional to the size of the dopant cation (Li⁺, Na⁺, Rb⁺, Cs⁺) relative to the host cation (K⁺). This relation was explained in terms of the axial perturbation of the impurity cation. The probability of orientational bleaching attributed to the RES saddle point ion configuration along the 〈110〉 axis was found to be inversely proportional to the size of the dopant cation, with activation energy barriers of ca. 0.44-3.34 eV. Surface relaxation energies of type II F_A centers were more important than those of type I F_A centers. In terms of defect formation energies, the products of type II F_A center imperfection were more stable than those of type I F_A. The difference between F or F_A band energies and exciton bands depended almost exclusively on the size of the positive ion species. As far as the adsorptivity of O and O⁻ is concerned, the results confirm that surface imperfection enhances the adsorption energies by ca. 4.38-16.37 eV. O and O⁻ penetrate through the defect-containing surface. The energy gap between the adsorbate and the defect containing surface and the donor-acceptor property of adsorbate play the dominant role in the course of adsorbate substrate interactions and the results were explained in terms of electrostatic potential curves and Mulliken population analysis.     

Scaling and performance of CO2 lasers at supra-atmospheric pressure

The performance of a compact uv photo-preionized TE laser is studied in the pressure range 1–5 bar. As the pressure is increased, the laser pulse shape is little altered, but both the peak power and the total output pulse energy increase significantly with pressure, even for constant input electrical energy. For various gas mixtures and excitation source capacitors the measurements suggest approximate output energy scaling with the product of the source charge per unit electrode area [C.m^–2] and the molecular partial pressure [CO₂+N₂+CO]. This is explained in terms of the pressure-dependent discharge impedance. An input-energy-related discharge instability limits the optimum laser pressure to 1.5–2.5 bar, and we show that, at constant input energy, the instability boundary depends on the molecular partial pressure alone. The pre-ionization photo-electron yield varies negligibly with pressure, but the discharge tolerance to added oxygen decreases asp ^–3 top ^–4, dependent on gas mixture. Nevertheless sealed operation for >10⁵ shots has been obtained with a 5% CO₂5% CO3% N₂2% H₂85% He gas mixture at a total pressure of 5 bar.     

Synthesis of chromium silicide with laser pulses

Thin chromium films, 60 nm thick, were deposited onto single-crystal silicon wafers. The samples were irradiated with 30 ns single pulses from a Nd: glass laser at fluences ranging from 0.4 to 2.25 J/cm². Rutherford backscattering spectrometry, transmission electron microscopy and electron diffraction measurements evidence the formation of CrSi₂ layers at the Cr/Si interface. The silicide thickness depends on the laser fluence.     

The photoluminescent properties of Eu in MgO-Ga2O3-SiO2 nanocrystalline glass-ceramic

The MgO-Ga₂O₃-SiO₂ glass-ceramic (GC) containing MgGa₂O₄ nanocrystals and glasses doped with Eu³⁺ ions were prepared by the sol-gel method. The down-conversion and up-conversion luminescence (UCL) properties were studied. The results indicated that the relative intensity of f-f transitions of Eu³⁺ decreased in contrast with that of charge transfer (CT) absorption with the increase in heating temperature. Using a Xe lamp and 800 nm femtosecond (fs) laser excitation, strong red luminescence of Eu³⁺ in MgO-Ga₂O₃-SiO₂ glasses and GC was observed.     

Spectroscopic study of a TEA CO2 laser of the CO2/X,X=H2; N2/H2 type

The spectral output of a TEA CO₂ laser utilizing nonconventional CO₂/H₂ and CO₂/N₂/H₂ gas mixtures was analysed. A simultaneous multiline output in P-branch of the 00°110°0 transition was recorded. A domination of the P(18) line is typical for the first, and that of the P(20) line for the second gas mixture. The enhancement of the P(18) line intensity in spectra (different from P(20) line domination-CO₂/N₂/H₂ mixture) is explained primarily by a smaller contribution of the R(23) line gain (01¹111¹0 CO₂ band), to P(20) line (00°110°0 band). All measurements were conducted with a nondispersive optical cavity.     

Negative ion effects in CO2 convection laser discharges

Negative ions are computed to be formed on a time scale and in quantities such that they may be a cause of plasma instability observed in low pressure electrical discharge convection CO₂ lasers. In a typical CO₂−N₂−He−H₂O laser mixture the principal ions are CO ₃ ⁻ , CO ₄ ⁻ and H⁻ with the total negative ion densityn ₋ given by 0.1n _e <n ₋<n _e , wheren _e is the electron density: but if the gases are re-cycled or if there is an air leak NO ₂ ⁻ and NO ₃ ⁻ are formed in significant amounts andn ₋ can become greater thann _e in a time considerably less than the gas dwell time in the electrical excitation discharge. CO is effective in reducingn ₋ in a system without re-cycling, but is ineffective in a re-cycled system with the oxides of nitrogen present.     

Structure, thermally and optically induced effects in amorphous As2Se3 films prepared by pulsed laser deposition

Thin amorphous As-Se films were prepared by pulsed laser deposition (PLD) and by classical thermal evaporation techniques. Raman spectra and optical properties (optical gap, E_g^opt, index of refraction, n, third-order non-linear susceptibility, χ⁽³⁾) of prepared films and their photo- and thermally induced changes were studied. The structure of laser deposited films was close to the corresponding bulk glasses contrary to thermal evaporated films. The composition of PLD films was practically unchanged during the process of deposition. The optically and thermally induced changes of n and of E_g^opt in PLD films are different from the changes in thermally deposited films. The differences are discussed.     

The suppression of structural phase transformation in LaVO3 and La1−xSrxVO3 thin films fabricated by pulsed laser deposition

Highly oriented (100) thin films of LaVO₃ and La_1−xSr_xVO₃ have been fabricated by pulsed laser deposition in a reducing atmosphere. The films show a transition from insulating to metallic behaviour in the composition region of x, 0.175<x<0.200. In the single crystals of the antiferromagnetic insulating phase, a first-order structural phase transition is observed few degrees below the magnetic transition, which manifests itself as a kink in the temperature dependence of resistivity. In the highly oriented thin films of LaVO₃ and La_1−xSr_xVO₃ fabricated on lattice matched substrates in this study, the structural phase transformation in the insulating phase has been suppressed. The electrical conduction is found to take place via hopping through localized states at low temperatures. The metallic compositions show a non-linear (T^1.5) behaviour in the temperature dependence of resistivity. V (2p) core level spectra of these films show a gradual change in the relative intensities of V³⁺ and V⁴⁺ ions as the value of x increases.     

Synthesis, characterization and optical limiting effect of nickel complexes of multi-sulfur 1,2-dithiolene

Three nickel complexes with a new multi-sulfur 1,2-dithiolene ligand, (n-Bu₄N)[Ni(cddt)₂] 1, (Ph₄P)[Ni(cddt)₂] 2 and [Ni(cddt)₂] 3 (cddt=4a, 6, 7, 7a-5H-cyclopenta[b]-1,4-dithiin-2,3-dithiolate), have been synthesized and characterized by electrochemical measurements, IR, EPR and UV-Vis-NIR spectroscopies. The crystal structure of complex 2 is determined. Their optical nonlinearities are measured by the Z-scan technique with an 8 ns pulsed laser at 532 nm and all exhibit NLO absorptive abilities. Complexes 1 and 2 both exhibit effective self-defocusing performance (n₂=−5.81×10⁻¹⁰ esu for 1 and −4.51×10⁻¹⁰ esu for 2). The optical limiting (OL) effects were observed with nanosecond and picosecond laser pulses. The OL capability of complex 3 is superior to C₆₀ at the same experimental condition in ns measurements.     

IR operated novel Ag0.98Cu0.02GaGe3Se8 single crystals

In this work, we report on the structural and optical properties of novel Ag_0.98Cu_0.02GaGe₃Se₈ single crystals that were synthesized by the Bridgman–Stockbarger technique. We have performed illumination by 10.6 μm CO₂ pulsed laser working in the microsecond time duration regime. Such illumination allows causing substantial changes for both pure electronic nonlinear optical effects like optical second harmonic generation as well as piezooptical effects described by the fourth rank tensors. The measurements of the piezo-optical effects were carried out at different temperatures. The effects are observed only during the IR CO₂ laser illumination and are disappeared after switching off the illumination. Simultaneously the IR induced optical second harmonic generation at Er:glass laser fundamental wavelength 1540 nm was performed during illumination by nanosecond Nd:YAG and Er³⁺:glass laser. The observed effects allow to use the studied materials as promising for IR-optoelectronic devices.     

An efficient lasing action from pyrromethene 556 dye-doped organically modified silicates

Organically modified solid-state silicates (ORMOSILS) doped with a new laser dye 1,3,5,7,8-pentamethylpyrromethene-2,6-disulfonate-BF₂ complex (pyrromethene 556) have been synthesized by a sol-gel method and the compositional effects on pore characteristics, fluorescence and lasing properties have been investigated. It is found that the use of dimethylsulfoxide and γ-glycidoxypropyltrimethoxysilane could greatly change the structure properties of sol-gel derived ORMOSILS cage, and thus the fluorescence and lasing properties of the materials could improve significantly. A successful laser oscillation from this dye-doped ORMOSILS sample has been achieved upon pumping with a Q-switched frequency-doubled Nd:YAG laser at 532 nm. A slope efficiency of 54% with a useful lifetime greater than 10,000 shots has been demonstrated at a pump repetition rate of 1 Hz and a pump intensity of 1 J/cm² by using the new ORMOSILS cage on our newly designed laser system. Our results have shown that it is possible to obtain a high-efficiency with a long-lifetime for a compact new laser device by low cost dye-doped solid-state ORMOSILS.     

Studies on the influence of lithium incorporation in the photoluminescence of Y2O3:Eu thin films

Nanostructured europium-doped yttrium oxide thin films with lithium as a co-dopant were prepared using pulsed laser ablation technique. X-ray diffraction studies of the films indicated amorphous nature of the as deposited films and a transformation to crystalline phase with increase of annealing temperature. In this transformation, lithium co-doped films showed early crystallization. Lithium substitution resulted not only in enhancement of photoluminescence at 612 nm, resulting from ⁵D₀-⁷F₂ transition within europium, but also found to reduce the required processing temperature for intense photoemission. The deviation observed in the value of lattice constant of films annealed at different temperatures is found to be sensitive to annealing temperature. In the light of this, the dependence of photoluminescence intensity on the magnitude of lattice imperfection is also discussed. The morphology and transmittance of the films are also found to be sensitive to annealing process and lithium doping.     

Laser-induced electron emission from an Au surface irradiated by single picosecond pulses at λ=2.94 μm. The intermediate region between multiphoton and tunneling effects

The photoinduced electron emission from the surface of a solid gold target irradiated by single picosecond pulses of an erbium laser is investigated. The applied laser intensity (5–120 GW/cm²) corresponds to the intermediate interaction region between the pure multiphoton and tunnel effects, where the decisive Keldysh-parameter, , is in the range 1<<12=n ₀. In the light intensity region which is free of surface heating (I _L<80 GW/cm²), the slope of the measured logarithmic intensity dependence of the photocurrent decreases from the n ₀=12 perturbative value down to n 5. Therefore the experiment shows that the Keldysh-type theories, which have recently been proved to describe correctly the ionization of atoms, are also valid to a certain extent in the case of the photoeffect in metals.     

XPS studies of short pulse laser interaction with copper

The effect of laser ablation on copper foil irradiated by a short 30 ns laser pulse was investigated by X-ray photoelectron spectroscopy. The laser fluence was varied from 8 to 16.5 J/cm² and the velocity of the laser beam from 10 to 100 mm/s. This range of laser fluence is characterized by a different intensity of laser ablation. The experiments were done in two kinds of ambient atmosphere: air and argon jet gas.The chemical state and composition of the irradiated copper surface were determined using the modified Auger parameter (α′) and O/Cu intensity ratio. The ablation atmosphere was found to influence the size and chemical state of the copper particles deposited from the vapor plume. During irradiation in air atmosphere the copper nanoparticles react with oxygen and water vapor from the air and are deposited in the form of a CuO and Cu(OH)₂ thin film. In argon atmosphere the processed copper surface is oxidized after exposure to air.