Laser damage and ablation of differently prepared CaF2(111) surfaces

Ablation thresholds and damage behavior of cleaved and polished CaF₂(111) surfaces produced by single shot irradiation with 248 nm/14 ns laser pulses have been investigated using the photoacoustic mirage technique and scanning electron microscopy. The standard polishing yields an ablation threshold of typically 20 J/cm². When surfaces are polished chemo-mechanically the threshold is raised to 43 J/cm². Polishing by diamond turning leads to intermediate values around 30 J/cm². Cleaved surfaces possess no well-defined damage threshold. The damage topography of conventionally polished surfaces shows ablation of flakes across the laser heated area with cracks along the cleavage planes. In the case of chemo-mechanical polishing only a few cracks appear. Diamond turned surfaces show small optical absorption, but cracks and ablation of tiles. The origin of such different damage behavior is discussed. Presented at the VIII-th Symposium on Surface Physics, Třešt’ Castle, Czech Republic, June 28 – July 2, 1999. This work was funded by the Deutsche Forschungsgemeinschaft, Sonderforschungsbereich 337. One of us (J.S.) acknowledges support from Deutscher Akademischer Austauschdienst and Gottfried Daimler- und Carl Benz-Stiftung.     

The role of defects in laser surface damage thresholds of fluoride crystals

The energy of the acoustic pulse generated by laser-surface interactions and measured by probe beam deflection was used to investigate laser surface damage thresholds of fluoride crystals with optical quality. It was found that damage thresholds decrease with increasing density of surface states. The defect density also controls the energy absorption mechanism: for surfaces with few defects, like polished MgF₂ and CaF₂, avalanche breakdown occurs at above 1 GW/cm₂, whereas for materials with lower damage thresholds, such as LiF, BaF₂, and roughened or incubated surfaces of CaF₂, multiphoton absorption across the band gap is observed.     

Dual-beam ablation of fused silica by multiwavelength excitation process using KrF excimer and F2 lasers

A new technique of dual-beam laser ablation of fused silica by multiwavelength excitation process using a 248-nm KrF excimer laser (ablation beam) coupled with a 157-nm F₂ laser (excitation beam) in dry nitrogen atmosphere is reported. The dual-beam laser ablation greatly reduced debris deposition and, thus, significantly improved the ablation quality compared with single-beam ablation of the KrF laser. High-quality ablation can be achieved at the delay times of KrF excimer laser irradiation shorter than 10 ns due to a large excited-state absorption. The ablation rate can reach up to 80 nm/pulse at the fluence of 4.0 J/cm² for the 248-nm laser and 60 mJ/cm² for the F₂ laser. The ablation threshold and effective absorption coefficient of KrF excimer laser are estimated to be 1.4 J/cm² and 1.2᎒⁵ cm^-1, respectively.     

Nonlinear processes in UV optical materials at 248 nm

Nonlinear processes in UV optical materials were investigated by using 280-fs, 248-nm pulses. Nonlinear absorption in CaF₂ was confirmed to be a two-photon process by using the luminescence of self-trapped excitons, which was also used for the single shot pulse width measurement. The absorption bands due to F centers were identified in CaF₂, MgF₂, and LiF after several hundred shots at 100 GW/cm². Absorption at 248 nm was considerable especially in MgF₂ and LiF. Self-focusing and self-phase modulation were observed in CaF₂.     

UV laser ablation of alumina ring faces for mechanical seal applications

Al₂O₃ seal ring faces were treated by KrF excimer laser irradiation. Surface characteristics induced by laser irradiation depend upon laser fluence, the number of laser pulses, the frequency and duration of the laser pulses, the rotation rate of the ring, and the processing atmosphere. Microstructural analyses of the surface and cross section of the laser-processed seal faces showed that, at low fluence (1.8 J/cm²), the surface is covered by scale due to the melting/resolidification processes. At high fluence (7.5 J/cm²), there is no continuous scaling on the surfaces. Material is removed by decomposition/vaporisation and the ablation depth is linearly dependent on the number of pulses; on the surface, a network of microcracks forms. The evolution of surface morphology and roughness is discussed with reference to composition, the microstructure and physical and optical properties of Al₂O₃, and laser processing parameters.     

晶面结构非均匀性对金刚石晶形的影响

 不仅考虑表面能，同时也引入表面结构非均匀性自由焓G_stⁱ，通过热力学分析，解释了随着晶体的逐渐长大而{110}面常在金刚石的外形中消失的机理，并用逆推法计算了有关数值。     

Ultrashort-laser-pulse damage threshold of transparent materials and the role of incubation

We present investigations of the surface damage threshold for transparent materials, e.g. a-SiO₂, CaF₂ and LiF, after single- and multiple-laser-pulse irradiation at 800 nm in the picosecond and sub-picosecond duration range. Our study shows clearly that the surface damage threshold drops dramatically during multiple-laser-shot irradiation, due to material-dependent incubation effects. This has important consequences for applications such as laser machining and for the lifetime of optical components. Different processes that can reduce the surface damage threshold with increasing laser shots are evaluated, such as sub-surface damage and defect formation. The mechanism of laser-induced defect formation, e.g. color centers, is believed to be mainly responsible for the observed reduction in the threshold for surface damage with increasing laser-shot numbers.     

Influence of Vacuum Organic Contaminations on Laser-Induced Damage of 1064 nm Anti-Reflective Coatings

We investigate the influence of vacuum organic contaminations on laser-induced damage threshold （LIDT） of optical coatings. Anti-reflective （AIR） coatings at 1064 nm made by Ta2 O5/SiO2 are deposited by the ion beam sputtering method. The LIDTs of AR coatings are measured in vacuum and in atmosphere, respectively. It is exhibited that contaminations in vacuum are easily to be absorbed onto optical surface because of lower pressure, and they become origins of damage, resulting in the decrease of LIDT from 24.5J/cm^2 in air to 15.TJ/cm^2 in vacuum. The LIDT of coatings in vacuum has is slightly changed compared with the value in atmosphere after the organic contaminations are wiped off. These results indicate that organic contaminations are the main reason of the LIDT decrease in vacuum. Additionally, damage morphologies have distinct changes from vacuum to atmosphere because of the differences between the residual stress and thermal decomposability of filmy materials.     

Excimer-laser ablation and micro-patterning of ceramic Si3N4

3 N₄ has been investigated. The ablation threshold in air, Φ_th, is around 0.3±0.1 J/cm² with ArF- and 0.9±0.2 J/cm² with KrF-laser radiation. With fluences Φ_th<Φ<4 J/cm² the irradiated surface is either very flat or it exhibits a cone-type structure, depending on the number of laser pulses employed. With fluences of 5 to 10 J/cm², the sample surface becomes very smooth, much smoother than the original mechanically polished surface. Pores, scratches, and cracks observed on the non-irradiated surface are absent within the illuminated area. In this regime, the ablation rates are typically 0.1 to 0.2 μm/pulse. Received: 10 April 1997/Accepted: 11 April 1997     

Ablation of transparent solids during self-limited deposition of diamond-like films from liquid hydrocarbons

A novel effect is studied of self-limitation of the diamond-like film thickness during laser irradiation of the interface of transparent substrates with liquid aromatic hydrocarbons. The interface is exposed through the transparent substrate to radiation of a copper vapor laser (wavelength of 510.6 nm, pulse duration of 20 ns). The thickness of diamond-like film increases linearly to 80-100 nm with the number of laser pulses and then saturates, while the substrate is ablated with nearly constant rate. This ablation rate depends on the thermal expansion coefficient of the substrate (glass, fused silica, sapphire, or CaF₂). The absorption of extinction coefficient of deposited films measured by ellipsometry is of order of 10⁴ cm^-1 and is sufficient to cause the significant heating of the interface. The ablation of the transparent substrates is due to their unequal thermal expansion compared to the diamond-like film having different thermal expansion coefficient. The measured ablation rates scale from 0.2 Å/pulse for glass to 4.5 Å/pulse for CaF₂. A 7m spatial resolution of the ablation process has been demonstrated for fused silica.     

Laser-induced positive ion and neutral atom/molecule emission from single-crystal CaHPO4·2 H2O: The role of electron-beam-induced defects

We examine laser-induced ion and neutral emissions from single-crystal CaHPO₄·2 H₂O (brushite), a wide-band-gap, hydrated inorganic single crystal, with 248-nm excimer laser radiation. Both laser-induced ion and neutral emissions are several orders of magnitude higher following exposure to 2 keV electrons at current densities of 200 7A/cm² and doses of 1 C/cm². In addition to intense Ca⁺ signals, electron-irradiated surfaces yield substantial CaO⁺, PO⁺, and P⁺ signals. As-grown and as-cleaved brushite show only weak neutral O₂ and Ca emissions, whereas electron-irradiated surfaces yield enhanced O₂, Ca, PO, PO₂, and P emissions. Electron irradiation (i) significantly heats the sample, leading to thermal dehydration (CaHPO₄ formation) and pyrolysis (Ca₂P₂O₇ formation) and (ii) chemically reduces the surface via electron stimulated desorption. The thermal effects are accompanied by morphological changes, including recrystallization. Although complex, these changes lead to high defect densities, which are responsible for the dramatic enhancements in the observed laser desorption.     

Enhanced Efficiency of Polymer： Fullerene Bulk Heterojunction Solar Cells with the Insertion of Thin TiO2 Layer near the LiF/A1 Electrode

The insertion layer of TiO2 between polymer-fullerene blend and LiF/AI electrode is used to enhance the shortcircuit current Isc and fill factor （FF）. The solar cell based on the blend of poly[2-methoxy-5-（2＇-ethylhexyloxy）- 1,4-phenylenevinylene] （MEH-PPV） and C60 with the modifying layer of TiO2 （about 20nm） shows the open- circuit Voc of about 0.62 V, short circuit current Isc of about 2.35 mA/cm^2, filling factor FF of about 0.284, and the power conversion efficiency （PCE） of about 2.4% under monochromatic light （50Onto） photoexcitation of about 17mW/cm^2. Compared to ceils without the TiO2 layer, the power conversion efficiency increases by about 17.5%. Similar effect is also obtained in cells with the undoped MEH-PPV structure of ITO/PEDOT：PASS/MEH- PPV/（TiO2）LiF/AI. The improved solar cell performance can be attributed to enhanced carrier extraction efficiency at the active layer/electrode interfaces when TiO2 is inserted.     

Interface kinetics during pulsed laser ablation

This work investigates evaporation kinetics -- the relation between the surface temperature and pressure during excimer laser ablation. Nickel targets are ablated by excimer laser pulses in a laser fluence range between 1 and 6 J/cm², with the upper limit exceeding the threshold of phase explosion (5 J/cm²). The surface pressure is determined with a polyvinylidene fluoride (PVDF) piezoelectric transducer. When phase explosion occurs, the surface temperature is known to be near the thermodynamic critical temperature, therefore, by measuring the surface pressure, the surface temperature-pressure relation is determined at the threshold fluence of phase explosion. The surface temperature and the threshold fluence of phase explosion are also estimated from the measured velocity of the vapor plume and gas dynamics calculations. It is shown that, during excimer laser ablation, the temperature and pressure relation deviates significantly from the equilibrium kinetic relation.     

Distinct orientation of AlN thin films deposited on sapphire substrates by laser ablation

We report the experimental characterization of the charged species produced in excimer laser ablation of a superconducting intermetallic compound (YNi₂B₂C). By using energy-selective time-of-flight mass spectrometry, we have obtained direct measurements of both mass spectra and kinetic energy distributions of ions. The investigation has been carried out in the laser fluence range 1-5 J cm^-2, which is typical of laser ablation thin film deposition. High kinetic energies of the charged component (up to 0.4 keV) have been observed even at moderate laser fluences.     

Cubic boron nitride deposition on silicon substrates at room temperature by KrF excimer laser ablation of h-BN

A hexagonal-BN target was ablated by high-fluence (6 and 12 J/cm²) KrF excimer laser pulses in low-pressure (5 Pa) N₂ atmosphere, without any aid of ion bombardment and heating of the Si substrate. Investigations of the deposited films show that the cubic-BN phase was deposited. The film deposited at 6 J/cm² appears to contain a higher cubic phase content with respect to the one deposited at 12 J/cm².     

The production of sub-micron sodium nitrate particles by laser ablation

Laser ablation is a useful source of particles for chemical analysis by inductively coupled mass spectroscopy in many applications. Optimum particle transport and ionization requires particles with sizes in the range 0.1-2 7m. Significant questions remain as to the mechanisms behind particle production by laser ablation. In this work, we collect and observe particles produced from single-crystal sodium nitrate during irradiation at 1.06 7m as a function of fluence and explore the possibility that laser-induced fracture may produce suitable particles. At fluences between 3 and 4 J/cm², single laser pulses incident on cleaved samples produce large numbers of fracture particles as a result of the fracture of undercut cleavage steps. Polished samples, lacking cleavage steps, yield few, if any, particles. As the fluence is raised to 5 J/cm², a small breakdown plume is observed and large melted droplets (~10 7m in diameter) are produced from both cleaved and polished targets, presumably by spallation of a thick melted layer. Particle generation by fracture has the potential to produce particles whose composition closely matches the local sample composition.     

Laser irradiation of SrTiO3 single crystals

Excimer laser irradiation (248 nm, 34 ns) of SrTiO₃(100)single crystals was studied in order to resolve the feasibility of obtaining well-defined patterned structures. Spots irradiated with a single pulse in the 0.4-1 J/cm² fluence range presented a pattern of cracks along crystallographic directions, which do not propagate beyond the borders, and no ejected material was observed on the non- irradiated areas. Arrays were patterned by translating the single crystal, and different morphologies were found depending on the width of the tracks. Tracks 10 7m wide or more developed a pattern of cracks, whereas 3 7m wide tracks did not. Artificial arrays in magnetoresistive La_2/3Sr_1/3MnO₃ thin films were prepared by using SrTiO₃(100) single-crystal substrates in which arrays had been previously patterned. The interfaces originated a substantial low-field tunnel magnetoresistance.     

Time-resolved shock-wave photography above 193-nm excimer laser-ablated graphite surface

Highly oriented pyrolytic graphite (HOPG) was ablated by a 193-nm ArF excimer laser in air. The fluence was varied in the range 1-25 J/cm². Every laser shot hit a pristine graphite surface. The emerging shock wave was recorded by a nanosecond-resolution photographic arrangement. The velocity of the shock wave as a function of time and laser fluence was measured. The amount of energy that generates the shock wave was determined and found to be about 5-7% of the incident laser energy. The shock wave is already present 10-15 ns after the maximum of the incident laser pulse. These facts imply that, even if high-energy (10-100 eV) ions, atoms, or clusters leave the surface, a layer several 10 nm thick has to be removed during this short period. The temperature of the shock front is ~2500-4000 K, as derived from the measured velocities. Measuring the ablation depth by atomic force microscopy as a function of fluence revealed that the single-shot ablation threshold is 1.4ǂ.2 J/cm², and the effective absorption coefficient is ~1.5᎒⁵ cm^-1.     

0.1~800 MPa压力下方解石拉曼光谱的实验研究

 利用石英的拉曼谱峰与温度和压力的关系，检验了在金刚石压腔中用方解石拉曼谱峰确定体系压力的可行性，并初步确定了在常温下方解石的拉曼谱峰与压力的关系。实验研究结果表明：在实验的压力范围内方解石稳定，且其1 085 cm^-1 谱峰约为石英464 cm^-1谱峰的3倍强度，因此非常适合作为热液金刚石压腔的压力标定物。在温度26 ℃、压力0.1~800 MPa条件下，方解石的拉曼谱峰（1 085 cm^-1）随着压力的增加，呈线形增大，其关系式为：p(MPa)=192×(ν_p－1 085)－21.8，1 085 cm^-1<ν_p<1 090 cm^-1。     

Shadowgraphic and emission imaging spectroscopic studies of the laser ablation of graphite in an Ar gas atmosphere

Laser ablation of graphite in an Ar atmosphere at 560 Torr was done using a nanosecond-pulse Nd:YAG laser (1064 nm) at a fluence of 12 J/cm². Dynamics in the ejection of carbon species and in their confinement near the graphite surface (<1 mm) due to their numerous collisions with Ar atoms were investigated by shadowgraphy, emission imaging, and emission spectroscopy at delay times of 0.01-100 7s following the laser irradiation. A shock wave was generated, and temporally and spatially dependent emissions from Ar⁺ and Ar were observed in addition to those from carbon species (C, C⁺, and C₂) and the Bremsstrahlung radiation from a hot plasma. We suggest that the dissipation of the kinetic and thermal energies of the carbon species, their backward motion, and their collisions with each other lead to the formation of clusters and particles through the interaction with Ar atoms.