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.     

Comparative studies of absorptance behaviour of alkaline-earth fluorides at 193 nm and 157 nm

Absorptance losses in MgF₂, CaF₂ and BaF₂ during 193-nm (DUV) and 157-nm (VUV) irradiation are investigated by employing a high-resolution laser calorimetric technique which allows the determination of both single- and two-photon absorptance at energy densities up to 110 mJ/cm². A strong wavelength dependence of the DUV and VUV absorption characteristics is observed: while effective two-photon absorption takes place at 193 nm, either no similar effect at all (in the case of BaF₂) or only a very minor effect (CaF₂) is observed at 157 nm. A first explanation for this absorption behaviour is given, implying the energetic band structure of CaF₂. In addition it is shown that, due to the strong nonlinear dependency, above a critical energy density the absorptance at 193 nm can exceed the absorptance at 157 nm. Furthermore, different single- and two-photon absorption coefficients are determined for different CaF₂ samples at 193 nm, indicating a two-step absorption mechanism. In addition, laser-induced aging is found in a MgF₂ sample at 193 nm, but not at 157 nm. Received: 21 June 2001 / Revised version: 2 November 2001 / Published online: 7 February 2002     

Absorption coefficient of infrared laser window materials

The absorption coefficient of a number of potential i.r. laser window materials has been measured in the region of multiphonon absorption. Absorptions as low as 0·004 cm^?1 have been detected by using thick samples and a differential technique with a dual-beam spectrophotometer. The absorption coefficients of LiF, CaF₂, BaF₂, SrF₂, MgF₂, Al₂O₃, KCl, NaCl and KBr all decrease exponentially with increasing wavenumber. The slope of the exponential bears a nearly constant relationship to the longitudinal optical phonon frequency, consistent with a multiphonon process whose strength depends on the number of phonons generated. Absorption data are also presented for Si, Ge, GaAs: Cr, GaAs: Fe, ZnSe and CdTe.     

Nanosecond UV laser damage and ablation from fluoride crystals polished by different techniques

Ablation thresholds and damage behavior of cleaved and polished surfaces of CaF₂, BaF₂, LiF and MgF₂ subjected to single-shot irradiation with 248 nm/14 ns laser pulses have been investigated using the photoacoustic mirage technique and scanning electron microscopy. For CaF₂, standard polishing yields an ablation threshold of typically 20 J/cm². When the surface is polished chemo-mechanically, the threshold can be raised to 43 J/cm², while polishing by diamond turning leads to intermediate values around 30 J/cm². Cleaved surfaces possess no well-defined damage threshold. When comparing different fluoride surfaces prepared by diamond turning it is found that the damage resistivity roughly scales with the band gap. We find an ablation threshold of 40 J/cm² for diamond turned LiF while the MgF₂ surface can withstand a fluence of more than 60 J/cm² without damage. The damage topography of conventionally polished surfaces shows flaky ablation across the laser-heated area with cracks along the cleavage planes. No ablation is observed in the case of chemo- mechanical polishing; only a few cracks appear. Diamond turned surfaces show small optical absorption but mostly cracks and ablation of flakes and, in some cases, severe damage in the form of craters larger than the irradiated area. The origin of such different damage behavior is discussed.     

Color centers in neutron-irradiated Y3Al5O12, CAF2 and LiF single crystals

The thermal annealing behavior of the Y₃Al₅O₁₂, CaF₂ and LiF single crystals bombarded at Algiers with reactor neutrons has been monitored by optical absorption spectroscopy. The irradiation was performed at about 315 K. On heating samples after irradiation, the optical absorption bands decrease and disappear completely at 873 and 523 K in the case of Y₃Al₅O₁₂ and CaF₂, respectively. Activation energies of 1.2±0.02 and 0.9±0.2 eV are estimated for Y₃Al₅O₁₂ and CaF₂, respectively. On the other hand, the LiF crystal shows a complex annealing behavior. Here, the optical absorption spectrum presents different shapes after each annealing temperature. Four steps are distinguished and discussed on heating samples from 300 to 673 K. Above 673 K, the absorption drops by about 50%; it completely disappears at 773 K.     

Laser conditioning and nonlinear absorption of LaF3/MgF2 dielectric multilayers at 193 nm

A sensitive and simple pulsed surface thermal lens (TL) technique is used in situ to investigate the laser conditioning and to measure the nonlinear absorption of LaF₃/MgF₂ dielectric multilayers deposited on CaF₂ substrates at 193 nm. Due to the high single-shot sensitivity of the surface TL technique, the laser conditioning can be monitored from the first shot of irradiation on a shot-by-shot basis. The LaF₃/MgF₂ multilayers show a very strong conditioning effect. The ratio of the absorption before and after the laser irradiation is in the range 4–8 for a highly reflective (LH)²⁰ LaF₃/MgF₂ multilayer, and 3–4 for (1L3H)⁷ and (3L1H)⁷ multilayers. In comparison, a (LH)²⁰ LaF₃/AlF₃ multilayer shows only a weak conditioning effect, with an absorption ratio of approximately 1.4. Our experimental results suggest that the strong conditioning effect of the LaF₃/MgF₂ multilayer is due to the absorption conditioning of the LaF₃ layers. However, the MgF₂ layers are shown to be responsible for the considerable increase in LaF₃ absorption as well as the ability to condition the absorption. The fluoride multilayers present non-negligible nonlinear absorption and the two-photon absorption coefficient of the multilayers is estimated to be approximately 5×10^-7 cm/W. Received: 11 December 2000 / Accepted: 20 February 2001 / Published online: 25 July 2001     

Ion-induced kinetic electron emission from 6LiF,7LiF and MgF2 thin films

<正>Secondary electron yields for Ar~+ impact on ~6LiF,~7LiF and MgF_2 thin films grown on aluminum substrates are measured each as a function of target temperature and projectile energy.Remarkably different behaviours of the electron yields for LiF and MgF_2 films are observed in a temperature range from 25℃to 300℃.The electron yield of LiF is found to sharply increase with target temperature and to be saturated at about 175℃.But the target temperature has no effect on the electron yield of MgF_2.It is also found that for the ion energies greater than 4 keV,the electron yield of ~6LiF is consistently high as compared with that of ~7LiF that may be due to the enhanced contribution of recoiling ~6Li atoms to the secondary electron generation.A comparison between the electron yields of MgF_2 and LiF reveales that above a certain ion energy the electron yield of MgF_2 is considerably low as compared with that of LiF.We suggest that the short inelastic mean free path of electrons in MgF_2 can be one of the reasons for its low electron yield.     

Laser oscillation by the F3− and F2− color centers in LiF crystal at room temperature

A stable, free-running LiF:F₃ ⁻ and LiF:F₂ ⁻ color center laser oscillation is achieved at room temperature by pumping with the 930-nm laser radiation. The LiF:F₃ ⁻ laser radiation has a peak at 1100 nm and shifts from the peak wavelength of the F₃ ⁻ luminescence band because of the absorption of the F₃ ⁻ luminescence by the F₂ ⁻ center, which co-exists with the F₃ ⁻ center. Received: 2 March 1999 / Revised version: 16 March 1999 / Published online: 12 May 1999     

Struktur und ultrarote Gitterschwingung von CaF2 und LiF

Thin films of CaF₂, LiF and LiF +20% KF are condensed on to substrates at 20 °K. Immediately after condensation and after annealing to 373 °K the infrared absorption is measured between 15 and 43 microns. Also Debye-Scherrer-diagrams are taken for investigating the structure of the layers. After condensation at 20 °K LiF has a high degree of disorder, CaF₂ and LiF+20% KF show amorphous phases. The infrared lattice vibration of such films is shifted to shorter wavelenghts.     

Superparamagnetic behaviour of cement clinker and its ferrite phase doped with different impurities

Two oxide mixtures of clinker and its ferrite phase of compositions (66.5 wt.% CaO+24.5 wt.% SiO₂+6.0 wt.% Al₂O₃+3.0 wt.% Fe₂O₃) and (60.4 wt.% CaO+15.4 wt.% Al₂O₃+24.2 wt.% Fe₂O₃) respectively were divided into protions and were mixed individually with 0.5, 1, 1.5 or 3 wt.% of LiF, MgF₂, CaF₂, CaCl₂ or ZnO. Each portion of clinker was fired at 1450°C and each portion of ferrite was fired at 1350°C for 30 min. then quenched in air. Mössbauer effect and X-ray diffraction measurements were performed on each sample. The impurities doping produced small particle size. The LiF doping gave the smallest particle size and the highest blocking temperature. The ferrite with LiF exhibited two Fe³⁺ sites while the other used impurities gave one site only. The superparamagnetic relaxation appeared only in the spectra of ferrite with impurities, which means that the impurities in clinker have a tendency to combine with the calcium silicate phases not with C₄AF.     

Fundamental absorption of calcium,magnesium, and lithium fluorides in the range of 8–12 μm

The fundamental absorption of calcium, magnesium, and lithium fluorides in the range of 8–12 μm has been studied experimentally by the spectrophotometric method. The known and experimentally obtained values of the absorption coefficients of these materials have been analyzed. It has been shown that the values experimentally measured in the range of 8–12 μm obey the Uhrbach rule and the multiphoton absorption determines the long-wavelength transmission cutoff of pure optical materials. The results presented make it possible to suggest that plates made of LiF and CaF₂ crystals and of optical ceramics based on MgF₂ can be used to fabricate calibrated radiation attenuators in the spectral range of 8–12 μm.     

Amplified spontaneous emission of F2− color center in LiF crystal

Intense, directional, and narrow-bandwidth 1130-nm emission was generated from LiF crystal at room temperature by the pumping with radiation in the wavelength region of the absorption band of the F₂ ⁻ color center. The emission was observed by the pumping with radiation of pulse energy above 5 mJ, and the divergence angle of the beam was about 16 mrad. This emission is attributed to the amplified spontaneous emission by the F₂ ⁻ center. Received: 8 April 1999 / Revised version: 18 May 1999 / Published online: 16 September 1999     

Effect of gamma irradiation on undoped and uranium doped calcium fluoride crystals

Color centers in undoped and U³⁺-doped CaF₂ crystals induced by γ-irradiation with different doses were studied by differential absorption and Raman spectra. Multiple color centers and conversion among them were caused in undoped CaF₂ with the creation of radiation defects. In U³⁺:CaF₂ crystal, trivalent uranium was demonstrated to act as hole trap in the process of γ-irradiation, which was ionized to tetravalent. This process was accompanied by the formation of F₂⁺ centers, but without additional background absorption due to radiation defects.     

A transferable representation of the induced multipoles in ionic crystals

Electronic structure calculations of the induced dipole and quadrupole moments on a fluoride ion at low symmetry sites in a model crystalline environments are described. The results are used to characterize the short range contributions to the induced multipoles on the F^? ion that arise from overlap between the wavefunctions of the ions. These are represented by general functions of the positions of the surrounding ions, suitable for use in a computationally tractable simulation model. The present calculations are designed to test the transferability to other classes of materials of this representation which, to date, largely has been deduced from calculations on distorted alkali halide crystals. First, the induced multipoles in mixed crystals of the alkali halides are considered, in order to check that they are predicted reliably by combining the representations deduced for the corresponding pure materials. Second, polarization effects in the alkaline earth fluorides MgF₂, and CaF₂, in a locally distorted fluorite crystal structure are examined. It is found that the material-specific parameters in the representation are related through simple functions of the ionic radii. This relationship holds between MgF₂, and CaF₂, in the fluorite structure, but also extends to the alkali fluorides previously studied. In order to illustrate the significance of polarization effects, the polarization model for CaF₂ is combined with an ab initio pair potential for the repulsion and dispersion interactions and used in a molecular dynamics simulation. Inclusion of the polarization term improves greatly the calculated phonon frequencies in the crystal.     

Laser-induced backside wet etching of fluoride and sapphire using picosecond laser pulses

Laser-induced backside wet etching (LIBWE) is a promising process for microstructuring of rigid chemical resistant and inert transparent materials. LIBWE with nanosecond laser pulses has been successfully demonstrated in a number of studies. LIBWE in a time scale of femtosecond and picosecond pulse durations has been investigated only in a few studies and just on fused silica. In the present study LIBWE of fluorides (CaF₂, MgF₂) and sapphire with a mode-locked picosecond (t _p=10 ps) laser at a UV wavelength of λ=355 nm using toluene as absorbing liquid has been demonstrated. The influence of the laser fluence and the pulse number on the etching rate and the achieved surface morphology was investigated. The etching rate grows linearly with the laser fluence in the low and high-fluence ranges with different slopes. The achieved etching rates for CaF₂ and for sapphire were in the same range. Contrary to CaF₂ and sapphire the etching rates of MgF₂ were one magnitude less. For backside etching on sapphire at high fluences smooth surfaces and at low fluences ripples pattern were found, whereas fluoride surfaces showed a trend towards crack formation.     

镱、钠共掺的氟化钙晶体在1050nm的可饱和吸收作用

通过在稳定连续波运转的Yb:YAG 激光器中插入不同掺杂浓度的新型钠、镱共掺的氟化钙晶体的对比性实验,证明了镱、钠共掺的氟化钙晶体在1050nm具有明显的可饱和吸收作用,从而解释了该晶体作为增益介质在该波段总是趋于自调Q运转的原因.Yb³⁺离子是该晶体可饱和吸收作用的主要因素,但是共掺入适当的Na离子可以明显改善晶体的调Q效果.优化共掺镱、钠离子的浓度和比例后的氟化钙晶体能够作为1050nm波段激光器的被动Q开关. 关键词： 镱、钠共掺氟化钙 可饱和吸收体 调Q     

A study on thermal degradation of LiF:Mg,Cu,P and LiF:Mg,Cu,Si

We investigated the thermal degradation of LiF:Mg,Cu,P (NTL-250) and LiF:Mg,Cu,Si (MCS) for the development of TL sheet. By thermogravimetry and differential scanning calorimetry (TG-DSC), the exothermic reaction was observed between 320 °C and 400 °C in MCS as well as NTL-250. The heat value of MCS was twice as large as that of NTL-250. This ratio corresponded with that of Mg amount in these TL materials measured by ICP-OES (inductively-coupled plasma optical emission spectrometry). X-ray diffraction (XRD) measurements were also carried out, and the peaks of MgF₂ phase were also observed in degraded MCS sample as well as NTL-250. Moreover, X-ray absorption near-edge structures (XANES) of Cu in these LiF TLDs were measured. The valences of Cu did not change before and after degradation. It indicates that the thermal degradation is caused by not Cu but Mg ion state change. The exothermic reaction is possible caused by the stabilization reactions, and then it was expected to correspond with MgF₂ precipitation. From these results, we concluded that the thermal degradations of these LiF TLDs are caused by the precipitation of MgF₂.     

Continuous-wave broadly tunable Cr:ZnSe laser pumped by a thulium fiber laser

We describe a compact, broadly tunable, continuous-wave (cw) Cr²⁺:ZnSe laser pumped by a thulium fiber laser at 1800 nm. In the experiments, a polycrystalline ZnSe sample with a chromium concentration of 9.5 × 10¹⁸ cm⁻³ was used. Free-running laser output was around 2500 nm. Output couplers with transmissions of 3%, 6%, and 15% were used to characterize the power performance of the laser. Best power performance was obtained with a 15% transmitting output coupler. In this case, as high as 640 mW of output power was obtained with 2.5 W of pump power at a wavelength of 2480 nm. The stimulated emission cross-section values determined from laser threshold data and emission measurements were in good agreement. Finally, broad, continuous tuning of the laser was demonstrated between 2240 and 2900 nm by using an intracavity Brewster cut MgF₂ prism and a single set of optics.     

Second-harmonic generation at the surface of cubic ionic crystals

The strength of the different source terms in the isotropic Second Harmonic (SH) response of the surface has been measured in different ionic crystals LiF, NaF, GGG(Gadolinium-Gallium-Garnet), and MgF₂, using as excitation beam the fundamental output of a Nd:YAG laser (1.06 m). The azimuthal dependences of the optical second-harmonic generation has also been measured at the LiF(100), LiF(111), GGG(100), and GGG(111) surfaces. No experimental evidence of a bulk contribution to the anisotropic part of the nonlineau polarization has been observed.