Effect of Vacuum on the Laser-Induced Damage of Anti-Reflection Coatings

In the comparison of damage modifications, absorption measurement and energy dispersive x-ray analysis, the effect of vacuum on the laser-induced damage of anti-reflection coatings is analyzed. It is found that vacuum decreases the laser-induced damage threshold of the films. The low laser-induced damage threshold in vacuum environments as opposed to air environments is attributed to water absorption and the formation of the O/Si, O/Zr sub-stoichiometry in the course of laser irradiation.     

Influence of Different Substrates on Laser Induced Damage Thresholds at 1064nm of Ta2O5 Films

Ta2O5 films are prepared on Si, BK7, fused silica, antireflection （AR） and high reflector （HR） substrates by electron beam evaporation method, respectively. Both the optical property and laser induced damage thresholds （LIDTs） at 1064 nm of Ta2O5 films on different substrates are investigated before and after annealing at 673 K for 12 h. It is shown that annealing increases the refractive index and decreases the extinction index, and improves the O/Ta ratio of the Ta2O5 films from 2.42 to 2.50. Moreover, the results show that the LIDTs of the Ta2O5 films are mainly correlated with three parameters： substrate property, substoichiometry defect in the films and impurity defect at the interface between the substrate and the films. Details of the laser induced damage models in different cases are discussed.     

Laser Induced Damage Threshold at 355 and 1064nm of Ta2O5 Films of Different Phases

Ta2O5 films axe deposited on fused silica substrates by conventional electron beam evaporation method. By annealing at different temperatures, Ta2 O5 films of amorphous, hexagonal and orthorhombic phases are obtained and confirmed by x-ray diffractometer （XRD） results. X-ray photoelectron spectroscopy （XPS） analysis shows that chemical composition of all the films is stoichiometry. It is found that the amorphous Ta2 O5 film achieves the highest laser induced damage threshold （LIDT） either at 355 or 1064nm, followed by hexagonal phase and finally orthorhombic phase. The damage morphologies at 355 and 1064nm are different as the former shows a uniform fused area while the latter is centred on one or more defect points, which is induced by different damage mechanisms. The decrease of the LIDT at 1064nm is attributed to the increasing structural defect, while at 355nm is due to the combination effect of the increasing structural defect and decreasing band gap energy.     

Effect of Process Parameters on Laser Damage Threshold of TiO2 Coatings

We investigate the laser damage behaviour of an electron-beam-deposited TiO2 monolayer at different process parameters. The optical properties, chemical composition, surface defects, absorption and laser-induced damage threshold （LIDT） of films are measured. It is found that TiO2 films with the minimum absorption and the highest LIDT can be fabricated using a TiO2 starting material after annealing. LIDT is mainly related to absorption and is influenced by the non-stoichiometric defects for TiO2 films. Surface defects show no evident effects on LIDT in this experiment.     

Impact of organic contamination on laser-induced damage threshold of high reflectance coatings in vacuum

The influence of organic contamination in vacuum on the laser-induced damage threshold (LIDT) of coatings is studied. TiO₂/SiO₂ dielectric mirrors with high reflection at 1064 nm are deposited by the electron beam evaporation method. The LIDTs of mirrors are measured in vacuum and atmosphere, respectively. It is found that the contamination in vacuum is easily attracted to optical surfaces because of the low pressure and becomes the source of damage. LIDTs of mirrors have a little change in vacuum compared with in atmosphere when the organic contamination is wiped off. The results indicate that organic contamination is a significant reason to decrease the LIDT. N₂ molecules in vacuum can reduce the influence of the organic contaminations and prtectect high reflectance coatings.     

Anti-Reflective Fluoride Coatings for Widely Tunable Deep-Ultraviolet Diode-Pumped Solid-State Laser Applications

An anti-reflective （AR） fluoride coating in the 170-230 nm spectral range is prepared by the thermal evaporation method for the applications of widely tunable deep-ultraviolet diode-pumped solid-state lasers. The transmittance of an AR coated calcium fluoride （CaF2） window in thickness 3mm is measured to be in the range of 95.8% at 170 nm to 97.1 % at 230 nm, with the maximum transmittance 99.2% and the minimum residual reflectance 0.04% appeared at 195 nm. The experimental results indicate that treating the AR coated window and the bare substrate with ultraviolet irradiation can significantly improve their optical performance.     

Development in Laser Induced Extrinsic Absorption Damage Mechanism of Dielectric Films

Absorption of host and the temperature-dependence of absorption coefficient have been considered in evaluating temperature distribution in films, when laser pulse irradiates on films. Absorption of dielectric materials experience three stages with the increase of temperature： multi-photon absorption; single photon absorption; metallic absorption. These different absorption mechanisms correspond to different band gap energies of materials, which will decrease when the temperature of materials increases. Evaluating results indicate that absorption of host increases rapidly when the laser pulse will be over. If absorption of host and the temperature-dependence of absorption are considered, the maximal temperatures in films will be increased by a factor of four.     

Design and Fabrication of Chirped Mirror

Chirped mirrors （CMs） are designed and manufactured. The optimized CM provides a group delay dispersion （ODD） of around -60fs^2 and average reflectivity of 99.4% with bandwidth 200 nm at a central wavelength of 800nm. The CM structure consists of 52 layers of alternating high refractive index Ta2O5 and low refractive index SiO2. Measurement results show that the control of CM manufacturing accuracy can meet our requirement through time control with ion beam sputtering. Because the ODD of CMs is highly sensitive to small discrepancies between the layer thickness of calculated design and those of the manufactured mirror, we analyze the error sources which result in thickness errors and refractive index inhomogeneities in film manufacture.     

Comparison of TiO2 and ZrO2 Films Deposited by Electron-Beam Evaporation and by Sol-Gel Process

TiO2 and ZrO2 films are deposited by electron-beam （EB） evaporation and by sol-gel process. The film properties are characterized by visible and Fourier-transform infrared spectrometry, x-ray diffraction analysis, surface roughness measure, absorption and laser-induced damage threshold （LIDT） test. It is found that the sol-gel films have lower refractive index, packing density and roughness than EB deposited films due to their amorphous structure and high OH group concentration in the film. The high LIDT of sol-gel films is mainly due to their amorphous and porous structure, and low absorption. LIDT of EB deposited film is considerably affected by defects in the film, and LIDT of sol-gel deposited film is mainly effected by residual organic impurities and solvent trapped in the film.     

Overcoat dependence of laser-induced damage threshold of 355 nm HR coatings

A series of HR coatings, with and without overcoat, were prepared by electron beam evaporation using the same deposition process. The laser-induced damage threshold (LIDT) was measured by a 355 nm Nd:YAG laser with a pulse width of 8 ns. Damage morphologies of samples were observed by Leica-DMRXE Microscope. The stress was measured by viewing the substrate deformation before and after coatings deposition using an optical interferometer. Reflectance of the samples was measured by Lambda 900 Spectrometer. The theoretical results of electric field distributions of the samples were calculate by thin film design software (TFCalc). It was found that SiO₂ overcoat had improved the LIDT greatly, while MgF₂ overcoat had little effect on the LIDT because of its high stress in the HR coatings. The damage morphologies were different among HR coatings with and without overcoats.     

Optical inhomogeneity model for evaporated Y2O3 obtained from physical thickness measurement

A multiparameter fitting with additional parameters for film inhomogeneity based on transmission results is used to get film inhomogeneity information and to compare different models for film structure. For a number of evaporated materials similar results from transmission fitting have been obtained by using a model consisting of two sublayers with a constant difference in refractive indices between them, either with a thin sublayer in the contact with a substrate or with air. As additional information, we obtained the film physical thickness result from step profile measurements for an oxygen-doped Y₂O₃ film on a fused silica and we compared it with the fit results for this coating. The result closest to the profilometry one has been achieved for a model with a thinner sublayer in contact with the substrate. The differences are great enough to assert that Y₂O₃ films on a fused silica possess a higher refractive index in the first stages of growth and then, after some transition, the main material with smaller refractive index grows on it.     

氧化物薄膜抗1064nm脉冲激光损伤的特性研究

对向种氧化物介质膜料的单层膜、增透膜及高反膜的激光损伤特性进行了实验研究,分别测试了单脉冲和多脉冲１０６４ｎｍ激光的损伤阈值,分析和讨论了实验的物理现象及其结果。研究表明ＳｉＯ２、Ｇｄ２Ｏ３、Ｅｒ２Ｏ３、Ｙ２Ｏ３具有激光损伤阈值高、损伤程度轻的特点,特别对１０Ｈｚ激光而言,更是明显;增透膜及高反膜加λ／２层ＳｉＯ２对其激光损伤程度和损伤几率。     

Effects of annealing on laser-induced damage threshold of TiO2/SiO2 high reflectors

The mechanism of improving 1064 nm, 12 ns laser-induced damage threshold (LIDT) of TiO₂/SiO₂ high reflectors (HR) prepared by electronic beam evaporation from 5.1 to 13.1 J/cm² by thermal annealing is discussed. Through optical properties, structure and chemical composition analysis, it is found that the reduced atomic non-stoichiometric defects are the main reason of absorption decrease and LIDT rise after annealing. A remarkable increase of LIDT is found at 300 °C annealing. The refractive index and film inhomogeneity rise, physical thickness decrease, and film stress changes from compress stress to tensile stress due to the structure change during annealing.     

Parameter Study on Far-Field Sub-Diffraction-Limited Hyperlens

Influence of hyperlens structure parameters, such as curvature radius and layer number, and incident wavelength on the beam propagation is numerically investigated by FDTD. It is found that the intensity and the full width at half maximum (FWHM) of the output beam can be controlled by the structure parameters and operating wavelength. The optimized structureparameters and operating wavelength are obtained, which will be of great significance in design and fabrication of optical hyperlens with high resolution.     

Stress, absorptance and laser-induced damage threshold properties of 355-nm HR coatings

A number of 355-nm Al₂O₃/MgF₂ high-reflectance (HR) coatings were prepared by electron-beam evaporation. The influences of the number of coating layers and deposition temperature on the 355-nm Al₂O₃/MgF₂ HR coatings were investigated. The stress was measured by viewing the substrate deformation before and after coating deposition using an optical interferometer. The laser-induced damage threshold (LIDT) of the samples was measured by a 355-nm Nd:YAG laser with a pulse width of 8 ns. Transmittance and reflectance of the samples were measured by a Lambda 900 spectrometer. It was found that absorptance was the main reason to result in a low LIDT of 355-nm Al₂O₃/MgF₂ HR coatings. The stress in Al₂O₃/MgF₂ HR coatings played an unimportant role in the LIDT, although MgF₂ is known to have high tensile stress.     

Influence of oxygen post-treatment on laser-induced damage of antireflection coatings prepared by electron-beam evaporation and ion beam assisted deposition

Antireflection coatings at the center wavelength of 1053 nm were prepared on BK7 glasses by electron-beam evaporation deposition (EBD) and ion beam assisted deposition (IBAD). Parts of the two kinds of samples were post-treated with oxygen plasma at the environment temperature after deposition. Absorption at 1064 nm was characterized based on surface thermal lensing (STL) technique. The laser-induced damage threshold (LIDT) was measured by a 1064-nm Nd:YAG laser with a pulse width of 38 ps. Leica-DMRXE Microscope was applied to gain damage morphologies of samples. The results revealed that oxygen post-treatment could lower the absorption and increase the damage thresholds for both kinds of as-grown samples. However, the improving effects are not the same.     

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     

Influence of substoichiometer on the laser-induced damage characters of HfO2 thin films

HfO₂ is one of the most important high refractive index materials for depositing high power optical mirrors. In this research, HfO₂ thin films were prepared by dual-ion beam reactive sputtering method, and the laser-induced damage thresholds (LIDT) of the sample were measured in 1-on-1 mode for laser with 1064 nm wavelength. The results indicate that the LIDT of the as-grown sample is only 3.96 J/cm², but it is increased to 8.98 J/cm² after annealing under temperature of 200 °C in atmosphere. By measuring the laser weak absorption and SIMS of the samples, we deduced that substoichiometer is the main reason for the low LIDT of the as-grown sample, and the experiment results were well explained with the theory of electronic-avalanche ionization.     

Thermal conductivity and interface thermal resistance of Si film on Si substrate determined by photothermal displacement interferometry

An in situ, noncontact, photothermal displacement interferometer for performing thermal diffusivity measurements on bulk and thin-film materials has been developed. Localized transient surface motion is generated through photothermoelastic coupling of a pulsed, heating laser beam to the sample under investigation. The maximum surface displacement is found to be linearly dependent on the laser power while the proportionality is a function of the thermal diffusivity. Both thin-film conductivity and film/substrate interface thermal resistance are derived from the measured, effective thermal conductivity by employing simple heat-flow analysis. Wedge-shaped Si films, vacuum deposited on single crystal Si wafers are studied with this technique. A sample with oxide layer removed by ion bombardment of the wafer surface prior to film deposition shows the same film conductivity as a sample film deposited on an as-cast wafer, while the uncleaned sample exhibits higher interface thermal resistance. It is found that the thin-film thermal conductivity is somewhat smaller than the bulk value. However, the existence of an interface thermal resistance, when combined with film thermal conductivity, can result in an effective thermal conductivity as low as two orders of magnitude lower than the bulk value.Currently supported by the LLE fellowship     

Random thickness errors-effect on reflectance of multilayer

In this paper the effect of thickness errors on the reflectance of multilayer is investigated. Using the matrix method, we deduced an expression for describing the dependence of reflectance on the film thickness errors. It is found that the degree of the thickness error-effect is affected by the ratio of low refractive index with high refractive index and the number of layer of multilayer. The computer simulations show that the degree of this effect is very small when the ratio is small and the number of layer is large. The thickness error hardly influences the reflectance of short-wave multilayer.