BK7 Substrates Research Articles

Laser damage properties of TiO_2/Al_2O_3 thin films grown by atomic layer deposition

Research on thin film deposited by atomic layer deposition (ALD) for laser damage resistance is rare. In this paper, it has been used to deposit TiO(2)/Al(2)O(3) films at 110 °C and 280 °C on fused silica and BK7 substrates. Microstructure of the thin films was investigated by x-ray diffraction. The laser-induced damage threshold (LIDT) of samples was measured by a damage test system. Damage morphology was studied under a Nomarski differential interference contrast microscope and further checked under an atomic force microscope. Multilayers deposited at different temperatures were compared. The results show that the films deposited by ALD had better uniformity and transmission; in this paper, the uniformity is better than 99% over 100 mm Φ samples, and the transmission is more than 99.8% at 1064 nm. Deposition temperature affects the deposition rate and the thin film microstructure and further influences the LIDT of the thin films. As to the TiO(2)/Al(2)O(3) films, the LIDTs were 6.73±0.47 J/cm(2) and 6.5±0.46 J/cm(2) at 110 °C on fused silica and BK7 substrates, respectively. The LIDTs at 11 °C are notably better than 280 °C.

Fabrication of mitigation pits for improving laser damage resistance in dielectric mirrors by femtosecond laser machining

Femtosecond laser machining is used to create mitigation pits to stabilize nanosecond laser-induced damage in multilayer dielectric mirror coatings on BK7 substrates. In this paper, we characterize features and the artifacts associated with mitigation pits and further investigate the impact of pulse energy and pulse duration on pit quality and damage resistance. Our results show that these mitigation features can double the fluence-handling capability of large-aperture optical multilayer mirror coatings and further demonstrate that femtosecond laser macromachining is a promising means for fabricating mitigation geometry in multilayer coatings to increase mirror performance under high-power laser irradiation.

Effects of deposition parameters on laser-induced damage threshold of Ta 2O 5 films

Ta 2O 5 films were deposited on BK7 substrates by e-beam evaporation with different deposition parameters such as substrate temperature (323–623 K), oxygen pressure (0.5–3.0×10 −2 Pa) and deposition rate (0.2–0.5 nm/s). Absorption, scattering and chemical composition were investigated by surface thermal lensing (STL) technique, total integrated scattering (TIS) measurement and X-ray photoelectron spectroscopy (XPS), respectively. The laser-induced damage threshold (LIDT) was assessed using pulsed Nd:YAG 1064 nm laser at a pulse length of 12 ns. The results showed that optical properties, absorption and LIDT were influenced by the deposition parameters and annealing. However, scattering was little correlated with the deposition parameters. On the whole, the LIDT increased with increasing substrate temperature and oxygen pressure, whereas it increased firstly and then decreased upon increasing deposition rate. After annealing at 673 K for 12 h, the LIDT of films improved significantly. The dependence of possible damage mechanism on deposition parameters was discussed.

Demonstration of the feasibility of a complete ellipsometric characterization method based on an artificial neural network

Ellipsometry is an optical technique that is widely used for determining optical and geometrical properties of optical thin films. These properties are in general extracted from the ellipsometric measurement by solving an inverse problem. Classical methods like the Levenberg-Marquardt algorithm are generally too long, depending on direct calculation and are very sensitive to local minima. In this way, the neural network has proved to be an efficient tool for solving these kinds of problems in a very short time. Indeed, it is rapid and less sensitive to local minima than the classical inversion method. We suggest a complete neural ellipsometric characterization method for determining the index dispersion law and the thickness of a simple SiO(2) or photoresist thin layer on Si, SiO(2), and BK7 substrates. The influence of the training couples on the artificial neural network performance is also discussed.

Preparation and characterization of polyvinyl butyral/silica hybrid antireflective coating: effect of PVB on moisture-resistance and hydrophobicity

A new modified antireflective coating was prepared by base catalyzed sol–gel process using tetraethylorthosilicate as precursor and polyvinyl butyral as modifier. The properties of the silica sols and AR coatings were characterized with Fourier-transfer infrared absorption spectroscopy, particle size analyzer, transmission electron microscope, programmable rheometer, UV–Vis spectrophotometry, ellipsometry, atomic force microscope and contact angle measurement. It was found that addition of 4% PVB greatly enhanced moisture-resistance of the AR coating. Optical transmittance of 4% PVB modified AR coating on BK7 substrate was found to be nearly 100%. The water contact angles of normal and 4% PVB modified AR coating were 51 and 53°, respectively, which indicates no significant increase of hydrophobicity of the modified coating. The peak transmittance of 4% PVB modified AR coating was almost unaffected after being exposed to the moist surroundings while that of normal silica coating decreased sharply from 99.8 to 96.5% within 2 weeks. The excellent moisture-resistance of PVB modified AR coating may be attributed to the adsorption of PVB on the surface of AR coating.

Synthesis and properties of silicon dioxide films prepared by pulsed laser deposition using ceramic SiO 2 target

Silicon dioxide (SiO 2) thin films were deposited on BK7 substrates by pulsed laser deposition (PLD) method using ceramic SiO 2 targets (C-SiO 2-Ts), which was sintered by solid state sintering. The reason for using C-SiO 2-T instead of the silicon target is to reduce the oxygen-deficiency phenomenon in deposited SiO 2 thin films. The influence of substrate-temperatures, oxygen-pressures and oxygen-plasma-assistance on the properties of synthesized films was studied. X-ray diffraction, atomic force microscopy, ultraviolet–visible–near-infrared scanning spectrophotometry were used to characterize the crystallinity, morphology and optical properties of deposited films. Results show that the root-mean-square roughness of films increased with the increase of oxygen-pressure, substrate-temperature and with the employment of oxygen-plasma. The transmittance of films increased with the increase of oxygen-pressure and decreased with the increase of substrate-temperature and with the employment of oxygen-plasma. Stoichiometric SiO 2 thin film with high optical quality was synthesized at room-temperature and 20 Pa oxygen-pressure using C-SiO 2-T.

Magnetic behaviors of amorphous Fe78Si9B13 thin films prepared by pulsed laser deposition

Amorphous magnetic thin films have been deposited onto BK-7 substrates using the pulsed laser deposition technique. The source material irradiated by the laser was a pack of amorphous ribbons of composition Fe78Si9B13. The structural properties of thin films were investigated and a large number of droplets were observed in the magnetic layers. Electric current passing through the films causes significant deformation of droplets and consequently changes the magnetic thin films characteristics. Magnetic properties evolution after electrical processing was investigated using magneto-optical Kerr effect.

Influence of Ytterbia Content on Residual Stress and Microstructure of Y2O3-ZrO2 Thin Films Prepared by EB-PVD*

Y2O3 stabilized ZrO2 (YSZ) thin films with different Y2O3 molar contents (0, 3, 7, and 12mol%) are deposited on BK7 substrates by electron-beam evaporation technique. The effects of different Y2O3 contents on residual stresses and structures of YSZ thin films are studied. Residual stresses are investigated by means of two different techniques: the curvature measurement and x-ray diffraction method. It is found that the evolution of residual stresses of YSZ thin films by the two different methods is consistent. Residual stresses of films transform from compressive stress into tensile stress and the tensile stress incre ases monotonically with the increase of Y2O3 content. At the same time, the structures of these films change from the mixture of amorphous and monoclinic phases into high temperature cubic phase. The variations of residual stress correspond to the evolution of structures induced by adding of Y2O3 content.

Influence of Different Substrates on Laser Induced Damage Thresholds at 1064 nm 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 673K 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.

SiO2保护层和退火对Ta2O5薄膜激光损伤阈值影响的对比研究

Ta2O5 films are prepared on BK7 substrates with conventional electron beam evaporation deposition. The effects of SiO2 protective layers and annealing on the laser-induced damage threshold (LIDT) of the films are investigated. The results show that SiO2 protective layers exert little influence on the electric field intensity (EFI) distribution, microstructure and microdefect density but increase the absorption slightly. Annealing is effective on decreasing the microdefect density and the absorption of the films. Both SiO2 protective layers and annealing are beneficial to the damage resistance of the films and the latter is more effective to improve the LIDT. Moreover, the maximal LIDT of Ta2O5 films is achieved by the combination of SiO2 protective layers and annealing.

引入过渡层提高介质减反膜的抗激光损伤阈值

A new method for increasing laser induced damage threshold (LIDT) of dielectric antireflection (AR) coating is proposed. Compared with AR film stack of H2.5L (H:HfO2, L:SiO2 on BK7 substrate, SiO2 interfacial layer with four quarter wavelength optical thickness (QWOT) is deposited on the substrate before the preparation of H2.5L film. It is found that the introduction of SiO2 interfacial layer with a certain thickness is effective and flexible to increase the LIDT of dielectric AR coatings. The measured LIDT is enhanced by about 50%, while remaining the low reflectivity with less than 0.09% at the center wavelength of 1064 nm. Detailed mechanisms of the LIDT enhancement are discussed.

Optical and electrical properties between 04 and 12 μm for Sn-doped In_2O_3 films by pulsed laser deposition and cathode sputtering

Optical properties of Sn-doped In(2)O(3) (ITO) have been studied in the optical range of 0.4-12 microm. A deposition has been made on BK7 glass, magnesium fluoride, sapphire, and zinc sulfide substrates. The layers have been characterized by their optical properties, DC electrical sheet resistivity, and Hall mobility. Sheet resistivity lies in the range of 6.8-318 Omega/sq for thicknesses between 16 and 280 nm. The best carrier mobility is obtained on BK7 and sapphire substrates, up to approximately 50 cm(2)/V s. The material shows good infrared transparency in the 3-5 microm range on magnesium fluoride and 0.4-4 microm on sapphire, and it is usable for practical applications up to 12 microm on zinc sulfide. Simulations have been carried out for optical indices and spectra calculations. The Drude model has been used to exploit the results in either direction: from electrical measured data to the simulation of optical spectra and indices, and from measured optical spectra to simulated optical indices and electrical parameters (mobility, carrier density). Hall mobility is considered a worthy and convenient material quality criteria for materials aimed at optics.

Effect of thickness on the structure, morphology and optical properties of sputter deposited Nb2O5 films

Nb2O5 films with the thickness (d) ranging from 55 to 2900nm were deposited on BK-7 substrates at room temperature by a low frequency reactive magnetron sputtering system. The structure, morphology and optical properties of the films were investigated by X-ray diffraction, atomic force microscopy and spectrophotometer, respectively. The experimental results indicated that the thickness affects drastically the structure, morphology and optical properties of the film. There exists a critical thickness of the film, dcri=2010nm. The structure of the film remains amorphous as d<dcri. However, it becomes crystallized as d>dcri. The root mean square of surface roughness increases with increasing thickness as d>1080nm. Widths and depths of the holes on film surface increase monotonously with increasing thickness, and widths of the holes are larger than 1000nm for the crystalline films. Refractive index increases with increasing thickness as d<dcri, while it decreases with increasing thickness as d>dcri. In addition, the extinction coefficient increases with increasing thickness as d>dcri.

Y 2O 3 stabilized ZrO 2 thin films deposited by electron beam evaporation: Structural, morphological characterization and laser induced damage threshold

Four kinds of Y 2O 3 stabilized ZrO 2 (YSZ) thin films with different Y 2O 3 content have been prepared on BK7 substrates by electron-beam evaporation method. Structural properties and surface morphology of thin films were investigated by X-ray diffraction (XRD) spectra and scanning probe microscope. Laser induced damage threshold (LIDT) was determined. It was found that crystalline phase and microstructure of YSZ thin films was dependent on Y 2O 3 molar content. YSZ thin films changed from monoclinic phase to high temperature phase (tetragonal and cubic) with the increase of Y 2O 3 content. The LIDT of stabilized thin film is more than that of unstabilized thin films. The reason is that ZrO 2 material undergoes phase transition during the course of e-beam evaporation resulting in more numbers of defects compared to that of YSZ thin films. These defects act as absorptive center and the original breakdown points.

Effect of sub-micrometer polymer gratings generated by two-beam interference on surface plasmon resonance

Sub-micrometer periodic gratings were generated on thin poly-carbonate films spin-coated onto thin flat silver layer covered BK7 substrates by two-beam interference realized with the fourth harmonic of the Nd:Yag laser. The 416 nm period of the laser-induced grating was half of the applied master grating, the modulation depth was increased by the number of laser pulses at a given intensity. Attenuated total reflection measurements were performed in case of plasmon propagation directions parallel and perpendicular to the grooves, and on rotated gratings oriented at angles making the grating coupling effect detectable. Atomic force microscopy (AFM) was applied to determine the modulation depth of the gratings. It was shown that the periodically corrugated polymer surfaces resulted in the coupling of the surface plasmons already when the structure period was very close to the plasmon wavelength. The small shift of the resonance positions proved that the modulation depth cannot be explained by material removal, the material rearrangement plays an important role in the grating formation. These experimental results correlate with our temperature model calculations revealing that there is softening on the upper polymer surface and melting down to the silver layer at the lowest and highest applied laser fluences, respectively.

Defect-induced failure of optical coating filters

Optical filters composed of Ag, Al 2 O 3 , and ZnSe films were prepared on BK7 substrates by evaporation. By employing spectrophotometer, microscope, scanning electron microscope (SEM), and energy dispersive spectrum (EDS) analysis, the moisture-dependent stability of the samples was tested. The experimental results revealed that filter failure often occurs initially at defect sites. Small sputtering particles and pinhole are found to be two types of defects that induced the optical coating filter failure. The mechanisms of the defect-induced failure of the filters also are discussed in the article.

Linear and third-order optical characterisation of polyDPCHD

We report here on linear and non-linear optical characterisation of the polydiacetylene poly[1,6-bis(3,6-dipalmitoyl- N-carbazolyl)-2,4-hexadiyne] (polyDPCHD). Thin films (14 and 32 nm) of good optical quality have been obtained by deposition of the monomer from chloroform solutions on glass by standard spin-coating technique, followed by thermal polymerisation at 115 °C. Complex index of refraction of a thin film deposited on a 1 mm thick fused silica substrate has been determined in the spectral range from 270 to 1700 nm by ellipsometry. Third-order optical susceptibility ( χ (3)) has been measured by Third Harmonic Generation (THG) technique on a thin polyDPCHD film deposited on a 160 μm thick BK7 substrate in the spectral range from 1000 to 1900 nm, using the Maker fringes method. The χ (3) dispersion shows the typical resonant behaviour due to multi-photon processes and is strictly related to the absorption spectra in the visible range. Values of χ (3) of the order of magnitude of 10 −11 esu have been obtained. A comparison between linear and non-linear optical properties of polyDPCHD and poly[1,6-bis(3,6-dihexaedcyl- N-carbazolyl)-2,4-hexadiyne] polyDCHD-HS will be presented.

Raman active E2 modes in aluminum nitride films

The vibrational spectrum of AlN thin films has been studied in recent years by theoretical and experimental methods, but there is still a lack of full knowledge of the Raman shift of the active E2 modes of this material. Early work by Carlone et al. [1] has indicated that the E2 modes of AlN deposited on Si occur at frequencies of 303cm-1 and 426cm-1. In the present study, Raman spectra of AlN deposited by magnetron sputtering on polished Si, quartz and BK7 substrates and also the Raman spectrum of pure Si, were compared to show that these peaks can actually be assigned as Raman peaks of Si, instead of AlN Raman peaks. We show that the Raman lines at 240cm-1 and 650cm-1 are the true AlN E2 Raman peaks in agreement with calculated and experimental works reported in the literature that indicate the occurrence of the low and high E2 Raman modes of AlN in the ranges (228-252)cm-1 and (631-665)cm-1, respectively.

Growth and characterization of pulsed laser deposited lead germanate glass optical waveguides

Lead germanate thin films have been grown on BK7 and silica substrates, using pulsed excimer laser deposition (PLD) at wavelengths of 193 and 248 nm, from a bulk lead germanate target under various partial pressures of oxygen. The films produced vary in colour from brown, through clear, to yellow, and a parametric study has been undertaken of the film properties as a function of the growth conditions. The measured losses of the lead germanate films grown on glass substrates varied from ∼2.5 to 7 dB cm −1 depending on the oxygen partial pressure used.

Design and experimental approach of optical reflection filters with graded refractive index profiles

A novel optical multilayer filter with graded refractive index profiles using a quarterwave stack was designed. This filter exhibits a high reflectance band, a sharp cutoff and a wide transmittance region as the result of sidelobe suppression. A 31 layer TiO2/SiO2 high reflectance filter with a graded refractive index profile was fabricated by helicon plasma sputtering on BK7 and Si (100) substrates. The deposition system was operated under ambient gas pressure of 1.8×10−1 Pa at room temperature. The measured transmittance of the spectrum was in good accord with calculated results. Transmission electron microscopic observations confirmed the expected microstructure of the filter.