The behaviour of interference optical filters for space applications has been investigated under low energy proton irradiation. In order to understand the behaviour of the interference coating subjected to proton irradiation, the interaction of protons with coating and substrate was simulated by the SRIM code. A beam of protons of 60 KeV with an integrated fluence of 1013 p+/cm2 was used. The spectral transmittances of fused silica, TiO2 and HfO2 single layers and interference coatings were measured before and after irradiation and, according to simulations, no significant effects were detected in the visible-near infrared spectrum, while some variations appeared at shorter wavelengths.
The study of lightning phenomena will be carried out by a dedicated instrument, the lightning imager, that will make use of narrow-band transmission filters for separating the Oxygen emission lines in the clouds, from the background signal. The design, manufacturing and testing of these optical filters will be described here.
The behaviour of interference optical filters for space applications has been investigated under low- and high-energy
proton irradiation. Low-energy protons are expected to be necessary to prove the effects on the coating, whereas the
high-energy proton tests shall verify mainly the substrate susceptibility to induced damage. The expected interaction of
protons with coating and substrate was simulated by software, to identify the most appropriate conditions for the
irradiation experiments. Two different accelerator facilities were used for low- and high- energy protons: 60 keV protons
with an integrated fluence of 1012 p+/cm2 and 30 MeV protons with an integrated fluence of 108 p+/cm2.
The spectral transmittance of the filters was measured before and after irradiation and, according to simulations, no
significant effects were detected in the visible-near infrared spectrum, while some variations appeared at short
wavelengths with low-energy irradiation.
KEYWORDS: Mirrors, Reflectivity, Reflectors, Gas lasers, Laser resonators, Optical components, Near field optics, Ion beams, Thin films, Sputter deposition
Multiple spot mirrors are non-conventional optical components which present a number of equally spaced circular spots with a high reflectance that decreases to zero outside the spots. It is believed that the use of multiple spot mirrors as full reflectors in plane-parallel laser cavities should allow to improve the laser beam optical quality and get a good fillimg of the active medium. A double-spot mirror and a four-spot mirror for a XeCl laser cavity (λ = 308 nm) have been fabricated by depositing HfO2 and SiO2 thin films by reactive ion beam sputtering. The design employed allows to obtain a maximum reflectance (79%) on the spot area and a reflectance lower than 0.5% outside the spots by shaping the thickness of one of the inner layer. To this aim, a properly shaped mask was put inside the vacuum chamber during the deposition of the graded layer. The realization and the optical characterization of these devices are presented in this paper.
Radiation resistance of optical materials against synchrotron radiation is important, if optical components for the high energetic regime have to be produced. In the framework of the European project EUFELE, which deals with the development of optical coatings for the free electron laser at ELETTRA (Trieste), a set of CaF2 substrates was irradiated with synchrotron radiation. The synchrotron radiation was varied by wavelength, dose, and high energetic background illumination. Before and after irradiation, the CaF2 substrates were investigated spectrophotometrically in the VUV, VIS and IR range. The surface topology was characterized by Nomarski microscope methods. Structure investigations were carried out with X-ray diffraction measurements. CaF2 shows different types of degradation like color center formation, surface modification, and increased VUV absorption bands. Defect formation will be presented in dependence of synchrotron irradiation conditions.
The aim of this work is to investigate the influence of the standing-wave electric field profile on the laser damage resistance of HfO2 thin films. To this end, HfO2 thin films of different optical thickness and deposited by the electron beam evaporation technique at the same deposition conditions have been analyzed. Laser damage thresholds of the samples have been measured at 308 nm (XeCl laser) by the photoacoustic beam deflection technique and microscopic inspections. The dependence of the laser damage threshold on the standing-wave electric field pattern has been analyzed.
The ion beam assistance during the film growth is one of the most useful method to obtain dense film along with improved optical and structural properties. Afnia material is widely used in optical coating operating in the UV region of the spectrum and its optical properties depend on the production method and the physical parameters of the species involved in the deposition process. In this work afnia thin films were evaporated by an e-gun and assisted during the growth process. The deposition parameters, ion beam energy, density of ions impinging on the growing film and the number of arrival atoms from the crucible, have been related to the optical and structural properties of the film itself. The absorption coefficient and the refractive index were measured by spectrophotometric technique while the microstructure has been studied by means of x-ray diffraction. A strictly correlation between the grain size, the optical properties and the laser damage threshold measurements at 248 nm was found for the samples deposited at different deposition parameters.
HfO2 is one of the most important high index thin film materials for the manufacture of interference coatings in the DUV spectral region down to 248 nm. High quality coatings and multilayer interference systems in conjunction with SiO2 as low index material can be deposited by various PVD technologies including reactive e-beam evaporation (RE), ion assisted deposition (IAD) and plasma ion assisted deposition (PIAD). Thin HfO2 films with optical thickness up to 3(lambda) /4 were deposited by RE, IAD and PIAD onto fused silica. The optical and structural properties of these films were investigated. The optical properties are related to the film structure and film density. The interaction of UV radiation with photon energies close to the band gap of HfO2 with different films was studied. LIDT at 248 nm were determined in the 1- on-1 and the 1000-on-1 test mode in dependence on the deposition technology and the film thickness. LIDT values of all investigated films decrease with increasing thickness due to the higher absorption and defect density. Additionally, data on the radiation resistance of interference coatings containing HfO2 will be presented.
Diamond like carbon (DLC) can be an ideal candidate as coating material for optical components owing to its high hardness value, chemical inertness and high transparency in the IR spectral region. DLC thin films have been deposited by RF magnetron sputtering from a graphite target in argon atmosphere without hydrogen on glass and silicon substrates in order to study the influence of some parameters on the mechanical and optical properties. Increasing the power input values, from 100W to 400W, the DLC films compactness increases too as it can be deduced from the raise in this range of power of both hardness and refractive index values, calculated by microindentations size and reflectance and transmittance spectrophotometer measurements respectively. Moreover, a set of sample was deposited at different substrate temperatures and at the highest used value a lower value of hardness and refractive index as well are obtained. The possibility to correlate the films compactness to their absorption humidity has been investigated by means of the IR transmittance spectra analysis. Densifying effect of RF sputtered DLC thin films examined in this work seems to be related, in conclusion, to the power input increasing rather than to the substrate temperature.
In this work, ZnSe thin films were deposited by radio frequency magnetron sputtering onto oriented silicon substrates. Three sets of samples were produced by varying the argon working gas pressure, and changing the sputtering power supply. The effect of the different growth conditions on the structural and optical properties of the ZnSe films was investigated by using XRD and FTIR spectroscopy. In particular, x-ray diffraction was used in order to study the residual strains and texture. The ZnSe coatings were grown in the cubic polymorph with the grains preferentially oriented for all the investigated pressures with the exception of 0.5 Pa. An in-plane residual stress reversal, which changed from compressive to tensile by going from the low to the high-pressure sample set, was observed. At the 0.5 Pa pressure, the ZnSe coatings were both in compressive and tensile state, according to the power supply value. These result have been correlated with the momentum of reflected neutral in order to find an exhaustive description of the deposition process. At low momentum values, the ZnSe films show tensile state, low refractive index, grains preferentially oriented, while at high momentum they were in compressive state, refractive index very alike to the bulk and grains randomly oriented.
The diamond like carbon (DLC) is very hard, inert to chemical attack and transparent in the IR region. In order to increase the transparency of DLC, the percentage of Sp3 bonds must be increased. In this work, thin film of DLC were deposited by the Dual Ion Beam Sputtering technique. An argon ion beam sputters a graphite target and a second one assists the film during the growth. A set of samples were produced at different momentum transfer varying the energy and current density of assisting ion beam and the growth rate. The momentum transfer is defined P equals nc divided by nat (root) 2 mc Ec where nc is the number of recoiling carbon atoms, mat is the number of the carbon atoms in the volume interested at the collision cascade process and Ec is the energy of the recoils after the collision with the Ar ions, nc, nat, and Ec were calculated by the TRIM program. THe optical properties as the optical gap of the samples were related to the momentum transfer value. As far as the mechanical properties were concerned, the hardness value was measured by Knoop microindentation for all samples.
One of the main responsibilities for adhesion failure of antireflection coatings on plastic substrates is the stress developing at the film/substrate interface during film deposition and subsequent system cooling. Coating a substrate by a different material always induces stress at the interface, because it enforces the film to copy the substrate structure. Further, non-equilibrium conditions during film growth and thermal deformations due to post- deposition cooling, also contributes to the total stress of the system. In particular, thermal stress plays an important role in adhesion failure of coated plastic,s because the expansion coefficient of polymers is about an order of magnitude greater than that of hard oxides. In order to prevent delamination of the film, this strain must be balanced by bond strength between the film and the substrate. In this work, we investigate the possibility of improving the film/substrate adhesion by reducing the total stress at that interface. In fact, the intrinsic stress of ion assisted hafnium oxide films is influenced by the assistance conditions, and consequently the total stress can be changed by varying the experimental parameters. Hafnium oxide layers were deposited on plastic substrate by dual ion beam sputtering, at different growth conditions. Stress values were obtained by measuring the curvature of the substrate before an dafter deposition. Practical adhesion properties were evaluated by mechanical tests.
Evaporated Afnia thin film suffer of high porosity that causes the variation of the optical parameters when the film is exposed to the atmosphere. The ion beam assisted deposition is a useful method to obtain dense and adherent thin film. In this work, the effect of the (Xe) low energy ion beam assistance on the optical properties of HfO2 thin film is investigated. The deposition parameters are expressed in terms of momentum transfer per arrival atoms. Dense films were obtained increasing the P value. The Sigmund's model was applied to describe the collision cascade mechanism and to calculate the threshold energy at which no sputtering occurs. The experimental measurements of sputtering yield were compared with the calculated values. Assisting the growing film by Xi ions at energy lower than the energy threshold, a high value of refractive index has been obtained, as well as for the sample assisted at high P values.
The dependence of the silicon oxynitride (SiOxNy) refractive index on chemical composition can be employed in producing graded refractive index optical coatings. In this work, SiOxNy thin films were deposited by dual ion beam sputtering. Samples with different composition were obtained by making use of SiO2 and Si targets. The energy and the ion to atom arrival ratio were also varied to study the correlation with the film stoichiometry. Infrared spectroscopy of the vibrational modes was used to investigate the composition of the samples. X-ray photoelectron spectroscopy and x-ray induced auger electron spectroscopy were combined to provide further insight into the chemical composition of the films.
Graded reflectance coatings are typically identified with optical coatings having a gaussian reflectance profile along the surface with circular symmetry. Indeed the first use of such coatings was as output couplers of gaussian laser resonators, then also super-gaussian reflectance profiles were considered. The variation of reflectance along the surface is obtained in most cases by a coating thickness variation, with an appropriate choice of both coating structure and thickness profile it is possible to obtain any maximum reflectance and any spatial behavior of the reflectance itself. New reflectance profiles that seem to be of interest are: graded reflectances with a central minimum, that is high transmittance at the coating center, and non- monotonic radial reflectance profiles as for example high reflectance on an annular area of the optical component. In addition, coatings with non-circular symmetry appear useful for particular laser applications. The methods for obtaining such reflectance profiles are discussed.
Titanium oxynitride (TiOxNy) thin films are of great interest for the fabrication of protective optical coatings. By varying the oxygen and nitrogen content in the films, the electrical, optical and mechanical properties of these coatings can be tailored properly. In this work, we fabricated TiN and TiOxNy films by dual ion beam sputtering technique. TiOxNy films were ion-assisted by a low energy oxygen and nitrogen mixed ion beam of variable O/N flux ratio. We observe that the incorporation of oxygen greatly improves the adhesion of the film on the glass substrate. Further, the optical extinction coefficient drastically decreases for increasing oxygen content, suggesting new applications of TiOxNy films as protective coatings on transparent substrates. The film composition by XPS analyses is in agreement with the results obtained by a simple model to describe the ion assistance phenomena. The crystallographic structure of the deposited films was characterized by using (Theta) - 2(Theta) x-ray diffraction and grazing incidence x-ray diffraction measurements. In the range up to 14 percent of the oxygen to nitrogen flux ratio, a TiN f.c.c. phase structure with preferred (111) growth-orientation of the grains is observed. For higher oxygen concentrations the absence of diffraction peaks suggests a more amorphous-like structure of the deposited film. Specular x-ray reflectivity measurements provide important and accurate information about the film-air and film-substrate interface roughness. The Kiessig fringes are caused by multiple internal interference of the x-ray beam and can be observed up to 3 degrees, which is a clear indication of the high homogeneity of the film thickness and of sharp interfaces.
Evaporated coatings need generally high substrate temperature to obtain a more dense structure of the layers avoiding the humidity adsorption when the coating itself is exposed to the atmosphere. Moreover, the employment of fluorides as low refractive index layers can increase the stress level of the coating because of their frequent hygroscopicity. Under severe operational conditions, the adsorbed water causes an expansion of the layer with a consequent destruction of the coating if the adhesion strengths is not strong enough to counter balance the increased stress. The ion beam assistance can be a useful method to increase the density of the coating produced by the evaporation technique. In fact, the ion bombardment during the thin film growth improves the packing density and enhances thin film resistance to moisture penetration. Magnesium fluoride (MgF2) and barium fluoride (BaF2) are two low refractive index materials employed in the optical coatings production whereas yttria material (Y2O3) can be used as high refractive index layer. In this work the effect of the ion beam assistance on the optical coating capability to offer resistance to atmosphere humidity penetration was investigated. Since the presence of an absorption band at a wavelength of 3 micrometers reveals the presence of the water in the layers, some of the investigated coatings were deposited on silicon substrates to measure the transmittance in the infrared region.
Thin film coatings having a radially variable reflectance with a circular symmetry are used for obtaining diffraction limited laser beams from unstable resonators. Different approaches are used for the design of such coatings with a consequent different final coating structure that contains one or more profiled thickness layers. Some design methods are discussed with reference to reflected intensity and phase profiles and the fabrication techniques for coatings at different wavelengths are illustrated.
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