MANUSCRIPT NOT RECEIVED VARIATION, VARIABILITY AND DIFFERENCES IN THE MEASUREMENT OF LASER-INDUCED DAMAGE THRESHOLDS R.M. Wood and R.J. Chad GEC Hirst Research Centre ABSTRACT The published literature on the subject of laser induced damage is full of well measured values which disagree with what other workers have found. This paper attempts to distinguish between the variations (described by physical laws and historical/statistical laws) and differences (values measured on similar samples at different laboratories). Examples will be presented of the three types of variance taken from recent measurements. In particular the variability of the damage threshold value versus percent of sites damaged curves will be presented for a series of coated substrates. MANUSCRIPT NOT RECEIVED THE CONSEQUENCE OF DOPING OPTICAL MATERIALS WITH D2O J.B. Franck, J.0. Porteus, L. F. Johnson, J.M. Pentony, and W.N Faith Physics Division, Research Department Naval Weapons Center China Lake, CA 93555-6001 and H. Angus Macleod ABSTRACT In general, water is ubiquitous in nature. In the production of optical thin films the point in the coating process at which the water is incorporated into the film often is unknown. In an attempt to produce a unique sample set for laser-induced desorption experiments, optical thin films were grown in the presence of copious quantities of D20. The results indicate that the uptake of water in optical thin films may not take place in the coating process as might be expected. Preliminary findings indicate that the laserdamage threshold can be improved significantly using this D20-doping process. Manuscript Received 1. Some Studies of Thin Film Distributed Bragg Reflectors KL Lewis, I T Muirhead*, A M Pitt, A G Cullis and G M Williams RSRE, Malvern, Worcs WR14 3PS, UK * OCLI, Dunfermline, Fife KY11 5FR, UK The Distributed Bragg Reflector (DBR) is of interest for a range of optical applications. This work is concerned with a study of the problems influencing the fabrication of such structures using molecular beam techniques. Many of the issues involved are concerned with the achievement of stable structures that do not shift under temperature cycling or laser irradiation. These centre around the fundamental properties of the coating materials selected, the degree of perfection of the films, and the control of microstructure and interface interdiffusion. Results have been obtained which show the effect of varying the thickness of the reflecting interfaces on the bandwidth and intensity of the fundamental reflection band. The degree of interface perfection in such structures has been examined using cross-sectional transmission electron microscopy, and correlated with the results of depth profiling X-ray photoelectron spectroscopy studies. Introduction The relationship between laser damage threshold and electric field distribution in thin film coatings has been the subject of many studies reported in the literature. Evidence has been obtained which suggests that coating designs which avoid high values of electric field intensity within layers, or at interfaces, tend to have significantly improved damage thresholds. In antireflection designs, the most vulnerable interface is generally that between the coating and substrate, since this is the likely site for incorporation of impurities. Apfel et al [1] found that the addition of a silica barrier layer between a glass substrate and first layer of a four-layer AR coating improved the average 1.06 m damage threshold. Previously, Newman et al [2] had studied the influence of electric field distribution on the damage resistance of thin fims of between 1/4 and 5/4 in thickness at 1.06 μm. For high index materials such as Ti02, the damage thresholds for odd 1/4 thicknesses were greater than for even A/4 thicknesses, as expected from calculated field values. This was further developed in a subsequent study of 1.06μm reflectors [3], where increases in damage threshold were obtained by using non-quarter wave thicknesses for the top few layers of a 1/4 stack. The designs were developed to minimise the standing wave field in the high index layer, which also served to reduce the field values at the interfaces between the successive layers. Reflectors incorporating 11 1/4 layers of TiO2/Si02 were found to have thresholds of about 1J/cm2 for 30ps pulses at 1.06 μm, whilst those with non-quarter wave layers added to the top of the reflector were able to resist twice these fluence levels. The optimum thicknesses for the top layers were derived by Apfel [4]. Similar improvements in damage threshold were found at other wavelengths. For example Newnam et al [5] showed the significant increase in damage resistance at 248nm (8 nsec pulses) possible in scandia/magnesium fluoride reflectors, with thresholds increased from about 3 J/cm2 to 5 J/cm2. A discussion was also presented highlighting the role that laser pulse width may have in determining the degree of enhancement in damage threshold, and the role that coating defects would have in masking any such effect. Carniglia et al [6] assessed the enhancement possible at 355mm using such suppressed electric field designs in scandia/magnesium fluoride, reporting a 40% increase in threshold compared with the basic HR stack. However a variant of the suppressed field design, in which the thickness ratio of the top two HL pairs was reversed, performed no better than the standard design, since the peak electric field in the high index scandia layer was equivalent to that in the standard case. The role of interfaces in such designs was not fully explored. In many fabrication processes it is not possible to guarantee a perfect interface, that is, one that is free from impurity species. Furthermore, due to the high coating temperatures frequently employed for oxide and fluoride materials, the possibility of interdiffusion effects between adjacent layers is increased, with the ensuing formation of hetero-species. Such effects can be explored by depth profiling techniques, providing that the specific analytical technique used is sufficiently sensitive to thin sections of material and also to different chemical environments for the constituent elements. The distributed Bragg reflector (DBR) design is an ideal candidate for distinguishing electric field and interface effects within multilayer structures. DBR structures are usually found in waveguide devices, but advanced growth and thin film deposition techniques such as molecular beam deposition [7] now make it possible to fabricate these from a range of materials to allow assessment at a variety of wavelengths. In general Bragg reflection arises from periodic variations in the dielectric constant of a medium. Various refractive index profiles can produce reflection bands. One example is that of the sinusoidal profile, which produces only one significant reflection band. The particular DBR structure considered in this work is characterised by an essentially uniform refractive index, with very thin, sharp discontinuities incorproated at /2n intervals. The electric field distribution in such a structrue is compared in figure (1) for a partial reflector design with that of a quarter wave stack, fabricated on a glass substrate. It can be seen that the peak electric field in the quarter wave case is situated at the interface between the high and low index layers, whereas in the DBR design, the peak field is within the high index layer. The relative position of the peak within the high index layer can be shifted simply by altering the phase of the structure by adding to, or subtracting a small amount of material from the multilayer at the air/film interface. For example the peak field can be placed close to an interface, allowing its resulting effect in laser damage threshold to be assessed. Alternatively, the peak field may be allowed to remain close to the centre of the high index layer, allowing an option of exploring the effect on laser damage threshold of incorporating a few atomic layers of different material at this position. The above arguments have largely been centred on a design based on material with a high average refractive index. Similar effects also occur in the reverse case, where the optical medium is primarily of a low index material, such as barium fluoride, with the periodic profile produced by /2n disposition of a small amount of high index material. This later design is likely to be closest to the ideal for ensuring maximum laser damage resistance, due to the wide energy gap of such fluorides. This DBR structure gives a reflection peak whose magnitude varies in intensity with total number of periods and with the average refractive index change an introduced at each x/2 position. The bandwidth of the reflection peak varies inversely with An. The design is therefore fairly flexible, and has the added advantage of allowing the use of techniques for preventing the propagation of columnar microstructure [8]. This work considers the case of partial reflector designs, as may be used in laser cavity mirrors, and highlights some of the results so far achieved. The reflectors were grown in a three chambered, load-locked UHV/MBE system, fitted with 3 Knudsen sources and in situ surface diagnostics (Auger, XPS) as described previously [7]. Substrates are cleaned before film growth using a raster scanned beam of argon ions (0.5 to 3KeV as appropriate). For film deposition, the substrates were transferred to the growth chamber where growth was arranged to occur from molecular beams of the constituent materials required for the multilayer. The availability of in-situ analysis techniques allows the examination of inter-film reaction at film-film interfaces. The analysis system incorporates a computer controlled depth-profiling facility, allowing continuous or discrete sputter-etching during XPS analysis and is capable of a depth resolution of about 10-20nm under ideal conditions. The assessment facilities have been supplemented by other techniques to provide further information on the structure and morphology of the films, including cross-sectional transmission electron microscopy (XTEM). Cross-sections were prepared by cleaving, epoxy mounting and abrasive thinning to 100 μm in thickness. Further thinning was carried out using argon ions before changing to reactive gases (iodine) for the final stages. This prevents any extrinsic dislocation loops and other ion-beam damage artefacts appearing in the thickness of the specimen being examined. Optical properties were determined by conventional transmission/reflection spectrophotometry. Laser damage thresholds were determined at the GEC Hirst Research Centre, Wembley, Middlesex using a Nd:YAG laser of pulse length 10 nsec. The laser beam was focussed to a spot of 1/e2 radius 59 um at 1064 nm. Measurements were made on a single shot basis as described in the paper by Wood et al at this conference [9]. The perfection of interfaces in the DBR structures was assessed in two different ways. In the first, a multilayer of BaF2/ZnSe was specially produced with 10nm thickness of each individual layer. This was then ion beam profiled, and the chemical composition of the layers determined at discrete intervals. Problems that can arise with such measurements are normally due to non-uniformities in the ion beam raster, preventing the formation of flat-bottomed etch pits, which can make the interface appear to more diffuse than it actually is. However the actual binding energies measured for constituent elements are a sensitive indicator of the chemical environment of those elements and are not critically dependant on whether the etching is absolutely uniform or not. Three spectral scans, expanded in the region of the Zn 2p state are shown in figure (2). The upper trace was measured at the interface between a ZnSe and BaF2 layer, near the edge of the ZnSe, whilst the lower was measured near the edge of the adjacent BaF2 layer. It is clear that a shoulder, present on the low energy side of the Zn 2p3/2 line, increases in intensity as the higher energy peak falls. The second peak is indicative of a different chemical environment for the Zn atom, with the spectral shift comparable to that expected for Zn in ZnF2. The second method used for the assessment of interface perfection involved fabricating DBR designs with increasingly thinner 1/2 distributor layers. The actual designs used were as listed in table (1) and were chosen to give a resonance near the band edge of the ZnSe. This would allow any change in position of the absorption edge to be revealed. Table 1. Details of DBR Designs used for Interface Studies The transmission spectra for the three reflectors are shown in figure (3), and values of optical density at the transmission minima are compared in intensity with those calculated for the different designs using conventional matrix techniques in table (1). It is found that agreement is not good, particularly in the 10nm distributor case. This suggests that the interface may not be as clearly delineated as expected, and that the optical properties are being significantly influenced by physical imperfection or chemical reaction. Examination of XTEM micrographs of the related system BaF2/ZnS as shown in figure (4a), shows that the interface roughness' is of the order of 25Å with a period of about 150-300A. This is largely fixed by the crystallite diameter of the ZnS layers of about 100Å. Since the morphology of ZnSe and its behaviour during growth is similar to that of ZnS, with similar crystallite diameters of about 100-200Å (figure 4(b)), it is reasonable to assume that an interface spread of approximately 25Å in amplitude also occurs for ZnSe/BaF2 structures. The effect of this interface spread on optical transmission at the Bragg resonance can be explored using a model in which the interface is approximated by a simple linear grading of refractive index from the ZnS to the BaF2 and vice-versa over the distance of 25Å referred to above. For the case of the DBR fabricated using 10nm BaF2 distributors, this means as a first approximation that half of the BaF2 layer is graded as shown in figure (5). Here the basic λ/? period has been divided into nine discrete sublayers layers of differing thickness. the high index material which forms the major part of the filter (ZnSe in this case), whilst layers 5 and 6 are the unaffected parts of the BaF2 low index material. Layers 2-4 and 7-9 are layers of intermediate refractive index chosen to represent the rough interface. In the first approximation referred to above, the discrete layer profile becomes as shown in table (2), column A. |