bluuebunny/crossref_metadata_2025 huggingface dataset:
Photothermal Measurement of Optical Coating Thermal Transport Properties
Authors: Swimm RT , Hou LJ
Published: February 2009

We report the present status of a project to measure the thermal transport properties of optical coating materials in thin-film format. The measurement technique employs a noncontact nondestructive method based on thermal diffusion-wave interferometry. Initial results confirm the feasibility of the method. Data on nickel coatings show thin film transport parameters slightly smaller than corresponding bulk values, and in addition clearly display the effects of a nonideal thermal bond between the coating and the substrate.
Mechanical Stability and Absorptance of Metal Films
Authors: McGuirk M , Perkin-Elmer
Published: February 2009

The absorptance of gold films deposited under a variety of conditions was measured to an accuracy of.0002 with an HF Laser Calorimeter. The absorptance was observed to vary from.0107 to.0091, which is an 18% change.

The specimens were analyzed with a Perkin-Elmer PHI Auger spectrometer and the absorptance was found to be directly correlated with the impurity content.

The impurities were determined to originate in the residual gas in the chamber. A Dycor residual gas analyzer was used to locate and eliminate the sources of the impurities.

Films deposited at the same rates and temperatures, but with essentially no impurities were found to have absorptance ten times greater than the original films. These films have a dramatically different structure. They have been commonly observed, and are generally described as “recrystallized.”

Impurities were intentionally added to produce an absorptance of approximately.0095. The doped films were observed to be substantially more stable than un-doped films under thermal cycling between room temperature and 225°C. The dopant atoms are believed to pin the grain boundaries. While no quantitative hardness measurements were made. The doped films are also expected to be harder.

Similar results were observed for silver. Partial recrystallization is hypothosized to explain the reflectance degradation of siver based EHR coatings reported by Dr. Don Decker in 1982.
Highly Damage Resistant Anti-reflection Coating on the Chemically Etched Surface for High Power Lasers
Authors: Yoshida K , Yoshida H , Kato Y , Yamanaka C , Ohtani M
Published: February 2009

When the surface of BK-7 glass is conventionally polished by CeO2 as polishing compound, the polishing compound is buried into the substrate. The buried compound can not be removed by standard optical cleaning techniques. Therefore, when the polished surface is anti-reflection (AR) coated and exposed to the focussed laser beam, the surface ionization occurs due to absorption at the isolated sites, leading to laser induced damages in the AR coating.

In order to improve the damage threshold, we have prepared the BK-7 substrate which was chemically etched after polishing. The damage threshold of the AR coating on the etched surface showed 1.5∼2 times improvement compared with that of un-etched surface, at the laser wavelength of 1.06 &m and 1 ns pulse width. Due to the special etching technique, the surface roughness of the etched surface was 10∼15 Årms, which was close to the surface roughness before etching (7∼12 Årms).
Influence of Solution Chemistry on the Microstructure of Sol-Gel Derived Films
Authors: Ferris KF , Exarhos GJ , Nguyen C
Published: February 2009

Recent work in this laboratory has demonstrated that the crystalline phase of titania deposited onto silica fibers by sol-gel techniques can be altered through changes in solution chemical equilibria brought about by variations in pH or chloride ion concentration. Molecular characterization of the solution microstructure and identification of the crystalline phase for deposited films is accomplished by Raman spectroscopic methods. The rutile phase of TiO2 has been identified in films deposited from solutions prepared with TiCl4 and HCl after a low-temperature densification (ca. 250°C) step. The more commonly observed anatase phase was formed without the addition of acid. Increased concentration of HCl in an alcoholic ethyl titanate solution results in increased stress in the anatase phase. A mixture of rutile and stressed anatase phases is found upon addition of LiCl to the acidic titaniumn alkoxide solution. These results suggest that the gel microstructure, which is a function of the solution chemistry, directly affects the crystal structure of the dehydrated, densified gel.
An Investigation of Laser Processing of Thin Film Coatings
Authors: Weber AJ , Stewart AF , Exarhos GJ
Published: February 2009

An initial set of experiments has been conducted to determine the practicality of laser processing of thin film coatings. A high average power CO2 laser was used to flood load the entire surface of each test sample. Thin film coatings of HfO2 and MgF2 on fused silica substrates were irradiated with power densities between 25 and 125 W/cm2.

Extensive pre- and post-test characterization revealed that some changes in the microstructure were observed, the coating optical thickness was reduced and short wavelength transmission was improved.
Influence of Post-deposition Treatment by UV Light and Oxygen (ozone) on 350 nm Damage Thresholds of SiO2 Films Deposited From Sols
Authors: Thomas I , Wilder J , Lee A , George D
Published: February 2009

Certain multilayer porous silica AR coatings on fused silica substrates prepared by the sol-gel process have been found to have a much lower laser damage threshold than single layer coatings prepared by the same method. Treatment with UV light in the presence of oxygen (which gives ozone) at low temperature was found to restore damage thresholds to the levels found in single layers. Damage thresholds were measured at 350 nm with a 25 ns pulse for 1000 shots at 25 Hz. The effect of exposure time and other factors such as replacement of oxygen with nitrogen and vacuum are described.
Laser Induced Damage of Dielectric Phase Retardation Mirrors at 1.064 Microns
Authors: Ristau D , Akhtar SMJ , Ebert J
Published: February 2009

Dielectric phase retarders are frequently used in high power laser systems for cutting and welding. The phase shift is commonly achieved by total reflection or optical tunneling.

In this paper laser induced damage thresholds of plane phase retardation mirrors are measured using single 14 ns Nd-YAG laser pulses. The results are compared to the properties of the corresponding nonshifting deflectors. We investigated in stacks of Titania, Tantala, Hafnia and Zirconia in conjunction with Silica designed for retardation angles of 45° and 90°. Laser induced damage thresholds are related to the field strength distribution in the layers.
Correlation of Damage Levels at 1.06μm Varying Modal Content and Irradiance History on Representative Coated Laser Optics
Authors: Arenberg JW , Frink ME , Mordaunt DW , Lee G , Seitel SC , Teppo EA
Published: February 2009

This paper reports on efforts to correlate two commercially oriented laser damage tests. The two test facilities are located at Hughes Aircraft Company and Montana Laser Optics.

A variety of coated laser optics have been tested under different test conditions and the damage levels compared. The test parameters varied were transverse modal content, spot size, sample conditioning history and, to a lesser degree, pulsewidth. The accumulated results were correlated by ascertaining the existence of a high probability functional relationship (linear fit) between pairs of damage levels for the same optics under different test conditions. A strong correlation was found between samples tested with the same irradiance history, regardless of the modal content and pulsewidth. Comparison of the samples of different irradiance conditioning history were found not to correlate in general.
Fabrication of Fluoride Thin Films Using Ultra-High Vacuum Techniques
Authors: Lewis KL , Pitt AM , Chew NG , Cullis AG , Wyatt-Davies TJ , Charlwood L , Dosser OD , Muirhead IT
Published: February 2009

Previous papers at this meeting have highlighted the potential afforded by molecular beam techniques in producing optical thin film structures with a high resistance to laser induced damage. These techniques have been recently applied to the growth of polycrystalline thin films of candidate fluoride materials suitable for use over a wide range of laser wavelengths. Here the chief materials of interest have been barium and lead fluoride. Despite sharing a common vaporisation process, significant differences exist in the behaviour of the two molecules on condensation. In the case of barium fluoride, the surface morphology is rough in films produced at room temperature and it is necessary to deposit at significantly elevated temperatures to obtain smooth fi1ms even in UHV. In comparison, lead fluoride films have an exceptionally smooth morphology, even when produced at ambient temperatures. Such factors have an important influence on the laser damage thresholds of the films.

In multilayer designs, some control of columnar microstructure is also possible by a stratification technique where two differing component materials are used to prevent microstructure propagation. The coating is built up by alternating the materials throughout the coating until the desired thickness is achieved. By continuously varying the relative thicknesses of the individual laminations, it is possible to produce graded index coating designs. These allow the exploration of interface effects, hitherto difficult using optical techniques.
Application of Sol-Gel Coating to Slab Laser
Authors: Inazumi T , Hara K , Izumitani T
Published: February 2009

The oscillation efficency of the slab laser has been improved with sol-gel coating film containing methyl radicals. Methyl radical in the silica coating film, effectively improves adherence and decreases refractive index(l.3756),removes pinholes and thereby improves chemical durability and mechanical strength. The sol-gel coating film was applied on the laser glass slab. The oscillation experiments showed the laser glass slab with the coating film has 1.85 times higher output energy and 2.3 times higher slope efficiency as the laser slab without the coating film.
Comparison of the Optical Properties of some High-Index Oxide Films prepared by Ion Beam Sputter Deposition with those of Electron Beam Evaporated Films
Authors: Pond B , Schmell RA , Carniglia CK , Raj T
Published: February 2009

A typical thin film of a high-index oxide coating material deposited by electron-beam (E-beam) evaporation has a refractive index lower than the bulk value for the material. This indicates that the film is porous, having a density less than the bulk density. In addition, such films are often inhomogeneous, having a higher refractive index near the substrate surface. Thin films of the same materials deposited by ion beam sputter deposition (IBD) techniques tend to have higher and more uniform refractive indices, indicating that IBD films are denser and more homogeneous. We have measured the index of refraction n, the extinction coefficient k and the degree of inhomogeneity of several high-index oxide materials deposited by IBD and by standard E-beam evaporation. The materials investigated were the oxides of zirconium, tantalum, hafnium, niobium and aluminum. In all cases, the IBD films were found to have higher refractive indices than their evaporated counterparts. The IBD films, in general, exhibited an inhomogeneity which was the reverse of the inhomogeneity found in the E-beam films. This indicates a difference in the nucleation and growth of the films. Tantala films show an absorption band in the visible spectral region resulting from electron trapping sites. Thus, this material is probably not suitable for coatings for visible and UV laser systems.
An Analysis of Rugate Filter Behavior
Authors: Becker RJ
Published: February 2009

An analysis is made of the performance of a Bragg filter of arbitrary modulation profile in terms of solutions to the wave equation for a single repeat unit. This approach has advantages over the standard multiplication of matrices corresponding to very thin but flat steps in that it allows for rapid calculations and analytic studies. Special attention is given to filters with sinusoidal profiles. The transfer matrix for the filter is expressed in terms of the wavefunction and its first derivative. In the case of sinusoidal modulation, the wavefunctions are Mathieu functions. In all cases, simple relationships exist between centerband frequencies and linewidths, and the basic characteristics of the filter, such as the mean index and modulation depth. The results of computer simulations used in parametric studies of filter performance are presented and compared with analytic predictions, especially those of Coupled Mode Theory. It is found that corrections to the simplified theory must be made to account for the finite width of the filter. One benefit of the adopted approach is that it gives directly the strength of the optical electric field as a function of position in the filter.
Photon Induced Desorption and Emission from Thin Film Dielectric Surfaces
Authors: Kardach JA , Stewart AF , Guenther AH , McIver JK
Published: February 2009

Results from 1-on-1 and N-on-1 laser induced damage experiments, together with associated charged particle emission from 22 dielectric thin films on fused silica, sapphire, and calcium fluoride substrates are reported. A Q-switched Nd:YAG laser (1.06 microns, 5 nsec (FWHM), 500 microns 1/e2 diameter) was used to irradiate the half wave (at 1.06 microns) thin films. The experiments were performed in a UHV vacuum chamber with a base pressure of 10-8 Torr, and were computer controlled.

Preliminary analysis indicates that the damage threshold for dielectric thin films increases with the substrates thermal conductivity, even for these very short laser pulses. Laser damage thresholds and charge emission curves are presented and characterized as a function of the incident laser energy density.
The Effect of Layer Thickness Errors in the Design of Coatings with Reduced Electric Field Intensity
Authors: Keller GL
Published: February 2009

Many papers have been written on the design of laser-damage-resistant high reflectors. Usually, reduction in the peak electric-field intensity in the outermost layers of a multilayer stack is studied.

In this work, the effect of random and systematic errors in the quarter-wave stack below the enhanced layers is investigated. Also, optical monitoring errors in the optimized layers and their relation to the standing-wave electric field through the multilayer stack are discussed.
Wavelength Dependence of Laser-Induced Sputtering from the (111) Surface of BaF2
Authors: Reif J , Fallgren H , Cooke WE , Matthias E
Published: February 2009

Using blue tunable pulse laser radiation of low fluence, we have investigated laser-induced sputtering from cleaved BaF2 (111) surfaces under ultrahigh vacuum conditions. Timecorrelated with the laser pulses we observed the positiveions Ba+, Ba++, (BaF)+, and F+. Practically no negative ions were found. Neutral atomic fluorine (F°) was desorbed abundantly. A pulse correlation of F° as well as the relative amount of F° and F+ could not be established at this stage. The emission yield of all positive ions as well as of F° was strongly wavelength dependent and showed a broad resonance around 2.9 eV.
Impact of Thermal Blooming on Laser Damage Measurement Accuracy
Authors: Newnam BE
Published: February 2009

The presence of thermal blooming in laser damage experiments conducted in air frequently goes unnoticed. For certain conditions, however, this effect can result in significant errors in determining the magnitude of damage thresholds.

Thermal blooming of a laser beam is a direct result of nonuniform heating of the transmitting medium (air) across the laser beam diameter. The resultant changes in refractive index cause the laser beam to diverge. In a laser damage experiment the test sample is often placed in the focal plane of a positive lens. With thermal blooming, however, the actual focus of the high-intensity laser beam can occur significantly in front of the sample, resulting in overestimates of the damage threshold.

Using the formulation of Skinner for a focused Gaussian beam, we calculate the magnitude of possible errors for damage thresholds of surfaces and thin films measured in air with various wavelengths. Measurements reported by several laboratories fall in the parameter space for which corrections for the effects of thermal blooming are recommended.
Characterization of Calcium Fluoride Optical Surfaces
Authors: Law M , Bender J , Carniglia CK
Published: February 2009

A calcium fluoride polishing study has been conducted in an effort to reduce those characteristics which contribute to optical scatter effects. Using both quantitative and qualitative diagnostic techniques, the surface and subsurface characteristics produced by various polishing procedures have been either measured or rank ordered to optimize the polishing process itself. Diagnostic techniques have included bright light illumination, surface profilometry, Nomarski microscopy, total internal reflection microscopy (TIRM), and total integrated scatter (TIS). These techniques have been applied to polished surfaces and to polished surfaces that had been etched with acid. The examination of acid-etched polished surfaces has revealed the presence of subsurface polishing damage which is concealed beneath the polished surface and is undetectable by routine observation.
An Investigation of Laser Processing of Silica Surfaces
Authors: Weber AJ , Stewart AF , Exarhos GJ , Stowell WK
Published: February 2009

An initial set of experiments has been conducted to determine the practicality of laser processing of optical substrates. In contrast to earlier work, a high average power CO2 laser was used to flood load the entire surface of each test sample. Fused silica substrates were laser polished on both surfaces at power densities ranging from 150 to 350 W/cm2. During each test sequence sample surface temperatures were recorded using a thermal imaging system.

Extensive pre- and post-test characterization revealed that surface roughness and scattering of bare silica surfaces were reduced while internal stress increased. Laser damage thresholds were found to increase only for certain conditions. Changes in the microstructure were observed. These preliminary experiments have demonstrated that laser processing can dramatically improve the optical properties of fused silica substrates. Optimization of processing parameters will be the subject of future work.
Spot Size Dependent Laser Materials Interactions Due to Surface Electromagnetic Waves
Authors: Swimm RT , Bass M , Fathe L , Lin JZ , Kurtz R
Published: February 2009

Measurements of the transient reflectivity change due to heating by a surface electromagnetic wave are reported. Initial results are inconclusive with regard to the existence of surface electromagnetic waves. Implications concerning the effects upon the laser-induced damage threshold are discussed.
Super-Polished Single Crystal Molybdenum Mirror for XUV ∼ X-Ray Radiation
Authors: Yoshida K , Takeuchi K , Kato Y , Fujiwara E , Yamanaka C , Yamashita K , Kurosawa K , Sasaki W , Uehara K , Okamoto H
Published: February 2009

High quality single crystal molybdenum (Mo) fabricated by secondary recrystallization method has been polished with a new polishing technique. The super-polished surface of the Mo single crystal has the surface roughness of 3.5Årms. THis super-polished single crystal Mo has been used for a high power electron-beam pumped argon excimer laser (λ=126nm) as a cavity mirror. Peak output power of more than 3MW has been achieved without any damage on the mirror surface for the first time.
Damage Morphologies and Cumulative Behavior of Laser Damage on Single Crystal Metal Surfaces
Authors: Jee Y , Becker MF , Walser RM
Published: February 2009

We have observed laser induced damage morphologies and the accumulation dependence of damage on single crystal metal surfaces under Q-switched Nd:YAG laser irradiation (1064 nm, 10 nsec pulses with a 240 μm spot diameter). The samples were chemically-polished copper and electro-polished aluminum of several crystallographic orientations.

Several different damage morphologies have been observed: slip-line formation, ripple patterns, flat-melting, and boiling. Damage probability versus fluence curves and accumulation curves were plotted to investigate damage behavior as it correlates with morphology. Flat-melting damage was observed near the 50% damage fluence on chemically polished copper surfaces, and slip-lines were found near the 50% damage fluence on electropolished aluminum surfaces. Surface defects produced during sample preparation greatly influenced the damage threshold of copper because of its high melting threshold. The influence of surface defects on copper decreased with increasing numbers of laser pulses. Accumulation curves show different damage behavior for crystals of different orientation. Accumulation was the largest on (111) Cu and Al surfaces, and the single shot damage threshold of these surfaces was less than the other crystal orientations for both Cu and Al.
Initiation of Laser Induced Damage by Small Metallic Particles
Authors: Becker W , McIver JK , Guenther AH
Published: February 2009

Laser induced damage in a dielectric material is triggered by the presence of a sufficient number of electrons in the conduction band. When these are accelerated by the laser field with phonon assistance, they can lead to an electron avalanche and, in turn, may initiate permanent detectable damage. We have investigated the precondition of the electron avalanche model, i.e. the initial creation of electrons in the conduction band. We assume the presence of small metallic inclusions (r ∼ 10 to 100Å) in the dielectric which we model by infinitely deep one-dimensional square well potentials. The dynamics of such a model are known to be chaotic if the laser field exceeds a certain threshold value. Under these conditions an electron can gain enough energy to bridge the gap between the valence band and the conduction band. The present model yields threshold fields associated with intensities of the order of 1010W/cm2. Unfortunately, the dependence on the wavelength is complicated, however, a threshold value exists for that wavelength above which no damage occurs no matter how strong the field is.
Some Comments On: Temperature Dependent Effects In Laser Induced Damage
Authors: Lange MR , McIver JK , Guenther AH
Published: February 2009

The temperature dependence of various processes can be extremely important in laser induced damage mechanisms. This has been well known since study in this field began. The ways in which temperature plays a role are both varied and many. However, one must be careful not to attribute too much of the responsibility for damage to temperature effects alone. Its specific roles must be carefully characterized and quantified.

On more than one occasion, there have been suggestions of temperature dependent absorption as an important pulsed laser damage mechanism in dielectrics. Similarly, there have been a collection of works supporting theories which do not significantly involve temperature. A comparison of both macroscopic and microscopic scales indicate that, in general, damage must consist of a natural series of evolutionary events that generally involve both temperature independent effects combined with those that are temperature dependent. The series of events that dominate in any process leading to damage depends critically upon the structure and state of the material involved and the photon energy that is incident, to name but the two most obvious. Thus, they must be tediously reconstructed for each given interaction, though certain cases, e.g., classes of material, may be generalized to some extent.

Recent works by the present authors have suggested the level of absorption involved in damage of certain optical films. Subsequently, some of the mechanisms for absorption were assumed and analyzed. In the present work some perspective is placed upon the role temperature plays in the steps leading up to damage.
Laser Induced Damage Threshold Derived from Chaotic Electron Dynamics
Authors: Becker W , McIver JK , Guenther AH
Published: February 2009

An important stage in the laser induced damage of a dielectric material is a mechanism-initiated triggering by the presence of sufficient electrons in the conduction band. When these electrons are subsequently accelerated by the laser field under photon assistance, they can lead to an electron avalanche, and, in turn, may initiate visible damage.

We have investigated the precondition or presetting of the electron model, i.e. the initial creation of electrons in the conduction band. We assume the presence of small metallic inclusions (r ∼10 to 100 Å) in the dielectric which we model by infinitely deep one-dimensional square well potentials. The dynamics of this model are known to be chaotic if the laser field exceeds a certain threshold value. Under these conditions the electrons can gain enough energy to bridge the gap between the valence band and the conduction band.

The model yields threshold fields of the order of 1010 W/cm2. However, the dependence on the wave length is complicated.
The Mechanism of Prebreakdown Nonlinear Energy Deposition From Intense Photon Fields at 532 nm in NaCl
Authors: Jones SC , Shen XA , Braunlich PF , Kelly P , Epifanov AS
Published: February 2009

We present direct experimental evidence that significant amounts of energy can be deposited in NaCl by short laser pulses of 532 nm wavelength without electron avalanche formation. The mechanism is four-photon free carrier generation and free carrier absorption with contributions by self-trapped holes. We also report the first reliable measurements of four-photon absorption cross sections in any wide-gap ionic solid.
Nonlinear Absorption Initiated Avalanche Breakdown in Dielectric ZrO2
Authors: Mansour N , Canto E , Soileau MJ , Van Stryland EW
Published: February 2009

We report direct measurements of nonlinear transmission in dielectric ZrO2 prior to the observation of catastrophic damage for nsec laser pulses at 532 nm. The tested cubic zirconia (ZrO2) specimens are stabilized with yttria (Y2O3) having concentrations of 9.4%, 12%, 15%, 18%, and 21%. Transmission measurements were performed for a broad range of pulsewidths from 40 psec to 18 nsec (FWHM) at 532 nm. Our results indicate that the nonlinearity prior to breakdown is mainly attributed to the formation of material defects (color centers) by two-photon absorption (2PA) at high irradiance. These induced color centers appear to be long-lived (the order of hours) but can be bleached by repeated irradiation with relatively low irradiance 532 nm pulses. The accumulated effects of color center absorption during the 2PA measurements makes extraction of the two-photon absorption coefficient difficult. However, the smallest value for the 2PA coefficient (∼0.04 cm/GW) was measured in the ZrO2 sample stabilized with 9.4% Y2O3 for the shortest pulsewidth (40 psec) with which the measurements were performed. The observed nonlinear transmission prior to breakdown is consistent with the interpretation that damage in this material is due to 2PA induced avalanche breakdown.
Influence of Impurities and Defects on 1064 nm and 532 nm Damage in Quartz
Authors: Merkle LD , Bandyopadhyay PK
Published: February 2009

It is axiomatic that even low concentrations of defects or impurities can control the damage behavior of transparent solids. Following the observation, reported in the 1985 Symposium, of strikingly different damage properties in two apparently similar samples of crystalline quartz, a study of several grades of quartz has been made to determine the influence of impurities and structural defects on their damage resistance. Data were taken at 1064 nm and 532 nm using a Q-switched YAG laser.

It has been found that considerable differences in single and multiple pulse damage “thresholds” exist among quartz samples of varying quality. These variations do not correlate with the concentrations of the most common impurities, Al and OH, but they appear to correlate with the density of coarsely spaced defects such as inclusions or etch channels (dislocations). These results tend to rule out damage mechanisms involving point-like defects, emphasizing instead the importance of larger impurities, even if submicroscopic.
Digital Fluence Profiling System and Software
Authors: Seitel SC , Rypma RL
Published: February 2009

An advanced digital fluence profiling system based on a windowless GE TN2502 or TN2509 silicon array camera and Poynting Products 505RM frame grabber is described. The camera array is located precisely in the target plane or equivalent, and synchronized to the pulsed laser source with a flashlamp radiation sensor circuit. Data acquired by the frame grabber are transferred to an HP Series 200 computer for analysis.

Analysis is performed with BEAMCODE 2.0 software which permits near real time extraction of the peak fluence to total energy ratio in a single pulse to better than 20 percent. When corrected to camera nonuniformity and compared to large-aperture calorimetry, accuracy exceeds 10 percent. Other functions include measurement of pointing stability (beam wander) and divergence, fit-to-gaussian, and graphics data displays in contour and pseudo-3D formats.
Simple Accurate Inversion of Knife-Edge Data from Radially Symmetric Laser Beams
Authors: O'Connell RM , Vogel RA
Published: February 2009

The intensity profile of a radially symmetric beam is shown to be related to the inverse Abel transform of knife-edge-scan data. Two inversion algorithms, previously applied to problems in plasma diagnostics, are described and applied to knife-edge data from a confocal unstable resonator laser. The resulting profiles are shown to be in good agreement with each other but significantly different from the profile obtained by assuming the beam to be approximately Gaussian. As a beneficial consequence, the results are shown to imply a significantly higher laser damage threshold for polymethylmethacrylate than previously reported.
Investigation of Optical Damage via Resonance Ionization Mass Spectrometry
Authors: Estler RC , Nogar NS
Published: February 2009

Resonance ionization mass spectrometry (RIMS) has been used as a diagnostic for interrogating optical damage events. RIMS involves multistep laser photoionization, generally through real intermediate states, followed by mass separation and detection. It is a sensitive and selective means of detection, and has the potential for detection of ultratrace inclusions in substrates and/or coatings.

In initial experiments, damage on uncoated CaF2 substrates was initiated by pulses of 1.06-μm light from a Q-switched Nd+3:YAG laser. Interrogation of the spalled plume revealed CaF radicals only when damage events occurred. No Ca atoms were observed, and no attempt was made to detect F atoms or CaF2. In subsequent experiments conducted at laser intensities below the damage threshold, we observed the presence of surface adsorbents and a correlation between the presence of the adsorbents and the occurrence of optical damage.
Laser Mirror Profilometry by High-Q Interferometry
Authors: Rotge JR
Published: February 2009

A novel interferometric technique is described which exploits the relationship between an optical resonator's length and resonant frequency. This concept has potential for measuring length variations of an order of « l Λ. This paper briefly describes the concept, the experiment performed to demonstrate the concept, and the experimental, results. Results obtained indicate a repeatability in the measured data to ∼/5 Λ rms.
Design and Construction of a Single Polarizer Infrared Ellipsometer/Reflectometer for Characterization of Highly Curved Surfaces
Authors: Stubbs JB , Loomis JS , Leonard TA
Published: February 2009

The University of Dayton has designed and built an Infrared Ellipsometer/ Reflectometer (E/R) to characterize curved surfaces in support of the ALPHA Laser Program. The instrument is tunable in wavelength and angle of incidence, is remotely controlled by a dedicated microcomputer, is positioned and scanned by a six-degree-of-freedom translation stage, and makes simultaneous measurements of Rp, Rs, Ψ, and Δ.

A return path configuration was chosen to provide the versatility necessary for the variety of curved surfaces to be used in the ALPHA laser. This E/R is capable of measuring Rp and Rs to 0.1% accuracy and Delta to better than 0.1° accuracy on curved surfaces to 1.0 centimeter radius of curvature.

The optical design and theory of operation as well as automatic alignment and focusing features are discussed. Data taken with this instrument are presented.
Laser-Induced Damage in Doped and Undoped Fluorozirconate Glass
Authors: Mansour N , Guha S , Soileau MJ , Bendow B , Martin D
Published: February 2009

Results of laser-induced damage studies in doped and undoped fluorozirconate (FZ) glass in the system of ZrF4-BaF2-LaF3-AlF3 are reported. Measurements were made at 1.06 and 0.53 μm for pulsewidths of 45 psec and 20 nsec (FWHM), respectively. The breakdown thresholds are significantly reduced for material doped with Cr, Mn, Ni, and Nd. The decrease in breakdown thresholds at the fundamental wavelength (λ) for the doped material was found to be roughly proportional to the linear absorption of the specimen at the second harmonic (λ/2). This suggests that two-photon absorption (2PA) is instrumental in initiating damage in the doped specimens.
Multiple Pulse Damage Studies of BK-7, KCl and SiO2 at 532 nm
Authors: Kitriotis D , Merkle LD , Dodson A
Published: February 2009

We report the results of damage studies at 532 nm on BK-7 glass, KCl, fused silica and two grades of crystalline quartz using a frequency doubled, Q-switched YAG laser. This work represents the initial stages of a program to investigate the mechanisms of multiple pulse laser-induced bulk damage in transparent solids. Comparison of BK-7 damage behavior with that of fused silica shows little effect due to the color center generation which occurs in BK-7 due to its small band gap. Commercially grown KCl exhibits multiple pulse catastrophic damage at fluences significantly below single pulse “threshold”, probably due to grown-in defects. Laser-induced damage behavior has been compared between two grades of crystalline quartz, with the surprising result that the nominally less perfect material is the more resistant to damage. The presence of “etch tunnels” may be involved, but considerably more study is required.
Infra-Red Absorption Spectroscopy of Nd:YAG and Nd:GSGG Surface Contaminants
Authors: Acharekar MA
Published: February 2009

Neodymium doped yttrium aluminum garnet (Nd:YAG) is a commonly used laser material for solid-state high power, high PRF lasers. Gadolinium scandium gallium garnett doubled doped with neodymium and chromium (Nd:Cr:GSGG) shows higher efficiency than the Nd:YAG laser. We previously reported the bulk and surface damage thresholds of the Nd:Cr:GSGG crystal. It was shown that the bulk damage threshold of the Nd:Cr:GSGG was as high as the Nd:YAG crystal.

The surface damage threshold measurements for the Nd:YAG and the Nd:Cr:GSGG are reported in this paper. Infrared absorption spectroscopy was used to analyze surface contaminants. The data show that the organic contaminants, CHn, derived from hydrocarbon (skin oil) associated with improper parts handling, signifantly reduce the surface damage threshold.
Excimer Radiation Colouration Processes and Surface Damage to Single Crystal CaF2 Substrates
Authors: Laidler I , Emmony DC
Published: February 2009

This paper considers the effects of a 249nm excimer pulse on CaF2 substrates and is concerned with both surface damage and bulk colouration processes.

The effect of different polishing procedures on the surface damage threshold of CaF2 has been investigated.

CaF2 has been shown to develop colour centres under prolonged U.V. irradiation. A technique has been developed to monitor the transient excited states created in the bulk material when irradiated with a 25ns excimer pulse. This involves a simultaneously pumped dye laser pulse probe operating at the absorption wavelengths of the excited species. From the results information about the photon absorption cross sections can be derived, the concentration of defect states at any time during the excimer pulse may be estimated and a knowledge of the maximum safe fluence levels established.
CO2 Laser Induced Damage to Germanium: Relation to Surface Preparation
Authors: Gibson DR , MacDonald CM , Wilson AD
Published: February 2009

The laser damage behaviour of germanium is examined in relation to surface preparation methods including alumina polishing, “Syton” polishing, diamond machining and plasma etching. Data on laser damage threshold (LDT), damage morphology, surface absorption, surface roughness and the transmittance of the laser pulse (TLP) are used in conjunction with a thermal model in an attempt to identify the various mechanisms responsible for surface damage in Ge. Alumina and “Syton” polishing result in significantly different LDTs, morphologies and TLPs which can be directly correlated with the predictions of a thermal model based on laser heating of embedded polishing particles. Plasma etching removes polishing debris and thus increases the LDT. Diamond machining gives a wide range of LDTs related to variations in surface topography and work damage. A practical ranking of the effect of surface preparation method on LDT is presented.
Glancing Incidence Absorption Measurements of Metal Mirrors
Authors: Kimura WD , Ford DH
Published: February 2009

Glancing incidence absorption measurements are performed on bare Cu, Mo, Au, Al, and Ag mirrors, and on Al and Ag mirrors with a protective dielectric overcoat. Data is obtained for angles of incidence from 0° to ⩾89°. Wavelengths studied are 0.5 μm and 10.6 μm at both s and p polarizations. The absorption is measured using a photoacoustic calorimetry technique in which the transducer is attached directly to the back of the test mirror. Absolute calibration of the absorption data is obtained by using a ratiometer to independently measure the absorption at near normal incidence.

The results are compared with the absorption predicted by Fresnel's equations for bare metals using values of n and k of the complex index of refraction measured by other sources. The absorption of a mirror at glancing incidence appears to be a sensitive function of its surface condition. New mirrors tend to agree with Fresnel's equations; while “old” mirrors have much higher absorption probably due to a layer of tarnish. The overcoated Al data deviates greatly from Fresnel's equations indicating that bare metal theory cannot be used and a thin film theory should be used instead. Representative results are given.
Multifacet, Metal Mirror Design for Extreme-Ultraviolet and Soft X-Ray Free-Electron Laser Resonators
Authors: Newnam BE
Published: February 2009

Resonator mirrors with reflectance greater than ~40% are required if free-electron laser (FEL) oscillators are to be extended to wavelengths below 100 nm. The principle of total-external reflection has been used to design multifacet mirrors of several metal films that surpass this requirement between 10 and 100 nm. These reflectors have high reflectance over a relatively broad band which suits well the inherent FEL wavelength tunability. The practical limitations to achieving and maintaining high reflectance include oxide- and carbon-contamination, scattering, and surface-plasmon absorption.
The Design of Optimum High Reflectivity Coatings for Grazing Angles of Incidence
Authors: Shellan JB
Published: February 2009

Although quarter-wave multilayer dielectric coatings (QW MLDCs) usually offer the highest reflectivity possible at a single wavelength, this may not be the case at grazing angles of incidence. For incident angles greater than the Brewster angle, the p polarization absorption can easily be several orders of magnitude higher than the s polarization absorption due to the destructive interference between the reflection from the superstrate — top layer interface and the rest of the coating. An analytic approach is developed for designing optimum reflectivity coatings at grazing angles of incidence, once the fraction of s and p polarized light is given. These coatings can give at least an order of magnitude reduction over QW MLDCs in total coating absorption, even for the case where less than 0.1% of the incident radiation is p polarized. The absorption of the designs found from the analytic results compare favorably with designs generated using numerical methods of non-linear optimization.
Thermal Conductivity of Enhanced High Reflectors
Authors: McGuirk M , Poirier RD
Published: February 2009

Perkin-Elmer has designed and fabricated a simple thermal conductivity tester for thin film coatings. The work is motivated by the results Don Decker, et al., reported at this conference last year. 1 Our measurements confirm his finding that the thermal conductivity of thin films is substantially less than the thermal conductivity of the corresponding bulk material. In one case the measured thermal conductivity of an enhanced high reflectance coating is a factor of 30 less than the conductivity predicted from bulk values.

Perkin-Elmer has considered the theoretical reasons for the low conductivity, and methods for increasing the conductivity. Increased thermal conductivity should increase the laser damage threshold for CW applications.
Properties of IAD Al2O3/SiO2 and Ta2O5/SiO2 Thin Film AR Structures
Authors: McNally JJ , Williams FL , Wilson SR , McNeil JR
Published: February 2009

We have investigated the properties of Al2O3/SiO2 and Ta2O5/SiO2 anti-reflection coatings which have been fabricated using ion-assisted deposition (IAD) techniques. The coatings were deposited with simultaneous 02+ bombardment during film growth. Preliminary laser damage results are presented. We describe the importance of oxygen-ion energy and flux in applying IAD to dielectric materials. Results illustrating increased values of film refractive index, reduction in optical scatter and improved environmental stability are reported.
Ion Assisted Deposition of Optical Films on Heavy Metal Fluoride Glass Substrates
Authors: McNally JJ , Al-Jumaily GA , McNeil JR , Bendow B
Published: February 2009

Heavy metal fluoride glass materials are attractive for optical applications in the near ultraviolet through infrared wavelength regions. However, many compositions are relatively soft and hygroscopic, and possess low softening temperature (250°C-300°C). We have applied ion assisted deposition (IAD) techniques to deposit MgF2, SiO2 and Al2O3/SiO2 thin film structures on fluoride glass substrates at ambient substrate temperature (̃100°C). The coatings deposited using IAD improve the environmental durability of the fluoride glass and appear to have reasonably good optical charateristics; without application of IAD, the deposited coatings are not durable and have poor adhesion.
Ion Beam Characterization of Multi-Layer Dielectric Reflectors
Authors: Beery JG , Hollander MG , Maggiore CJ , Redondo A , Westervelt RT , Taylor TN
Published: February 2009

Energetic ion beams were used to characterize multilayer dielectric reflectors. Alpha-particle beams with beam spot sizes between 10 microns and a few millimeters were scattered from reflectors consisting of 32-layer SiO2/HfO2 and 38-layer MgF2/ThF4. The RBS spectra reveal the nature of the laser damage processes by providing information on diffusion, mixing, and loss of material in the coatings. The particle-induced x-ray emission (PIXE) technique gave complimentary results on low-concentration impurities in the coatings.
Raman Microprobe of Laser-Induced Surface Damage Regions in TiO2 and ZrO2 Coatings
Authors: Friedrich DM , Exarhos GJ
Published: February 2009

The anatase-to-rutile phase transition, which attends pulsed laser-induced damage of anatase TiO2 thin films, is studied by simultaneous microphotography and Raman microprobe. The damage is distributed across the damage area in regions of distinct morphology and spectra. In multilayer coatings, a mechanical surface-fracture zone surrounds a set of thin, layered flat rings of distinct but irregular boundaries. These rings surround a rough, pitted transparent central crater in the quartz substrate. Raman difference spectroscopy is used to reveal damage regions containing the rutile phase and regions devoid of TiO2. Similarly, Raman microprobe spectra reveal the monoclinic crystalline form of ZrO2 in damaged regions of amorphous ZrO2 coatings. Finally, frequency shifts of the anatase v6 line (nominally 143 cm-1) are used to identify stress in laser-pulse irradiated anatse coatings.
Probing Structural Properties of Amorphous Films With Raman Spectroscopy
Authors: She CY , Hsu LS
Published: February 2009

Using an oblique geometry, we have previously demostrated that Raman spectroscopy be used to characterize submicron-thick optical films. With this technique, we have discovered and studied amorphous-crystalline and anatase-rutile transformations in various types of titania films. These studies were however done with treatment and diagnostics carried out in a sequential manner. One objective of this paper is therefore to establish the feasibility of using in-situ Raman spectroscopy to monitor the process of crystallization and structural transformation of optical thin-films in real time. A second objective is to demonstrate that Raman spectroscopy can be used to detect microscopic differences existed in amorphous thin films. We show that the microscopic differences in three otherwise similar types of amorphous titania films may be correlated with their obvious structural differences upon thermal annealing.
Time-Resolved Temperature Determinations From Raman Scattering of TiO2 Coatings During Pulsed Laser Irradiation
Authors: Exarhos GJ
Published: February 2009

Simultaneous measurement of Stokes/anti-Stokes Raman intensitities for a particular vibrational mode allows instantaneous surface temperatures to be determined. Time-resolved Raman spectra were measured on a specially constructed spectrometer using pulsed laser excitation and gated detection techniques. Measurements on equilibrated coatings and uncoated substrates at several temperatures served to confirm the utility of the technique for temperature determination as well as calibrate the intensity response of the instrument.

Pulsed irradiated SiO2 and Y(PO3)3 glass substrates yield no temperature increase at laser energies where surface damage is not apparent. At higher pulse energies, an increase in anti-Stokes scattered radiation is observed, signifying a temperature rise. Analogous results are seen in Raman spectra of low energy pulse-irradiated TiO2 coatings recorded at delays of 0 and 72 nsec from the excitation pulse. At higher energies, increased coating temperatures are observed at these times as well as at short times preceding the peak maximum of the excitation pulse. Raman results are used to model the thermal response of pulsed laser-irradiated dielectric coatings.
Influence of Deposition and Post Deposition Conditions on Laser Induced Damage in Electron Beam Evaporated TiO2 and SiO2 Films
Authors: Rao KN , Murthy MA , Mohan S , Rao MR
Published: February 2009

Composition and structure of oxide films vary with different deposition techniques, conditions and starting materials, resulting in different damage thresholds. In this investigation, single and multi layer films of TiO2 and SiO2 were deposited with TiO and SiO2 as starting materials. The substrate temperatures were maintained 25, 75, 125 and 175 ° C and post heat treatment was carried out at 75, 125, 175 and 225° C in air. These films were subjected to laser damage threshold studies, using Q-switched Nd:YAG Laser of 8 to 9ns. pulse width.

Maximum threshold values obtained were 19 J/cm2 for TiO2 films deposited at 25 °C and post heated at 75 °C; 44J/cm2 for SiO2 film deposited at 25 °C and 18J/cm2 for multilayer stacks of TiO2/SiO2 deposited at 25 °C. Variation in damage threshold values is explained on the basis of film composition, structure and optical properties.