Gary E. Loomis

Reactive evaporation of low-defect density hafnia

Motivation for this work includes observations at Lawrence Livermore National Laboratory of a correlation between laser damage thresholds and both the absorption and the nodular-defect density of coatings. Activated oxygen is used to increase the metal-oxidation kinetics at the coated surface during electron-beam deposition. A series of hafnia layers are made with various conditions: two µ-wave configuations, two sources (hafnium and hafnia), and two reactive oxygen pressures. Laser damage thresholds (1064-nm, 10-ns pulses), absorption (at 511 nm), and nodular-defect densities from these coatings are reported. The damage thresholds are observed to increase as the absorption of the coatings decreases. However, no significant increase in damage thresholds are observed with the coatings made from a low nodular-defect density source material (hafnium). Hafnia coatings can be made from hafnium sources that have lower nodular-defect densities, lower absorption, and damage thresholds thatare comparable with coatings made from a conventional hafnia source.

Laser conditioning characterization and damage threshold prediction of hafnia/silica multilayer mirrors by photothermal microscopy

Laser conditioning has been shown to improve the laser damage threshold of some optical coatings by greater than 2x. Debate continues within the damage community regarding laser-conditioning mechanisms, but it is clear that nodular ejection is one of the byproducts of the laser conditioning process. To better understand why laser conditioning is so effective, photothermal microscopy was used to measure absorption of coating defects before and after laser exposure. Although a modest absorption reduction was expected due to the lower electric field peaks within a pit and the absence of potentially absorbing nodular seeds, surprisingly, absorption reductions up to 150x were observed. Photothermal microscopy has also been successfully used to correlate laser-induced damage threshold and absorption of defects in hafnia/silica multilayer optical coatings. Defects with high absorption, as indicated by high photothermal signal, have low damage thresholds. Previously a linear correlation of damage threshold and defect photothermal signal was established with films designed and damage tested at 1(omega) (1053 nm) and Brewsters angle (56.4 degree(s)), but characterized by photothermal microscopy at 514.5 nm and near-normal angle of incidence (10 degree(s)). In this study coatings designed, characterized by photothermal microscopy, and damage tested at the same wavelength, incident angle, and polarization did not have a correlation between defect photothermal signal and absorption.

High-efficiency dielectric multilayer gratings optimized for manufacturability and laser damage threshold

Ultrashort pulse, high-intensity lasers offer new opportunities for the study of light-matter interaction and for inertial confinement fusion. A 100 Terawatt laser operating at 400 fs and 1.053 micrometers is operational at LLNL, and a 1000 Terawatt (Petawatt) laser will come online in early 1996. These lasers use large-aperture (40 cm and 94 cm diameter, respectively) diffraction gratings to compress the amplified laser pulse. At present, holographically produced, gold overcoated photoresist gratings are used: these gratings represent the fuse in the laser chain. Higher laser damage thresholds and higher diffraction efficiencies are theoretically possible with multilayer dielectric gratings (MDGs). A number of design parameters regarding both the multilayer stack and the etched grating structure can be optimized to maximize the laser damage threshold and also improve the processing latitude for the interference lithography and reactive ion etching steps used during manufacture of these gratings. This paper presents model predictions for the behavior of hafnia/silica MDGs both during processing and in operation, and presents experimental data on the diffraction efficiency and short-pulse laser damage threshold for optimized witness gratings.

Optical multilayer films based on an amorphous fluoropolymer

Multilayered coatings were made by physical vapor deposition (PVD) of a perfluorinated amorphous polymer, Teflon AF2400, and with other optical materials. A high reflector for 1064 nm light was made with ZnS and AF2400. An all‐organic 1064 nm reflector was made from AF2400 and polyethylene. Oxide (HfO2 and SiO2) compatibility with AF2400 was also tested. The multilayer morphologies were influenced by coating stress and unintentional temperature rises from the PVD process. Analysis by liquid nuclear magnetic resonance of the thin films showed slight compositional variations between the coating and starting materials of perfluorinated amorphous polymers.

Damage thresholds of fluoride multilayers at 355 nm

Fluoride multilayer coatings were evaluated for use in 355 nm high reflector applications. The LaF3/Na3AlF6, NdF3/Na3AlF6, and GdF3/Na3AlF6 multilayers had laser damage thresholds of 20 J/cm2, 17.9 J/cm2, and 7.4 J/cm2 (measured at 10 ns pulsewidths), respectively. High tensile stresses in the coatings restricted this evaluation to only 5-layer-pair partial reflectors (49 - 52%). The LaF3/Na3AlF6, NdF3/Na3AlF6, and GdF3/Na3AlF6 multilayers had tensile stresses of approximately equals 1.1 X 109, 1.3 X 109, and 9.3 X 108 dynes/cm2, respectively. Substrate material and glow-discharge processing of the substrates were found to influence the density of stress-induced coating fractures and damage thresholds in some cases. If stress fracturing and scatter can be controlled, these fluoride material combinations are suited for 3(omega) applications.

Integrated laser with low-loss high index-contrast waveguides for OEICs

Photonic integrated circuits require the ability to integrate both lasers and waveguides with low absorption and coupling loss. This technology is being developed at LLNL for digital logic gates for optical key generation circuits to facilitate secure communications. Here, we demonstrate an approach of integrating InGaAs DQW edge emitting lasers (EEL) with electron beam evaporated dielectric waveguides. The EELs are defined by electron cyclotron resonance etching (ECR). This approach results in highly anisotropic etched mirrors with smooth etched features (sidewall rms roughness = 28 Å, surface rms roughness = 10 Å). The mirror is etched to form both the laser cavity and define the waveguide mesa, which accommodates a dielectric stack, where the core is aligned with the active region of the laser to achieve maximum vertical mode overlapping. The waveguides are based on SiO2/Ta2O5/SiO2 which yields a high index contrast of 0.6, resulting in low loss guides (~2-3dB/cm). The design of the interface has taken into account the waveguide transmission loss, air gap spacing and tilt between the laser and waveguide. The critical feature for this deposition technique is its required high directionality or minimal sidewall deposition and corner effects. In the butt coupled EEL/waveguide system we have measured a slope efficiency to be as high as 0.45 W/A. We have in conclusion demonstrated a technology that allows direct coupling of a dielectric optical interconnect to a semiconductor laser monolithically fabricated on the semiconductor substrate.

Absorption and damage thresholds of low-defect-density hafnia deposited with activated oxygen

Motivation for this work included observations at Lawrence Livermore National Laboratory and elsewhere of a correlation between increasing laser damage thresholds (DT) and both decreasing nodular-defect density and absorption of coatings. We reduced the nodular-defect densities by a factor of over 4x in hafnia (HfO2) coatings deposited by reactive e-beam evaporation from a Hf target source. In order to increase the metal oxidation kinetics at the coated surface, Hf was e-beam deposited reactively with O2 activated by a (mu) -wave discharge. The effect of using activated O2 during the evaporation of a HfO2 target source was also evaluated. A series of HfO2 layers were made with various conditions; we alternated between two (mu) -wave configurations, Hf and HfO2 targets and two reactive O2 pressures. Laser DTs (1064 nm - 10 ns pulses), absorption (at 511 nm), and nodular- defect densities from these coatings are reported. The DT correlated inversely with the coating absorption.

Electrostatic reduction of particulates for laser resistant hafnia coatings

We have reduced by 50% particulate defect density of hafnia coatings deposited onto silicon substrates through the use of electric fields, physical barriers and deposition rates. In an effort to reduce the number of hafnia particulates deposited onto silicon wafers, parallel plate electrodes were placed on either side of the evaporant plume. The particulate level was determined as the deposition rate was varied from 0.75 angstroms/sec to 12 angstroms/sec. Then, parallel plate electrodes were placed on either side of the plume as a way of electrostatically deflecting hafnia particulates away from the substrates. Later a single plate electrode was used in conjunction with a physical barrier placed over the hearth. The results of our study indicate that minimal defects occur when a parallel plate electric field is applied in conjunction with a fast deposition rate. Using a screen as a physical barrier, and/or a single electrode had little or no effect. This data may be useful in the manufacture of multilayer optical coatings with high laser damage thresholds.

Characterization Of Physically Vapor Deposited Af2400 Thin Films

Anti-reflective coatings made with Teflon AF2400 had the highest damage thresholds recorded for physical vapor deposited coatings at the Lawrence Livermore National Laboratory damage facility. Physical vapor deposited layers of Teflon AF2400, a perfluorinated amorphous polymer, maintained the bulk optical properties of a high transmittance from 200 nm to 1600 nm, and a low refractive index. In addition, the refractive index can be intentionally reduced by control of two common deposition parameters, deposition rate and substrate temperature. Scanning electron microscopy and nuclear magnetic resonance observations indicated that morphological changes caused the variations in the refractive index rather than compositional changes. The coatings adhered to fused silica and silicon wafers under normal laboratory handling conditions.

Amorphous fluoropolymer: next generation optical coating candidate

Anti-reflective (AR) and high reflector (HR) optical coatings were made by physical vapor deposition (PVD) of Teflon AF2400, a perfluorinated amorphous polymer. The AR had the highest laser damage thresholds recorded for PVD coatings at the Lawrence Livermore National Laboratory damage facility. The HR was a multilayer of ZnS and AF2400. The bandwidth was 550 nm, centered at 1064 nm. Single layers of Teflon AF2400 deposited by PVD were characterized optically. The refractive index could be intentionally reduced below the bulk value by varying either deposition rate or substrate temperature. Scanning electron microscopy and nuclear magnetic resonance observations indicated that morphological changes caused the variations in the refractive index rather than compositional changes.