Richard A. Hoffman

Moisture resistant optical films: their production and characterization

We have used an accelerated humidity test consisting of a 24-h cycle between 27-50 degrees C at 95% relative humidity to rate the moisture resistance of typical protective films such as CaF(2), LaF(3), and fluorinated polymers (TFE, FEP) deposited on BaF(2) and NaCl substrates. The test life of BaF(2) specimens was increased from about 4 cycles to over 9, 26, and 42 cycles when protected by CaF(2), TFE, and LaF(3), respectively. Heat treating the as-deposited CaF(2) films further improved specimen life by a factor of 2. The lifetime of NaCl specimens was improved from a fraction of a cycle to 1 and 3 cycles, respectively, by CaF(2) and TFE coatings. No adherent LaF(3) films could be deposited on the salt. Microhardness measurements of the CaF(2) films showed a direct correlation with humidity resistance. The effect of heat treatment was apparently to sinter the fine CaF(2) particles composing the film, thus increasing film density, refractive index,and hardness while reducing moisture permeability. Caf(2) and TFE films function as antireflective, as well as protective layers, on BaF(2) and NaCl.

Pulsed laser damage characteristics of vapor-deposited copper mirrors on silicon carbide substrates

Pulsed 100-nsec 10.6-microm laser damage characteristics of composite bare copper mirrors were determined. The mirrors were prepared by vapor deposition of a copper film on polished silicon carbide substrates formed by chemical vapor deposition, Comparisons are made with a bulk oxygen-free high-conductivity (OFHC) copper mirror prepared by precision diamond turning. Vapor-deposited Cu on SiC was found to be inherently superior in resisting surface damage related to defects and to thermal stress. However, melt thresholds are comparatively lower. The overheating problem was analyzed by a thermal modeling of thresholds measured on mirrors having different film thicknesses. Results are consistent with reduced thermal conductivity in the SiC substrate as compared with bulk Cu. The effect of the interface was evaluated from samples that had been ion polished prior to deposition and on others where an intermediate chromium layer was deposited. The scattering and figure of the mirrors are also reported.

Ion polishing of copper: some observations.

The change in absorptivity at 10.6 microm of copper samples, at various stages of polishing including ion polishing, is reported. The ion polishing apparatus is also described; it employs a low energy Xe ion beam incident at a glancing angle to maximize the removal rate while minimizing ion penetration and damage. The sample to be polished is rotated and the beam scanned across the surface to produce uniform removal of material. Starting with single crystal copper that had been mechanically polished and by using combinations of ion polishing and vacuum annealing, the absorptivity, determined by calorimetry, was reduced from 0.92% to 0.76%. Although optical figure and scatter were not monitored, SEM pictures show that the surface features become smoother with ion polishing. A similar experiment was performed on a single crystal copper sample that had been electropolished, and the combination of ion polishing and annealing lowered the absorptivity from 0.97% to 0.89%.