Linda F. Johnson

Optical and thermal properties of spinel with revised (increased) absorption at 4 to 5 μm wavelengths and comparison with sapphire

Abstract. Infrared absorption of high-quality, commercial, polycrystalline MgAl2O4 spinel is ∼40% greater in the range of 3.8 to 5.0 μm than the value predicted by the computer code OPTIMATR®, which has been used for window and dome design for more than 20 years. As a result, spinel and a-plane sapphire windows designed to support the same external pressure with the same probability of survival have approximately the same infrared absorptance in the range 3.8 to 5.0 μm. c-Plane sapphire has greater absorptance than spinel in the range 3.8 to 5.0 μm. Spinel has two weak absorption bands near 1.8 and 3.0 μm. At 1.064 μm, the laser calorimetric absorption coefficient of spinel is 10 to 50 times greater than that of sapphire. New measurements of specific heat capacity, thermal expansion, thermal conductivity, elastic constants, and refractive index (including dn/dT) of spinel are reported.

Slope distribution of a rough surface measured by transmission scattering and polarization

Transmission scattering from medium to air was used to measure the slope distribution of the rough plane surface of a transparent glass hemisphere. A facet model successfully explained the measured results of refraction, scattering, and polarization: Transmission scattering existed for incident angles greater than the critical angle, all measured curves for the normalized scattered intensity versus the facet slope angle for different detection directions overlapped, and the measured polarization of scattering was approximately constant for >99% of the facets. The slope distribution obtained by transmission scattering agrees with those of the surface profiles in the valid range of the profiler and can represent the slope distribution of the rough surface.

Effect of ion implantation on the strength of sapphire at 300–600°C

Ion implantation with 11B+ or 28Si+ at 1000°C doubled the ring-on-ring flexure strength of c-plane sapphire disks tested at 300°C but had little effect on strength at 500 or 600°C. Disks were implanted on the tensile surface with 2 × 1017 B/cm2 (half at 40 keV and half at 160 keV) or 1 × 1017 Si/cm2 (80 keV). Sapphire implanted with 1 × 1018 B/cm2 had only half as much flexure strength at 300° or 500°C as sapphire implanted with 2 × 1017 B/cm2. Implantation with B, Si, N, Fe or Cr had no effect on the c-axis compressive strength of sapphire at 600°C. Boron ion implantation (2 × 1017 B/cm2, half at 40 keV and half at 160 keV) induced a compressive surface force per unit length of 1.9 × 102 N/m at 20° and 1.4 × 102 N/m at 600°C. The infrared emittance at 550–800° of B-implanted sapphire at a wavelength of 5 μm increased by 10–15% over that of unimplanted sapphire. Infrared transmittance of sapphire implanted with B, Si or N at either 1000°C or 25°C is within ∼1–3% of that of unimplanted material at 3.3 μm. Implantation with Fe or Cr at 25°C decreases the transmittance by 4–8% at 3.3 μm, but implantation at 1000°C decreased transmittance by only 2–4% compared to unimplanted material.

Effective permittivity near zero in nanolaminates of silver and amorphous polycarbonate

Experiments were conducted to demonstrate a material with epsilon near zero (ENZ). Dimensions estimated by effective medium theory guided the fabrication of nanolaminate composites of silver and amorphous polycarbonate. This approach ensures that the ordinary component (not the extraordinary component) of the relative permittivity of a uniaxial material equals zero. The nanolaminates were characterized for optical properties using spectroscopic ellipsometry, reflectance, and transmittance. Simulations using both, a new scattering retrieval method, and an effective-medium approximation (EMA) were compared to the experimental results. These results indicate that nanolaminates should enable further exploration into the new optical phenomena predicted for ENZ materials.

Compressive coatings for strengthened sapphire

Micron-size cracks and voids produced during grinding and polishing may be responsible for sapphires loss of flexure strength at elevated temperatures. The ability to fill these micron-size voids and cracks with a compressive coating may remove the crack-initiating defects that lead to loss of strength. It is well known that compression of c-axis sapphire can cause twinning on rhombohedral crystal p;lanes, especially at elevated temperatures. If twins on two different planes intersect, a crack will form and the sapphire will fail in tension. In ring-on-ring biaxial flexure test, microtwins could form where the load rings contact the c-axis sapphire. A coating should mitigate the very high compressive stress produced at the surface of the sapphire by the load rings. Results presented here for ring- on-ring biaxial flexure test show that compressive coatings increase the fracture strength of c-axis sapphire by a factor of about 1.95 at 600 degrees C. Additional results show that the coating do not significantly strengthen sapphire at ambient temperature. This is not surprising since sapphire already is very strong in compression and tension at ambient temperature.

Protective optical coatings for diamond infrared windows

Some applications of diamond infrared windows will require protective optical coatings which not only boost IR transmittance but also protect the diamond surface from oxidation, as diamond surfaces are observed to oxidize in air above 700 degree(s)C. Three different concepts of protective optical coatings on diamond were evaluated using a calibrated plasma heating source, to determine how well they protect diamond surfaces from oxidation at high temperatures. Two of the three coating designs protected diamond surfaces from oxidation to peak temperatures exceeding 1300 degree(s)C, and to temperatures exceeding 800 degree(s)C for periods up to 10s. Microscopic examination of the films shows that the coatings underwent some degree of morphological change, but afforded diamond excellent oxidation protection nonetheless.

Development of low-OPD windows for airborne laser

We begin with a brief review of prior work relating to optical windows for use with high power laser beams. A typical window must provide pressure separation between system segments, ultra-low loss, and small wavefront distortion of the many outgoing laser beams and signal returns despite heating by the high energy laser beam. Historically, two approaches have been examined to improve such windows.

Infrared transparent conductive oxides

A novel class of complex metal oxides that have potential as transparent conducting oxides (TCOs) for the electromagnetic-interference (EMI) shielding on IR-seeker windows and missile domes has been identified. These complex metal oxides exhibit the rhombohedral (R3m) crystalline structure of naturally occurring delafossite, CuFeO2. The general chemical formula is ABO2 where A is a monovalent metal (Me+1 such as Cu, Ag, Au, Pt or Pd, and B is a trivalent metal (Me3+) such as Al,Ti,Cr,Co,Fe,Ni,Cs,Rh,Ga,Sn,In,Y,La,Pr,Nd,Sm or Eu. By adjusting the oxygen content, the conductivity can be varied over a wide range so that the delafossites behave as insulators, semiconductors or metals. This paper presents results for films of p-type CuxAlyOz and n-type CuxCryOz deposited by reactive magnetron co-sputtering from high-purity-metal targets. Films have been deposited using conventional RF- and DC-power supplies, and a new asymmetric-bipolar-pulsed- DC-power supply. Similar to the high-temperature-copper- oxide superconductors, the presence of Cu-O bonds is critical for the unique properties. Fourier transform infrared (FTIR) and electron spectroscopy for chemical analysis (ESCA) are used to understand the relationship between the optoelectornic properties and the molecular structure of the films. For example, FTIR absorption bands at 1470 and 1395cm-1 are present only in CuxAlyOz films that exhibit enhanced electrical conductivity. When these bands are absent, the CuxAlyOz films have high values of resistivity. In addition to the 1470 and 1395cm-1 bands observed in CuxAlyOz films, another pair of bands at 1040 and 970cm-1 is present in CuxCryOz films.

Examining epsilon near zero structures through effective medium theory and optical thin film analysis

Epsilon near zero (ENZ) structures are of increasing interest with developments initially directed at metal-dielectric material combinations and recently extended to doped semiconductor-dielectric combinations - all in an effort to drive the permittivity and wave number of the structure near zero. Of further interest is the effective theoretical characterization of these multi-layered material structures. We investigate increasing the number of layers - from one to four - of a visible ENZ design structure. Theoretical predictions are compared with experimental material properties collected from ellisometry; the region where effective medium theory breaks down and optical thin film analysis succeeds are examined.

Optical coatings for deep concave surfaces

A method of antireflection coating the interior and exterior surfaces of a deep concave optic is under development and is described. The challenges of coating such an optic include obtaining uniform performance, good mechanical and optical performance across a temperature range of ambient to 1000oC, and the transition to cost effective production. The coating process utilizes a tuned cylindrical magnetron sputtering source which sits inside the nose cone to coat the inner surface and a complementary cylindrical sputtering source to coat the outside surface. The flux from the sputtering source is tuned along the length of the cylinder by stacking an inner core of magnets in such a way as to produce a spatially variant magnetic field which allows the source distribution to approximate a uniform deposition on the surface of the optic. A deposition occulting mask provides fine tuning of source uniformity.