Mark B. Moran

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.

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.

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.

Optical characterization of MEMS deformable mirror array structures

Surface properties and optical properties of several deformable mirror arrays (DMA) without actuators were characterized. The mirror arrays are micro-electronic- mechanical system (MEMS) devices which were fabricated by Boston University for wavefront correction in adaptive optics. The surface properties measured for the samples agree with the properties specified for the BU-MEMS-DMA structures. Scattering and diffraction by the mirror arrays were measured at a wavelength of 632.8nm. The DMA with the etching pattern generates a diffraction pattern full of special structures. The broadening is serious for a rough sample while it is negligible for a smooth continuous membrane DMA. The diffraction pattern demonstrates that the DMA with an RMS roughness of 300nm is not suitable for the adaptive optics to correct for wavefront error. The continuous membrane DMA with roughness less than 10 nm are useful for adaptive optics.

IR-Transparent Electrically Conductive CuAl x O y Deposited by Reactive Magnetron Co-Sputtering

This paper reports on progress in the fabrication of IR-transparent electrically conductive copper aluminum oxide (CuAl x O y ) by reactive magnetron co-sputtering from high-purity-Cu and -Al targets in an argon-oxygen-gas mixture. Recent equipment modifications have resulted in much better control of deposition parameters like forward and reflected power and, consequently, much better control of film composition. Applying the correct amount of power to each target and adjusting the oxygen-partial pressure have significantly reduced the growth of surface-oxide layers on the metal targets. Equipment and process improvements have eliminated the sputterrate inconsistencies and arcing that led to lack of composition control in our earliest films. We now have much better control of film composition and are beginning to understand the relationship between the electro-optical properties and the molecular structure of the films using Fourier transform infrared (FTIR) spectroscopy, electron spectroscopy for chemical analysis (ESCA), inductively coupled plasma (ICP) emission spectroscopy, high resolution electron microscopy (HREM) and electron energy loss spectroscopy (EELS). A pair of weakly intense FTIR absorption bands at 1470 and 1395cm −1 is present in films that have enhanced electrical conductivity and IR transparency. Understanding the origin of these bands could speed development of CuAlO 2 as a wide-bandgap-conductive oxide since these bands are clearly associated with enhanced conductivity and carrier mobility. The best film to date transmits about 80% in the mid-wave IR and has a sheet resistance of 160Δ;/sq.

Aluminum nitride coatings by reactive pulsed dc magnetron sputtering

Aluminum nitride (AIN) has the potential to meet the requirements of hypersonic-seeker windows that operate in the visible to mid-wave infrared and rf frequencies. With dielectric properties and thermal-shock resistance similar to those of hot-isostatic-pressed silicon nitride, but with a thermal conductivity eight times higher, AIN is a primary candidate for high-power-microwave windows and for high- speed radomes. The true potential of AIN has yet to be demonstrated because single crystals of AIN with purities high enough to be considered intrinsic have never been fabricated. Carbon and oxygen are the two most significant impurities and are the most difficult to eliminate. Impurities and voids present in AIN fabricated from nanocrystalline powders degrade the thermal-mechanical properties and produce optical scatter that limits the useful wavelength range. A large amount of scatter could be removed by eliminating the carbon and oxygen impurities and by reducing the crystallite size in polycrystalline AIN to sub-micron and nanometer sizes. Oxygen incorporation has yet to be avoided using conventional powder processing techniques because the newly formed AIN powders react spontaneously with trace amounts of oxygen. Over the past 50 years, the solid-state-chemistry approach has been used by several research groups and has produced useful material for electronic heat-sink applications but never the high- optical-quality material needed for 2-mm-thick missile domes and windows. This suggests that research aimed at producing high-quality-microcrystalline-AIN coatings and freestanding films would be productive. This paper will present results for films of AIN deposited using reactive asymmetric- bipolar-pulsed-dc-magnetron sputtering.