C. E. Rice

Proton exchange for high‐index waveguides in LiNbO3

We describe the fabrication and characterization of optical waveguides formed in LiNbO3 by proton exchange in benzoic acid melts at 200–250 °C. Proton exchange, in LiNbO3 the replacement of lithium ions with protons, takes place when the substrate is immersed in the molten acid. We observe a surface increase in the refractive index of 0.12, for the extraordinary polarization only, with a step function index profile. This is the highest index increase obtainable to date for LiNbO3. Measured diffusion rates for x‐cut crystals are 1.37 μm2/h at T = 249 °C and 0.37 μm2/h at T = 217 °C, so that very deep guides can be formed in short times. Diffusion is somewhat slower in the z direction. The process as described is not useful for y‐cut crystals, since it damgaes this surface. Losses, measured on x‐cut samples, were <0.5 dB/cm. All measurements were made at 0.633 μm.

Preparation of superconducting thin films of Ba2YCu3O7 by a novel spin-on pyrolysis technique

A technique is described for preparing superconducting cuprate thin films using wet chemical precursors. This new method is simple and inexpensive, and allows facile manipulation of film stoichiometry. A solution of metal acetates in aqueous acetic acid is spun on a substrate, pyrolyzed, and heat treated. A 1600‐A film of Ba2YCu3O7 prepared by this method had a room‐temperature resistivity of 500 μΩ cm, Tc(onset) of 90 K, and R=0 at 58 K. It showed preferential orientation with the orthorhombic c axis perpendicular to the film, and random azimuthal orientation. The preparation and electrical and microstructural characterization of these films are discussed.

Preparation of superconducting thin films of calcium strontium bismuth copper oxides by coevaporation

Superconducting films of Ca‐Sr‐Bi‐Cu oxides have been prepared by coevaporation of CaF2, SrF2, Bi, and Cu, followed by post‐oxidation in wet O2. The films were characterized by four‐probe resistivity measurements, Rutherford backscattering, transmission electron microscopy, x‐ray diffraction, and Hall measurements. Zero resistance was achieved at ∼80 K, although evidence of traces of superconductivity at higher temperatures was seen in resistivity and Hall data. The critical current at 4.2 K was 1.0×106 A cm−2. The films were epitaxial on 〈100〉 and 〈110〉 SrTiO3 substrates. The electrical and structural properties of the films were insensitive to film composition over a wide range of stoichiometries.

High‐resolution patterning of high Tc superconductors

We have used a 20 keV Ga focused ion beam to pattern superconducting submicrometer bridge structures in thin films of Ba2YCu3O7 material by physical sputtering. The technique can produce structures down to 0.5 μm or less in epitaxial films with no degradation in superconducting transition temperature (Tc) or critical current density (Jc). Photolithography was used to define a coarse pattern of 20‐μm‐wide and 50‐μm‐long strips, each wired for four‐terminal resistance measurements. Submicrometer constrictions were then milled by the focused ion beam to form weak‐link junctions with roughly 0.3 μm separating the superconducting banks. We have demonstrated that focused ion beam micromachining is capable of producing submicrometer‐sized superconducting structures.We have used a 20 keV Ga focused ion beam to pattern superconducting submicrometer bridge structures in thin films of Ba2YCu3O7 material by physical sputtering. The technique can produce structures down to 0.5 μm or less in epitaxial films with no degradation in superconducting transition temperature (Tc) or critical current density (Jc). Photolithography was used to define a coarse pattern of 20‐μm‐wide and 50‐μm‐long strips, each wired for four‐terminal resistance measurements. Submicrometer constrictions were then milled by the focused ion beam to form weak‐link junctions with roughly 0.3 μm separating the superconducting banks. We have demonstrated that focused ion beam micromachining is capable of producing submicrometer‐sized superconducting structures.

Ion beam thinning and polishing of YBa2Cu3O7 films

Successive ion beam milling at grazing angles of a 2400‐A‐thick, c‐axis‐oriented YBa2Cu3O7 film is shown to give smooth films which superconduct at thicknesses on the order of tens of angstroms. The thinning and polishing process is characterized at successive milling stages using resistance transitions, x‐ray analysis, scanning electron microscopy, and Rutherford backscattering and channeling analysis of composition and surface quality. As thinning proceeds, scanning electron microscopy and diffraction features associated with a‐axis texture are removed and crystalline quality, as measured by x rays and channeling, markedly improves.

Proton exchange in LiNbO3

Abstract This paper reviews the recently described process of proton exchange in LiNbO3 and reports some current results. Only results directly applicable to integrated optics will be described, although other applications are possible, and much of the work is interesting from a more fundamental point of view.

Variation in waveguides fabricated by immersion of LiNbO3 in AgNO3 and TlNO3: The role of hydrogen

Optical waveguides fabricated in LiNbO3 by what has previously been described as silver‐lithium or thallium‐lithium ion exchange in the molten nitrates AgNO3 or TlNO3 have not generally been reproducible. We show that the large increases of the extraordinary index (Δne = 0.12), which are observed intermittently, result not from introduction of the heavy ions, Ag+ or Tl+, but from hydrogen‐lithium exchange, which occurs when hydrogen is present as an impurity in the melts.

Observation of electrochromism in solid‐state anodic iridium oxide film cells using fluoride electrolytes

Thin film cells of composition SnO2/AIROF/fluoride/Au have been constructed, where AIROF is anodic iridium oxide film and fluoride is PbF2 on PbSnF4. These devices exhibit reversible electrocoloration and bleaching, with response times as low as 0.1 sec. The behavior of these cells is consistent with the anion insertion mechanism for AIROF electrochromism.

A comparison of the behaviors of tungsten trioxide and anodic iridium oxide film electrochromics in a nonaqueous acidic medium

The performance of anodic iridium oxide film (AIROF) and WO3 electrochromic electrodes was tested in a water‐free perchloric acid/dimethylsulfoxide solution. WO3 could be colored and bleached electrochemically to nearly the same extent as in water solutions, albeit much more slowly. By contrast, no electrochromic effect could be produced with AIROF electrodes. This proves that the electrochromic processes for these two materials are fundamentally different and that a simple proton‐electron injection mechanism cannot account for AIROF electrochromism.

Superconducting thin films of high Tc cuprates prepared by spin‐on/pyrolysis

Thin films of high Tc cuprate superconductors have been prepared by spinning on metal organic precursors from solution, followed by pyrolysis to oxide films. This new technique allows easy manipulation of film stoichiometry, and gives excellent compositional uniformity throughout the films. A 1600 A film of Ba2YCu3O7 on 〈100〉 MgO had a room temperature resistivity of 500 μΩcm, Tc(onset) of 90K, and R=0 at 58K. The film morphology was essentially a single planar array of grains with lateral dimensions of 2–5μm, oriented with the orthorhombic c axis perpendicular to the substrate. The preparation and electrical and microstructural characterization of thin films of Ba2YCu3O7 and other high Tc cuprates will be discussed.