John J. Larkin

Hydrothermal growth of bismuth silicate (BSO)

Bismuth silicate (Bi 12 Si 20 ) is a promising material for use in optical signal processing. It is a photorefractive material with good response at argon laser wavelengths (488 nm) and is well suited to holography and four wave mixing applications. It exhibits a high response speed and good sensitivity, although the gain is far less than that of the ferroelectric materials (barium titanate, etc.). Most of the currently available material is obtained by the Czochralski growth process or the directional gradient freeze process. Nominally undoped crystals from these processes yield material of acceptable research quality but crystal uniformity and reproducibility have been a problem. Improved growth techniques are needed for advanced applications. Hydrothermal growth of this material dramatically changes the “intrinsic” optical properties. There are indications that lower temperature aqueous growth prevents the formation of defects created in the melt processes. This yields a baseline material ideally suited for the study of these defects. In addition, by modifying the optical properties with dopants, material tailored to specific wavelengths and applications may possibly be produced. Growth procedures and preliminary optical characterization results are reported.

The growth of high purity, low dislocation quartz

Abstract Hydrothermal growth runs were conducted in research size autoclaves, employing both sodium hydroxide and sodium carbonate as mineralizers, in an effort to achieve higher purity quartz with a minimum of dislocations. It was found that the impurity concentration of aluminum can be routinely reduced to below 0.5 parts per million without a liner. The average dislocation count can be reduced by at least an order of magnitude with the use of special seeds.

Low‐defect colorless Bi12SiO20 grown by hydrothermal techniques

Colorless single crystals of bismuth silicon oxide (BSO) have been grown using a pressure‐balanced hydrothermal technique. The absorption shoulder which causes the yellow coloration observed in conventional BSO was completely missing in the hydrothermal material. The 10 K absorption edge was found to be 3.45 eV for the hydrothermal samples. Because of the absence of a low‐temperature photochromic response and the observation of only very weak TSC peaks, it appears that the hydrothermal crystals represent near‐intrinsic BSO. When a crystal was pulled from a melt of hydrothermal material, the yellow coloration returned.

The low‐temperature photochromic response of bismuth silicon oxide

Exposing the photorefractive material bismuth silicon oxide (BSO) at low temperatures to 2.4–3.3 eV light produces photochromic absorption bands. In undoped and Fe‐doped BSO these bands appear to consist of a series of overlapping bands ranging from around 1.5 eV in the infrared to near the band edge. The infrared component is always weaker than the visible range contributions. The infrared portion anneals just above 100 K; in some samples this anneal is accompanied by the appearance of additional structure in the visible region. In undoped BSO the major anneal of the photochromic bands takes place above 200 K. If iron is present the photochromic bands are weaker and an anneal stage in the 120–150 K range appears. Bleaching with either 1.51 or 2.28 eV laser light uniformly lowered the photochromic bands in both undoped and Fe‐doped BSO. In BSO:Al the aluminum electronically compensates the deep donor centers responsible for the yellow coloration observed in undoped crystals. At low temperatures, photoexci...

Deep defect levels of photorefractive sillenites

The photorefractive effect in materials such as bismuth silicon oxide (BSO) depends on photoionizing deep defect levels inadvertently present rather than controllably introduced. Using thermal stimulated conductivity measurements, a preliminary attempt has been made at associating specific levels with a particular sillenite member and impurity dopants. While many of the features prevail throughout, significant changes occur when Ge is substituted for Si to give BGO and when p‐ (Al) and n‐type (P) impurities are added to dope BSO.

Optical studies of Czochralski and hydrothermal bismuth silicate

Abstract Bismuth silicate in the sillenite structure, (Bi 12 SiO 20 ), is a photorefractive material of interest for applications in two-wave and four-wave mixing, optical correlation and holographic storage devices. It has higher sensitivity and speed than other visible light photorefractive materials. Currently available material is obtained by the Czochralski growth process or the directional gradient freeze technique. These methods procedure material of adequate quality for most purposes but problems with crystal uniformity and reproductibility limit the material for some applications. The hydrothermal growth technique will potentially yield highly reproducible material of large size and excellent uniformity. Doping studies on both Czochralski and hydrothermal crystal indicate that the photorefractive properties may be tailored to individual applications.

Purification and analysis of α-quartz

Abstract Hydrothermal growth runs were conducted in research size autoclaves employing both sodium hydroxide and sodium carbonate as mineralizers in an effort to achieve higher purity quartz with a minimum of dislocations. It was found that the impurity concentration of aluminum can be routinely reduced to below 0.5 parts per million without a liner.

Photoluminescence in Czochralski and hydrothermally grown bismuth silicon oxide

Excitation with band edge light at low temperatures produces an emission near 2.9 eV in both hydrothermally and Czochralski grown Bi 12 SiO 20 samples. The emission band is much stronger in the hydrothermal samples. It disappears when the samples are heated to 50 K. A weak 1.9 eV emission is also observed in the Czochralski samples. This emission grows when the 2.9 eV decays near 50 K and then goes out between 100 and 150 K