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Nominally undoped p-type Barium titanate was grown that exhibits interesting intensity dependent photorefractive and photochromic properties. When its photochromism is activated the absorption increases at the activating wavelength and there is a concurrent and proportional reduction in photorefractive response time. Three photoactivated and thermally reversible absorption conditions and the difference spectra of the photochromic crystal are shown in Figure 1., This article is from 'Organization of the Optical Society of America Photonic Science Topical Meeting Series. Volume 14. Photorefractive Materials, Effects, and Devices Held in Beverly, Massachusetts on 29-31 July 1991,' ADA253001, p351-354.

The photorefractive properties of cobalt doping barium titanate have been recently reported by Rytz et al. High beam coupling gains were reported therein. Similarly, we have grown a series of cobalt doped p-type barium titanate crystals, (undoped, 17 ppm, 50 ppm and 75 ppm), that also have relatively large beam coupling gains, e.g. for Ella and k sub g//c the gain is 7.4/cm. at 514.5 nm in the 75 ppm sample. We have further characterized these crystals by determining the sublinear dependence of the response time on intensity, the asymmetry of the beam coupling gain for + or - C-axis crystal orientations, (i.e. electrooptic and absorptive coupling), intensity dependence of the gain, and light-induced dark decays. Experimental results indicate that, as the cobalt concentration is increased, the photorefractive character of the crystals change from 'type B' to 'type A' as defined by Mahgerefteh and Feinberg. Type A crystals have filled hole-shallow traps and type B crystals have partially filled hole shallow traps and thus low and high dark conductivity respectively.

Variable-valent iron impurities have been suggested as the source of photorefractive charge carriers in BaTiO3. Very high-purity BaTiO3 crystals were grown with transition-metal impurity levels below the 10 to the 16th power/cc baseline estimate for photorefractivity. Iron-doped crystals also were grown with concentrations of 5, 50, 250, 500, 750, and 1000 ppm Fe. A study of some growth problems effecting purity and perfection of TSSG BaTiO3 crystals was performed. Some preliminary growth experiments in an rf heated Czochralski furnace indicated that it might be possible to utilize the cleaner growth environment of this type of equipment for BaTiO3. Studies were also conducted to investigate the problem of capping after seeding in the TSSG process. It was concluded that convective instabilities in the melt were probably the limiting factor. Keywords: Chemical purification; Crystal growth; Photorefractive effect; Iron doped; Barium titanate.

Skull Melting and top seeded solution growth of YVO4 were studied. A computerized optical spectroscopy system was designed and implemented. Laser pump laser gain measurements were made of Nd:YAG and CAMGAR.

Within the realm of crystalline laser materials, the class of fluorides distinguishes itself mostly by the wide variety of laser wavelengths displayed. Laser operation has now been reported from 3.9 micrometers in the infrared to 286 nm in the ultraviolet. Many are operated flash-lamp pumped, while others have shown high utility as linear down conversion lasers and rare earth ion, while others are sensitized by other co-dopants which absorb the pump energy and transfer it to the active laser ions. The potential of large spectral diversity for laser operation is due both to the wide window of transparency that fluorides possess and the lower rates of non-radiative decay. The high band gap in the ultraviolet also leads to low linear absorption, low non-linear refractive indices and multiphoton absorption. Additionally, the good chemical stability displayed by high purity stoichiometric fluoride compounds allows their use with ultraviolet pump sources at high energies, without incurring UV induced damage. We review the most recent research associated with such materials, particularly the host crystal, lithium yttrium fluoride, LiYF4 (YLF). (Author)

An efficient laser operating near the o.94 micrometer wavelength is of interest because with frequency doubling one can have a 470 nm laser with potentially higher overall efficiency than any primary laser at this wavelength. Nd(3+) in most oxide garnet host crystals has a 0.94 micrometer transition. What is required for 0.94 micrometer transition and a large branching ratio of 0.94 micrometer emission relative to the 1.06 emission. Detailed spectroscopy was performed on Nd:CAMGAR in order to determine the relevant cross sections. The results show that the 1.06 micrometer cross section in CAMGAR is about 0.25 of the 1.06 micrometer cross section in YAG whereas the ratio of the 0.94 micrometer cross sections is 0.5. Laser experiments on both Nd:YAG and Nd:CAMGAR by longitudinal pumping with an Ar laser substantially verified the relative cross sections of the two materials, The 0.94 micrometers results were limited however and the resulting cross section for Nd:CAMGAR is higher than the spectroscopically determined value. HO(3+) is an alternate ion for a 0.94 micrometer laser. The Ho 5F5 level to 5I7 level has been reported lased at 0.98 epsilon micrometer both in 2% in LiYF4 and in LiHoF4. An analysis of Ho:YLF has been done to evaluate its potential as a useful laser material.

Prepared in cooperation with Massachusetts Inst. of Tech., Cambridge.