Transverse-periodic-oriented nematic liquid crystals (LCs) are a special type of optical axis grating that are capable of very high efficiency diffraction (theoretically, 100%) in thin layers of materials with thickness comparable to the radiation wavelength. In particular, they fully diffract linearly polarized input beam into circularly polarized +1st and -1st diffraction orders. We experimentally demonstrate switching between diffraction orders of such gratings when the polarization of the incident beam changes from right-circular to left-circular and vice versa with the aid of an electrically controlled LC phase retarder. Such a setup in which the diffraction efficiency and direction are controlled externally, without application of an electric field to the transverse-periodic grating, provides additional control opportunities and does not compromise the quality of the grating. The grating used in the experiment was 1.5 microm thick and had a period of 4 microm. The contrast ratio of switching between the +1st and -1st orders was as high as 267:1 for a He-Ne laser beam with a switching time of 6.6 ms.

Holographic UV mirrors were recorded in a volume of photothermorefractive glass. This photosensitive silicate glass doped with silver, cerium, and fluorine is transparent in the near-UV, visible, and near-IR spectral regions, and its induced refractive-index change reaches 10(-3). UV radiation of a He-Cd laser at 325 nm was used for Bragg grating recording with a spatial frequency of approximately 9200 mm(-1). The absolute diffraction efficiency of the recorded mirror reached 12.6% at 325 nm and did not deteriorate under the long-time effects of optical irradiation and heating up 400 degrees C.

In some circumstances, the mechanical and optical properties of multiphase brittle materials strongly depend on the level of residual micromechanical stresses that arise upon cooling due to thermal and elastic mismatch between the constituent phases. Here we study the residual internal stress in a partially crystallized oxyfluoride glass, best known as photothermorefractive ͑PTR͒ glass. This material is composed of a glass matrix with embedded nanosize sodium fluoride ͑NaF͒ crystals. Using both the Selsing model and solid-state nuclear magnetic resonance in combination with first principles calculations we found that the crystals are under a tensile stress field of approximately 610-800 MPa. For this stress level the estimated critical crystal diameter for spontaneous cracking is about 2300-1900 nm, which greatly exceeds the observed diameters of 7-35 nm. Hence no spontaneous cracking is expected for the PTR glasses. First principles calculations indicate that the stress induced change of the refractive index of the NaF crystals is about −0.08%, which agrees with the observed refractive index changes.

Photo-thermo-refractive glass containing nanosized NaF crystals embedded in the glassy matrix shows a significant shift of X-ray diffraction (XRD) lines to lower angles resulting from large residual (tensile) stresses within the crystals. This is thus an excellent ''model'' system to test residual stresses models in glass-ceramics and composites because: the estimated stresses are high-about 1 GPa-the precipitates are nearly spherical, the NaF crystals structure is cubic and their volume fraction is quite small, which eliminates overlap between the stress fields of neighbor crystals. Samples treated at a sufficiently high temperature to develop larger (micrometer size) crystals revealed microcracking of the glassy matrix around the crystals, which partially relieved the residual stresses and decreased the shift of the XRD peaks. The experimental results for the magnitude of the residual stresses and the critical crystal diameter for microcracking agree with theoretical values calculated by the Selsing and the Davidge & Green models, respectively. These results suggest that these two models can be used for stress estimates and as a first approach for the design of tough glass-ceramics. R. Bermejo-contributing editorThis work was financially supported by CNPq/Brazil-process no. 151917/2006-0, FAP-ESP/Brazil-contract no. 07/08179-9, the Brazilian Synchrotron Light Laboratory/MCT research proposals XRD2-5322 and XRD1-5824 and DARPA/ADHELS program.