The fabrication and characterization of ultraviolet photodetectors based on GaN p-n junctions is reported. The devices are grown by metalorganic vapour phase epitaxy on basal-plane sapphire substrates. These detectors are visible-blind with a sharp wavelength cut-off at 360 nm. The photocurrent is linear with incident power from 10 mW m −2 up to 2 kW m −2 , with a responsivity of 145 mA W −1 at 360 nm. The device time response is dominated by the effective resistance-capacitance time constant, and a 105 ns response is estimated for very low load resistances. A comparison with the response of GaN photoconductor detectors is also presented. The application of these high-performance photodetectors for solar ultraviolet monitoring is described.

We present the temperature dependence of the green up-converted luminescence in submicron-sized LiNbO 3 :Er 3þ /Yb 3þ particles in the range between 12 and 180 C. It is shown that LiNbO 3 :Er 3þ /Yb 3þ exhibits a very high sensitivity (S ¼ 0:007/ C at physiological temperature and a maximum value of S ¼ 0:014/ C at 350 C) that surpasses those previously found for other materials based on Er 3þ /Yb 3þ luminescence, indicating that LiNbO 3 :Er 3þ /Yb 3þ is a suitable probe for optical temperature sensing through the fluorescence intensity ratio technique. This high sensitivity can be explained using the intrinsic spectroscopic parameters of Er 3þ ions in this host.

International audienceThe highly visible and infrared (up to 6 mu m) transparent Sr3Al2O6 polycrystalline ceramic was obtained by full crystallization of the corresponding glass composition. The glass synthesis and the direct congruent crystallization processes are described, and the material transparency is discussed in light of its microstructure. This new transparent ceramic exhibits a high density (i.e., complete absence of porosity) and micrometer-scale crystallites with very thin grain boundaries. These microstructural characteristics, inherent to the preparation method, minimize light scattering and demonstrate the advantages of this synthesis route compared to the high-pressure process used for the few reported transparent polycrystalline materials. This Sr3Al2O6 ceramic shows a H = 10.5 GPa hardness, a E-r = 150 GPa reduced elasticity modulus, and a 9.6 x 10(-6) K-1 thermal expansion coefficient. Such a transparent strontium aluminate ceramic opens the way to a wide range of applications, especially photonics when doped by various doping agents. As examples, the luminescence of Sr3Al2O6:Eu3+ and Sr3Al2O6:Er3+, which show strong emissions in the visible and infrared ranges, respectively, is presented. Moreover, the Sr3Al2O6:Ce3+ material was found to exhibit scintillation properties under X-ray excitation. Interestingly, the analogous Sr3Ga2O6 transparent polycrystalline ceramic material could equally be prepared using the same elaboration method, although its hygroscopicity prevents the preservation of its high transparency under normal conditions. The establishment of the key factors for the transparency of this economical and innovative synthesis method should enable the prediction of new classes of technologically relevant transparent ceramics

In this work, continuous-wave broadly tunable simultaneous generation of red ͑650-690 nm͒, green ͑520-575 nm͒, and blue ͑425-495 nm͒ light in aperiodically poled Zn-diffused LiNbO 3 :Er 3ϩ /Yb 3ϩ channel waveguides is reported after Ti:sapphire excitation in the 850-990 nm range. The red and green emissions arise from energy transfer and upconversion mechanisms between Yb 3ϩ and Er 3ϩ ions, while the blue light with a maximum efficiency of 0.04% W Ϫ1 cm Ϫ1 is produced by quasi-phase matching processes. © 2003 American Institute of Physics. ͓DOI: 10.1063/1.1617367͔ Lithium niobate (LiNbO 3 ) compares favorably with other ferroelectric crystals for nonlinear optical applications not only because of its large nonlinear coefficients but also because of the possibility of modulating them, offering the option of nonlinear frequency conversion via quasi-phase matching ͑QPM͒ processes. Several nonlinear devices based on periodic ͑PPLN͒ or aperiodically ͑APPLN͒ poled LiNbO 3 have been reported, including the additional advantage of using waveguiding structures, where high power densities are easily available. [1][2][3][4][5][6][7][8] In this work, nonlinear channel waveguides fabricated by Zn diffusion in Er 3ϩ /Yb 3ϩ codoped APPLN are used to generate simultaneous red, green, and blue ͑RGB͒ light. AP-PLN crystals were grown by the off-centered Czochralski method, along the a axis, with automatic diameter control by a crucible-weighting technique. The initial melts containing congruent LiNbO 3 (͓Li͔/͓Nb͔ϭ0.945) were doped with Er 2 O 3 and Yb 2 O 3 with a purity grade of 99.99%.The growth conditions, pulling and rotation rates and the seed crystal shift, were adjusted appropriately to favor the formation of modulated ferroelectric domain distributions 9 in order to achieve red, green, and blue light under excitation in the 880-1040 nm spectral range (Yb 3ϩ absorption band͒. Using this procedure chirped-period-poled domain structures are obtained. The domain pattern from a y-cut wafer was revealed by etching in a diluted solution of HF:HNO 3 ͑1:2 by volume͒ for 1 h and then checked by conventional optical microscopy. As illustrated in Fig. 1, the wafer exhibits several chirped-period-poled domain structures with periods ranging from 5 to 16 m, being the averaged period of ϳ7 m.The wafer was then polished to optical grade in order to fabricate the channel waveguides by Zn diffusion. 7,10,11 In the present work, the metal diffusion was performed following a two-step procedure ͑exchange and diffusion͒ [12][13][14] which preserves the initial wafer ferroelectric pattern. 15 The exchange process at 550°C for 2 h was performed followed by annealing in open atmosphere at 850°C for 4 h. These conditions produce a 4 m depth Gaussian index profile, with a maximum index change of 0.15% and 0.20% in the extraordinary and ordinary refractive indices, respectively. These waveguides support two modes in the visible range and are single mode at the IR pump wavelengths used in the experiments.The experimental configuration used in the mea...

No abstract