M. O. Ramírez

Directional dependence of the second harmonic response in two-dimensional nonlinear photonic crystals

A two-dimensional periodic arrangement of alternating ferroelectric domains in LiNbO3, with asymmetric domain duty cycle has been used to demonstrate that counterpropagating beams along the polar axis can generate disparate second harmonic patterns, which are nonsymmetrical with respect to the source and the point of observation. These findings provide alternative routes to generate dissimilar light-matter interaction processes in two-dimensional structures assembled onto polar surfaces, including metals for plasmonics or biological compounds.

Two dimensional ferroelectric domain patterns in Yb3+ optically active LiNbO3 fabricated by direct electron beam writing

We report on highly controllable ferroelectric domain inversion in Yb3+ doped LiNbO3 laser crystal. The ferroelectric domain patterns are fabricated by direct electron beam writing without any previous masking process. Square lattices of inverted domains with diameters and distance between domains as low as 1 μm are demonstrated. The lateral growth of the inverted domains is analyzed as a function of the applied charge and the threshold values for domains in the 1–10 μm length scale are determined. Spatially resolved low temperature fluorescence spectroscopy and non-collinear second harmonic generation experiments are also employed to evaluate the optical properties of the system.

Thermal hysteresis in the luminescence of Cr3+ ions in Sr0.6Ba0.4 (NbO3)2

The temperature dependence of the emission spectrum of Cr3+ ions in Sr0.6Ba0.4 (NbO3)2 has been systematically investigated around the ferroelectric phase transition of this crystal (≈370 K). In spite of the strong thermal quenching of the luminescence occurring at these temperatures, the emission spectrum is still clearly detectable and shows dramatic changes in both line shape and intensity when passing through the phase transition temperature. A redshift of about 6 nm occurs when the crystal is driven from the polar to the nonpolar phase. In addition, this shift displays a thermal hysteresis, then providing the possibility of realizing a bistable spectral system based on this material.

Nd3+ ion shift under domain inversion by electron beam writing in LiNbO3

Ferroelectric domain inversion has been obtained in Nd3+ doped lithium niobate by means of direct electron beam writing. The local effects of the polarization inversion on the optical transitions of Nd3+ ions have been studied by low temperature high resolution site selective spectroscopy. Inverted regions present different axial crystal field acting over Nd3+ ions compared with noninverted (original) regions. The results can be interpreted in terms of slight shifts of Nd3+ ions along the ferroelectric c axis within the Li+ octahedrons, as a result of the lattice rearrangement after the domain inversion processes.

Neodymium doping in UV-IR transparent ferroelectric BaMgF4

The incorporation of Nd3+ in the ferroelectric fluoride BaMgF4 crystal is investigated for its potential application as self-frequency-converter solid state laser. Low temperature high-resolution optical spectroscopy shows that Nd3+ is located at two well distinguished sites referred as A and B. The whole set of energy levels in the excited F43/2, I411/2, and ground I49/2 states are identified for both Nd3+ sites and their crystallographic location in BaMgF4 lattice is discussed. The radiative lifetime of the F43/2 metastable laser level is obtained to be 1430 and 800 μs for site A and B, respectively, and the luminescence quantum efficiency of the F43/2 metastable state for the dominant A site is 87%. The possibility of annihilating the contribution of the B site in the optical spectrum is demonstrated by means of the incorporation Na+ into the crystal.

Optical spectroscopy of Er3+-doped Bi12SiO20 piezoelectric crystal

Abstract Photoluminescence of Er 3+ ions in a Bi 12 SiO 20 crystal is investigated under excitation in the near infrared region ( 4 I 15/2 → 4 I 9/2 transition) at low temperature. Two non-equivalent Er 3+ centers are detected in the crystal. Besides the one photon infrared emission ( 4 I 11/2 → 4 I 15/2 ), up-converted luminescence in the green ( 4 S 3/2 → 4 I 15/2 ) and red ( 4 F 9/2 → 4 I 15/2 ) spectral regions and down-converted emission ( 4 S 3/2 → 4 I 13/2 ) in the near infrared region have been observed and resolved for each type of Er center. The mechanisms responsible for the conversion process are analyzed. The position of the Stark energy levels are obtained for each center for different Er 3+ multiplets.

Transparent glass-ceramics: crystallization mechanisms and optical properties

Rare earth (RE)-doped oxyfluoride glass-ceramics possess optical properties of interest for telecommunications and optoelectronics. Specifically, these materials combine the transparency and mechanical and chemical resistance of aluminosilicate glasses with the low phonon energy and facile incorporation of RE ions in the fluoride crystalline phase, which enhances optical emission intensity. Oxyfluoride nano-glass-ceramics have been extensively investigated ever since Wang and Ohwaki1 developed ytterbium-erbium (Yb3C=Er)-doped lead fluoride nanocrystals within an oxide glassy matrix. Studies have been focused on exploring new crystalline phases and adding different RE ions.2–7 A complete overview of this research field is available elsewhere.8 In medicine, sodium yttrium fluoride (NaYF4) nanoparticles doped with Yb=Er3C have been reported as analytes (molecules of interest) for detecting DNA.9, 10 Zhang and Huang11 reviewed down-conversion applications of IR quantum cutting glass-ceramics containing a range of fluoride-based nanocrystals, including materials used for solar energy. Richards12 described the advantages of applying upconversion to photovoltaics, in other words, achieving photon energies that can be more readily absorbed by silicon (Si) solar cells. Lanthanum fluoride .LaF3) is a commonly used fluoride for hosting RE ions in the crystalline phase. Moreover, because to date NaYF4 is known to be one of the most efficient hosts for green and blue up-conversion (e.g., two IR photons giving rise to one visible photon),13, 14 RE-doped fluorides of the type AREF4 Figure 1. (a) Dark-field scanning transmission electron microscopy image of 70Si7La glass-ceramic obtained at 520C for 90h, with the spot analysis location indicated. (b) Normalized silicon (Si) intensity along the trace of the line scan indicated in (a). NaLaF4: Sodium lanthanum fluoride.

Micrometric spatial control of rare earth ion emission in LiNbO3: A two-dimensional multicolor array

We report on the preparation and optical characterization of a two-dimensional multicolor emission arrangement obtained by embedding high refractive-index Er3+ doped CaTiO3 nanoparticles into a Nd3+ doped LiNbO3 crystal substrate prepatterned with an array of microvoids. By controlling the spatial location of the rare earth ions at the micrometer scale, we show the possibility of simultaneous spatial and spectral control of the spontaneous emission in a two-dimensional rare earth optically activated array. The results can be useful for the development of microcomposite rare earth based photonic devices, such as multicolor emission displays or pixelated color structures.

Optical spectroscopy of Yb3+-doped Ca3Sc2Ge3O12 garnet crystal

Good quality Ca3Sc2Ge3O12 garnet crystals activated with Yb3+ have been grown by means of the flux growth method and characterized using optical spectroscopy techniques. The low-temperature absorption and emission spectra give evidence of the presence of nonequivalent optical centers, as a consequence of the distribution of the Yb3+ ions in different lattice sites. The effect of codoping with Na+ or Mg2+ on the Yb3+ site distribution has been investigated. The energy-level schemes of the most important centers have been determined on the basis of site-selective spectroscopy experiments. Cooperative luminescence has also been observed and reliably compared with the self-convolution of the NIR luminescence.

Rare earth ion doped non linear laser crystals

We show how non linear crystals activated with Yb3☎ or Nd3☎ ions can be used to develop diode pumped solid state lasers emitting in the visible region of the electromagnetic spectrum. For this purpose we have selected relevant examples of systems investigated in our laboratory.