P. Molina

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.

Nonlinear prism based on the natural ferroelectric domain structure in calcium barium niobate

We show the ability of calcium barium niobate as a nonlinear material to produce conical second harmonic generation in a broad frequency range. The nonlinear properties of this system are based on the broad sized microdomain distribution appearing naturally in the as-grown crystal. The large angular deflection obtained for conical second harmonic generation allows considering this system as a nonlinear prism with variations in the second harmonic generation angle as large as 35° in the wavelength range of 430–660 nm.

Spontaneous Emission and Nonlinear Response Enhancement by Silver Nanoparticles in a Nd3+‐Doped Periodically Poled LiNbO3 Laser Crystal

Plasmonic nanostructures are attracting great interest because they can provide mechanisms for remarkable optical brightness enhancement and sub-wavelength light control, which are crucial features for a broad range of scientifi c and technological applications. [ 1,2 ] An extremely large variety of metal/dielectric combinations (types of materials and confi gurations) are currently the subject of intense studies, revealing new fundamental properties and leading to novel devices with improved performances, which in many cases has been possible owing to advances in complex synthesis and fabrication techniques. [ 3 ]

Simultaneous generation of second to fifth harmonic conical beams in a two dimensional nonlinear photonic crystal.

Broadly tunable multiple high-harmonic conical beams have been generated by means of a multistep χ(2) cascade processes in a two dimensional nonlinear photonic crystal. The nonlinear structure consists of a square lattice of inverted hexagonal domains with diameters and distances between domains as low as 1 μm. The large number of reciprocal lattice vectors provided by both the square nonlinear structure and the hexagonal shaped domains, along with imperfections on the size and shape of the individual domains make possible the simultaneous generation of second up to fifth harmonic conical beams in a single nonlinear structure by using different types of phase matching geometries. The frequency response can be tuned in an extremely large spectral range, and continuous generation of nonlinear conical beams covering the whole visible spectral region can be achieved. Further, the same photon energy can be generated at different orders, so that concentrically emitted conical beams with angular dispersion as large as Δθ = 50° can be observed. The results highlight the significance of highly controlled engineered 2D nonlinear structures to generate advanced multi-photon devices with large spatial and spectral tunable response.

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.

Plasmon-Assisted Nd3+-Based Solid-State Nanolaser

Solid-state lasers constitute essential tools in a variety of scientific and technological areas, being available in many different designs. However, although nanolasing has been successfully achieved for dyes and semiconductor gain media associated with plasmonic structures, the operation of solid-state lasers beyond the diffraction limit has not been reported yet. Here, we demonstrate room temperature laser action with subwavelength confinement in a Nd(3+)-based solid-state laser by means of the localized surface plasmon resonances supported by chains of metallic nanoparticles. We show a 50% reduction of the pump power at threshold and a remarkable 15-fold improvement of the slope efficiency with respect to the bulk laser operation. The results can be extended to the large diversity of solid-state lasers with the subsequent impact on their applications.

Tb3+-Yb3+ cooperative down and up conversion processes in Tb0.81Ca0.19F2.81:Yb3+ single crystals

The energy transfer processes between Tb3+ and Yb3+ ions are studied in Tb-based fluoride single crystals. These are very transparent from the UV to the IR wavelength regions, except for the characteristic absorption lines of the rare-earth ions under study. In contrast with previously reported Tb3+-doped glasses and oxide powders, these crystals containing a high Tb3+ concentration present two major advantages for the study of the energy transfer processes in the Tb3+-Yb3+ ion pair. Firstly, the adverse influence of host defects is minimized with the use of high quality crystals. Secondly, the high Tb3+ concentration guarantees a much higher absorption cross-section of UV light, and consequently these crystals have real potential for practical applications. Photoluminescence spectra in the visible-IR wavelength region demonstrate the existence of efficient down and up conversion processes by crossed excitation and emission characteristics of Tb3+ and Yb3+ ions. In the down conversion process, Tb3+(5D4) →...

Selective plasmon enhancement of the 1.08 μm Nd3+ laser Stark transition by tailoring Ag nanoparticles chains on a PPLN Y-cut.

Selective photoluminescence enhancement of the specific Nd(3+) Stark transition for which laser gain has been obtained in Nd(3+)/LiNbO3 is demonstrated by means of plasmonic resonances with the appropriate symmetry configuration. By using the nonpolar Y-cut of a periodically poled LiNbO3 crystal as platform for photoreduction of metallic nanostructures, periodically distributed chains of Ag nanoparticles oriented parallel to the ferroelectric c-axis are obtained. This alternative metallic nanostructure configuration supports the resonance between the localized surface plasmon and exclusively the π-polarized Stark laser line of Nd(3+) ions at 1.08 μm, while maintaining the remaining crystal field transitions unchanged. The work provides the experimental proof on how plasmonic-based optical antennas can be used to influence selectively rare earth optical Stark transitions to improve the performance of solid state laser gain media.

Luminescence of Rare Earth Ions in Strontium Barium Niobate Around the Phase Transition : The Case of Tm3+ Ions

The photoluminescence of ferroelectric Sr 0.6 Ba 0.4 (NbO 3 ) 2 congruent crystals doped with four different trivalent rare earth ions, Nd 3+ , Eu 3+ , Tm 3+ and Yb 3+ , has been examined in the temperature range around the Curie temperature, when the system undergoes the transition to the paraelectric phase. While the luminescence of large ions is not sensitive (Nd 3+ ) or weakly sensitive (Eu 3+ ) to the phase transition, that of small ions (Tm 3 + and Yb 3 + ) is highly sensitive to temperature changes around the Curie temperature. Particular emphasis is given to the Sr 0.6 Ba 0.4 (NbO 3 ) 2 :Tm 3 + system, for which the luminescence spectra around the phase transition and low temperature laser site selective spectra are systematically investigated. The different spectral sensitivity of the rare earth ions to the phase transition is interpreted in terms of the lattice sites occupied by the different rare earth ions in the Strontium Barium Niobate crystal host.

Blue SHG Enhancement by Silver Nanocubes Photochemically Prepared on a RbTiOPO4 Ferroelectric Crystal

Silver nanocubes with low size dispersion have been selectively photo-deposited on the positive surface of a periodically poled RbTiOPO4 ferroelectric crystal. The obtained nanocubes show preferential orientations with respect to the substrate suggesting epitaxial growth. The plasmonic resonances supported by the nanocubes are exploited to enhance blue SHG at the domain walls.