Luis Mateos

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.

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.

Micro-spectroscopic characterization of ferroelectric domain structures in Yb 3+ :LiNbO 3 prepared by electron beam writing

Sub-micrometer inverted domains in Yb3+ doped bulk LiNbO3 crystals are reported by using direct electron beam writing (DEBW) as a tool to reverse the spontaneous polarization in a two dimensional geometry. The effect of electron bombardment within the domain inversion process is analyzed at the micrometer scale by combining spatially resolved confocal Raman, Fluorescence and Second Harmonic Generation (SHG) imaging techniques. The obtained results not only confirms the feasibility of DEBW on the inversion procedure -the linear and nonlinear optical properties of the system remain unaltered after the process, but they also show that the slight structural changes associated with the polarization reversal in LiNbO3 are independent on the employed switching mechanism. The possibility of obtaining complementary non-destructive spectroscopic images of domains is also shown. Together, the results highlight the outstanding opportunities offered by confocal spectroscopy as a non invasive tool to probe the interaction between intrinsic defects and ferroelectric domain reversal structures in LiNbO3. Additionally, they provide valuable information to further decrease the size and distance between adjacent inverted domains in a solid state system in which, IR laser action, self-frequency conversion processes and laser tunability have been demonstrated.

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.

Second Harmonic Conical Waves for Symmetry Studies in

In this work, we further exploit the usefulness of quadratic second harmonic generated patterns from a χ(2) two-dimensional ferroelectric domain structure to reveal the symmetry of trigonal ferroelectric crystals. Via conical-type second harmonic generation we show that the intrinsic symmetry properties of the crystal can be visualized to the naked eye by simply rotating the crystal around its polar axis. We demonstrate how this approach can goes a step further in crystallographic studies allowing us to distinguish changes on the sign of the crystallographic axis in bulk crystals.

\chi^{(2)}

Hexagonal plasmonic necklaces of silver nanoparticles organized in 2D superlattices on functional ferroelectric templates are fabricated in large-scale spatial regions by using a surfactant-free photo-deposition process. The plasmonic necklaces support broad radiative plasmonic resonances allowing the enhancement of second harmonic generation (SHG) at the ferroelectric domain boundaries. A 400-fold SHG enhancement is achieved at the near-UV spectral region with subsequent interest for technological applications.

Nonlinear Photonic Crystals

Triply coincident nonlinear second harmonic interactions continuously tunable in an extremely large spectral range (800–1400 nm) are demonstrated in strontium barium niobate random quadratic crystal. The spectral dependence of the triple process is in agreement with a first order quasi-phase-matching collinear interaction. The analysis of the polarization states of the generated beams reveals mutually coherent interactions between different processes simultaneously contributing to the total intensity. The results constitute a necessary step in the implementation of ultra-broadband entangled photon pairs from randomly poled structures and are relevant to a wide range of applications, for which broadband polarization controlled light is required.