M. O. Ramirez

Optical characterization and laser gain modeling of a NdAl3(BO3)4 (NAB) microchip laser crystal

In this work a systematic optical characterization of the NdAl3(BO3)4 (NAB) crystal has been carried out by means of spectroscopic measurements. The energy levels (118) of the Nd3+ constituent ions have been identified and labeled by appropriate crystal field quantum numbers (μ=1/2 or μ=3/2). Laser site selective experiments revealed the presence of two nonequivalent environments for the Nd3+ ions. From room temperature absorption and emission spectra and using the Judd–Ofelt formalism the emission cross sections and quantum efficiency (η=0.1) of the metastable state 4F3/2 have been determined. By further modeling we have estimated the crystal length and output coupler transmittance which optimize laser efficiency of a diode pumped NAB laser. The optimum crystal length found in all cases is around 100 μm. Finally, the high laser efficiencies predicted (up to 61% and 45% for laser oscillation at 1.06 and 1.34 μm, respectively) make NAB an excellent candidate to be incorporated in a diode pumped microchip l...

Phase transition in SrxBa1−xNb2O6 ferroelectric crystals probed by Raman spectroscopy

In this paper the temperature evolution of the main Raman peak at about 630 cm−1 has been studied in order to detect the ferroelectric to paraelectric phase transition in strontium barium niobate (SrxBa1−xNb2O6) crystals with different stoichiometries (x = 0.33, 0.5 and 0.61). Both its peak wavenumber and width have been shown to be strongly dependent on temperature and display clear anomalies around the Curie temperature. The obtained results manifest the usefulness of simple unpolarized Raman spectra to probe the changes in Tc as a result of modifications in the crystal composition.

Luminescence of trivalent rare earth ions in the yttrium aluminium borate non-linear laser crystal

Single crystals of YAl 3 (B0 3 ) 4 doped with the rare earth ions Sm 3+ , Eu 3+ , Tb 3 +, Dy 3+ ,Tm 3+ and Yb 3 * were grown using the flux growth technique. Luminescence spectra and decay curves were recorded in the visible range at room and low temperatures. The observed transitions were assigned and analysed on the basis of group theory for the D 3 point symmetry. Evidence for the presence of non-equivalent centres has been found for Yb 3+ and possibly for Sm 3+ and Dy 3+ doped crystals.

Evaluation of ytterbium doped strontium barium niobate as a potential tunable laser crystal in the visible

A first spectroscopic characterization of Yb3+ ions in Sr0.6Ba0.4(NbO3)2 nonlinear crystal is presented. From low temperature emission and absorption spectra the energy level diagram of Yb3+ ions in this crystal has been built up. The ground state splitting has been found to be as high as 940 cm−1 allowing for potential quasi-four-level laser operation. In order to assess the ability of this system as a tunable solid state laser the main parameters of relevance in laser dynamics have been obtained and compared to those reported for other Yb3+ doped lasers. Optical microscopy has been used to study the ferroelectric domains appearing in the as-grown crystals. A relatively broad distribution of domain sizes (around 3 μm) will, in principle, allow for broadband noncritical second-harmonic generation of fundamental radiation by quasiphase matching. The obtained results make this crystal a promising candidate for tunable laser generation in the visible domain.

Optical spectroscopy and crystal-field analysis of YAl3 (BO3)4 single crystals doped with dysprosium

YAl3 (BO3)4 crystals doped with Dy3+ were grown from a potassium trimolybdate flux. Their absorption and visible emission spectra and decay curves were measured at temperatures ranging from 10 to 298 K. The complete energy level scheme has been deduced from the low temperature measurements and reproduced by theoretical calculations based on a parametric Hamiltonian including coulombic, spin–orbit and crystal-field terms. The Judd–Ofelt parametrization scheme has been applied to the analysis of the room temperature absorption spectra. The calculated radiative lifetime of the 4F 9/2 state is 344 μs; this value is reasonably consistent with the experimental data.

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 ]

Temperature dependence of Nd3+↔Yb3+ energy transfer in the YAl3(BO3)4 nonlinear laser crystal

The temperature dependence of the Nd3+→Yb3+ energy-transfer rate in the YAl3(BO3)4 nonlinear laser crystal has been investigated from the analysis of fluorescence decay curves recorded in the 10–600 K range. Three different regimes, independent on the dopant concentration, have been observed in the thermal behavior of the Nd3+→Yb3+ energy-transfer rate. By comparing experimental results with theoretical predictions based on the Dexter model [J. Chem. Phys. 21, 836 (1953)], the origin of these different regimes has been explained. In addition, the influence of temperature and of both Nd3+ and Yb3+ concentrations on the Nd3+←Yb3+ energy back-transfer rate has been also investigated, concluding that it is a migration-assisted energy-transfer process. Finally, the populations of both Nd3+ and Yb3+ metastable states achieved after continuous-wave Nd3+ excitation have been calculated and measured and results have been explained in terms of the thermal behavior of both forward- and back-transfer rates.

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.

Spectroscopic study of Yb3+ centres in the YAl3(BO3)4 nonlinear laser crystal

A spectroscopic study of Y b3+ ions in Y Al3(BO3)4 laser crystals is presented. Polarized absorption and site selective spectroscopy experiments at low temperature have been used to determine the presence of two different Y b3+ centres in this host crystal in the concentration range 0.2–9 at.%. The contribution of these centres to the absorption spectra has been found to be dependent on the total Y b3+ concentration, and the Stark energy level diagrams corresponding to the different Y b3+ centres have been determined. The importance of electron–phonon coupling in the optical transitions of Y b3+ ions has been also pointed out.