Carlos Zaldo

Spectroscopy and Lasing of Yb-Doped

Tetragonal (space group f4 macr) single crystals of NaY(WO4)2 doped with Yb to a density of 4.52 times 1020 cm-3 have been employed as laser active materials for the 1-mum spectral range, operating at room temperature. Using Ti:sapphire laser pumping, slope efficiencies as high as 74.6% were achieved without special cooling for the pi-polarization. The Yb-laser was continuously tunable from 1003.7 to 1073.0 nm with a birefringent filter. Pulses as short as 53 fs were obtained at 1035 nm by SESAM passive mode-locking with intracavity dispersion compensation and additional extracavity pulse compression using analogous prism pairs. Experimental data on the spectroscopic properties of Yb3+ in the 5-300 K temperature range and the room temperature optical properties of this novel Yb-host is also presented.

{\hbox {NaY}}{({\hbox {WO}}_{4})}_{2}

Lasing of Yb3+ in a disordered single crystal host, NaGd(WO4)2, is reported. Pump efficiencies as high as 20% and slope efficiencies as high as 30% are achieved for both sigma- and pi-polarizations with Ti:sapphire laser pumping. The emission of Yb:NaGd(WO4)2 is centered near 1030 nm. Tunability between 1016 and 1049 nm is obtained with a Lyot filter.

: Tunable and Femtosecond Mode-Locked Laser Operation

Local piezoelectric deformation is investigated in (Pb,La)TiO3 (PLT) and Pb(Zr,Ti)O3 (PZT) thin films via scanning force microscopy (SFM) as a function of the ac voltage Vac applied between the conducting tip and the bottom electrode. Thus obtained voltage dependence of the effective piezoelectric coefficient (local piezoelectric nonlinearity) is compared with the corresponding macroscopic piezoelectric behavior determined by laser interferometry. As expected, the local piezoresponse of PLT films measured inside uniformly polarized areas (ferroelectric domains) remains almost linear with increasing Vac until the driving voltage becomes comparable with the coercive one. The corresponding macroscopic response is substantially nonlinear, suggesting significant contribution from the motion of 90° domain walls. On the contrary, in PZT films the local piezoelectric behavior is strongly nonlinear, whereas the macroscopic piezoelectric coefficient is almost field independent. Moreover, depending on the polarity o...

Tunable laser operation of ytterbium in disordered single crystals of Yb:NaGd(WO4)2.

We report, for the first time to our knowledge, femtosecond-pulse operation of a Tm,Ho:NaY(WO(4))(2) laser at around 2060 nm. Transform-limited 191 fs pulses are produced with an average output power of 82 mW at a 144 MHz pulse repetition frequency. Maximum output power of up to 155 mW is generated with a corresponding pulse duration of 258 fs. An ion-implanted InGaAsSb quantum-well-based semiconductor saturable absorber mirror is used for passive mode-locking maintenance.

Nonlinear local piezoelectric deformation in ferroelectric thin films studied by scanning force microscopy

Abstract The high-temperature solution growth of KGd(WO 4 ) 2 single crystals doped with trivalent rare earth (RE 3+ ) ions has been investigated by using the top seeded solution growth (TSSG) technique and K 2 W 2 O 7 as solvent. Electron probe microanalyses were used to determine the RE concentration in the crystals grown. For the 1% doping level in the melt the distribution coefficients of Nd, Er, Ho and Tm are in the 0.8–1.1 range. Yb and Pr have smaller distribution coefficients of 0.7 and 0.5, respectively. The crystal field splittings of the Ho 3+ , Er 3+ and Yb 3+ electronic multiplets have been determined at 4.2 K. Some evidence of the presence of several Yb 3+ centres has been obtained.

Femtosecond (191 fs) NaY(WO4)2 Tm,Ho-codoped laser at 2060 nm.

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...

Growth and characterisation of monoclinic KGd1−xREx(WO4)2 single crystals

Output powers as high as 300mW were obtained at 1925nm in the cw regime with a Tm laser operating at room temperature, either with Ti-sapphire or diode laser pumping, using a new single crystal of NaGd(WO4)2 grown by the Czochralski method and doped with 5mol.% of Tm3+ in the melt. This crystal belongs to the I4¯ tetragonal space group and exhibits a locally disordered structure due to the random occupancy of the same lattice sites by Na and Gd (or Tm) ions. The local disorder results in large bandwidths of the Tm3+ optical transitions (e.g., FWHM≈60cm−1 at 5K for the H63-->F43 transition involved in the laser emission), which allows one to obtain one of the broadest laser tunability ranges, from 1813 to 2025nm(≈17THz), achieved with a Tm3+-doped crystalline material. A detailed characterization of the Tm3+ optical spectroscopy in this novel host was performed at 5 and 300K.

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

The disordered crystal LiGd0.936Yb0.064(MoO4)2 was grown with high optical quality by the flux method using Li2MoO4 as a solvent. The crystal possesses tetragonal symmetry belonging to the space group I4¯, with two nonequivalent sites occupied by the dopant. Spectroscopic studies at 5 and 300 K provided information on the Stark energy-level splitting and the absorption and emission cross sections of the Yb3+ ion. Laser operation of Yb3+ was obtained for the first time in such a Li-containing double tungstate or molybdate. The tuning range with a Ti:sapphire laser pumping extended over ∼32nm for the σ polarization and 23nm for the π polarization. Without a tuning element the laser performance was similar for the two polarizations. By using a 10% transmission output coupler, a maximum output power of ≈470 mW was obtained with a slope efficiency η=64.5% and the absorbed pump power at threshold was 520 mW. Laser operation was also achieved by pumping with a tapered diode laser and a fiber-coupled diode laser module, with a Yb laser output power of 0.66W in the latter case.

Broadly tunable laser operation near 2μm in a locally disordered crystal of Tm 3+ -doped NaGd(WO 4 ) 2

The polarized optical absorption (OA) and photoluminescence of Pr3+ doped KGd(WO4)2 (KGW) single crystals have been measured at selected temperatures between 7 and 300 K. For the studied Pr concentrations, [Pr] = 0.03×1020-1.9×1020 cm-3, a unique site is observed. 74 energy levels were experimentally determined for this site and labelled with the appropriate A or B irreducible representations corresponding to the C2 symmetry of the Gd point site in KGW. The set was fitted to a Hamiltonian of adjustable parameters including free-ion as well as real Bqk and complex Sqk crystal-field parameters. A good simulation of the experimental crystal field energies was achieved with a root mean square deviation σ = 15.3 cm-1. Distortions in the oxygen bonds to Pr3+ are found to contribute to the broadening of some OA bands, particularly those related to the 1D2 multiplet. The OA edge is determined by the interconfigurational 4f→4f15d1 Pr3+ transition peaking at 34 200 cm-1. From the average 300 K OA cross sections the radiative lifetimes of the Pr3+ multiplets have been calculated considering the standard and modified Judd-Ofelt (JO) theories. A better agreement with the experimental results is obtained by the standard theory: the average JO parameters obtained at 2 = 12.0×10-20 cm2, 4 = 8.15×10-20 cm2 and 6 = 2.64×10-20 cm2. Electrons excited to the 3P0 multiplet decay very efficiently to the 1D2 multiplet even at 15 K. In samples with [Pr]≥0.3×1020 cm-3 the excitation of the 1D2 multiplet decays non-radiatively by an electric dipole-dipole transfer between Pr neighbours.

Growth, spectroscopy, and tunable laser operation of the disordered crystal LiGd(MoO 4 ) 2 doped with ytterbium

Continuous wave and femtosecond mode-locked laser operation of Yb3+ in the tetragonal NaLu(WO4)2 crystal host is demonstrated by pumping with a Ti:sapphire laser. Pumping with 1.8W at 974nm, a maximum output power of 650mW was achieved at 1029.6nm. The slope efficiency was in excess of 60%. The laser performance was similar for the two polarizations. By inserting a birefringent filter the output wavelength was tunable from 1010to1055nm. Pulses as short as 90fs with an average power of 50mW were generated by passive mode locking at a repetition rate of 95MHz. These attractive laser properties of NaLu1−xYbx(WO4)2 are related to the inhomogeneous broadening of the Yb3+ spectral features resulting from the local disorder of the host crystal. We report the spectroscopic properties of Yb3+ in the 5–300K temperature range and the optical properties of the host at room temperature.