Mauricio Rico

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

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

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.

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

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.

Optical spectroscopy of Pr3+ in KGd(WO4)2 single crystals

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.

Polarization and local disorder effects on the properties of Er 3+ -doped XBi(YO 4 ) 2 , X=Li or Na and Y=W or Mo, crystalline tunable laser hosts

Pure and Er-doped ([Er]crystal≤2.4 × 1020 cm−3), NaBi(WO4)2 (NBW), NaBi(MoO4)2, and LiBi(MoO4)2 crystals have been grown by the Czochralski method. The three crystal hosts have similar structural and optical properties. The noncentrosymmetric space group I4¯ (No. 82) crystallographic structure has been established through 300 K single-crystal x-ray and neutron (for NBW only) diffraction. Er3+ energy levels were determined experimentally and simulated in the S4 symmetry through a crystal-field analysis. With this background, the large spectroscopic bandwidths observed were ascribed to the presence of two 2b and 2d sites for Er3+ and to different short-range Na+ and Bi3+ distributions around both sites. The radiative properties of Er3+ are described by the Judd-Ofelt theory achieving branching ratios and radiative lifetimes for transitions useful as laser channels. The 4I13/2—>4I15/2 laser channel (λ≈1.5 μm) shows a peak emission cross-section σEMI(λ≈1530 nm)≈0.5×10−20 cm2 and a quantum efficiency η∞0.68 to 0.74. The laser emission is envisaged to be tunable by Δλ≈100 nm.

Emission cross sections and spectroscopy of Ho/sup 3+/ laser channels in KGd(WO/sub 4/)/sub 2/ single crystal

The spectroscopic properties of Ho/sup 3+/ laser channels in KGd(WO/sub 4/)/sub 2/ crystals have been investigated using optical absorption, photoluminescence, and lifetime measurements. The radiative lifetimes of Ho/sup 3+/ have been calculated through a Judd-Ofelt (JO) formalism using 300-K optical absorption results. The JO parameters obtained were /spl Omega//sub 2/=15.35/spl times/10/sup -20/ cm/sup 2/, /spl Omega//sub 4/=3.79/spl times/10/sup -20/ cm/sup 2/, /spl Omega//sub 6/=1.69/spl times/10/sup -20/ cm/sup 2/. The 7-300-K lifetimes obtained in diluted (8/spl middot/10/sup 18/ cm/sup -3/) KGW:0.1% Ho samples are: /spl tau/(/sup 5/F/sub 3/)/spl ap/0.9 /spl mu/s, /spl tau/(/sup 5/S/sub 2/)=19-3.6 /spl mu/s, and /spl tau/(/sup 5/F/sub 5/)/spl ap/1.1 /spl mu/s. For Ho concentrations below 1.5/spl times/10/sup 20/ cm/sup -3/, multiphonon emission is the main source of non radiative losses, and the temperature independent multiphonon probability in KGW is found to follow the energy gap law /spl tau//sub ph//sup -1/(0)=/spl beta/exp(-/spl alpha//spl Delta/E), where /spl beta/=1.4/spl times/10/sup -7/ s/sup -1/, and /spl alpha/=1.4/spl times/10/sup 3/ cm. Above this holmium concentration, energy transfer between Ho impurities also contributes to the losses. The spectral distributions of the Ho/sup 3+/ emission cross section /spl sigma//sub EM/ for several laser channels are calculated in /spl sigma/- and /spl pi/-polarized configurations. The peak a /spl sigma//sub EM/ values achieved for transitions to the /sup 5/I/sub 8/ level are /spl ap/2/spl times/10/sup -20/ cm/sup 2/ in the /spl sigma/-polarized configuration, and three main lasing peaks at 2.02, 2.05, and 2.07 /spl mu/m are envisaged inside the /sup 5/I/sub 7//spl rarr//sup 5/I/sub 8/ channel.

Measurement and crystal-field analysis of Er3+ energy levels in crystals of NaBi(MoO4)2 and NaBi(WO4)2 with local disorder

Abstract The polarised optical absorption and photoluminescence of Er 3+ -doped tetragonal I 4 NaBi(MoO 4 ) 2 (NBMo) and NaBi(WO 4 ) 2 (NBW) single crystals have been measured at 10 K. In these hosts Na + and Bi 3+ cations are statistically distributed over 2b and 2d crystallographic sites both with S 4 symmetry. Er 3+ is expected to replace Bi 3+ , therefore inhomogeneous broadening of its spectral bands may arise simultaneously from the occupancy of the above two sites and from the different short-range Na and Bi distributions around them. Despite the broad bands observed, well-defined polarisation rules have been determined, indicating that no reduction of host site symmetry occurs. The positions of 52 and 66 observed Kramers doublets in NBMo and NBW crystals have been established and labelled with their corresponding irreducible representations. Detailed single-electron Hamiltonians combining together free-ion and crystal-field interactions have been used to reproduce the energy levels and associated wavefunctions of the 4f 11 configuration of Er 3+ in both hosts. Very satisfactory correlations were obtained between experimental and calculated crystal-field levels, with rms deviations σ =9.5 and 13.3 cm −1 for NBMo and NBW, respectively. In an attempt to account for the linewidths observed, the energy separation due to the two non-equivalent 2b and 2d sites where Er 3+ can be found has been calculated taking into account only the first oxygen shell (i.e., neglecting the Na and Bi disorder). The site-associated calculated energy separation is generally lower but close to the experimental 10 K linewidths.

Yb sensitising of Er3+ up-conversion emission in KGd(WO4)2:Er:Yb single crystals

KGd(1−x−y)ErxYby(WO4)2 (x/y=0.024/0, 0.02/0.022 and 0.019/0.066) single crystals have been grown by the top seeded solution growth slow cooling method using K2W2O7 as solvent. Erbium concentration was selected at [Er]=1.2–1.5×1020 cm−3 in order to minimise Er–Er non-radiative losses and the ytterbium concentration was varied in the range [Yb]=0–4.2×1020 cm−3. The optical absorption in the 850–1100 nm spectral range is characterised by the overlap between ytterbium 2F5/2 and erbium 4I11/2 manifolds. At room temperature the green 4S3/2 Er3+ emission has been excited by energy transfer from the 2F5/2 (934.2, 953.2 and 980.8 nm) ytterbium multiplet. The up-converted emission is weakly polarised and within the concentration range studied its intensity increases with the Yb3+ concentration.

Progress in crystal growth and characterisation of rare-earth doped non-linear KTP crystals for laser applications

The incorporation of rare earth ions into KTiOPO4 crystals and into other isomorphous phases is more eAcient for impurities of small ionic radius. Some of the Nd 3a and Er 3a spectroscopic properties in KTiOPO4 (KTP) and RbTiOPO4 (RTP) are discussed to illustrate the contribution of several centres to the optical absorption and photoluminescence properties. The Nd 3a centres observed in KTiOPO4 crystals grown in a W-rich flux have larger optical absorption cross sections than those observed in KTP crystals grown in a self-flux. ” 1999 Elsevier Science B.V. All rights reserved.

Optical emission properties of Nd3+ in NaBi(WO4)2 single crystal

Room temperature optical absorption measurements of Nd3+ in NaBi(WO4)2 single crystals grown by the Czochralski method have been used to calculate the following Judd-Ofelt intensity parameters Ω2 = 30.9 × 10−20, cm2, Ω4 = 12.0 × 10-20 cm2 and Ω6 = 9.3 × 10−20 cm2. Using these parameters the emission properties of Nd3+ in this host have been estimated with particular attention to the 4F3/2 energy level responsible for stimulated emission channels. The experimental branching ratios of this level are obtained from photoluminescence and agree with those calculated using the above Ωk parameters. The 4F3/2 emission has neligible non-radiative intrinsic losses and the 10 K lifetime at low Nd concentration, 0.9 × 1018 cm−3, is τexp = 143 μs. Even at 300 K and for a much higher Nd concentration, 4.0 × 1019 cm−3, non-radiative losses are moderate, giving a quantum efficiency n ≊ 0.85. The 4F3/2 emissions show a stronly polarized character. The largest emission cross-section occurs at λ = 1060.9nm for the 4F3/2 → 4I11/7 laser channel with a π-polarized character, σEMI = 16 × 10−20 cm2.