Mode-locked and Q-switched laser operation of the Yb-doped Li<Subscript>6</Subscript>Y(BO<Subscript>3</Subscript>)<Subscript>3</Subscript> crystal

We report for the first time pulsed laser operation of the Yb-doped Li₆Y(BO₃)₃ (Yb-LYB) crystal in the nanosecond as well as in the femtosecond regime. Pulse durations as short as 355 fs have been obtained in passively mode-locked operation and pulse energies up to 140 μJ in Q-switched operation. PACS 42.70Hj; 42.55Xi; 42.60Gd; 42.65Re     

Comparative spectroscopic and laser properties of Yb<Superscript>3+</Superscript>-doped CaF<Subscript>2</Subscript>, SrF<Subscript>2</Subscript> and BaF<Subscript>2</Subscript> single crystals

We present the spectroscopic properties and room-temperature cw tunable laser operation of Yb³⁺-doped CaF₂, SrF₂ and BaF₂ single crystals grown and studied in the same conditions. Emission cross sections, lifetimes, laser thresholds, laser slope efficiencies and laser wavelength tuning ranges are compared. It appears that Yb³⁺-doped BaF₂ might be more promising for diode-pumped high power laser operation. PACS 42.55.Rz; 42.70.Hj     

Photoconductivity in LiNbO<Subscript>3</Subscript> crystals codoped with MgO and Cr<Subscript>2</Subscript>O<Subscript>3</Subscript>

Polymer stabilized cholesteric texture cells with various chiral and monomer concentrations were fabricated. Electro-optical measurement indicates that the chiral concentration and the monomer concentration significantly influence the hysteresis, threshold voltage and field-on and field-off response times of the cell. The chiral dopant is found to increase the hysteresis width and to decrease the field-off response time of the cell. The monomer is found to decrease the hysteresis width and to increase the field-off response time of the cell. The field-on response time is independent of the monomer concentration and the chiral concentration of the cell. A thinner cell has a smaller hysteresis width. PACS 61.30.-v; 42.70.Df; 42.79.Kr     

Non-critical phase-matched second harmonic generation in Gd<Subscript>x</Subscript>Y<Subscript>1-x</Subscript>COB

We have characterized non-critical phase-matching (NCPM) for both Type I and Type II second harmonic generation (SHG) in y-cut Gd_xY_1-xCOB using a nanosecond optical parametric oscillator (OPO). The variation of the NCPM wavelength with temperature was investigated for different values of the compositional parameter x. Efficient SHG of 1064 nm was achieved by choosing the suitable compositional parameter x=0.28 and by tuning the temperature of the crystal to 52 °C. Using a 25-mm-long Gd_0.28Y_0.72COB crystal, conversion efficiencies of 41 and 43% were obtained respectively from a mode-locked Nd:YAG and a Q-switched Nd:YAG laser. PACS 42.25.Lc; 42.65.Ky; 42.70.Mp; 42.79.Nv     

CW and tunable laser operation of Yb<Superscript>3+</Superscript> doped CaF<Subscript>2</Subscript>

We present the spectroscopic properties and room temperature of a cw tunable laser operation with Yb³⁺ doped CaF₂ single crystals grown in our laboratory. A laser slope efficiency of 50% with respect to the absorbed 920 nm pump power was obtained, and the laser wavelength could be tuned between 1000 and 1060 nm. PACS 42.55R; 42.70An erratum to this article can be found at     

Temperature and wavelength dependent refractive index equations for MgO-doped congruent and stoichiometric LiNbO<Subscript>3</Subscript>

We present wavelength- and temperature-dependent refractive index equations for 5% MgO-doped congruent PPLN and for 1% MgO-doped stoichiometric PPLN crystals valid for a wide spectral and temperature range. The dispersion equations were derived from quasi-phase-matched nonlinear interactions with these two crystal compositions in the near and mid-infrared. The results show a good agreement with previously published frequency conversion experiments. PACS 42.65.Ky; 42.65.Yj; 42.70.Mp     

Actively mode-locked Tm-Ho:LiYF<Subscript>4</Subscript> and Tm-Ho:BaY<Subscript>2</Subscript>F<Subscript>8</Subscript> lasers

We report on the generation of mode-locking pulse trains with high average output powers from diode-pumped Tm-Ho:LiYF₄ and Tm-Ho:BaY₂F₈ lasers emitting at around 2 μm. The highest output power of 365 mW was obtained with the Tm-Ho:YLF₄ laser, whereas the shortest pulse duration of 120 ps and the widest tunability range of 59 nm was achieved with the Tm-Ho:BaY₂F₈ laser. PACS 42.55.Xi; 42.60.Fc; 42.72.Ai; 42.55.Rz; 42.70.Hj     

Amplitude gratings generation by selective defect bleaching in KCl:CO<Subscript>3</Subscript>K<Subscript>2</Subscript> crystals

Volume grating formation by spatially-selective defect bleaching is demonstrated in coloured KCl:CO₃K₂ crystals. The sample is coloured by a train of pulses with a wavelength of λ= 266 nm and bleached with a cw Ar-laser with a wavelength of λ= 514.5 nm. Diffraction efficiency for a 1.6 μm grating period is studied both experimentally and theoretically, using the evolution of colour centre concentration. Hologram recording parameters are obtained and the evolution of the spatial grating profile is discussed. Diffraction efficiencies three times higher than previously published efficiencies are obtained. PACS 42.40.Ht; 42.40.Lx; 42.70.Ln     

Electronic and structural properties of zincblende Al<Subscript>x</Subscript>Ga<Subscript>1-x</Subscript>N

Numerical calculations based on first-principles are applied to study the electronic and structural properties of zincblende Al_xGa_1-xN. The results show that the lattice constant has a very small deviation from the linear Vegard’s law. The direct and indirect bowing parameters of 0.295±0.034 eV and -0.125±0.060 eV are obtained, respectively, and there is a direct-indirect crossover near x=0.692. Besides, the bulk moduli and their pressure derivatives are monotonically increased with an increase of the aluminum composition x. The deviation parameter of the bulk modulus of -5.32±1.60 GPa is obtained. PACS 71.15.-m; 71.15.Nc; 71.55.Eq; 71.20.Nr; 42.70.Qs     

Performance of photorefractive crystals as derived from EPR-based defect studies: application to BaTiO<Subscript>3</Subscript>:Rh and Ba<Subscript>0.77</Subscript>Ca<Subscript>0.23</Subscript>TiO<Subscript>3</Subscript>:Rh

A method is introduced which allows us to predict the performance of a photorefractive material quantitatively using electron paramagnetic resonance (EPR)-based defect studies. This includes the determination of the defect densities and the parameters governing their light-induced charge changes. On this basis the effective trap densities and the photorefractive response times are calculated. All quantities can be determined without theoretical simplifications such as employed in previous approaches to the problem. The method is applied to BaTiO₃ and congruently melting Ba_0.77Ca_0.23TiO₃, both doped with rhodium. The iron defects, present as background contaminations, are fully taken into account. Their influence on the intensity saturating the space charges is calculated on this basis. The complete energy dependences of the absorption cross sections of all optically active Rh and Fe defects are given. PACS 42.70.Nq; 76.30.Mi; 78.40.-q     

Interface injection-limited carrier-transport properties of Alq<Subscript>3</Subscript>

We have studied the carrier-transport properties of the amorphous organic material tris (8-hydroxyquinoline) aluminium (Alq₃) for Al and LiF/Al cathodes at room temperature. The investigation was made by the current–voltage characteristic measurements for different Alq₃ film thicknesses. It is found that the current–voltage characteristic has a linear dependence on the thickness of the Alq₃ film. The current density increases by several orders for a LiF/Al cathode over that of Al at a given bias voltage. The carrier-injection processes at the metal/organic contact dominate the current–voltage characteristics. The carrier injection seems to be limited by the charge hopping of interfacial molecular sites. PACS 73.50.Gr; 73.40.-c; 73.61.Ph; 78.45.+h; 42.70.Jk     

Si/SiO<Subscript>2</Subscript> superlattice based optical planar microcavity

Si/SiO₂ Fabry–Pérot microcavities with a silicon nanocrystal (Si-nc) active spacer have been realized using a novel process based on a reactive magnetron sputtering of a pure silica target. Spectral, spatial and temporal behaviours of the quantum dots confined inside the resonator are detailed. Compared with a reference sample, the spectral and spatial emission distributions are significantly narrowed and the forward emission intensity is enhanced. Time resolved photoluminescence measurements also revealed an increase of the spontaneous emission rate. PACS 42.70.Qs; 78.55.-m; 78.66.-w     

Polar Fe(IO<Subscript>3</Subscript>)<Subscript>3</Subscript> nanocrystals as local probes for nonlinear microscopy

We have investigated nanocrystals of Fe(IO₃)₃ by polarization-sensitive second harmonic generation (SHG) microscopy. As the nonlinear optical properties of this material were only poorly characterized, we have first determined the relative values of the elements of its second-order susceptibility tensor, by the global fitting of the polarization-resolved SHG response of an ensemble of nanocrystals. This information allows one to optically retrieve the orientation of individual particles in the sample. The high SHG efficiency measured for nanocrystals of Fe(IO₃)₃ and their polar nature could make them very attractive for nonlinear microscopy of biological samples. PACS 42.70.Mp; 78.67.Bf     

Spectroscopic characteristics of Pr<Superscript>3+</Superscript> in biaxial La<Subscript>2</Subscript>(WO<Subscript>4</Subscript>)<Subscript>3</Subscript> crystal

A Pr³⁺-doped La₂(WO₄)₃ crystal grown by the Czochralski method has been investigated as a promising laser material. The principal axes of the optical indicatrix and Pr³⁺ concentration of the crystal were determined. The polarized absorption, fluorescence spectra and fluorescence decay curves of the main emission multiplets of the crystal were measured at room temperature. The spectroscopic parameters were obtained by the modified Judd–Ofelt theory combined with the normalized method. The peak stimulated emission cross-sections of the major emission lines were estimated. The good spectroscopic properties imply that the Pr³⁺:La₂(WO₄)₃ crystal is a potential laser gain medium for solid-state laser and self-stimulated Raman laser applications. PACS 78.20.-e; 42.70.Hj     

Linear and nonlinear optical properties of yttrium formate dihydrate,Y(HCOO)<Subscript>3</Subscript>·2H<Subscript>2</Subscript>O

Yttrium formate dihydrate, Y(HCOO)₃·2H₂O (point group 222), a promising new material for Raman laser frequency converters, has been reinvestigated with respect to the linear and nonlinear optical (second-harmonic generation, SHG) properties. High-precision data of refractive indices and their dispersion in the wavelength range 365–1083 nm are given, together with the three independent components of the tensor of nonlinear optical susceptibility (SHG) [d_ijk ^SHG]. A calculation of the macroscopic nonlinear optical susceptibility from the hyperpolarizabilities of the formate groups results in good agreement with the experimental values. This signals that the nonlinear optical interaction in Y(HCOO)₃·2H₂O can mainly be attributed to the formate groups. PACS 42.65.Ky; 42.70.Mp; 78.20.Ci     

Spherical shape BaO-ZnO-B<Subscript>2</Subscript>O<Subscript>3</Subscript>-SiO<Subscript>2</Subscript> glass powders prepared by spray pyrolysis

Fine-sized BaO-ZnO-B₂O₃-SiO₂ (BZBS) glass powders were directly prepared by high temperature spray pyrolysis. The hollow glass powders prepared at low preparation temperature of 1000 °C had a low density of 2.65 g/cm³. However, the densities of the BZBS powders obtained at preparation temperatures of 1200 and 1400 °C were each 3.92 and 4.13 g/cm³. The mean size of the BZBS glass powders prepared by spray pyrolysis at preparation temperature of 1400 °C was 0.98 μm. The glass transition temperature (T_g) of the prepared BZBS glass powders was 518.9 °C. The dielectric layers formed from the prepared BZBS glass powders with a dense structure had a clean surface and a dense inner structure without voids at the firing temperature of 580 °C. The transparencies of the dielectric layers formed from the prepared BZBS glass powders were higher than 90% within the visible range. PACS 42.70.Ce; 85.60.Pg; 71.55.Jv     

Growth and optical properties of paraelectric K<Subscript>1-y</Subscript>Na<Subscript>y</Subscript>Ta<Subscript>1-x</Subscript>Nb<Subscript>x</Subscript>O<Subscript>3</Subscript> single crystals

We report the successful growth of an electroholographic crystal, potassium sodium tantalate niobate (KNTN), by a top-seeded solution growth method. Both blue and colorless crystals were obtained. The structure, optical absorption, and refractive dispersion properties of the as-grown crystals have been investigated. Furthermore, the Kerr coefficients R₁₁ and R₁₂ of paraelectric K_0.95Na_0.05Ta_0.61Nb_0.39O₃ single crystal were determined by using an automated scanning Mach–Zehnder interferometer. The crystal has large Kerr coefficients with R₁₁= 2.8×10^-16 m²/V² and R₁₂= -0.3×10^-16 m²/V² at the wavelength of 632.8 nm near its cubic–tetragonal phase boundary. PACS 81.10.Dn; 42.70.Nq     

Spectroscopy and laser properties of Tm<Superscript>3+</Superscript>:KY(WO<Subscript>4</Subscript>)<Subscript>2</Subscript> crystal

The polarized absorption spectra of Tm³⁺-doped potassium yttrium tungstate (Tm:KY(WO₄)₂) crystal at room temperature were measured. The emission spectrum and lifetime of the ³ F ₄ excited state were determined. Using standard and modified Judd–Ofelt theories, the intensity parameters and the radiative lifetimes were calculated and good agreement with the experimental results was obtained for both theories. Continuous-wave laser operation in Tm:KYW crystal under laser diode pumping at 802 nm and 1750 nm was demonstrated with slope efficiency of 53% and 28% and output power of about 555 mW and 86 mW, respectively. PACS 42.55.Xi; 42.60.Pk; 42.70.Hj     

Broadband infrared luminescence from transparent glass–ceramics containing Ni<Superscript>2+</Superscript>-doped <Emphasis Type="BoldItalic">β</Emphasis> -Ga<Subscript>2</Subscript>O<Subscript>3</Subscript> nanocrystals

Transparent Ni²⁺-doped β-Ga₂O₃ glass–ceramics were synthesized. The nanocrystal phase in the glass–ceramics was identified to be β-Ga₂O₃ and its size was about 3.6 nm. It was confirmed from the absorption spectra that the ligand environment of Ni²⁺ ions changed from the trigonal bi-pyramid fivefold sites in the as-cast glass to the octahedral sites in the glass–ceramics. The broadband infrared emission centering at 1270 nm with full width at half maximum (FWHM) of more than 250 nm was observed. The fluorescence lifetime was about 1.1 μs at room temperature. The observed infrared emission could be attributed to the ³ T _2g (³ F )→³ A _2g (³ F ) transition of octahedral Ni²⁺ ions. It is suggested that the Ni²⁺-doped transparent β-Ga₂O₃ glass–ceramics with broad bandwidth and long lifetime have a potential as a broadband amplification medium. PACS 42.70.-a; 42.70.Ce; 81.40.Tv     

Nonlinear optical properties of Li<Subscript>2</Subscript>B<Subscript>4</Subscript>O<Subscript>7</Subscript> (LB4) crystal for the generation of tunable ultra-fast laser radiation by optical parametric amplification

Using a very simple and straightforward approach we derive the condition to be satisfied for achieving wavelength-insensitive broadband phase matching in a type-I noncollinear optical parametric amplifier (NOPA), required for the generation of ultra-fast laser radiation. Nonlinear optical properties of a relatively newly grown crystal Li₂B₄O₇ (LB4) have also been studied and we found that this crystal satisfies the condition required to realize the broadband phase matching and is suited for the generation of tunable visible–near-infrared ultra-fast laser radiation employed in a 395-nm-pumped type-I NOPA. The phase-matching bandwidths of type-I NOPAs in different borate-group crystals, such as BBO, CLBO, and KABO, are also numerically estimated. The values are 157, 164, 152, and 174 THz for 1-mm-thick BBO, CLBO, KABO, and LB4 crystals, with the noncollinear angles between the input pump and the signal beams 3.7°, 3.0°, 3.4°, and 2.9°, respectively, for the signal wavelengths centered at 630 nm. In addition to the largest bandwidth, LB4 crystal has several other attractive properties to be used in optical parametric applications, such as high laser damage threshold, wide optical transmission, easy crystal growth to excellent optical quality with large sizes, easy treatment of cutting and polishing, and nonhygroscopicity. PACS 42.65.Yj; 42.65.Re; 42.70.Mp