Sum-frequency generation in a cooledβ-BaB2O4 crystal

Sum-frequency generation in -BaB₂O₄ has been studied by mixing the unpolarized output of an excimer laser pumped dye laser with its second harmonic. The shortest wavelength obtained at 95 K is 195.3 nm. A lower limit of 194.4±0.2 nm at the temperature of liquid helium can be expected.     

Characteristics of a new-type SHG crystala β-BaB2O4 in the femtosecond region

The optical characteristics of a new nonlinear crystal β-BaB₂O₄, including the group-velocity dispersion and the nonlinear coefficient in the femtosecond region, are investigated through the second-harmonic generation (SGH). The performances of β-BaB₂O₄ relative to KDP are evaluated from both calculations and measurements. From the measurements using the crystals with different thicknesses, the influence of the group-velocity mismatch of β-BaB₂O₄ on the broadening of the correlation width is revealed, and is found to be smaller than that of KDP. Overall performance of the SHG conversion efficiency of β-BaB₂O₄ including all affecting factors is evaluated to show higher values by a factor of 10 to 10² than that of KDP depending on operating conditions.     

A tunable UV-light source for laser spectroscopy using second harmonic generation in β-BaB2O4

A -BaB₂O₄ (BBO) crystal is used to generate UV-light tunable in the range 230–303 nm by frequency doubling of an excimer-laser-pumped dye laser. This interval can be covered by a series of continuous scans of 0.25 nm (in the UV). During each scan the -BaB₂O₄ phase-matching angle is synchronized with the rotation of grating and etalon in the dye laser. The bandwidth of the frequency-doubled light is about 2 or 4 GHz depending on the dye laser etalon. The application is illustrated by a section of the one-photon excitation spectrum of Li₂ Rydberg molecules.     

EPR Study of Nd3☎ Ions in β-BaB2O4 Single Crystals

An Electron Paramagnetic Resonance (EPR) study of Nd^3? ions in single crystals of the low-temperature phase of barium borate (β-BaB₂O₄ or BBO) is presented. The EPR spectra show the existence of a minimum of three different Nd^3? centres. The g-matrices for three of the centres have been determined from the analysis of the angular dependencies of the spectrum in two planes of the crystal. This study allows us to conclude that neodymium is incorporated in the low-symmetry Ba^2? site. Possible reasons for the appearance of various Nd^3? centres are discussed.     

Generation of continuous-wave coherent radiation tunable down to 190.8nm in β-BaB2O4

Continuous-wave coherent radiation tunable in the wavelength range 190.8–196.1 nm has been generated by sum-frequency generation in -BaB₂O₄. The fundamental beams were supplied by a Ti:Al₂O₃ laser and a frequency-doubled argon-ion laser.     

Picosecond third-harmonic light generation inβ-BaB2O4

The type-II phase-matched third-harmonic light generation in a -BaB₂O₄ crystal is studied experimentally. A passively mode-locked Nd: phosphate glass laser is used as a pump source. At a pump pulse peak intensity ofI ₁₀=5×10¹⁰W/cm² a third-harmonic conversion efficiency of a percent is obtained. A theoretical discussion of phase-matched third-harmonic generation in crystals of the symmetry group of -BaB₂O₄ (trigonal class 3) is given. The effective nonlinear susceptibility _eff for type-II phase-matching is determined.On leave from the Shanghai Institute of Optics and Fine Mechanics, Academia Sinica, Shanghai, P.R. China     

Spontaneous emission spectrum of a four-level atom in a double-band photonic crystal

The spontaneous emission spectrum from a four-level atom in a double-band photonic crystal has been investigated.We use the model which assumes three atomic transitions. One of the transitions interacts with the free vacuum modes,and the other two transitions couple to the modes of the isotropic photonic band gap (PBG), the anisotropic PBG and another free vacuum. The effects of the fine structure of the lower levels on the spontaneous emission spectrum of an atom are investigated in detail in the three cases. New features of four (two) transparencies with two (one) spontaneous emission peaks, resulting from the fine structure of the lower levels of an atom, are predicted in the case of isotropic PBG modes.     

Experiment study of wavelength conversion in a dispersion-flattened photonic crystal fiber

Wavelength conversion based on four-wave mixing(FWM)has been demonstrated using a 40-m dispersion flattened highly nonlinear photonic crystal fiber(HNL-PCF).A conversion efficiency of-26 dB for a pump power of 19.5 dBm and a conversion bandwidth of 28 nm have been obtained,which are limited by the continuous wave(CW)laser wavelength range and tunability of optical band pass filters(OBPFs).     

YAG laser-induced β-BaB2O4 crystalline dot formation in Sm2O3–BaO–B2O3 glasses

A continuous-wave (CW) YAG laser (power: 0.75–0.9 J/s, irradiation time: 15 s–15 min) with a wavelength of 1064 nm is irradiated to 11.1Sm₂O₃·44.4BaO·44.4B₂O₃ glass, and the formation of β-BaB₂O₄ (β-BBO) crystalline dots with a diameter of 30–150 μm is confirmed from micro-Raman spectra. β-BBO crystals with around 200 μm length grow towards the interior of the glass. The incorporation of Sm³⁺ into β-BBO crystalline dots is suggested from micro-Raman and fluorescence spectra. The second harmonic generation is detected from the array (10×10=100 dots) of β-BBO crystalline dots, indicating that each crystalline dot formed by YAG laser irradiation is a nonlinear optical crystal. CW YAG laser irradiation to glass with Sm³⁺ ions is a nice technique for a spatially controlled crystal growth.     

Spontaneous emission spectrum of a three-level atom embedded in photonic crystal

The two models of three-level (one upper level and two lower levels, or two upper levels and one lower level) atom embedded in a double-band photonic crystal are adopted. The atomic transitions from the upper levels to the lower levels are assumed to be coupled by the same reservoir which are respectively the isotropic photonic band gap (PBG) modes, the anisotropic PBG modes and the free vacuum modes. The effects of the fine structure of the atomic ground state levels in the model with one upper level and two lower levels, and the quantum interferences in the model with two upper levels and one lower level on the spontaneous emission spectrum of an atom are investigated in detail. Most interestingly, it is shown that new spontaneous emission lines are produced from the fine splitting of atomic ground state levels in the isotropic PBG case. The quantum interferences induce additional narrow spontaneous lines near the transition from the empty upper level to the lower level.     

Synthesis and optical properties of Co2+ and Ni2+ ions doped β-BaB2O4 nanopowders

Co²⁺ and Ni²⁺ ions doped β-BaB₂O₄ nanopowders have been prepared by co-precipitation method and their structural properties are studied by spectroscopic techniques. Powder XRD data reveals that the crystal structure belongs to monoclinic and the average crystallite size is calculated. Optical absorption spectra data reveal octahedral site symmetry for Co²⁺ and Ni²⁺ ions. Crystal field (Dq) and inter-electron repulsion (B and C) parameters are evaluated for Co²⁺ doped β-BaB₂O₄ nanopowders as Dq=960, B=900 and C=3850 cm^?1 and for Ni²⁺ doped β-BaB₂O₄ nanopowders, Dq=900, B=850 and C=3500 cm^?1. FT-IR spectra showed the characteristic vibrational bands related to BO₃ and BO₄ molecules. Photoluminescence spectra contain the emission bands in ultraviolet and blue regions.     

Cascaded carbon monoxide laser frequency conversion into the 4.3-4.9 μm range in a single ZnGeP2 crystal

Collinear cascaded mid-IR frequency conversion in a single nonlinear optical crystal was accomplished. Concurrent collinear generation of the sum frequency of multiline fundamental band carbon monoxide laser radiation as a first frequency conversion cascade resulted in collinear difference frequency generation within the 4.3 to 4.9 μm spectral range when mixing this sum frequency radiation with the fundamental one as the second cascade in the same ZnGeP(2) nonlinear optical crystal.     

Amplified spontaneous emission of F2− color center in LiF crystal

Intense, directional, and narrow-bandwidth 1130-nm emission was generated from LiF crystal at room temperature by the pumping with radiation in the wavelength region of the absorption band of the F₂ ⁻ color center. The emission was observed by the pumping with radiation of pulse energy above 5 mJ, and the divergence angle of the beam was about 16 mrad. This emission is attributed to the amplified spontaneous emission by the F₂ ⁻ center. Received: 8 April 1999 / Revised version: 18 May 1999 / Published online: 16 September 1999     

Suppressing the preferential o-polarization oscillation in a high power Nd：YVO4 laser with wedge laser crystal

We observe the phenomenon of priority oscillation of the unexpected a-polarization in high-power Nd：YVO4 ring laser. The severe thermal lens of the a-polarized lasing, compared with the n-polarized lasing, is the only reason for the phenomenon. By designing a wedge Nd：YVO4 crystal as the gain medium, the unexpected a-polarization is completely suppressed in the entire range of pump powers, and the polarization stability of the expected zc-polarized output is enhanced. With the output power increasing from threshold to the maximum power, no a-polarization lasing is observed. As a result, 25.3 W of stable single-frequency laser output at 532 nm is experimentally demonstrated.     

Enhanced emission of the laser generated plume in Ar gas

We report an effect of enhanced emission of laser generated plume in gas ambi-ent.Nonreactive argon gas was used in the experiment of UV laser ablation of copper plate.The emission of Cu atoms from the plume was increased more than the times.The emission in-tensity is sensitive to the gas pressure,but in a certain region of gas pressure,the enhanced ef-fect can be controlled stably.For the emission lines from other element,Zn,in the plume,the intensities are subjected to the same factor of increasement.     

Yb:CaF2 — a new old laser crystal

Here is presented and discussed the history, the spectroscopic, the thermo-mechanical and the laser properties of the new old laser crystal Yb:CaF₂, of its Sr and Ba isotypes as well as of an Yb and Na codoped compound. It will be shown that Yb:CaF₂ is a very particular luminescent material, in which the laser-active center probably consists of a complex hexameric cluster, that it is the most promising Yb-doped fluoride material for large-scale, high-power and high-energy laser systems and that it can compete in several aspects with the currently used Yb-doped oxide crystals and glasses. The opportunity of operating the crystals at cryogenic temperatures and the recently achieved improvements in the field of femtosecond pulsed laser operation and high peak power laser amplification is highlighted and evaluated.     

Determination of λ and κ in a mosaic of single crystal YBa2Cu3O6.95

We present transverse fieldSR measurements of the magnetic field distribution in a mosaic of single crystal YBa₂Cu₃O_6.95 in high fields (1.9–6.5 T) applied parallel to the crystallographicc-axis. TheSR lineshapes are shown in detail and the results of fits to the lineshapes are summarized.     

Topology of spin Hamiltonian and crystal field tensors for Mn2+ in ZnSeO4·6H2O crystal

The expansion of the {B ₂} and {B ₄} tensors of the spin Hamiltonian (SH) for Mn²⁺ (⁶S-state) is performed in terms of the irreducible tensor products {V _{L 1} ?V _{L 2}}₂ and {V _{L 1} ?V _{L 2}}₄ of the crystal field (CF) tensors {V _{L 1}} and {V _{L 2}}. The EPR spectra of Mn²⁺ in the ZnSeO₄·6H₂O crystal are studied and the SH tensors {B ₂} and {B ₄} are calculated. The tensors {V ₄?V ₄}₂ and {V ₄?V ₄}₄ are computed using the point-charge model (PCM) of the [Zn(H₂O)₆] complex with the C₂ symmetry and are compared with the SH tensors {B ₂} and {B ₄}, respectively. The correct signs of the elements and the pseudo-symmety axes of the tensors are obtained both for the {B ₂} tensor and {V ₄?V ₄}₂ and for the {B ₄} tensor and {V ₄?V ₄}₄. It is concluded that the irreducible tensor products {V ₄?V ₄}₂ and {V ₄?V ₄}₄ provide the predominant contribution respectively to the SH tensors {B ₂} and {B ₄} of Mn²⁺.     

Suppressing the preferential σ-polarization oscillation in a high power Nd:YVO_4 laser with wedge laser crystal

We observe the phenomenon of priority oscillation of the unexpected σ -polarization in high-power Nd:YVO 4 ring laser. The severe thermal lens of the σ -polarized lasing, compared with the π-polarized lasing, is the only reason for the phenomenon. By designing a wedge Nd:YVO 4 crystal as the gain medium, the unexpected σ -polarization is completely suppressed in the entire range of pump powers, and the polarization stability of the expected π-polarized output is enhanced. With the output power increasing from threshold to the maximum power, no σ -polarization lasing is observed. As a result, 25.3 W of stable single-frequency laser output at 532 nm is experimentally demonstrated.     

Isotopic effect in Brillouin light-scattering spectra of a Cs5H3(SO4)4 · xH2O crystal

A Brillouin scattering study of the behavior of hypersonic longitudinal acoustic phonons in crystalline pentacesium trihydrogen tetrasulfate Cs₅H₃(SO₄)₄ · xH₂O (PCHS) and its deuterated analog Cs₅D₃(SO₄)₄ · xD₂O (DPCHS) at temperatures ranging from 420 to 120 K is reported. The effect of deuteration on the crystal lattice dynamics is investigated. The differences in the behavior of hypersonic acoustic phonons in the PCHS and DPCHS crystals suggest the existence of a hydrogen isotope effect in both the high-and low-temperature phases. A possible model of the isotopic effect in the high-temperature phase is discussed. An analysis is made of the acoustic response of the PCHS and DPCHS crystals in the region of the transition to the glasslike phase.