Fabrication of Microstructures in LiF Crystals by a Femtosecond Laser

We report the space-selective formation of colour centres and refractive index change in LiF crystals at room temperature by an 800nm Ti:sapphire femtosecond laser irradiation. Optical absorption and photoluminescence spectra have been measured for the crystal before and after the laser irradiation. The absorption spectra indicate F and F3^ colour centres are induced in LiF after the laser irradiation. When the temperature of heat treatment reaches 300℃, the absorption peak disappears, indicating that the induced colour centres are bleached. The induced refractive index change is also evaluated. A mechanism for the formation of colour centres and refractive index change is proposed.     

Influence of Gamma-Ray Irradiation on Absorption and Fluorescent Spectra of Nd：YAG and Yb：YAG Laser Crystals

We investigate the influence of gamma-ray irradiation on the absorption and fluorescent spectra of Nd^3＋：Y3A15 O12 （Nd： YAG） and Yb^3＋ ： Y3A15 O12 （Yb： YAG） crystals grown by the Czochralski method. Two additional absorption （AA） bands induced by gamma-ray irradiation appear at 255nm and 340nm. The former is contributed due to Fe^3＋ impurity, the latter is due to Fe^2＋ ions and F-type colour centres. The intensity of the excitation and emission spectra as well as the fluorescent lifetime of Nd：YAG crystal decrease after the irradiation of 100 Mrad gamma-ray. In contrast, the same dose irradiation does not impair the fluorescent properties of Yb：YAG crystal. These results indicate that Yb： YA G crystal possesses the advantage over Nd： YA G crystal that has better reliability for applications in harsh radiant environment.     

Space-Selective Precipitation of Ba2TiSi2O8 Crystals in Sm3^＋-Doped BaO-TiO2-SiO2 Glass by Femtosecond Laser Irradiation

The ferroelectric crystal Ba2TiSi2O8 with high second-order optical nonlinearity is precipitated in Sm^3＋-doped BaO-TiO2-SiO2 glass by a focused 800hm, 250 kHz and 150fs femtosecond laser irradiation. No apparent blue and red emissions are observed at the beginning, while strong blue emission due to second harmonic generation and red emission due to the f-f transitions of Sm^3＋ are observed near the focal point of the laser beam after irradiation for 25s. Micro-Raman spectra confirm that Ba2 TiSi2O8 crystalline dots and lines are formed after laser irradiation. The mechanism of the phenomenon is discussed.     

Primary experimental studies on mid—infrared FEL irradiation on dental substances at BFEL

A free electron laser (FEL) with its characteristics of wide wavelength tunability,ultrashort pulse time structure,and high peak power density is predominantly superior to all other conventional lasers in applications.Several experimental studies on mid-infrared FEL irradiation on dental enamel and dentine were performed at the Beijing FEL. EXperimental aims were to investigate changes in the hardness,ratios of P to Ca and Cs before and after irradiation on samples with a charateristic absorption wavelength of 9.66μm,in the colors of these sample surfaces after irradiation with different wavelengths around the peak wavelength.The time dependence of temperature of the dentine sample was measured with its ps pulse effects compared to that with a continuous CO2 laser.FTIR absorption spectra in the range of 2.5-15.4μm for samples of these hard dental substances and pure hydroxyapatite were first examined to decid their chemical components and absorption maximums.Primary experimental resulte will be presented.2001 Elsevier Science B.V.All rights reserved.     

Effect of proton and laser irradiation on the elastic and inelastic properties of a V-Ti-Cr alloy

The behavior of Young’s modulus E and the decrement of ultrasonic vibrations δ in a V-4Ti-4Cr alloy is studied during proton (8-MeV protons, dose rate 10⁴ Gy/s) or IR laser (YAG: Nd³⁺ laser, wavelength 1.06 μm, intensity up to 10² W/cm²) irradiation. Measurements are performed using the method of a composite piezoelectric oscillator (longitudinal 100-kHz resonance vibrations). The sizes of the irradiated surface regions of a sample in the proton and laser experiments are the same in order to provide the same thermal conditions in the sample-quartz transducer system. The amplitude, time, and temperature dependences of E and δ are measured before and after preliminary plastic deformation, as well as before, during, and after irradiation of a sample. The process of postdeformation aging (the kinetics of recovery of internal friction after deformation) during proton irradiation is shown to differ substantially from that during laser irradiation. The specific features detected can be explained by the more intense evolution of the defect structure during proton irradiation. Analysis shows that radiation annealing is related to the ionizing component of proton irradiation, which excites the electronic subsystem of the metallic alloy and, thus, creates hot electrons and plasmons. The electron excitations relax at lattice defects (dislocations) and increase the dislocation mobility; this results in a relatively rapid decrease in the dislocation density and in a more significant (as compared to the laser irradiation) decrease in the level of internal stresses in the material. __________ Translated from Fizika Tverdogo Tela, Vol. 46, No. 8, 2004, pp. 1409–1415. Original Russian Text Copyright ? 2004 by Kardashev, Plaksin, Stepanov, Chernov.     

Growth and characterization of high-quality Mn-doped LiAlO_2 single crystal

<正>The growth of a Mn-doped LiAlO_2 single crystal by the Czochralski(CZ) method and the characterization of its spectroscopy and thermoluminescence(TL) are presented.The X-ray rocking curve and chemical etching analysis show that the as-grown crystal has good crystallinity.The full-width at half-maximum (FWHM) of the LiAlO_2(200)ωrocking curve is 23.2 arcsec and the etching pits density of the(100) plane is(1.6-4.0)×10~4 cm~(-2).The transmission spectrum indicates that the crystal is highly transparent in the 200-1500-nm wavelength range.The emission spectrum of the crystal consists of a peak around 579 nm when excited with 428-nm light.The TL spectra show that the LiAlO_2:Mn crystal has glow peaks at 150 and 172℃.The change of TL characteristics of the crystal before and after thermal annealing in the air is discussed,and the effect of annealing and irradiation on the evolution of defect types is analyzed.     

Femtosecond laser-induced long-lasting phosphorescence in Pr^3＋-doped ZnO-B2O3-SiO2 glass

This paper studies the phenomenon of long-lasting phosphorescence induced by a femtosecond laser in Pr3 -doped ZnO-B2O3-SiO2 glass. With the glass irradiated by a focused femtosecond laser for a short time, the emission of strong reddish long-lasting phosphorescence from the irradiated part of the glass can be observed. The emission peaks are located at 495 and 603 nm in wavelength, showing that the long-lasting phosphorescence originates from the emission of Pr^3 . The intensity of the phosphorescence decreases in inverse proportion to time after the removal of the laser. By analysing the absorption and electron spin resonance spectra of the glass, we find that colour-centres are induced in the glass matrix after the irradiation of the femtosecond laser. A possible mechanism has been provided to account for the generation of long-lasting phosphorescence.     

Structural and optical investigation of nonpolar a-plane GaN grown by metal-organic chemical vapour deposition on r-plane sapphire by neutron irradiation

Nonpolar (1120) a-plane GaN films are grown by metal-organic chemical vapour deposition (MOCVD) on r-plane (1102) sapphire.The samples are irradiated with neutrons under a dose of 1 × 10 15 cm 2.The surface morphology,the crystal defects and the optical properties of the samples before and after irradiation are analysed using atomic force microscopy (AFM),high resolution X-ray diffraction (HRXRD) and photoluminescence (PL).The AFM result shows deteriorated sample surface after the irradiation.Careful fitting of the XRD rocking curve is carried out to obtain the Lorentzian weight fraction.Broadening due to Lorentzian type is more obvious in the as-grown sample compared with that of the irradiated sample,indicating that more point defects appear in the irradiated sample.The variations of line width and intensity of the PL band edge emission peak are consistent with the XRD results.The activation energy decreases from 82.5 meV to 29.9 meV after irradiation by neutron.     

A Gain-Switched Fe:ZnSe Laser Pumped by a Pulsed Ho,Pr:LLF Laser

We demonstrate a gain-switched Fe:ZnSe laser pumped by a 2958 nm pulsed Ho,Pr:LLF laser. The maximum single pulse energy is 16.4uJ with a minimum pulse duration of 13.9 ns at the pulse repetition frequency of1 Hz when the Fe:ZnSe crystal is cooled to 77 K by liquid nitrogen, corresponding to a slope efficiency of 22.9%.The central wavelength and FWHM linewidth are 3957.4 nm and 23.2 nm, respectively. The output energy monotonically decreases as the crystal temperature increases in the range 77–293 K.     

Watt-level Pr~(3+):YLF deep red laser pumped by a fiber-coupled blue LD module or a single-emitter blue LD

A power-scaled laser operation of Pr:YLi F4(YLF)crystal at 720.9 nm pumped by a 443.6 nm laser diode(LD)module was demonstrated.The 20 W module was used to pump the Pr:YLF crystal,and a maximum output power of 3.03 W with slope efficiency of 30.04%was obtained.In addition,a 5 W blue LD was also used to pump the Pr:YLF laser,and a maximum output power of 0.72 W was obtained at room temperature.The output power was limited by the wavelength mismatch between the single-emitter LD and the absorption peak of the crystal.     

Effect of S Substitution for P Point Defects in KDP Crystals： First-Principles Study

The electronic structure and geometric distribution of phosphor replaced by sulfur in potassium dihydrogen phosphate （KDP） are investigated by first-principles calculations. The point defect narrows down the energy gap to about 4.9eV, corresponding to a two-photon absorption of 355nm after correction. This can explain the decrease of the laser damage resistance in KDP crystals. Moreover, the defects twist the crystal structure and weaken bonds, especially the O-H bonds, so these bonds may be the first sites to crack under laser irradiation.     

A High-Power Continuous-Wave Laser-Diode End-Pumped Nd：YVO4 Laser of Single-Frequency Operation

We design and build a cw high quality and high power Nd:YV04 laser of single frequencv operation with a laser-diode dual-end-pumped geometry. The influence of the Nd^3 -doping concentration in the Nd:YVO4 crystal on the output performance of laser is theoretically and experimentally studied. With a Nd:YVO4 crystal of the Nd^3 -doping concentration 0.3 at. % and at pump power of 45 W, the output power of the single frequency laser is 18 W, and the slope efficiency is 48%.     

Femtosecond pulse laser-induced self-organized nanostructures on the surface of ZnO crystal

This paper reports self-organized nanostructures observed on the surface of ZnO crystal after irradiation by a focused beam of a femtosecond Ti：sapphire laser with a repetition rate of 250kHz. For a linearly polarized femtosecond laser, the periodic nanograting structure on the ablation crater surface was promoted. The period of self-organization structures is about 180 nm. The grating orientation is adjusted by the laser polarization direction. A long range Bragglike grating is formed by moving the sample at a speed of 10μm/s. For a circularly polarized laser beam, uniform spherical nanoparticles were formed as a result of Coulomb explosion during the interaction of near-infrared laser with ZnO crystal.     

Laser-induced damage and periodic stripe structures of a CaF_2 single crystal by an ArF excimer laser

The laser-induced damage threshold of a calcium fluoride(CaF2)single crystal was obtained by a 193 nm ArF excimer laser.The damage morphology of the crystal was analyzed.The results showed that the surface of CaF2 single crystal broke along the natural cleavage plane under ArF excimer laser irradiation,some fragments fell off,and Newton’s rings were observed on the curved fragments.Laser-induced periodic stripe structures(LIPSS)appeared on the surface layer beneath the fragments that peeled off.The spacing of LIPSS was measured,and the formation mechanism of LIPSS was analyzed based on the interference model.     

A high-power high-stability Q-switched green laser with intracavity frequency doubling using a diode-pumped composite ceramic Nd：YAG laser

We successfully obtain a high-average-power high-stability Q-switched green laser based on diode-side-pumped composite ceramic Nd:YAG in a straight plano-concave cavity. The temperature distribution in composite ceramic Nd:YAG crystal is numerically analyzed and compared with that of conventional Nd:YAG crystal. By using a composite ceramic Nd:YAG rod and a type-II high gray track resistance KTP (HGTR-KTP) crystal, a green laser with an average output power of 165 W is obtained at a repetition rate of 25 kHz, with a diode-to-green optical conversion of 14.68%, and a pulse width of 162 ns. To the best of our knowledge, both the output power and optical-to-optical efficiency are the highest values for green laser systems with intracavity frequency doubling of this novel composite ceramic Nd:YAG laser to date. The power fluctuation at around 160 W is lower than 0.3% in 2.5 hours.     

Optical Switch Formation in Antimony Super-Resolution Mask Layers Induced by Picosecond Laser Pulses

Sb is a classic material of a super-resolution near field structure （super-RENS） mask layer in which the optical switch formation is often realized by nanosecond laser pulse stimulation. We achieve fast and repeatable optical switching driven by picosecond laser pulses in a proper fluence range on Sb thin films. The optical properties of Sb thin films before and after switching are studied by surface-sensitive micro-area ellipsometry. The change of optical constants after switching is less than 2% in the whole visible range. The Sb mask layer is shown to be very promising for ultrafast super-resolution optical storage applications.     

Self-formation of void femtosecond array in Al2O3 crystal by laser irradiation

The femtosecond laser induced void array inside Al2O3 crystals was discussed. The void array was formed spontaneously under the irradiation of a single beam of infrared femtosecond laser which was focused at a fixed point inside the Al2O3 crystal sample. It was found that the regular voids only could be fabricated near the sample surface, which was different from the situation in CaF2 single crystal reported before. The possible mechanism of the phenomena was also discussed.     

Optical property measurements of 235 mm large-scale Ti:sapphire crystal

A Ti:sapphire crystal with a diameter of 235 mm and thickness of 72 mm was grown by the heat exchange method(HEM). The absorption intensity of the crystal at 532 nm averaged at 91%. The figures of merit(FOMs)at different positions of the crystal were measured and the FOM value in the central region was found to reach 90.The transmittance laser beam was intact with no obvious distortions and had only a small deformation compared with the incident laser beam. A small-signal amplification experiment was performed on the Ti:sapphire crystal and a gain of more than 6 times was achieved with a pump energy density of 1.98 J∕cm~2. These tests indicate that the 235 mm Ti:sapphire crystal has excellent optical qualities and will further improve the energy output of a 10 PW laser system.     

Self-formation of void array in Al_2O_3 crystal by femtosecond laser irradiation

The femtosecond laser induced void array inside Al_2O_3 crystals was discussed.The void array was formed spontaneously under the irradiation of a single beam of infrared femtosecond laser which was focused at a fixed point inside the Al_2O_3 crystal sample.It was found that the regular voids only could be fabricated near the sample surface,which was different from the situation in CaF_2 single crystal reported before. The possible mechanism of the phenomena was also discussed.     

Fenestration operation in middle ear bone with pulsed infrared lasers: an in-vivo study

The feasibility of fenestration operation in middle ear bone with pulsed infrared laser is evaluated. Healthy male New Zealand rabbits in vivo are used in the experiment. Middle ear mastoid bone of animal model is completely exposed with conventional methods, and then a pulsed CO2 laser （10.6 μm） and an Er：YAG laser （2.94 μm） are used to perform the fenestration operation. Diamond drill is also used as a control group. The total operation time and light irradiation time are recorded and the opening efficiency is assessed. The morphological changes and thermal damage around the opening window on the middle ear bone are examined. It is shown that both laser systems are suitable for the fenestration operation in middle ear bone, and this no-touch technique has a lot of benefits compared with traditional methods. The bleeding during operation has an important effect on operation time and thermal injury and needs to be controlled efficiently in further study.