Sm3+ photoreduction in BaCl2 nanophases precipitated fluoroaluminate glasses under femtosecond laser irradiation

Photoreduction of samarium-doped BaCl(2)-modified aluminofluoride glass by femtosecond laser irradiation and x-ray irradiation were investigated. Photoluminescence of samarium ions indicated that photoreduction of Sm(3+)→Sm(2+) efficiently occurred in glass samples containing more than 5 mol.% BaCl(2) after femtosecond laser irradiation, while dramatic change was not observed by x-ray irradiation. Transmission electron microscope results revealed that BaCl(2) nanophases only precipitated from glass matrix with a high BaCl(2) content by focusing femtosecond laser irradiation. Samarium ions were selectively incorporated into the precipitated nanophases, resulting in the enhancement of Sm(3+) photoreduction under lower laser power.     

共掺Na~ 的Yb~(3 )∶CaF_2晶体的荧光分析与低阈值激光运转

基于975nm激发的室温下荧光光谱测量,系统地研究了在新型钠、镱共掺的氟化钙晶体中钠离子的作用机理,分析了掺钠离子浓度与激光阈值的关系,获得了具有最低激光阈值的掺杂浓度优化配比.激光实验表明对于2%Yb3 离子浓度的氟化钙,掺入3%的钠离子能够获得最低阈值的激光运转,这与理论和荧光分析完全一致.     

Optical properties of Sm3+:CaO-Li2O-B2O3-BaO glass and codoped Sm3+:Eu3+:

Calibo glasses of Sm³⁺ with and without BaO have been synthesized and the optical properties (like absorption and photoluminescence) analyzed in terms of Judd–Ofelt theory. The fluorescence yield of Sm³⁺ enhances with BaO, and is optimum at 10% BaO. The effect of temperature on the energy transfer probability in (Sm³⁺:Eu³⁺) codoped glass has been reported and it can be inferred that the combination may serve as a temperature sensor. PACS 42.70.Ce     

Residual stress relaxation in the core of optical fiber by CO(2) laser irradiation

We observed residual stress relaxation by CO(2) laser irradiation in the cores of optical fibers by direct stress measurement. It was demonstrated that the mechanical stress was fully relaxed by CO(2) laser irradiation and that the remaining stress in the core was thermal stress that was due to a mismatch of the thermal expansion coefficients of the fiber core and cladding. The net core stresses after relaxation were 17, 68, and 203 MPa in Ge-B-codoped fibers drawn at 0.53, 1.38, and 3.48 N, respectively. Changes in the refractive indices of the cores as a result of residual stress relaxation were also estimated.     

Temperature dependence of spectral and laser properties of Er~(3+)/Al~(3+) co-doped aluminosilicate fiber

Using a heavily erbium-doped aluminosilicate fiber prepared by the sol-gel method combined with high temperature sintering, the temperature dependence of the spectrum around the 1.55 nm band and single-mode fiber laser properties were investigated, respectively. The absorption cross section increases 29.2% at ～1558 nm with the temperature increasing from 20°C to 140°C, while the emission cross section slightly increases4.3%. In addition, the laser slope of the heavily erbium-doped aluminosilicate fiber at 1558 nm decreases4.4% from 10.8% to 6.4% with the temperature increasing from 18°C to 440°C. Meanwhile, an experiment lasting3 h proves that the fiber laser has excellent stability below 440°C.     

Improvement in refractive-index change in LiNbO3：Ce：Cu by applying an external electric field

By jointly solving two-centre material equations with a nonzero external electric field and coupled-wave equations, we have numerically studied the dependence of the non-volatile holographic recording in LiNbO3：Ce：Cu crystals on the external electric field. The dominative photovoltaic effect of the non-volatile holographic recording in doubly doped LiNbO3 crystals is directly verified. And an external electric field that is applied in the positive direction along the c-axis （or a large one in the negative direction of the c-axis） in the recording phase and another one that is applied in the negative direction of the c-axis in the fixing phase are both proved to benefit strong photorefractive performances. Experimental verifications are given with a small electric field applied externally.     

Formation of TiO2 film with lower electrical resistance by aerosol beam and fiber laser irradiation

Titanium dioxide (TiO₂) is a functional ceramic with unique photoconductive and photocatalytic properties. In our previous study, a TiO₂ film was formed by aerosol beam irradiation. The films were darkened by femtosecond laser irradiation in air. Then electrical resistance of the darkened area on the film decreased. The heating process is also a useful method to vary the TiO₂ film property. Local heating can be performed by using a continuous wave (CW) fiber laser. In this study, the film was irradiated with a commercial CW fiber laser in vacuum. Laser irradiated area on the film was also darkened after CW fiber laser irradiation. The electrical resistance of the darkened area on the films was decreased as laser fluence was increased. Electrical resistance of the darkened area after CW fiber laser irradiation in vacuum was much smaller than that after femtosecond laser irradiation.     

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.     

Self-formation of void array in Al2O3 crystal by femtosecond 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.     

Amplitude-detuning characteristic of a CO2 laser in loss and refractive-index modulations

The nonlinear amplitude-detuning characteristic of a CO₂ laser in loss and refractive-index modulations with frequencies that differ by a factor of two is theoretically investigated. The response of the laser, in this case, is practically symmetric relative to the center of the line of the working transition while generation dynamics is complicated due to the appearance of spikes of quadrupling of the oscillation period on the wings of the line and doubling of the oscillation period in the remaining range of detunings. B. I. Stepanov Institute of Physics, National Academy of Sciences of Belarus, 68, F. Skorina Ave., Minsk, 220072. Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 66, No. 2, pp. 185–189, March–April, 1999.     

Optical phase change in bismuth through structural distortions induced by laser irradiation

ABSTRACT

Semimetal bismuth (Bi) is known to possess a wide range of peculiar properties, owing to its unique electronic band structure. Its electronic band can easily be distorted by structural changes, and thereby undergo transitions between semimetal to either semiconductor or metal states. Utilising a focused laser beam, one can easily introduce structural defects, along with phase changes, oxidation, and morphological modifications. Confocal Raman microscopy indicated that the as-fabricated Bi droplets inhibit the Raman signal from the underlying silicon (Si) substrate. After a laser flash heating step, the intensity of Si optical phonons was strongly enhanced at the positions of Bi droplets, and exceeding the intensity from the bare Si substrate. Thus, such laser irradiating step on the Bi droplets induces an optical phase change. The optical phase change was detected as going from inhibition to strong enhancement of the underlying Si substrate Raman signal. From the observed Bi optical phonon modes (E_g and A_1g), alterations in the Raman peaks due to laser exposure indicated that the ordered crystallinity in pristine Bi droplets became deteriorated. The effects of atomic displacements and loss of structural order in Bi droplets impacts its dielectric response. The observed Si Raman signal enhancement is similar to the surface-enhanced Raman scattering effect typically known for noble metals.     

Improvement of electrospun polymer fiber meshes pore size by femtosecond laser irradiation

Polymer meshes have recently attracted great attention due to their great variety of applications in fields such as tissue engineering and drug delivery. Poly(?-caprolactone) nanofibers were prepared by electrospinning giving rise to porous meshes. However, for some applications in tissue engineering where, for instance, cell migration into the inner regions of the mesh is aimed, the pore size obtained by conventional techniques is too narrow. To improve the pore size, laser irradiation with femtosecond pulses (i.e., negligible heat diffusion into the polymer material and confined excitation energy) is performed. A detailed study of the influence of the pulse energy, pulse length, and number of pulses on the topography of electrospun fiber meshes has been carried out, and the irradiated areas have been studied by scanning electron microscopy, contact angle measurements and spectroscopic techniques. The results show that using the optimal laser parameters, micropores are formed and the nature of the fibers is preserved.     

Fourth-harmonic generation of a continuous-wave CO(2) laser by use of an AgGaSe(2)/ZnGeP(2) doubly resonant device

We report a doubly resonant continuous-wave CO(2) laser frequency-quadrupling device that generates 200nW of 2.55-mum (4?) and as much as 2mW of 5.1-mum (2?) radiation out of 1.7-W fundamental radiation at 10.2 mum (?). The quadrupling process results from two resonant cascading second-harmonic generations by use of a walk-off-compensated twin AgGaSe(2) device (??2?) and a ZnGeP(2)nonlinear crystal (2??4?).     

In situ spectroscopic measurement of structural change in SiO2 during femtosecond laser irradiation

Spectra of light transmitted through SiO₂ specimens were measured during irradiation of amplified 800 nm ultrashort (400 fs) laser with a repetition rate of 100 kHz. The spectra exhibit a broad dispersion including a peak at around 450 nm, and the peak wavelength and the intensity are found to significantly depend on the irradiation time. Positively charged oxygen vacancies (E center) and oxygen-deficiency centers (ODCs) with neutral charge were observed by electron spin resonance (ESR) and photoluminescence (PL), respectively, after irradiation. The formation of the E center and the subsequent transformation to ODCs significantly depend on the laser power density. The correlation between the intensity of the peak at around 450 nm and the ESR signal intensity of the E center, and also the observation of PL spectrum due to ODCs suggest that self-trapped excitons exhibiting the peak at around 450 nm are created through multi-photon absorption, followed by the formation of the E center, and this finally leads to the formation of ODCs. PACS 78.47.+p; 61.80.Ba; 76.30.Mi     

Fabrication of long-period fiber gratings by focused irradiation of infrared femtosecond laser pulses

We have fabricated long-period fiber gratings by use of a novel technique using focused irradiation of infrared femtosecond laser pulses. We investigate the thermal stability of the fabricated fiber gratings. The values of the loss peak wavelength and the transmittance of the fiber gratings after heat treatment below 500 degrees C are the same as initial values before heat treatment. The fiber gratings that were fabricated by this technique have a high resistance to thermal decay. We propose that this technique will be useful for fabrication of fiber gratings with a superior aging characteristic.     

CO2激光写入长周期光纤光栅的折变理论及实验研究

理论分析了利用高频CO₂激光脉冲在不同类型光纤中写入长周期光纤光栅（LPFG）时光纤包层和纤芯发生折射率改变的机理和计算方法.结果表明,残余应力释放、快速固化、光纤致密化和熔融变形是高频CO₂激光能成栅的主要原因,但采用不同的写入方法、不同的激光写入能量在不同的光纤类型上制作光栅时各种机理的重要性不同.利用长周期光纤光栅对构成的M-Z干涉仪实验测量了高频CO₂激光以不同辐射能量在光纤不同作用位置所引起的光纤纤芯或包层的折射率平均变化量,并通过实际制作光栅的过程来初步验证光纤折射率的变化情况.CO₂激光与光纤相互作用机理的分析和实验研究为利用这种方法制作折变型全光纤器件提供了实验制作的基础和完善工艺的方法. 关键词： 光纤器件 长周期光纤光栅 2激光脉冲')" href="#">CO₂激光脉冲 折变     

Phase change in CoTi2 induced by MeV electron irradiation

The phase change induced by MeV electron irradiation in the intermetallic compound E9₃–CoTi₂ was investigated using high-voltage electron microscopy. Under MeV electron irradiation, CoTi₂ was first transformed into an amorphous phase and, with continued irradiation, crystallite formation in the amorphous phase (i.e. formation of crystallites of a solid-solution phase within the amorphous phase) was induced. The critical temperature for amorphisation was around 250 K. The total dose (dpa) required for crystallite formation (i.e. that required for partial crystallisation) was high (i.e. 27–80 dpa) and, even after prolonged irradiation, the amorphous phase was retained in the irradiated sample. Such partial crystallisation behaviour of amorphous Co₃₃Ti₆₇ was clearly different from the crystallisation behaviour (i.e. amorphous-to-solid solution, polymorphous transformation) of amorphous Cr₆₇Ti₃₃ reported in the literature. A possible cause of the difference is discussed.     

Structure and properties of CaNb2O6:Sm3+ thin films by pulsed laser deposition

CaNb₂O₆:Sm³⁺ films were prepared on quartz glass and α-Al₂O₃(001) substrates by pulsed laser deposition. The structural, morphological, and optical properties of the CaNb₂O₆:Sm³⁺ films were characterized by X-ray diffraction (XRD), atomic force microscopy (AFM), emission-scan electron microscopy (SEM), Raman spectroscopy, X-ray photoelectron spectroscopy (XPS) and photoluminescence spectroscopy (PL) measurements. The results show that the structure and properties of CaNb₂O₆:Sm³⁺ films were dependent on substrates. The CaNb₂O₆:Sm³⁺ films on Al₂O₃(0001) substrate have better crystallinity. The full-width at half-maximum (FWHM) of (131) peak are 0.45 and 0.32 for the CaNb₂O₆:Sm³⁺ film on glass and Al₂O₃(001), respectively. The crystallite size of CaNb₂O₆:Sm³⁺ films grown on glass and Al₂O₃(001) was about 8.22 and 9.98 nm, respectively. The oxidation state of the Sm element on the films was Sm³⁺ state. The photoluminescence (PL) spectra were measured at room temperature, the CaNb₂O₆:Sm³⁺ films on Al₂O₃(001) substrate have a better PL intensity, the identified emission bands were by the intra 4f transitions of Sm³⁺ from the excited level to the lower levels at 567 nm for ⁴G_5/2→⁶H_5/2 transition, at 609 nm for ⁴G_5/2→⁶H_7/2 transition, and at 657 nm for ⁴G_5/2→⁶H_9/2 transition.     

Amplification of spontaneous emission in Nd~(3 )-doped fiber pumped by Ti: Al_2O_3 tunable laser

Amplification of spontaneous emission(ASE) in 4m Nd~(3 )-doped fiber pumped by Ti : Al~2O_3 tunable laser is studied theoretically and experimentally. The output of 2.64mW from ASE source pumped at 833 nm is obtained and the experimental results of ASE source are compared with that of Nd~(3 )-doped fiber ring laser.     

Thermal characteristics of Nd~(3 )-doped fiber ring laser pumped by Ti:sapphire tunable laser

A Nd(3 )-doped fiber ring laser pumped by a CW Ti:sapphire tunable laser is reported. The characteristics of the system are studied for the first time at 77K and 300K. The results show a lower threshold and a higher slope efficiency at 77K than that at 300K. Applications of the Nd~(3 )-doped fiber ring laser in a multiplexing fiber sensor system and a fiber-ring-laser type temperature sensor are discussed.