Color center formation in KCl and KBr single crystals with femtosecond laser pulses

Because of their extremely high instantaneous powers, femtosecond lasers can color many nominally transparent materials. Although the excitations responsible for this defect formation occur on subpicosecond time scales, subsequent interactions between the resulting electronic and lattice defects complicate the evolution of color center formation and decay. These interactions must be understood in order to account for the long-term behavior of coloration. In this work, we probe the evolution of color centers generated by femtosecond laser radiation in potassium chloride and potassium bromide single crystals on time scales from microseconds to hundreds of seconds. By using an appropriately chosen probe laser focused through the femtosecond laser spot, we follow the changes in coloration due to individual or multiple femtosecond pulses and the evolution of that coloration for long times after femtosecond laser radiation is terminated.     

Activation of color centers in bismuth glass by femtosecond laser radiation

The activation of color centers in the bismuth aluminum-boron-phosphate glass by high-intensity femtosecond laser radiation is experimentally studied. The dynamics of the laser-induced emitting centers in the bulk of sample is characterized. The photoactivation of bismuth glasses is possibly related to the recharging of structural precursors, which serve as effective electron traps and whose possible configuration is discusses. The effect of the femtosecond irradiation involves the initiation of the nonlinear ionization of the glass matrix and the generation of plasma with the concentration of carriers that is sufficiently high to provide almost complete recharging of precursors over several laser pulses.     

Effect of cerium oxide on the precipitation of silver nanoparticles in femtosecond laser irradiated silicate glass

We investigated the effect of cerium oxide on the precipitation of Ag nanoparticles in silicate glass via a femtosecond laser irradiation and successive annealing. Absorption spectra show that Ce³⁺ ions may absorb part of the laser energy via multiphoton absorption and release free electrons, resulting in an increase of the concentration of Ag atoms and a decrease of the concentration of hole-trapped color centers, which influence precipitation of the Ag nanoparticles. In addition, we found that the formed Ag⁰ may reduce Ce⁴⁺ ions to Ce³⁺ ions during the annealing process, which inhibits the growth of the Ag nanoparticles. PACS 78.55.Qr; 78.67.Bf; 81.05.Pj     

Luminescence, intrinsic photoeffect, and color-center conversion in anisotropic crystals under femtosecond laser excitation

The spatial-temporal dynamics of the photochemical conversion of color centers with varying multipolarity and orientation of quantum transitions in irradiated anisotropic crystals exposed to pulsed polarized femtosecond optical radiation and the resulting spatial distribution of the residual and newly-formed color centers are studied. The properties of the concurrent luminscence and photoconductive processes closely related to the photochemical transformation of light-absorbing quantum systems are also examined. A femtosecond laser technique is proposed, which allows extended spatial-periodic structures or narrow profiles of color-center concentrations, including single thin layers or dips, to be formed at a specified depth from the crystal surface through photochemical transformations. Institute of Applied Physics at the Irkutsk University. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 3, pp. 31–42, March, 2000.     

飞秒激光诱导玻璃内析出BaTiO3晶体

使用800nm、120fs、200kHz的高重复频率超短脉冲激光照射BaO-TiO2-SiO2系玻璃，玻璃经飞秒激光照射数秒钟后，激光会聚点的发光由800nm波长的红色突然转变为非常强的400nm的蓝色，对激光辐照点的发光光谱和X射线衍射分析表明，具有倍频效应的BaTiO3晶体已经产生。     

Femtosecond-laser-induced-crystallization and simultaneous formation of light trapping microstructures in thin a-Si:H films

We report the observation of crystallization and simultaneous formation of surface microstructures in hydrogenated amorphous silicon (a-Si:H) thin films as one step laser processing. Light trapping microstructures of around 300 nm in height were formed on a-Si:H films of thickness in the range of 1.5 μm to 2 μm deposited on soda lime glass after exposure to femtosecond laser pulses. Scanning electron microscope (SEM) images show the formation of spikes that are around 1 μm part and their heights could be controlled by the laser fluences. Atomic force microscope (AFM) images were taken to study the roughness created on the surface. The mean roughness of the textured surface increases with laser fluence at smaller power densities, and for power densities beyond 0.5 J/cm² the film removal deteriorates the texturing. X-ray diffraction results indicate the formation of a nano-crystalline structure with (111) and (311) crystal orientation after the laser treatment. The observed black color and enhanced optical absorption in the near infrared region in laser treated films may be due to a combined effect of light trapping in the micro-structured silicon surface because of multiple total internal reflections, phase change in the film, possible defect sites induced after laser treatment and formation of SiO_x. Demonstration of light trapping microstructures in thin a-Si:H films and simultaneous crystallization could provide new opportunities for optoelectronic devices. PACS 42.55.Px; 42.62.Cf; 81.05.Ge     

Ultra-Fast Decay Process of Electrons in LiNbO3 Crystal Induced by Femtosecond Pulse

Temporal evolution of absorption induced by single femtosecond pulse （13Ors, 800nm） with high intensity in LiNbO3 is obtained using the probe shadow imaging technique in order to investigate light-induced electron relaxation processes. By saturating the polaron density with a high intensity laser pulse, ultra-fast decay process on picosecond time scale is observed. The decay time constant is about 141 ps and it is attributed to the direct interband electron-hole recombination process.     

On a Spatially Selective Phototransformation Method for Investigation of Diffusion of Quantum Systems

A new method of determining the diffusion characteristics of quantum systems in crystals is investigated in the present paper on the example of color centers. The method is based on a spatially selective photochemical crystal transformation upon exposure to optical radiation pulses. The principal feasibility of implementation of this method for the determination of the diffusion coefficient of color centers based on the observed dynamics of spatially modulated distribution of their concentrations induced by optical radiation with femtosecond coherence is demonstrated.     

玻璃空间电离辐照着色损伤考核方法

针对空间电离辐照的复杂性,在玻璃空间电离辐照着色损伤动力学研究基础上,讨论了玻璃空间电离辐照着色损伤考核面临的问题,提出了一种动力学的考核方法。基于该方法,以K9-HL玻璃为研究对象,研究了特定轨道(近地点350km,远地点425km,轨道倾角51.6°)下玻璃空间电离辐照着色损伤过程,分析了特定轨道下玻璃空间电离辐照着色损伤特征,结果显示:K9-HL玻璃特征轨道在轨20年后蓝光区域透过率降低严重导致其必然呈现淡黄色,严重影响了其作为高性能无色光学材料的使用。此外,对玻璃空间电离辐照着色损伤~(60)Co-γ射线辐照考核方法的适用性进行了分析讨论,结果表明,采用~(60)Co-γ射线作为辐照源分析玻璃空间电离辐照着色损伤可能引入较大误差,造成该现象的原因是~(60)Co-γ射线在玻璃中的作用深度极大及考核方法未考虑色心弛豫消失。     

A high-stability medium based on CaF<Subscript>2</Subscript>:Na crystals with colloidal color centers: I. Photothermochemical conversion of colloidal color centers in CaF<Subscript>2</Subscript>:Na crystals

Photothermochemical conversion of simple color centers (which include from one to four anionic vacancies) and highly aggregated ones in additively colored crystals of calcium fluoride doped by sodium is studied. The annealing of crystals with a low sodium content in a reducing atmosphere (additive coloration) leads to the predominant formation of simple color centers, which convert into highly aggregated centers under the joint action of heating and irradiation in absorption bands of simple centers. The irradiation of highly aggregated centers into their absorption bands and simultaneous heating causes these centers to convert into simple centers. The additive coloration of crystals with a relatively high sodium content leads to the predominant formation of highly aggregated centers. The heating of these crystals along with the irradiation in absorption bands of highly aggregated centers causes these centers to convert into simple centers. The formation of different color centers in the course of additive coloration of crystals with different impurity content and different results of photothermochemical conversion of centers in these crystals are connected with the dual action of impurities. Anion vacancies, which compensate the charge of the impurity alkali metal, facilitate the aggregation of color centers. At the same time, the alkali impurity stabilizes simple color centers.     

强辐照场下氟化铈（CeF3）的荧光特性

研究了国产ＣｅＦ３晶体在４．３ｋＧｙ的辐照剂量下的荧光特性及其在室温下辐照损伤的恢复性能，测量显示辐照对荧光光谱和衰减时间不产生影响，但色心的形成使晶体透光性能变差。对辐照损伤的机理进行了探讨。     

Femtosecond pump-probe investigation of ultrafast silver nanoparticle deformation in a glass matrix

Single-color femtosecond pump-probe experiments were performed to investigate the time dependence of laser-induced ultrafast desorption and deformation processes of silver nanoparticles in glass. After laser excitation at wavelengths close to the surface plasmon resonance, transient extinction changes were found to exhibit dynamics on quite different time scales ranging from sub-picoseconds to some hundred ps. The slowest observed decay component is identified as characteristic for the deformation/desorption processes. Possible mechanisms for these processes are discussed. Received: 3 April 2000 / Revised version: 3 July 2000 / Published online: 20 September 2000     

Oscillation of the refractive index at the focal region of a femtosecond laser pulse inside a glass

The temporal evolution of refractive-index change produced by a tightly focused femtosecond (fs) laser pulse inside a soda-lime glass plate was investigated by use of a transient lens method with subpicosecond time resolution. An oscillating behavior of the light intensity in the central region of the probe beam was observed 0-1500 ps after irradiation of the plate. The oscillation was interpreted in terms of a rapid temperature increase and the ensuing propagation of the pressure wave. This study is to our knowledge the first real-time observation of refractive-index change inside a glass induced by a fs laser pulse.     

Tuning the frequency of ultrashort laser pulses by a cross-phase-modulation-induced shift in a photonic crystal fiber

Femtosecond pulses of fundamental Cr:forsterite laser radiation are used as a pump field to tune the frequency of copropagating second-harmonic pulses of the same laser through cross-phase modulation in a photonic crystal fiber. Sub-100-kW femtosecond pump pulses coupled into a photonic crystal fiber with an appropriate dispersion profile can shift the central frequency of the probe field by more than 100 nm, suggesting a convenient way to control propagation and spectral transformations of ultrashort laser pulses.     

Are neutral sodium atoms produced on glass surfaces by electron bombardment?

Recent papers have speculated that neutral sodium atoms are produced in significant quantities in glass by electron bombardment. XPS measurements on an electron-bombarded soda lime silicate glass revealed no evidence for neutral atoms at the surface or at depths up to the maximum penetration of the electron beam.     

Characterization of high-order harmonic radiation on femtosecond and attosecond time scales

We characterize the temporal structure of high-order harmonic radiation on both the femtosecond and attosecond time scales. The harmonic emission is characterized by mixed-color two-photon ionization with an infrared femtosecond laser using a Mach–Zehnder interferometer where both pump and probe arms travel completely separate paths. In a first experiment, we measure the duration and chirp of individual harmonics. In a second experiment, we resolve, for the first time with this type of setup, the attosecond beating of several harmonics generated under conditions similar to the first experiment. We suggest that the results of both measurements can be combined to determine the full attosecond time structure of the harmonic emission. PACS 32.80.Rm; 42.65.Ky     

Surface damage and color centers generated by femtosecond pulses in borosilicate glass and silica

Color center generation by femtosecond laser pulses (30 fs) is observed in a fluence range below the damage threshold in an alkali-free barium borosilicate (BBS) glass, and in a thin layer of SiO₂ on a fused silica substrate. The color centers are characterized spectroscopically. The optical density of the color centers in BBS glass is by two orders of magnitude higher than that in silica. A healing process with a time constant of about 30 h can be found. PACS 42.62.-b; 61.80.Ba; 71.55.Jv     

On the nature of “coherent artifact”

The coherent interaction of femtosecond laser pulses in the pump-probe regime has been experimentally studied in the time domain by monitoring light reflection from a tellurium single crystal. The optical response of the probed medium exhibits periodic variations at a frequency equal to that of the exciting laser radiation. Experimental dependences of the observed “coherent artifact” on the pump/probe intensity ratio, the number of accumulated pulses, and the mutual orientation of the polarization vectors of electromagnetic fields and the crystallographic axes are well described by the proposed phenomenological model.     

Storing energy in lithium fluoride crystals irradiated with femtosecond laser pulses

The release of energy in the form of the light sum of thermally stimulated luminescence (TSL), stored under conditions of self-focusing and the multiple filamentation of femtosecond laser radiation during the interaction between model wide-bandgap dielectric crystals of lithium fluoride is studied. It is shown that F₂ color centers are important centers of emission in the TSL process.     

Optical properties of structurally modified glasses doped with gold ions

We report on the optical properties of a structurally modified silicate glass doped with Au ions. The area in the vicinity of the focal point of an 800-nm femtosecond laser in a glass sample became gray as a result of the formation of color centers after laser irradiation and turned red because of precipitation of Au nanoparticles after further annealing at 550 degrees C for 30 min. When the glass was excited by UV light at 365 nm, yellowish-white and orange-yellow emissions were observed in the laser-irradiated and the Au-nanoparticle-precipitated area, respectively. An optical Kerr shutter experiment showed that the Au nanoparticle-precipitated glass had an ultrafast nonlinear optical response, and the third-order nonlinear susceptibility was estimated to be approximately 10(-11) esu.