Ultrashort high repetition rate exposure of dielectric materials: laser bonding of glasses analyzed by micro-Raman spectroscopy

We report on the joining of different glass types with dissimilar optical, thermal and mechanical properties by ultrashort laser welding at high repetition rates. Femtosecond laser pulses were focused at the interface of two optically contacted transparent samples. Using nonlinear absorption processes and heat accumulation of successive pulses, we achieved strong bonds between the samples. We used a three-point bending test to determine the breaking strength. With this technique, we achieved for instance for a borosilicate glass a breaking strength of up to 95 % of the bulk material. In addition, we even welded different material combinations. Although the welded glasses exhibit different thermal and mechanical properties, we obtained breaking strengths which are comparable to the utilized bulk materials. Using Raman spectroscopy we mapped the laser-processed material along the welded interface. Thereby, we determined that the welds consist of a mixture of both species, which is formed during the laser induced melting of the materials.     

Femtosecond laser micromachining and biological therapy

Nonlinear micromachining with femtosecond laser pulses has attracted much attention in microphotonics and biophotonics. In this paper, we demonstrate the fabrication of optical elements in bulk transparent materials, direct welding between transparent substrates, and subcellular ablation in living cells by focusing femtosecond laser pulses.     

Control of femtosecond laser written waveguides in silica glass

Writing conditions for the fabrication of optical waveguides in bulk fused silica glass by use of 1 kHz focused femtosecond laser pulses at 800 nm were systematically determined for different focusing geometries. The results demonstrate that waveguides can be formed based on optical breakdown, filamentation (single or multiple), or a combination of both processes, when using pulse energies lower than the threshold of structural damage. The mechanisms of laser-induced index change are also discussed. PACS 42.65.Jx; 42.70.Ce; 42.79.Gn     

Threshold effect in femtosecond laser induced nanograting formation in glass: influence of the pulse duration

Recently, femtosecond laser direct writing in porous glass is emerging as a powerful technique for building arbitrary 3D hollow micro/nanostructures in bulk glass materials. In this study, we investigate the pulse duration dependence of laser intensity window for inducing a single nanocrack inside porous glass by femtosecond laser direct writing. We find that the window for a single nanocrack increases with the pulse duration, while the roughness of side walls in the nanocracks becomes higher for pulses longer than ~300 fs. When the femtosecond laser pulses of an optimized duration of ~200 fs are chosen, a sufficiently broad range of laser intensity (~44 % of the structuring threshold) for creating a single nanocrack can be obtained, while smooth sidewalls required by nanofluidic applications can still be maintained. The reported results will be beneficial not only for the development of the 3D femtosecond laser micro/nanostructuring techniques, but also for gaining a deeper understanding of the physical mechanism behind the nanograting formation induced by femtosecond laser irradiation in glass and other transparent materials.     

Nonlinear imaging of embedded microstructures inside transparent materials with laser-induced ionization microscopy

We present a novel nonlinear imaging method that utilizes femtosecond laser-induced plasma emission to probe microscopic structures embedded inside transparent materials. This nonlinear diagnostic tool can resolve either elemental or structural variations of the sample of interest and provide significant improvements over the ordinary linear microscopes by having much higher contrast ratios for the observed areas of different refractive indices. Examples of using this technique to examine the microstructures fabricated by ultrashort laser pulses inside optical glass are presented.     

Pulse width effect in ultrafast laser ionization imaging

The effects of different laser pulse widths on laser-induced ionization imaging of microstructures embedded in transparent materials are investigated. It is shown that a femtosecond laser-induced ionization probe can detect the variation of elemental composition of the sample materials with a higher contrast ratio, whereas the ionization probe generated by picosecond laser pulses is more sensitive to the structural change inside optical materials, which can be well explained by the different roles of multiphoton ionization and avalanche ionization involved in material breakdown. These results also suggest that an optimum diagnosis could be obtained if well-selected laser parameters are employed in ultrafast laser ionization imaging.     

Bulk heating of transparent materials using a high-repetition-rate femtosecond laser

Femtosecond laser pulses can locally induce structural and chemical changes in the bulk of transparent materials, opening the door to the three-dimensional fabrication of optical devices. We review the laser and focusing parameters that have been applied to induce these changes and discuss the different physical mechanisms that play a role in forming them. We then describe a new technique for inducing refractive-index changes in bulk material using a high-repetition-rate femtosecond oscillator. The changes are caused by a localized melting of the material, which results from an accumulation of thermal energy due to nonlinear absorption of the high-repetition-rate train of laser pulses. Received: 21 November 2001 / Accepted: 9 July 2002 / Published online: 25 October 2002 RID="*" ID="*"Corresponding author. Fax: +1-858/534-7697, E-mail: cschaffer@ucsd.edu RID="**" ID="**"Current address: University of California, San Diego, Department of Physics, La Jolla, CA 92 093, USA     

Ultrafast laser written active devices

Direct‐write optical waveguide device fabrication is probably the most widely studied application of femtosecond laser micromachining in transparent dielectrics at the present time. Devices such as buried waveguides, power splitters, couplers, gratings, optical amplifiers and laser oscillators have all been demonstrated. This paper reviews the application of the femtosecond laser direct‐write technique to the fabrication of active waveguide devices in bulk glass materials.     

Discrete damage traces from filamentation of Gauss-Bessel pulses

Filamentation of Bessel-Gauss pulses propagating in borosilicate glass is found to produce damage lines extending over hundreds of micrometers and consisting of discrete, equidistant damage spots. These discrete damage traces are explained by self-regeneration of Gauss-Bessel beams during propagation and are potentially applicable in laser microfabrication of transparent materials.     

Estimation of the Refractive Index Change in Glass Induced by Femtosecond Laser Pulses

It is known that local refractive index change occurs when femtosecond laser pulses with extremely high peak power are launched into glass. We focused 130-femtosecond laser pulses of 800 nm into the bulk of glass and examined the shape of the induced refractive index change. We found that the length of the spot of the refractive index change along the optical axis reached about 30 μm despite the diameter being about 2 μm. To estimate the distribution of induced refractive index change, we fabricated Bragg grating by scanning the focused spot and calculated the amount of the change by applying Kogelnik’s coupled mode theory to the measured diffraction efficiencies of the higher order diffracted beams.     

Color center formation in soda lime glass and NaCl single crystals with femtosecond laser pulses

The high instantaneous powers associated with 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 produced by femtosecond laser radiation in soda lime glass and single crystal sodium chloride on different time scales, from microseconds to hundreds of seconds. By using an appropriately chosen probe laser focused through the femtosecond laser spot, we can follow the changes in coloration due to individual or multiple femtosecond pulses, and follow the evolution of that coloration for a long time after femtosecond laser radiation is terminated. For the soda lime glass, the decay of color centers is well described in terms of bimolecular annihilation reactions between electron and hole centers. Similar processes appear to operate in single crystal sodium chloride. PACS 82.50.Pt; 78.55.Qr; 78.55.Fv; 78.47.+p     

Sub-picosecond ultraviolet laser filamentation-induced bulk modifications in fused silica

We present experiments with sub-picosecond ultraviolet laser pulses (248 nm, 450 fs) tightly focused in the bulk of fused-silica samples. The high laser intensities attained generate plasma through multi-photon absorption and electron avalanche processes in the bulk of the material. Depending on the initial experimental conditions three distinct types of structural changes in the material are observed, from small changes of the refractive index to birefringence, and even cracks and voids. We also observe the creation of micro-channels, up to 115 m in length, inside the material due to self-guiding and filamentation of the laser pulses in the transparent material. The selective change of the refractive index is a promising method for the fabrication of photonic structures such as waveguides and three-dimensional integrated optical devices. PACS  52.38.Hb; 42.65.-k; 42.70.-a     

飞秒激光脉冲在正色散固体材料中的自压缩

实验研究了正色散固体介质中的激光脉冲自压缩现象,证明了无需任何外加色散补偿情况下，固体透明介质中的自聚焦传输过程可使高功率飞秒激光脉冲实现时域脉冲压缩，并详细研究了输出脉冲的时域和频域特性随入射脉冲强度的演化规律.实验结果表明脉冲自压缩量随入射脉冲强度的增加呈递增趋势，然而当入射光强增大到足以引起超连续谱及锥形辐射产生时，脉冲时域形状会发生分裂.此外还发现发散光束入射情况下同样可以观察到脉冲自压缩现象. 关键词： 超短激光脉冲 脉冲压缩 非线性传输     

Cascaded highly nondegenerate four-wave-mixing phenomenon in transparent isotropic condensed media

Broad-bandwidth light pulses with different frequencies, extending from the IR to the UV, are simultaneously generated when two noncollinear, ultrafast laser pulses from a visible dual-frequency laser propagate through bulk isotropic transparent media such as common glass. This phenomenon, which is believed to have been previously unreported, can be explained by a cascade of highly nondegenerate four-wave-mixing processes and corresponds to a coherent scattering effect with geometrically minimized phase mismatch. Frequency-upconverted beams were observed up to the 11th order.     

Femtosecond laser direct writing of embedded optical waveguides in aluminosilicate glass

Using tightly focused femtosecond laser pulses to irradiate lines in aluminosilicate glass, embedded lines with increased refractive index, which function as optical waveguides were observed. The pulse energy (4.5–11.2 μJ) and writing speed (50–700 μm/s) were shown to affect the resultant optical properties of the waveguides such as the magnitude of refractive index change, core diameter and propagation mode. At pulse energies above 5 μJ, two types of structures were observed, namely an inhomogeneous void-like structure and a cross-sectional crack-like structure. These structures were found to affect significantly the resultant waveguiding properties of the irradiated lines. Using pulse energy of 5 μJ or below, single mode waveguides were fabricated. Raman spectroscopy showed that the fs laser pulses generated structural changes to the aluminosilicate glass. The fabrication of a 1×4 splitter was also demonstrated. PACS 42.62.-b; 42.82.-m; 81.05.Kf     

Excimer laser-assisted annealing of silicate glass with ion-synthesized silver nanoparticles

The effect of KrF excimer laser radiation on a composite layer consisting of sodium-potassium silicate glass with silver nanoparticles is studied as a function of the number of laser nanosecond pulses. The silver nanoparticles are synthesized by ion implantation. The measured optical absorption of the composite layer demonstrates that the silver nanoparticle size decreases monotonically as the number of laser pulses increases. Rutherford backscattering shows that laser annealing is accompanied by silver diffusion into the bulk of the glass and partial metal evaporation from the sample surface. The detected decrease in the silver nanoparticle size is discussed in terms of simultaneous melting of silver nanoparticles and the glass matrix due to the absorption of laser radiation.     

Femtosecond laser application for high capacity optical data storage

A femtosecond (fs) laser application for multi-layer optical recording is investigated. Information patterns at different layer depths were written inside a transparent glass substrate due to micro-void formation by fs laser ablation, which causes re-distribution in glass materials and a refractive index modification. The information bits recorded in a single layer can be retrieved clearly without interference from the neighboring layers. A fs laser irradiation of a transparent polymer matrix (doped with fluorescent materials for use as low-cost recording media) is also studied. A fs laser induced photo-chemical reaction changes the chemical properties of the fluorescent materials and records information bits inside the matrix. With an ultra-fast laser as a new light source, 3D optical recording can be available for high capacity data storage up to 1 TB per disc. PACS 82.50.-m; 42.65. Re; 72.70.Jk.     

Three-dimensional hole drilling of silica glass from the rear surface with femtosecond laser pulses

By moving silica glass in a preprogrammed structure, we directly produced three-dimensional holes with femtosecond laser pulses in single step. When distilled water was introduced into a hole drilled from the rear surface of the glass, the effects of blocking and redeposition of ablated material were greatly reduced and the aspect ratio of the depth of the hole was increased. Straight holes of 4-mu;m diameter were more than 200 microm deep. Three-dimensional channels can be micromachined inside transparent materials by use of this method, as we have demonstrated by drilling a square-wave-shaped hole inside silica glass.     

Spatially resolved measurement of femtosecond laser induced refractive index changes in transparent materials

We present a practical method to determine femtosecond laser induced refractive index changes in transparent materials. Based on an iterative Fourier transform algorithm, this technique spatially resolves the refractive index of complex structures by combining the dimensions of the modified region with the corresponding phase change extracted from far-field intensity measurements. This approach is used to characterize optical waveguides written by a femtosecond laser in borosilicate glass.     

飞秒脉冲在透明材料中的三维光存储及其机理

使用经钛宝石啁啾脉冲放大的脉冲宽度为200fs、波长为800nm、重复频率为1kHz的超短脉冲激光束,紧聚焦到熔融石英中实现了三维逐位式光数据存储,记录下20层三维数据位点,利用CCD和数码相机对数据位进行了观察讨论了飞秒超短脉冲与透明介质的相互作用,以及产生等离子体的雪崩电离和多光子吸收电离的机理实验结果表明:在飞秒超短脉冲与透明光学介质的相互作用中起主要作用的是多光子吸收.