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.     

Water-assisted femtosecond laser ablation for fabricating three-dimensional microfluidic chips

When the femtosecond laser focused in the water, the breakdown will be induced. The generated high-speed jet and shock wave can be used to etch silica glass for fabricating three-dimensional (3D) microfluidic chips. We present a simple and practical method to produce 3D multilayer microfluidic chips in silica glass. This method offers high design flexibility and fabricating feasibility. We also introduce a convenient cleaning method for diluting and ejecting the ablated debris from microchannel. Therefore, the femtosecond laser induced high-speed jet and shock wave can be used to fabricate complex microfluidic chips in silica glass. Experimental results show that the diameter of microchannel is uniform and the complexity of the microfluidic chip is under control. As a proof of principle, we demonstrate the feasibility of the fabricating process by using the water-assisted femtosecond laser ablation.     

Morphology studies of the metal surfaces with enhanced absorption fabricated using interferometric femtosecond ablation

Recently, the enhancing of bulk metals optical absorption with focused femtosecond pulses was demonstrated. This absorption enhancement is caused by different nano- and micro-structures which are formed during laser ablation with ultrashort pulses. In this paper we study the evolution of the surface structures using interferometric ablation and compare it to normal fs-ablation. Previously we have shown that interferometric femtosecond ablation is an efficient method to fabricate absorbing metal surfaces. In this study we ablated large areas of hole-array structures with different pulse numbers in polished stainless steel and copper samples. The evolution of surface morphology and the depth of the holes for these structured surfaces are presented. In addition, the reflectance of laser generated surface structures are measured at the wavelength range of 200–2300 nm using a standard spectrophotometer.     

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     

Femtosecond laser induced microripple on PDMS surface

We find the femtosecond laser induced microripple beside the focused femtosecond laser spot and along the movement direction of the laser spot on polydimethylsiloxane （PDMS） surface. The microripple may be due to the melting of PDMS induced by femtosecond laser pulses and the subsequent cool-down solidification of the melting PDMS along with the movement of the femtosecond laser spot. This result will be helpful to understand the interaction between the femtosecond laser and the polymer.     

Fabrication of Micro -Optical Devices by a Femtosecond Laser

Femtosecond laser is a perfect laser source for materials processing when high accuracy and small structure size are required. Due to the ultra short interaction time and the high peak power, the process is generally characterized by the absence of heat diffusion and, consequently molten layers. Various induced structures have been observed in materials after the femtosecond laser irradiation. Here, we report on fabrication of micro-optical devices by the femtosecond laser. 1) formation of optical waveguide with internal loss less than 0.5dB/cm in the wavelength region from 1.2 to 1.6 mm, by translating a silica glass perpendicular to the axis of the focused femtosecond laser beam; 2) nano-scale valence state manipulation of active ions inside transparent materials; 3) space-selective precipitation and control of metal nanoparticles inside transparent materials; The mechanisms and applications of the femtosecond laser induced phenomena were also discussed.     

Desorption of an organic conducting polymer by soft X-ray radiation created by a femtosecond laser

The possibility of the desorption of complicated molecular complexes by soft X rays resulting from a solid target irradiated by a single sharply focused femtosecond laser pulse with an energy of several millijoules has been experimentally demonstrated for polyaniline, which is an organic conducting polymer. X-ray desorption and photodesorption of polyaniline by femtosecond laser pulses have been compared using a time-of-flight mass spectrometer. The results provide the possibility of studying surfaces with spatial nanoresolution and high elemental (chemical) selectivity, as well as observing the photodesorption with a high temporal resolution.     

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.     

Femtosecond laser processing of indium-tin-oxide thin films

Micro- and nano-scale crystalline indium-tin-oxide (c-ITO) patterns fabricated from amorphous ITO (a-ITO) thin films on a glass substrate using a (low NA 0.26) femtosecond laser pulse that is not tightly focused are demonstrated. Different types of c-ITO patterns are obtained by controlling the laser pulse energies and pulse repetition rate of a femtosecond laser beam at a wavelength of 1064 nm: periodic micro c-ITO dots with diameters of ~1.4 μm, two parallel c-ITO patterns with/without periodic-like glass nanostructures at a laser scanning path and nano-scale c-ITO line patterns with a line width ~900 nm, i.e. ~1/8 of the focused beam׳s diameter (7 μm at 1/e²).     

Femtosecond laser-induced stimulation of a single neuron in a neuronal network

We demonstrated the stimulation of neurons at a single-cell level in cultured neuronal network by a focused femtosecond laser. When the femtosecond laser was focused on a neuron loaded with a fluorescent calcium indicator, the fluorescence intensity immediately increased at the laser spot, suggesting that intracellular Ca²⁺ increases in the neuronal cell due to the femtosecond laser irradiation. The probability of Ca²⁺ elevation at the laser spot depended on the average laser power, irradiation time, and position of the focal point along the optical axis, indicating that the femtosecond laser activates neurons because of multiphoton absorption. Moreover, after laser irradiation of a single neuron cultured on multielectrode arrays, the evoked electrical activity of the neurons was demonstrated by electrophysiological systems, which concluded that the focused femtosecond laser could achieve stimulating a single neuron in a neuronal network with high spatial and temporal resolution.     

Formation mechanism of element distribution in glass under femtosecond laser irradiation

We report on the formation mechanism of element distribution in glass under high-repetition-rate femtosecond laser irradiation. We simultaneously focused two beams of femtosecond laser pulses inside a glass and confirmed the formation of characteristically shaped element distributions. The results of the numerical simulation in which we considered concentration- and temperature-gradient-driven diffusions were in excellent qualitative agreement with the experimental results, indicating that the main driving force is the sharp temperature gradient. Since the composition of a glass affects its refractive index, absorption, and luminescence property, the results in this study provide a framework to fabricate a functional optical device such as optical circuits with a high-repetition-rate femtosecond laser.     

Gene delivery process in a single animal cell after femtosecond laser microinjection

Microinjection of extracellular molecules into a single animal cell was performed by an amplified femtosecond laser irradiation. When a single-shot laser pulse was focused on the plasma membrane of a single fibroblast from the mouse cell line NIH3T3 with a high-numerical aperture objective lens, a transient hole with a diameter of 1 μm was formed. The delivery process of extracellular molecules immediately after the hole formation was monitored by a fluorescence staining with fluoresceinisothiocyanate-dextran (FITC-dextran). Then the gene expression was confirmed using a DNA plasmid of an enhanced green fluorescent protein (EGFP). The gene expression was observed when the laser pulse was focused first on the cellular membrane and then on the nuclear membrane, while the gene was not expressed when the laser was focused only on the cellular membrane. On the basis of these results, the efficiency of gene delivery by the femtosecond laser microinjection and the subsequent gene expression were clarified.     

Nonlinear optical effects of near-IR femtosecond laser radiation on the morphology and structure of a nerve cell in the field of an optical trap

Changes in the morphology and structure of a neuron cell in the process of manipulating it with an femtosecond laser tweezers operating at a wavelength of 800 nm are examined. The changes in the morphology and structure are caused by the nonlinear optical absorption and multiphoton excitation of cell biomacromolecules by femtosecond pulses of light. The cell nucleus is demonstrated to be destroyed by focused femtosecond laser radiation (FLR). Changes in the state of the cytoplasmic nucleoproteins and the hydrophobicity of the plasmatic membrane under the action of FLR focused inside the cell are observed. By the example of the simultaneous displacement of the neuron with a cw laser and cutting of a neuron with FLR, the operation of holographic optical manipulator and a scalpel based on the use of femtosecond and cw lasers is considered. The possibility of the simultaneous microsurgical operation with several optical foci of FLR is demonstrated.     

单次飞秒激光脉冲在玻璃内部产生多次微爆的研究

在入射能量不同的情况下，用近红外飞秒激光脉冲在重钡火石玻璃（ZBaF15）内部产生了色心和微爆，在熔融石英的内部产生了多次微爆现象.实验表明：入射能量较高时，色心的中间会产生微爆；在松聚焦条件下，一个飞秒激光脉冲在透明介质内会激发多次微爆.基于飞秒激光脉冲在透明介质内的自聚焦效应和自由电子等离子体的自散焦效应，理论分析了多次微爆产生的原因；也讨论了飞秒激光脉冲诱导玻璃折射率改变的机理.     

Polarization control of multi-photon absorption under intermediate femtosecond laser field

It has been shown that the femtosecond laser polarization modulation is a very simple and well-established method to control the multi-photon absorption process by the light–matter interaction. Previous studies mainly focused on the multiphoton absorption control in the weak field. In this paper, we further explore the polarization control behavior of multiphoton absorption process in the intermediate femtosecond laser field. In the weak femtosecond laser field, the secondorder perturbation theory can well describe the non-resonant two-photon absorption process. However, the higher order nonlinear effect(e.g., four-photon absorption) can occur in the intermediate femtosecond laser field, and thus it is necessary to establish new theoretical model to describe the multi-photon absorption process, which includes the two-photon and four-photon transitions. Here, we construct a fourth-order perturbation theory to study the polarization control behavior of this multi-photon absorption under the intermediate femtosecond laser field excitation, and our theoretical results show that the two-photon and four-photon excitation pathways can induce a coherent interference, while the coherent interference is constructive or destructive that depends on the femtosecond laser center frequency. Moreover, the two-photon and fourphoton transitions have the different polarization control efficiency, and the four-photon absorption can obtain the higher polarization control efficiency. Thus, the polarization control efficiency of the whole excitation process can be increased or decreased by properly designing the femtosecond laser field intensity and laser center frequency. These studies can provide a clear physical picture for understanding and controlling the multi-photon absorption process in the intermediate femtosecond laser field, and also can provide a theoretical guidance for the future experimental realization.     

High resolution scanning microanalysis on material surfaces using UV femtosecond laser induced breakdown spectroscopy

Femtosecond lasers together with high resolution optics have given us the ability to achieve submicron ablation spots which can play an important role in specific micromachining applications. Light emitted from the plasma at the sample surface created by a focused femtosecond laser pulse can also be used in laser induced breakdown spectroscopy (LIBS) and allows us to characterize the chemical composition of the target surface with micron-level lateral resolution. The spatial resolution using LIBS has often been defined by measuring the FWHM of the crater size. In this report, we study the application of femtosecond 266 nm laser pulses with very low energies of 10׳s of nanojoules. We have investigated spatial resolution using the detection of thin strips of chromium on silicon substrates and compared the actual width of the chromium versus the experimentally obtained width using LIBS detection. The variation of signal levels for low pulse energies is investigated on chromium surfaces. A spatial resolution of ~1 μm was obtained for detection of chromium from the emission.     

Efficient generation of vibrational stimulated Raman emission in the ultraviolet region using a femtosecond pump beam

The third harmonic emission (261 nm, 30 μJ) of a femtosecond Ti:sapphire laser is focused into molecular hydrogen to generate vibrational stimulated Raman emission. High-order emission lines are efficiently generated by using a high-beam-quality nearly-transform-limited laser and high-pressure hydrogen (40 atm) as a Raman medium.     

Lidar measurement of constituents of microparticles in air by laser-induced breakdown spectroscopy using femtosecond terawatt laser pulses

We experimentally demonstrated remote sensing of the constituents of microparticles in air by combining laser-induced breakdown spectroscopy (LIBS) and lidar, using femtosecond terawatt laser pulses. Laser pulses of 70 fs duration and 130 mJ energy generated filaments when focused at a focal length of 20 m and the pulses irradiated artificial saltwater aerosols in air at a 10 Hz pulse repetition rate. Na fluorescence was observed remotely at a distance of 16 m using a 318 mm diameter Newtonian telescope, a spectrometer, and an intensified CCD camera. These results show the possibility of remote measurement of the constituents of atmospheric particles, such as aerosols, clouds, and toxic materials, by LIBS-lidar using femtosecond terawatt laser pulses.     

Cellular response to near-infrared femtosecond laser pulses in two-photon microscopes

The influence of femtosecond near-infrared (NIR) microirradiation on cell vitality and cellular reproduction has been studied. Chinese hamster ovary cells exposed to a highly focused 150-fs scanning beam at 730, 760, and 800 nm (80 MHz, 80-mus pixel dwell time) of /=6 -mW mean power, cells were unable to form clones. They died or became giant cells. Complete cell destruction, including cell fragmentation, occurred at mean powers >10 mW. Cell death was accompanied by intense luminescence in the mitochondrial region. When we consider the diffraction-limited spot size in the submicrometer region, intensities and photon flux densities of 0.8-kW pulses (10-mW mean power) are of the order of terawatts per square centimeter (10(12)W/cm (2)) and 10(32) photons cm(-2) s(-1) , respectively. Extremely high fields may induce destructive intracellular plasma formation. The power limitations should be considered during NIR femtosecond microscopy of vital cells and in the design of compact NIR femtosecond solid-state lasers for two-photon microscopes.