飞秒激光诱导钛表面形成高空间频率周期条纹结构

利用波长为800 nm的飞秒激光,在空气和去离子水中诱导钛表面形成不同的周期条纹结构。在空气中,激光能量密度为0.265 J/cm2时,钛表面主要形成周期为500~560 nm低空间频率条纹结构;激光能量密度为0.102 J/cm2时,主要形成的是周期为220~340 nm高空间频率条纹结构。两种条纹均垂直于入射激光偏振方向,且条纹周期随着脉冲重叠数的增大而增大。在水中,除形成垂直激光偏振方向、周期为215~250 nm的高空间频率条纹结构,还形成了平行于激光偏振方向且周期约为入射激光波长八分之一的高空间频率条纹结构。利用表面等离子体理论、二次谐波及Sipe理论对各种周期条纹结构的形成机理进行分析,发现周期条纹结构的形成与钛表面氧化层有密切的关系。     

飞秒激光诱导钛表面形成高空间频率周期条纹结构

利用波长为800 nm的飞秒激光,在空气和去离子水中诱导钛表面形成不同的周期条纹结构。在空气中,激光能量密度为0.265 J/cm2时,钛表面主要形成周期为500~560 nm低空间频率条纹结构;激光能量密度为0.102 J/cm2时,主要形成的是周期为220~340 nm高空间频率条纹结构。两种条纹均垂直于入射激光偏振方向,且条纹周期随着脉冲重叠数的增大而增大。在水中,除形成垂直激光偏振方向、周期为215~250 nm的高空间频率条纹结构,还形成了平行于激光偏振方向且周期约为入射激光波长八分之一的高空间频率条纹结构。利用表面等离子体理论、二次谐波及Sipe理论对各种周期条纹结构的形成机理进行分析,发现周期条纹结构的形成与钛表面氧化层有密切的关系。     

High-order harmonics in static gas target by 795-nm Ti:sapphire femtosecond laser pulses

The 5th - 23sd high-order harmonics generation in rare gases in static gas target with 120-fs, 85-m J/pulse,10-Hz laser system was investigated. Compared with the traditional gas target, static gas target is simple to be used in experiment, and the experimental parameters can be easily controlled. The effects on highorder harmonics due to laser intensities (energy), polarization, gas densities, confocal parameter, and phase mismatch were studied in this paper.     

Dynamics of the formation of laser-induced periodic surface structures on dielectrics and semiconductors upon femtosecond laser pulse irradiation sequences

The formation of laser-induced periodic surface structures (LIPSS) upon irradiation of fused silica and silicon with multiple (N _DPS) irradiation sequences consisting of linearly polarized femtosecond laser pulse pairs (pulse duration ～150 fs, central wavelength ～800 nm) is studied experimentally. Nearly equal-energy double-pulse sequences are generated allowing the temporal pulse delay Δt between the cross-polarized individual fs-laser pulses to be varied from ?40 ps to +40 ps with a resolution of ～0.2 ps. The surface morphologies of the irradiated surface areas are characterized by means of scanning electron and scanning force microscopy. Particularly for dielectrics in the sub-ps delay range striking differences in the orientation and spatial characteristics of the LIPSS can be observed. For fused silica, a significant decrease of the LIPSS spatial periods from ～790 nm towards ～550 nm is demonstrated for delay changes of less than ～2 ps. In contrast, for silicon under similar irradiation conditions, the LIPSS periods remain constant (～760 nm) for delays up to 40 ps. The results prove the impact of laser-induced electrons in the conduction band of the solid and associated transient changes of the optical properties on fs-LIPSS formation.     

Fiber delivery of femtosecond pulses from a Ti:sapphire laser

We propose a way to deliver nanojoule-energy, 100-fs pulses at 800 nm through a few meters of standard optical fiber. Pulses from a mode-locked laser are compressed temporally, and then spectrally, to produce the desired pulses at the end of the fiber. Initial experiments agree well with calculations and demonstrate the benefits of this technique: For an energy of ~0.5 nJ , the delivered pulses are ~5 times shorter than those delivered by other techniques. The issues that must be addressed to scale the technique up to delivered pulse energies of 5 nJ are identified, and the apparatus employs only readily available components. Thus we expect it to find use in the many applications that would benefit from fiber delivery of femtosecond pulses.     

Generation of synchronized femtosecond and picosecond pulses in a dual-wavelength femtosecond Ti:sapphire laser

We obtain synchronized 45-fs and 0.848-ps pulses by achieving cross-mode locking in a double-cavity dual-wavelength femtosecond Ti:sapphire laser. Autocorrelation and cross correlation show that the femtosecond and picosecond pulses are well synchronized, with a timing jitter of 41 fs. Cross-phase modulation dominates the processes of cross-mode locking and synchronization.     

Experimental research of laser-induced periodic surface structures in a typical liquid by a femtosecond laser

A constant elastic alloy is a widely used material with a high elastic modulus and an excellent wave velocity consistency. Different morphologies on the constant elastic alloy surface are observed through femtosecond laser irradiation. When the laser average fluence is set to 0.58 J∕cm2and 200 laser pulses, with the increasing depth of distilled water, the period of the laser-induced periodic surface structures(LIPSS) becomes shorter accordingly.The higher the ethanol concentration is, the more spot-shaped structures will be formed among the surface structures when the depth of the coverage of ethanol is 2 mm. The period of the LIPSS reaches its maximum when the concentration of ethanol is 80%.     

Generation of synchronized femtosecond and picosecond laser pulses in a two-beam-pumped Ti:sapphire laser

<正>Two operating modes,independent self-mode-locking and cross-mode-locking,are presented in a two-beampumped double-cavity dual-wavelength femtosecond Ti:sapphire laser.Synchronization of femtosecond and picosecond laser pulses is achieved by properly adjusting the cavity length matching and distributing the pump laser powers in the two laser cavities,and moreover,a timing jitter of 517 fs between femtosecond and picosecond pulses is obtained,with wavelength tuning ranges around 36 and 22 nm in the femtosecond and picosecond cavities,respectively.     

Regenerative amplification of femtosecond laser pulses in Ti:sapphire at multikilohertz repetition rates

We have generated and applied noncoherent x-ray radiation in an all-solid-state laser system operating at repetition rates up to 20 kHz. Based on a model that takes into account the strong thermal loading of the Ti:sapphire rod, a laser cavity with low sensitivity to thermal lensing was chosen. With a maximum pump power of 80 W, an output power as high as 27 W was obtained in gain-switched operation, and, with a seeding from a femtosecond oscillator, 60-fs, 0.8-mJ (8-W) pulses at 10 kHz and 0.32-mJ (6.5-W) pulses at 20 kHz were generated. High power femtosecond output was used to generate x-ray continuum radiation up to 5 keV from a liquid-gallium jet target.     

Formation of periodic surface structures by ultrashort laser pulses

The formation of periodic surface structures by ultrashort laser pulses was observed experimentally and explained theoretically. The experiments were performed on graphite with picosecond laser pulses. The spatial period of the structures is of the order of the wavelength of the incident radiation, and the orientation of the structures is correlated with the direction of polarization of the light. The key point of the theoretical model proposed is resonance excitation of surface electromagnetic waves, which under conditions such that the temperature of the electronic subsystem is decoupled from the temperature of the crystal lattice causes a “temperature grating” to be written on the flat solid surface of the sample while the laser pulse is being applied on account of the temperature dependence of the surface impedance. The formation of a periodic surface profile from the temperature grating occurs by the volume expansion of a melted layer near the surface of the material. For typical values of the surface tension and viscosity for metals, there is not enough time for the periodic profile to be resorbed before the liquid layer solidifies. The formation of periodic surface structures is delayed in time relative to the laser pulse. Zh. éksp. Teor. Fiz. 115, 675–688 (February 1999)     

飞秒钛宝石放大激光脉冲的载波包络相位测量与控制

通过对重复频率为1 kHz的放大飞秒激光脉冲的光谱干涉实验,结合傅里叶变换进行了载波包络相位漂移的实验研究.在此基础上利用锁相环反馈控制技术实现了对载波包络相位的精密锁定,锁定后的激光脉冲稳态相位均方根误差小于80mrad,锁定时间超过3h.同时在理论上分析了光谱干涉测量放大激光脉冲载波包络相位的原理,给出了光谱干涉信号与载波包络相位的关系. 关键词： 飞秒钛宝石放大器 载波包络相位 光谱干涉 超连续     

Multipulse irradiation of silicon by femtosecond laser pulses: Variation of surface morphology

The multipulse interaction of ultraviolet femtosecond laser pulses with silicon and generation of surface structures in a large area spot (?1 mm²) has been studied. The evolution of multiscale structures at the constant fluence strongly depends on the number of pulses, N. For N < 200, the “carpet-like” pattern of nano-, and micro-spikes is generated by the bubble explosion in a thin surface foam layer. The accumulation of bubbles and their explosion due to repetition of laser pulses cause damped membrane-like oscillations of the silicon surface. For 200 ≤ N, bifurcation of surface morphology takes place: (i) the surface tension waves of the wavelength ∼200 μm appear in the peripheral region of the spot. Generated by the surface thermal gradient in the liquid foam layer, they spread from the hot centerline towards the periphery of the spot. The change of their wavelength with propagation distance indicates onset of the Eckhaus instability caused by the phase modulation in multipulse interaction. (ii) Deep caverns appear in a highly superheated silicon layer in the central region of the spot due to the fast gas-liquid phase separation and the fragmentation process.     

In-bulk and surface structuring of sapphire by femtosecond pulses

The actual space-time dependent intensity distribution of a tightly focused (numerical aperture NA = 1.35) Gaussian femtosecond pulse is modeled inside dielectric material. Such focusing is typically used for recording with sub-wavelength resolution inside dielectrics. The multi-pulse structuring inside the bulk and on the surface of sapphire are demonstrated. Formation of nano-cracks and nano-crystals is revealed inside the crystalline sapphire. Ripple formation on the surface is discussed in terms of the efficacy map calculated by theory given in ref. [J.E. Sipe, J.F. Young, J.S. Preston, H.M. van Driel, Laser-induced periodic surface structure. I. Theory, Phys. Rev. B 27 (2) (1983) 1141-1154.].     

Amplification of femtosecond laser filaments in Ti:sapphire

Femtosecond laser filamentation is studied in a broadband amplifying medium, sapphire doped with electronically excited Ti ions. Evidence for fluence amplification of self-guided pulses, increase of filamentation length, as well as a lowering of the input laser power necessary for filamentation is reported.     

Ultrafast time-resolved imaging of femtosecond laser-induced periodic surface structures on GaAs

We report the formation dynamics of periodic ripples on Ga As induced by femtosecond laser pulses（800 nm, 50 fs） via a collinear time-resolved imaging technique with a temporal resolution of 1 ps and a spatial resolution of 440 nm. The onset of periodic ripples emerges in the initial tens of picoseconds in the timescale of material ejection. The periodic ripples appear after irradiation of at least two pump pulses at surface defects produced by the first pulse and the ripple positions kept stable until the formation processes complete. The formation mechanisms of laser-induced periodic ripples are also discussed.     

SEM and Raman spectroscopy analyses of laser-induced periodic surface structures grown by ethanol-assisted femtosecond laser ablation of chromium

The effect of fluence and pulse duration on the growth of nanostructures on chromium (Cr) surfaces has been investigated upon irradiation of femtosecond (fs) laser pulses in a liquid confined environment of ethanol. In order to explore the effect of fluence, targets were exposed to 1000 pulses at various peak fluences ranging from 4.7 to 11.8?J?cm^–2 for pulse duration of ～25?fs. In order to explore the effect of pulse duration, targets were exposed to fs laser pulses of various pulse durations ranging from 25 to 100?fs, for a constant fluence of 11.8?J?cm^–2. Surface morphology and structural transformations have been analyzed by scanning electron microscopy and Raman spectroscopy, respectively. After laser irradiation, disordered sputtered surface with intense melting and cracking is obtained at the central ablated areas, which are augmented with increasing laser fluence due to enhanced thermal effects. At the peripheral ablated areas, where local fluence is approximately in the range of 1.4–4?mJ?cm^–2, very well-defined laser-induced periodic surface structures (LIPSS) with periodicity ranging from 270 to 370?nm along with dot-like structures are formed. As far as the pulse duration is concerned, a significant effect on the surface modification of Cr has been revealed. In the central ablated areas, for the shortest pulse duration (25?fs), only melting has been observed. However, LIPSS with dot-like structures and droplets have been grown for longer pulse durations. The periodicity of LIPSS increases and density of dot-like structures decreases with increasing pulse duration. The chemical and structural modifications of irradiated Cr have been revealed by Raman spectroscopy. It confirms the formation of new bands of chromium oxides and enol complexes or Cr-carbonyl compounds. The peak intensities of identified bands are dependent upon laser fluence and pulse duration.     

Morphology control of laser-induced periodic surface structure on the surface of nickel by femtosecond laser

An interesting transition between low spatial frequency laser-induced periodic surface structure(LIPSS) and high spatial frequency LIPSS(HSFL) on the surface of nickel is revealed by changing the scanning speed and the laser fluence. The experimental results show the proportion of HSFL area in the overall LIPSS(i.e., K) presents a quasi-parabola function trend with the polarization orientation under a femtosecond(fs) laser single-pulse train.Moreover, an obvious fluctuation dependence of K on the pulse delay is observed under a fs laser dual-pulse train.The peak value of the fluctuation is found to be determined by the polarization orientation of the dual-pulse train.     

Determination of the temporal structure of femtosecond laser pulses by means of laser-induced air plasma

A new approach is presented to reveal the temporal structure of femtosecond laser pulses by recording the corresponding time-resolved shadowgraphs of the laser-induced air plasma. It is shown that the temporal structures of femtosecond laser pulses, normally not observable by the ordinary intensity autocorrelator, can be detected through intuitively analyzing the ultrafast evolution process of the air plasma induced by the femtosecond laser pulses under examination. With this method, existence of pre- and post-pulses has been clearly unveiled within the time window of ±150 fs in reference with the main 50-fs laser pulses output from a commercial 1-kHz femtosecond laser amplifier. The unique advantage of the proposed method is that it can directly provide valuable information about the pulse temporal structures' effect on the laser-induced ionization or material ablation.     

Implications of transient changes of optical and surface properties of solids during femtosecond laser pulse irradiation to the formation of laser-induced periodic surface structures

The formation of laser-induced periodic surface structures (LIPSS) upon irradiation of silicon wafer surfaces by linearly polarized Ti:sapphire femtosecond laser pulses (pulse duration 130 fs, central wavelength 800 nm) is studied experimentally and theoretically. In the experiments, so-called low-spatial frequency LIPSS (LSFL) were found with periods smaller than the laser wavelength and an orientation perpendicular to the polarization. The experimental results are analyzed by means of a new theoretical approach, which combines the widely accepted LIPSS theory of Sipe et al. with a Drude model, in order to account for transient (intra-pulse) changes of the optical properties of the irradiated materials. It is found that the LSFL formation is caused by the excitation of surface plasmon polaritons, SPPs, once the initially semiconducting material turns to a metallic state upon formation of a dense free-electron-plasma in the material and the subsequent interference between its electrical field with that of the incident laser beam resulting in a spatially modulated energy deposition at the surface. Moreover, the influence of the laser-excited carrier density and the role of the feedback upon the multi-pulse irradiation and its relation to the excitation of SPP in a grating-like surface structure is discussed.