Spot-size dependence of the ablation threshold in dielectrics for femtosecond laser pulses

The multi-pulse ablation threshold of barium borosilicate glass was measured using 30-fs pulses of a high repetition rate (1 kHz) laser system. The threshold fluence was found to decrease with increasing beam radius ranging from 20 to 400 m. Two existing models are applied by considering thermal accumulation and point defects, respectively . PACS 42.62.C; 44.10.+i; 77.22.Jp; 77.84.Bw; 79.20.D     

Polarization-dependent ablation of silicon using tightly focused femtosecond laser vortex pulses

We demonstrate experimentally that, in a tight focusing geometry, circularly polarized femtosecond laser vortex pulses ablate material differently depending on the handedness of light. This effect offers an additional degree of freedom to control the shape and size of laser-machined structures on a subwavelength scale.     

Single-shot front-side nanoscale femtosecond laser ablation of a thin silver film

Nano- and microscale holes, as well as related sub-ablative nanospikes and sub-micron bumps, were produced in a 30-nm thick silver film on a silica substrate by single femtosecond laser pulses with variable pulse energies, focused by different strong focusing optics. Characteristic laser energy deposition dimensions exceed the expected focal spots by nearly 2 microns, indicating the considerable lateral thermal transport in the film, while the effective hole formation thresholds decrease versus increasing numerical aperture of focusing optics. Morphologies of the sub-ablative solidified surface nanostructures and numerical estimates of deposited volume energy density undermine blowing-off the molten film due to subsurface boiling and near-critical phase explosion at lower and higher sub-threshold fluences, respectively.     

Lithographical laser ablation using femtosecond laser

Lithographical laser ablation was demonstrated using a femtosecond laser with a lithographical optical system. In this method, a femtosecond laser beam passes through a mask and the pattern is imaged on a film by a coherent optical system. As a result, the film is lithographically ablated, and a micron-sized pattern can be generated in a single shot. The resolution of generation was 13 m, and the narrowest width of a generated line was about 4 m. Moreover, the system was applied to transmission gratings as masks, and nano-sized periodic structures such as nano-sized hole matrices and nano-meshes were generated in a single shot. PACS 52.38.Mf; 42.25.Hz; 42.82.Cr; 81.16.-c     

飞秒激光激发空气电离的阈值研究

报道在脉宽50fs—22ps,波长800nm脉冲激光作用下的空气电离阈值的研究结果.利用探测等离子体发光信号的方法,实验测量了激发空气电离所需的阈值激光强度.结果表明,当激光脉冲宽度从50fs增加到22ps时,阈值光强I_th从8.7×10¹⁴W/cm²下降到2.7×10¹³W/cm²;I_th经历了由迅速降低逐渐发展为缓慢降低的过程.在50fs—1p 关键词： 飞秒激光 阈值 多光子电离 碰撞电离     

Nanocomposites based on globular photonic crystals grown by laser ablation using femtosecond laser pulses

The prepared samples were characterized by reflection and transmission spectra of broadband visible-range radiation, and the volume fraction of substances introduced into pores of artificial opals is estimated. It is found that the introduction of solid-state nanoparticles with negative real part of the permittivity into opal pores results in a decrease in the effective refractive index and a short-wavelength shift in the spectral position of the stop band in artificial opal.     

飞秒激光激发空气电离的阈值研究

报道在脉宽50fs—22ps，波长800nm脉冲激光作用下的空气电离阈值的研究结果.利用探测等离子体发光信号的方法，实验测量了激发空气电离所需的阈值激光强度.结果表明，当激光脉冲宽度从50fs增加到22ps时，阈值光强I_th从8.7×10¹⁴W/cm²下降到2.7×10¹³W/cm²；I_th经历了由迅速降低逐渐发展为缓慢降低的过程.在50fs—1p     

Non-thermal ablation of neural tissue with femtosecond laser pulses

2 ; for 100 fs pulses from the same laser the experimental threshold was at 1.5 J/cm². Histopathological examinations and scanning electron micrographs confirm the high quality of the excisions. No sign of significant thermal damage was observed. Received: 29 January 1997/Revised version: 14 July 1997     

Ablation of microstructures applying diffractive elements and UV femtosecond laser pulses

A new method for simple and economic fabrication of diffractive optical elements (DOEs) with three and four phase levels, by UV nanosecond (ns) laser ablation is presented. The technique is based on the combination of two sequentially generated complementary 2-level phase elements. During the fabrication, complete ablative removal of a highly absorbing silicon suboxide layer by pixelated backside illumination ensures the necessary high precision and optical quality. Full functionality of the new DOEs is demonstrated by fabricating micro-structures using UV femtosecond pulses.     

Absorption and ionization of molecular nitrogen by UV femtosecond laser pulses

Mechanisms of non-linear absorption and ionization of molecular nitrogen gas by UV femtosecond laser pulses were studied using photogalvanic and photoacoustic techniques. The effect of the intermediate Rydberg resonance, its dynamic Stark perturbation and ponderomotive upshift of the first ionization potential of nitrogen molecules by the intense laser pulses has been revealed by observing an increase of a power slope of ion yield from three to four at increasing laser intensity.     

飞秒激光烧蚀硅表面产生微纳结构过程中激光脉冲数目的影响

为了研究飞秒激光脉冲数目与硅表面形貌之间的关系,在相同的SF6气体氛围下,改变照射硅表面的飞秒激光脉冲数,发现在飞秒激光照射下由硅表面形成的微型锥状尖峰的高度与飞秒激光脉冲数呈现一种非线性关系.通过对该关系的研究有利于找出在制造具有较高吸收效率的高微型锥状尖峰的"黑硅"的实验条件,有利于基于"黑硅"材料的光电器件转化效...     

Modeling of femtosecond ablation of aluminum film with single laser pulses

Detailed investigation of pulsed laser ablation dynamics is performed for aluminum target under action of 100 fs pulses with peak intensity 3.95 × 10¹² W/cm² and wavelength 0.8 μm.Non-equilibrium two-temperature model with hydrodynamic Stephan problem was used for modeling. Explicit tracking of moving interphase boundaries permits exact determination of their velocity and amount of removed and evaporated material. Detailed ablation process is analyzed using the study of temperature, pressure and density evolution in the target. High phase front velocities (melting up to 5 km/s and evaporation up to 350 m/s) are caused by strong overheating of solid and liquid phases.     

Tailored ablation processing of advanced biomedical hydroxyapatite by femtosecond laser pulses

The micromachining of hydroxyapatite (HAp) is highly important for orthopedics and dentistry, since human bone and teeth consist mainly of HAp. We demonstrate ultrashort Ti:sapphire laser ablation of HAp, using pulse-widths of 50 fs, 500 fs, and 2 ps at a wavelength of 820 nm and at 1 kpps. The crucial medical issue is to preserve the chemical properties of the machined (ablated) surface. If the chemical properties of HAp change, the human bone or tooth cannot re-grow after laser processing. Using X-ray photoelectron spectroscopy, we observe chemical properties of HAp ablated in air. The HAp is ablated at laser fluences of 3.2 J/cm² (6.4×10¹³ W/cm² at 50 fs), 3.3 J/cm² (6.6×10¹² W/cm² at 500 fs), and 9.6 J/cm² (4.8×10¹² W/cm² at 2 ps), respectively. As a result it is found that the ablated surface is unchanged after laser ablation over the pulse-width range used in this experiment. Received: 7 October 2002 / Accepted: 20 January 2003 / Published online: 28 May 2003 RID="*" ID="*"Corresponding author. Fax: +81-45/566-1533, E-mail: obara@obara.elec.keio.ac.jp     

Surface damage threshold and structuring of dielectrics using femtosecond laser pulses: the role of incubation

We present results on the surface damage threshold of a-SiO₂ and YLF after single and multiple laser pulse irradiation at a pulse duration of 100 fs and radiation wavelength of 800 nm. The surface damage threshold drops dramatically after the first laser shots until reaching an almost constant level. The threshold reduction at low shot numbers is attributed to laser induced defect formation. This has important consequences for applications, such as laser machining and the lifetime of optical components. As an example of relevance to applications, we discuss the generation of high quality micro pockets in a-SiO₂ and YLF.     

紫外超短脉冲激光的单次测量

利用瞬态光栅衍射法研制了一台单次频率分辨光学开关激光参数测量仪,可以对脉宽范围在10 ps以内的紫外超短脉冲激光进行单次测量。利用该仪器对重复频率为10 Hz的放电泵浦和单次运行的电子束泵浦KrF准分子激光器的输出脉冲分别进行了单次测量,结果表明：对于放电泵浦,当系统工作在零啁啾附近时,脉冲波形和光谱分布较规则,相位分布起伏较小,当系统偏离零啁啾状态时,脉冲波形和光谱分布不再规则,并且相位分布为抛物线结构;对于单次运行的电子束泵浦,脉冲波形具有多峰结构,光谱具有复杂的调制,脉宽约为2 ps,带宽约为1.3 nm,相位分布为抛物线结构。     

紫外超短脉冲激光的单次测量

 利用瞬态光栅衍射法研制了一台单次频率分辨光学开关激光参数测量仪,可以对脉宽范围在10 ps以内的紫外超短脉冲激光进行单次测量。利用该仪器对重复频率为10 Hz的放电泵浦和单次运行的电子束泵浦KrF准分子激光器的输出脉冲分别进行了单次测量,结果表明：对于放电泵浦,当系统工作在零啁啾附近时,脉冲波形和光谱分布较规则,相位分布起伏较小,当系统偏离零啁啾状态时,脉冲波形和光谱分布不再规则,并且相位分布为抛物线结构;对于单次运行的电子束泵浦,脉冲波形具有多峰结构,光谱具有复杂的调制,脉宽约为2 ps,带宽约为1.3 nm,相位分布为抛物线结构。     

Thermodynamic evolution of materials during laser ablation under pico and femtosecond pulses

Using molecular-dynamics, we study the thermodynamic evolution of a simple two-dimensional Lennard–Jones system during laser ablation for pulse durations ranging from 200 fs to 400 ps. We briefly review results previously obtained for fs pulses where the evolution of the material was shown to be solely a function of the locally absorbed energy (provided that only thermal effects are important), i.e., is adiabatic. For longer pulses (100 and 400 ps) the situation becomes more complex, as the relaxation path also depends on the position in the target and on the timescale on which expansion occurs. We show that, in contrast to fs pulses, the material ejected following ps laser irradiation does not enter the liquid–vapor metastable region before ablation occurs, hence showing that phase explosion is not the dominant mechanism in this regime. Following on from previous work, we propose that trivial fragmentation is the main ablation mechanism. PACS 79.20.Ds; 79.20.Ap; 61.80.Az     

Direct creation of black silicon using femtosecond laser pulses

Using a direct femtosecond laser surface structuring technique, an array of equally spaced parallel nanostructure-textured microgrooves on silicon was produced that causes a dramatic reduction of the treated silicon reflectance. The processed area appears velvet black at all viewing angles. Throughout the visible region, the reflectance of the blackened surface is less than 5%. The antireflection effect of the processed surface also extends to the mid-infrared wavelength range. Furthermore, this technique has a potential in reducing silicon reflectance at terahertz frequencies and even in millimeter wavelength range.