Theory of ultra-short laser pulses

Using generalized Ginzburg-Landau equations derived recently we solve the laser equations analytically for the case of spontaneous ultra-short laser pulses. The results are compared with previous computer solutions by Risken and Nummedal and very good agreement is found.     

Interaction of picosecond laser pulses with a thin target

Interaction between a picosecond laser pulse of nonrelativistic intensity with a thin target is studied in terms of the kinetic theory of laser plasma, which is based on constructing propagators for the plasma particle distribution functions. Allowance is made for both the self-consistent plasma field and plasma particle correlations (collisions). The interaction causes charge separation in the target and strong heating of electrons.     

超短脉冲激光对岩石的烧蚀作用研究

对干燥状态和浸水后的红砂岩、花岗岩、大理岩在不同超短脉冲激光入射频率和烧蚀发次等条件下的烧蚀情况进行了研究.结果发现,浸水后的岩石烧蚀质量损失明显大于干燥状态下的质量损失.同时质量损失与激光入射频率和烧蚀发次有着密切的关系.     

Chromium carbide thin films deposited by ultra-short pulse laser deposition

Pulsed laser deposition performed by a laser with a pulse duration of 250 fs has been used to deposit films from a Cr₃C₂ target. Due to the different processes involved in the laser ablation when it is performed by an ultra-short pulse source instead of a conventional short pulse one, it has been possible to obtain in vacuum films containing only one type of carbide, Cr₃C₂, as shown by X-ray photoelectron spectroscopy. On the other hand, Cr₃C₂ is not the only component of the films, since a large amount of amorphous carbon is also present. The films, deposited at room temperature, are amorphous and seem to be formed by the coalescence of a large number of particles with nanometric size. The film composition can be explained in terms of thermal evaporation from particles ejected from the target.     

Selective ablation of thin films with short and ultrashort laser pulses

Micromachining of CuInSe₂ (CIS)-based photovoltaic devices with short and ultrashort laser pulses has been investigated. Therefore, ablation thresholds and ablation rates of ZnO, Mo and CuInSe₂ thin films have been measured for irradiation with nanosecond laser pulses of ultraviolet and visible light and subpicosecond laser pulses of a Ti:sapphire laser. The experimental results were compared to the theoretical evaluation of the samples heat regime. In addition, the cells photo-electrical properties were measured before and after laser machining. Scanning electron microscopy (SEM) and energy dispersive X-ray (EDX) analyses were employed to characterise the laser-induced ablation channels. Using nanosecond laser pulses, two phenomena were found to limit the laser-machining process. Residues of Mo that were projected onto the walls of the ablation channel and the metallization of the CuInSe₂ semiconductor close to the channel lead to a shunt. The latter causes the decrease of the photovoltaic efficiency. As a consequence of these limiting effects, only subpicosecond laser pulses allowed the selective or complete ablation of the thin layers without a relevant change of the photo-electrical properties.     

超短激光脉冲对宽带光学物质的微加工

通过讨论超快飞秒激光脉冲和长脉冲宽度的激光脉冲紧导致宽能隙透明电介质的损伤机理和比较超短激光脉冲与长激光脉冲对宽能隙透明电介质的损伤程度，得出超短激光脉冲是一种可对透明宽带电介质进行加工的有效工具的结论。当波长为800nm，脉冲宽度为150fs的激光脉冲紧聚焦到不同的宽能隙透明电介质（K9玻璃和ZK6玻璃）体内时，可制作不同光栅常数的光栅，并在波长为635nm的He-Ne连续激光的垂直照射下，对光栅的远场相对衍射效率和光栅的衍射效率进行了测量。     

Self-organization of thin metal films by irradiation with nanosecond laser pulses

Modifications in thin metal films under intensive laser irradiation were studied. Gold, silver, copper, chromium and aluminum films with the thickness of 100 nm were deposited on the glass substrate. Back-side irradiation through the substrate with a burst of nanosecond pulses tightly focused to a line was applied. The film removal threshold with a single pulse F_th was estimated for every material and laser fluence was kept above it in the range of 1.5-3 F_th during experiments. Diverse behavior of the films depending on the metal, the shift between pulses and laser fluence was observed. In chromium, the regular structures were developed in a quite wide range of processing parameters. In gold, three kinds of ripples were observed: transverse (similar to ripples in chromium), longitudinal and a structure of ripples oriented at 60° to each other. The combination of physical properties facilitated the regular assembly of the molten metal in chromium and to some extent in gold.     

Nd:YAG laser ablation characteristics of thin CIGS solar cell films

This work reports that the ablation characteristics of thin CuIn_1?x Ga _x Se₂ (CIGS) solar cell film differ significantly with elemental composition and laser pulse energy. From in situ shadowgraphs measured during Nd:YAG laser (1,064 nm) irradiation of CIGS films and crater morphologies, it was found that strong surface evaporation is dominant for low Ga concentration films of which band gap is well below the photon energy. As the band gap of CIGS film becomes close to or over the laser photon energy due to increased Ga content, surface absorption diminishes and at low laser energy, laser heating of the film plays an important role. It is demonstrated that for the CIGS films with Ga/(Ga + In) ratio being approximately over 0.2, the laser irradiation leads to solid phase removal of the film due to thermomechanical fracture at low laser energy but to ablative evaporation at elevated energy.     

Nanostructuring of surfaces by ultra-short laser pulses

Nanostructuring of an extended surface area is performed by ultra-short-pulse laser ablation in the low-fluence regime. A layer of micrometer-sized quartz spheres is used as a lens array in direct contact with the sample. The thickness of a transparent spacer layer under the spheres is adjusted so that the sample is struck by an array of well-focused spots. The threshold character of the ablation process allows the formation of sub-diffraction-limited structures, down to 500-nm holes with 800-nm light. The deposition of the lens array directly on the surface makes the technique broadly applicable, also to samples that show great variations in height. PACS 61.80.Ba; 78.47.+p; 81.16.Rf; 81.65.Cf     

Nanostructured thin films obtained by ultra-short pulse laser deposition of vanadium carbide

In this paper we will report the results obtained by femtosecond Pulsed Laser Deposition on vanadium carbide. These results, compared with those obtained for another group 5 carbide, tantalum carbide, evidence large analogies between the two systems. Optical emission spectroscopy shows in both cases the presence of particles in the secondary plume and in both cases the films are formed by nanoparticles and present a stoichiometry corresponding to the hemicarbide.     

Self-induced mismatching effects in harmonic generation with ultra-short laser pulses

Relying on exact numerical calculations, we report some simple formulae by which it is possible to take into account any self-induced mismatch effect in second harmonic generation with high power ultra-short laser pulses. In particular, we have considered the following effects: bandwidth effect, thermal effect, frequency chirping, angular tilting, pulse lengthening.     

Nonlinear absorption of ultrashort laser pulses in thin metal films

Self-consistent simulations of the ultrafast electron dynamics in thin metal films were performed. A regime of nonlinear oscillations was observed that corresponds to ballistic electrons bouncing back and forth against the films' surfaces. When an oscillatory laser field is applied to the film, the field energy is partially absorbed by the electron gas. Maximum absorption occurs when the period of the external field matches the period of the nonlinear oscillations, which, for sodium films, lies in the infrared range. Possible experimental implementations are discussed.     

Low-threshold high-dynamic-range optical limiter for ultra-short laser pulses

We demonstrate an optical limiter for ultra-short (∼100-fs) laser pulses. The device has a dynamic range (= damage energy/onset-of-limiting energy) of more than 10000 and an onset-of-limiting energy of only ∼10 nJ. The output-pulse energy is kept below 1.3 μJ. The limiting mechanism is based on two-photon absorption and refractive nonlinearities in a 20-mm piece of ZnSe. We discuss the importance of the different nonlinearities, damage issues, and guidelines for the construction of the device. Received: 20 December 2001 / Revised version: 25 March 2002 / Published online: 8 May 2002     

Patterning of CIGS thin films induced by rear-side laser ablation of polyimide carrier foil

Laser patterning of thin films is essential for the future development of flexible electronic devices. The damage-free scribing of thermally sensitive thin films such as copper–indium–gallium diselenide (CIGS) that is required for solar module fabrication by integrated interconnections is still challenging. In this study a new approach for non-thermal, damage-free scribing of CIGS films on polyimide foil is proposed and demonstrated. In contrast to the usually used direct laser ablation of the thin-film stack, laser ablation of the polyimide carrier foil at laser fluences higher than 3 J/cm² is utilized to achieve CIGS film delamination and thus the patterning of the thin film. The edges of the patterned CIGS films do not show typical laser-ablation-induced modifications like melting, debris contamination, or crack formation. The mechanism of the thin-film removal is of non-thermal origin and is probably due to stress formation at the CIGS/Mo interface resulting from secondary processes of polyimide laser ablation like shock-wave formation or local sample deformation.     

Accurate description of ultra-short tightly focused Gaussian laser pulses and vacuum laser acceleration

High-order correction terms to the expression of the field of ultra-short tightly focused Gaussian laser pulses are derived. Terms up to seventh order in the small dimensionless spatial parameter s=1/(k₀w₀₀) and first order in the small dimensionless temporal parameter ε=1/(ω₀t₀) are explicitly presented (ω₀=ck₀ the central oscillatory frequency, 2t₀ the pulse duration, w₀₀ the beam waist radius at the central frequency ω₀). To evaluate the correction efficacy, both the corrected and the paraxial field equations are used in detailed simulation studies of laser/electron interaction dynamics. Special attention is given to the vacuum laser acceleration scheme. The influence on the electron dynamics due to the diffraction edge field of a tightly focused laser beam is also investigated. PACS  42.25.Bs; 42.60.Jf; 41.75.Jv     

Selective ablation of thin Mo and TCO films with femtosecond laser pulses for structuring thin film solar cells

We report on recent results on selective ablation of TCO (SnO₂, ZnO) and metallic layers as a possible process for structuring thin film solar cells. The multipulse ablation thresholds determined for substrate and various thin films show a parameter window where the films can be completely removed by a single scan without at the same time damaging the underlying material. By employing ultrashort pulsed lasers, nonthermal ablation at repetition rates up to the megahertz regime enables high quality structuring combined with process speeds meeting industrial demands. PACS 42.65.Re; 42.55.Xi; 42.62.-b; 84.60.Jt     

Laser damage studies of silicon surfaces using ultra-short laser pulses

A comparative study of damage morphology in GaAs induced by s- , p- and linearly polarized laser light (1.064 μm, 35 ps) is presented. For linearly polarized light damage initiates in the form of pits. This material damage occurs below the surface. For s- or p-polarized light material damage involves only the surface layer. For larger fluences or number of pulses the differences are less marked and the damage morphology occurs in a similar manner either for linearly polarized or s- or p-polarized light. Ripples are formed when multiple irradiation is used due to interference between the front and back faces of the test sample. The spacing of these ripples is 3 μm, which is in good accordance with the reported work of Guosheng et al. (Phys. Rev. B 26 (1982) 5366).     

Nanostructuring of thin Au films by means of short UV laser pulses

The particle size distribution, morphology and optical properties of the Au nanoparticle (NP) structures for surface enhanced Raman signal (SERS) application are investigated in dependence on their preparation conditions. The structures are produced from relatively thin Au films (10–20 nm) sputtered on fused silica glass substrate and irradiated with several pulses (6 ns) of laser radiation at 266 nm and at fluencies in the range of 160–412 mJ/cm². The SEM inspection reveals nearly homogeneously distributed, spherical gold particles. Their initial size distribution of the range of 20–60 nm broadens towards larger particle diameters with prolonged irradiation. This is accompanied by an increase in the uncovered surface of the glass substrate and no particle removal is observed. In the absorption profiles of the nanostructures, the broad peak centred at 546 nm is ascribed to resonant absorption of surface plasmons (SPR). The peak position, halfwidth and intensity depend on the shape, size and size distribution of the nanostructured particles in agreement with literature. From peak intensities of the Raman spectra recorded for Rhodamine 6G in the range of 300–1800 cm⁻¹, the relative signal enhancement by factor between 20 and 603 for individual peaks is estimated. The results confirm that the obtained structures can be applied for SERS measurements and sensing.