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).     

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.     

Estimation of electron density and temperature of semiconductor surfaces excited by ultra-short laser pulses

A simple technique, based on the interference principle, to obtain simultaneously the instantaneous electron density and temperature of ultra-short laser-excited semiconductor surface plasma is proposed and demonstrated. The interference of the incident laser and the surface plasmons forms nano-ripples on the surface. From the observed nano-ripple period, one can easily retrieve the density and temperature information. As a demonstration of the technique, the electron density and temperature are obtained for various band gap semiconductor materials based on the experimentally observed nano-ripples using 800 and 400 nm light in various ambient media and incident angles. The electron density estimated varied in the range of 2 $n_{\mathrm{c} }$ –10 $n_{\mathrm{c}}$ and the corresponding electron temperature in the range 10 $^{4}$ –10 $^{5}$ K, depending on the material band gap, the incident laser intensity, the ambient medium, the angle of incidence, and the laser wavelength. The information of the electron density and temperature is useful for choosing laser parameters (like fluence, wavelength, angle of incidence, ambient medium) and target materials (different band gap semiconductors) for obtaining a better size controllability of the nanostructure production. The information can also help one in obtaining essential plasma parameter inputs in the quest for understanding ultra-fast melting or understanding the pre-plasma conditions created by the pre-pulse of ultra-high intensity laser pulses.     

Nanostructuring with spatially localized femtosecond laser pulses

Spatially localized femtosecond pulses have been produced by a combination of scanning near-field optical microscopy with ultrashort pulse lasers. With these pulses direct ablative writing on metal surfaces is demonstrated. Possible applications of this technique for nanostructuring, repair, and production of lithographic masks are discussed.     

Morphology of ablation craters generated by ultra-short laser pulses in dentin surfaces: AFM and ESEM evaluation

In this study, the surface morphology and structure of dentin after ablation by ultra-short pulses were evaluated using environmental scanning electron microscopy (ESEM) and atomic force microscopy (AFM). The dentin specimens examined were irradiated by a chirped-pulse-amplification (CPA) Ti:sapphire laser (800 nm) and the optimal conditions for producing various nanostructures were determined. Based on the ESEM results, it was possible to identify an energy density range as the ablation threshold for dentin. The laser-induced damage was characterized over the fluence range 1.3-2.1 J/cm². The results demonstrate that by selecting suitable parameters one can obtain efficient dentin surface preparation without evidence of thermal damage, i.e., with minimized heat affected zones and reduced collateral damage, the latter being normally characterized by formation of microcracks, grain growth and recrystallization in the heat affected zones.     

Nanostructuring of semiconductor surfaces under the action of femtosecond pulses

The nanostructures with dimensions (110–250 nm), much smaller than the wavelength of laser light, obtained on surfaces of various semiconductors under the action of laser ablation by femtosecond pulses are studied. Nanostructures obtained at large angles of incidence and under the action of interference of two femtosecond pulses on the surface of the samples are analyzed. Original Russian Text ￠ R.A. Ganeev, T.Q. Jia, 2008, published in Optika i Spektroskopiya, 2008, Vol. 105, No. 1, pp. 154–159.     

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.     

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.     

Molecular dissociation of SF6 by ultra-short CO2 laser pulses

We have measured the absolute dissociation probability of SF₆ gas irradiated by CO₂ laser pulses varying in duration from 0.5 nsec to 100 nsec. We find a threshold for dissociation of ? 1.4 J/cm², independent of pulse duration. For fixed energy density, the dissociation probability increases as the pulse duration decreases, but not nearly to the degree expected from theory. We have also determined the dissociation probability in the ν₂ + ν₆ combination band and find it to be 10³ times less than in the ν₃ band, in contradiction to some conclusions recently reported by Ambartzumian et al. The dissociation mechanism takes place without collisions, and if collisions occur, they tend to be detrimental to the dissociation rate.     

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     

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。     

超短脉冲高次谐波谱的截止频率分析

本文数值计算了超短脉冲(脉冲持续时间为两个光学周期)的高次谐波发射功率谱.研究结果表明,超短脉冲谐波谱的截止频率不再符合IP 3.17UP的截止规则(IP为原子电离势,UP为有质动力能).对此,本文采用"三步"模型给出了合理解释,并用小波变换验证了结果的正确性.     

Micro-hole processing of polyimide film by ultra-short laser pulses and its applications

Micro machining of a polyimide film with ultra-short pulsed laser was demonstrated. A through-hole with a diameter of less than 10 μm could be produced by optimizing the pulse duration, repetition rate, and number of shots. Thermal damage around the through-hole can be made negligibly small when the pulse duration was shorter than 140 fs. We also found that the minimum shot number needed for the creation of a through-hole becomes smaller as the repetition rate of laser shot increases. On the basis of the optimum irradiation condition, we demonstrated laser cutting of a polyimide film without thermal damage and copper filling in through-holes with diameters of 9 μm by a plating process.     

Creation of periodic subwavelength ripples on tungsten surface by ultra-short laser pulses

Formation of periodic subwavelength ripples on a metallic tungsten surface is investigated through a line-scribing method under the irradiation of 800?nm, 50 fs to 8 ps ultra-short laser pulses. The distinctive features of the induced ripple structures are described in detail with different laser parameters. Experimental measurements reveal that with gradual decrease of the laser fluence, the pulse duration or the scanning speed, the ripple period is inclined to reduce but the ripple depth tends to become pronounced. Theoretical analyses suggest that the transient dielectric function change of the tungsten surface mainly originates from the nonequilibrium distribution of electrons due to the d-band transitions. A sandwich-like physical model of air?Cplasma?Ctarget is proposed and the excitation of a surface plasmon polaritonic (SPP) wave is supposed to occur on the interface between the metallic target and the electron plasma layer. Formation of ripples can be eventually attributed to the laser?CSPP interference. Theoretical interpretations are consistent with the experimental observations.     

Sub-half-cycle polarization gates in ultra-short laser pulses induced by non-linear propagation effects

We demonstrate the possibility of imprinting a controlled ultra-fast time-varying ellipticity on few-cycle laser pulses through the non-linear propagation in a thin anisotropic crystal. The method is based on the self- and cross-phase modulation induced by the third-order (Kerr) non-linearity when the pulse propagates through the crystal. The feasibility of this technique is demonstrated through the numerical integration of the Maxwell equations, showing the possibility of producing a sub-half-cycle temporal gate of linear polarization. In comparison with alternative approaches, our method results in a polarization gate with higher efficiency, and also it can be controlled by changing the polarization axis of the laser pulse at the entrance of the crystal. We finally demonstrate the efficiency of this scheme in the generation of single attosecond pulses. PACS 42.81.Gs; 42.65.Re; 32.80.Wr     

Heating of opaque surfaces by picosecond laser pulses

Received: 5 June 1997/Accepted: 13 June 1997     

Recrystallization of germanium surfaces by femtosecond laser pulses

The damage morphology of germanium surfaces using femtosecond laser pulses of various fluences and number of pulses is reported. The single pulse damage threshold in the present experiment was 9.7±4.0×10⁻¹³ W/cm². The experimental threshold value was compared with theory, considering the damage threshold as the melting threshold. The cooling rate calculated on the basis of present results is 2.4×10^15°C/s. Recrystallization was the common feature of the damage morphology. For fluences greater than the single pulse damage-threshold micropits and spherical grains of micron size were formed in the damaged surface. Ablation (surface removal) was also observed at higher fluences (at two or three times of damage threshold value). The damage morphology, induced by multiple pulses, was unaffected for linear and circular polarization.     

超短脉冲激光产生等离子体通道寿命研究

利用强飞秒激光在大气中产生等离子体通道内带电粒子的动力学模型,综合考虑二体吸附、三体吸附、离解和复合等因素,分析后续激光退吸附作用对等离子体通道寿命的影响,给出有效延长通道寿命的后续激光控制参量。计算表明,选择不同的激光注入形式和注入时间,对通道寿命的延长有着不同的效果,适当优化的激光可以延长通道寿命达25 s以上。     

超短脉冲激光产生等离子体通道寿命研究

利用强飞秒激光在大气中产生等离子体通道内带电粒子的动力学模型,综合考虑二体吸附、三体吸附、离解和复合等因素,分析后续激光退吸附作用对等离子体通道寿命的影响,给出有效延长通道寿命的后续激光控制参量。计算表明,选择不同的激光注入形式和注入时间,对通道寿命的延长有着不同的效果,适当优化的激光可以延长通道寿命达25 s以上。     

Roadmap to ultra-short record high-energy pulses out of laser oscillators

Highly-chirped dissipative solitons of the cubic-quintic Ginzburg-Landau equation found in this work may provide a roadmap to design passively mode-locked laser oscillators that generate pulses of extremely high energy. We provide a region in the space of the system parameters where high-energy dissipative solitons are found, along with their typical spectral and temporal features.