Single- and multi-pulse formation of surface structures under static femtosecond irradiation

Femtosecond surface structure modifications are investigated under irradiation with laser pulses of 150 fs at 800 nm, on copper and silicon. We report sub-wavelength periodic structures formation (ripples) with a periodicity of 500 nm for both materials. These ripples are perpendicular to the laser polarization and can be obtained with only one pulse. The formation of these ripples corresponds to a fluence threshold of 1 J/cm² for copper and 0.15 J/cm² for silicon. We find several morphologies when more pulses are applied: larger ripples parallel to the polarization are formed with a periodicity of 1 μm and degenerate into a worm-like morphology with a higher number of pulses. In addition, walls of deep holes also show sub-wavelength and large ripples.     

Femtosecond laser induced submicrometer structures on the ablation crater walls of II-VI semiconductors in water

Femtosecond laser ablations (100 fs, 800 nm, 0.2 mJ/pulse) were performed to produce craters on CdS, ZnS:Cu and ZnSe wafers in water. On the surface of the crater walls, a variety of submicrostructural formations were presented, such as the ripples and network structures for CdS, the subwavelength ripples and columnar structures for ZnS:Cu, even the regular cubic-shaped submicron rods for ZnSe. Based on the field-emission scanning electron microscope (FE-SEM) study of the different characteristic surface morphologies, the possible formation mechanisms were discussed correspondingly. For example, two distinct mechanisms are contributing to the different styles of ripples formed on CdS and ZnS:Cu. The former is the interference effects between the incoming laser beam and scattered surface wave, while the latter is the self-organization structure formation. In addition, the re-crystallization of the water-confined hot plasma would play an important role in the formation of ZnS:Cu column structures and ZnSe rods.     

Crater geometry characterization of Al targets irradiated by single pulse and pulse trains of Nd:YAG laser in ambient air and water

High intensities laser pulses are capable to generate a crater when irradiating metal targets. In such condition, after each irradiation significant ablation occurs on the target surface and as a result a crater is formed. The crater characterization is very important specifically for some applications such as micromachining. In this paper, the crater formation in metal targets was studied experimentally. The planar aluminum 5052 targets were irradiated by frequency doubled (532 nm), Q-switched Nd:YAG (∼6 ns) laser beam in ambient air and distilled water. A crater was produced after each irradiation and it was characterized by an optical microscope. Different laser intensities as well as pulse trains were applied for crater formation. The effects of laser characteristics in crater geometry were examined. The depth of the craters was measured by optical microscope and the diameter (width) was characterized by processing of the crater image. The results were explained in terms of ablation threshold and plasma shielding. The results show that the crater geometry extremely depends on the laser pulse intensity, the number of laser pulses, and ambient.     

Periodic surface structures on crystalline silicon created by 532 nm picosecond Nd:YAG laser pulses

Creation of laser-induced morphology features, particularly laser-induced periodic surface structures (LIPSS), by a 532 nm picosecond Nd:YAG laser on crystalline silicon is reported. The LIPSS, often termed ripples, were produced at average laser irradiation fluences of 0.7, 1.6, and 7.9 J cm⁻². Two types of ripples were registered: micro-ripples (at micrometer scale) in the form of straight parallel lines extending over the entire irradiated spot, and nano-ripples (at nanometer scale), apparently concentric, registered only at the rim of the spot, with the periodicity dependent on laser fluence. There are indications that the parallel ripples are a consequence of the partial periodicity contained in the diffraction modulated laser beam, and the nano-ripples are very likely frozen capillary waves. The damage threshold fluence was estimated at 0.6 J cm⁻².     

Investigation of different surface morphologies formed on AISI 304 stainless steel via millisecond Nd:YAG pulsed laser oxidation

Different surface morphologies on AISI 304 stainless steel have been obtained after millisecond Nd:YAG pulsed laser oxidation. The effects of laser processing parameters, especially pulse width and laser energy density on the surface morphologies of the stainless steel were emphatically investigated. The results showed that surface morphologies were significantly changed with increasing laser pulse widths and laser energy densities. When the pulse width was 0.2–1.0 ms and laser energy density was 4.30×10⁶–7.00×10⁶ J/m², the surface was obviously damaged and the morphologies varied gradually from craters to ripple structures. However, when the pulse width was longer than 1 ms and the laser energy density was increased from 1.90×10⁷ to 3.16×10⁷ J/m², the sizes of craters got smaller until disappeared and the surface became flatter and smoother. Nevertheless, the smooth surface was not obtained under overhigh laser energy densities. In addition, the schematic relationship was used to describe the formation process and mechanism of different surface morphologies.     

Subwavelength ripple formation induced by tightly focused femtosecond laser radiation

Subwavelength ripples (<λ/4) are obtained by scanning a tightly focused beam (∼1 μm) of femtosecond laser radiation (λ = 800 nm, t_p = 100 fs) over the surface of either bulk fused silica and silicon and Er:BaTiO₃. The ripple pattern extends coherently over many overlapping laser pulses parallel and perpendicular to the polarisation. Investigated are the dependence of the ripple spacing on the spacing of successive pulses, the direction of polarisation and the material. The evolution of the ripples is investigated by applying pulse bursts with N = 1 to 20 pulses. The conditions under which these phenomena occur are specified, and some possible mechanisms of ripple growth are discussed. Potential applications are presented.     

Damage performance of TiO2/SiO2 thin film components induced by a long-pulsed laser

In order to study the long-pulsed laser induced damage performance of optical thin films, damage experiments of TiO₂/SiO₂ films irradiated by a laser with 1 ms pulse duration and 1064 nm wavelength are performed. In the experiments, the damage threshold of the thin films is measured. The damages are observed to occur in isolated spots, which enlighten the inducement of the defects and impurities originated in the films. The threshold goes down when the laser spot size decreases. But there exists a minimum threshold, which cannot be further reduced by decreasing the laser spot size. Optical microscopy reveals a cone-shaped cavity in the film substrate. Changes of the damaged sizes in film components with laser fluence are also investigated. The results show that the damage efficiency increases with the laser fluence before the shielding effects start to act.     

An investigation of long pulsed laser induced damage in sapphire

The formation of keyhole and transverse section of a laser-cut kerf with slight stripe undulations by a 1064 nm ms pulse laser on (0 0 0 1) sapphire was investigated. The morphologies of keyhole and transverse section surfaces were evaluated by SEM, and the composition of transverse section of laser-cut kerf was evaluated by EDS, XRD and XPS. The time scale for onset of vaporization and the keyhole depth with different laser pulse energies were calculated. The result suggests that the depth of keyhole is approximately directly proportional to laser pulse energy. On sapphire transverse section surface, the element ratio of Al to O deviates from the stoichiometry of sapphire, perhaps due to the oxygen removal from surface.     

Structure formation on the surface of alloys irradiated by femtosecond laser pulses

Laser-induced periodic surface structures with different spatial characteristics have been observed after multiple linearly polarized femtosecond laser pulse (120 fs, 800 nm, 1 Hz to 1 kHz pulse repetition frequency) irradiation on alloys. With the increasing number of pulses, nanoripples, classical ripples and modulation ripples with a period close to half of classical ripples have all been induced. The generation of second-harmonic has been supposed to be the main mechanism in the formation of modulation ripples.     

Morphology of ablation craters generated by fs laser pulses in LiNbO3

The surface morphology of the ablation craters generated in LiNbO₃ by 130 fs laser pulses at 800 nm has been investigated by AFM/SNOM microscopy. The single pulse fluence corresponding to the ablation threshold has been estimated to be ≈1.8 J/cm².A complex structure including random cone-shaped protrusions is observed inside the ablated crater. The scale of the protrusion spacing is in the submicron range and the heights are typically of a few tens of nanometers. At and outside the crater rim a novel quasi-periodic wave-like topography pattern is observed in both types of microscopy techniques. The average wavelength, that is slightly dependent on pulse fluence, is (500-800 nm) comparable to the light wavelength. This novel topography feature keeps a close similarity with a Fresnel diffraction pattern by an absorbing circular obstacle or impact wave pattern produced by a combination of heat and shock wave (resemble that of impact crater). It is proposed that the obstacle is associated to the strongly nonlinear multiphoton absorption at the peak of the pulse profile. The energy deposited by nonlinear absorption of such profile causes ablation of both the crater and the rippled structure.     

溶胶-凝胶光学薄膜的激光损伤研究

 采用溶胶-凝胶工艺制备了SiO₂与ZrO₂单层介质膜，用输出波长1.06μm，脉宽15ns的电光调Q激光系统产生的强激光进行辐照实验。观察了光学薄膜经强激光辐照后的损伤情况，讨论了溶胶 凝胶光学薄膜在强激光照射下的损伤机理，提出了溶剂替换、紫外光处理、添加有机粘接剂等提高溶胶 凝胶光学薄膜激光损伤阈值的方法。     

Formation of periodic surface nanostructures on Ti:Al2O3 crystals using femtosecond laser pulses

Periodic surface nanostructures are observed on Ti³⁺:Al₂O₃ single crystals that have been irradiated by a single focused beam from a femtosecond pulsed laser (wavelength: 800 nm; pulse duration: 130 and 152 fs). Atomic force microscopy images of single-ablated zones and modified structures created by fixing and translating samples through the focal region of a linearly polarized laser beam reveal self-organized periodic surface nanostructures (ripples) with a subwavelength spacing, which are oriented perpendicular to the electric-field vector of the laser beam. The period of the subwavelength ripples obtained by linearly polarized laser irradiation varies from ∼λ/5 to 2λ/5 (λ: incident laser wavelength) depending on the laser pulse energy. This phenomenon can be explained by assuming that the incident light field interferes with the electric field of electron plasma waves propagating inside the material; this interference periodically modulates the electron plasma density and modifies the surface ablation. In addition, for the first time, we observe screw-shaped nanostructures in the focal spot of circularly polarized beam irradiation. The morphology of these nanostructures appears to reflect the circular polarization of the laser light.     

Ultraviolet laser microstructuring of silicon and the effect of laser pulse duration on the surface morphology

The study of the laser pulse duration effect on the silicon micro-spikes morphology is presented. The microcones were produced by ultraviolet (248 nm) laser irradiation of doped Si wafers in SF₆ environment. The laser pulse duration was adjusted at 450 fs, 5 ps and 15 ns. We have analyzed the statistical nature of the spikes’ morphological characteristics, such as periodicity and apex angle by exploiting image processing techniques, on SEM images of the irradiated samples. The correlation of the quantitative morphological characteristics with the laser parameters (pulse duration, laser fluence and number of pulses) provides new insight on the physical mechanisms, which are involved on the formation of Si microcones.     

Optimization of laser patterning of textured gallium-doped zinc oxide for amorphous silicon photovoltaics

Laser scribing process of in-house textured gallium-doped zinc oxide (GZO) is optimized, aiming to improve the performance of amorphous silicon (a-Si:H) photovoltaic (PV) modules. The reasons for different scribing quality of textured GZO and SnO₂:F scribed at 1064 nm with pulse duration of 40 ns were analyzed. Apart from separation resistance, quality of the scribed lines was evaluated by laser scan microscopy from three-dimensional images. Other types of lasers, such as laser with shorter pulse duration, laser at 355 nm and laser with Gaussian-to-tophat converter, were used to smooth the edges and flatten the bottoms of the scribed lines. The proper laser scribing realizes the advantages of textured GZO films used as front contacts in PV modules. A short-circuit current density of 14.3 mA/cm² and an initial aperture area efficiency of 8.8% were obtained on 16 cm × 16 cm textured GZO coated glass scribed at 355 nm with pulse duration of 40 ns.     

Femtosecond laser-induced damage in reflector

We have investigated the damage for ZrO₂/SiO₂ 800 nm 45° high-reflection mirror with femtosecond pulses. The damage morphologies and the evolution of ablation crater depths with laser fluences are dramatically different from that with pulse longer than a few tens of picoseconds. The ablation in multilayers occurs layer by layer, and not continuously as in the case of bulk single crystalline or amorphous materials. The weak point in damage is the interface between two layers. We also report its single-short damage thresholds for pulse durations ranging from 50 to 900 fs, which departs from the diffusion-dominated scaling. A developed avalanche model, including the production of conduction band electrons (CBE) and laser energy deposition, is applied to study the damage mechanisms. The theoretical results agree well with our measurements.     

The influence of ultra-fast temporal energy regulation on the morphology of Si surfaces through femtosecond double pulse laser irradiation

The effect of ultra-short laser-induced morphological changes upon irradiation of silicon with double pulse sequences is investigated under conditions that lead to mass removal. The temporal delay between 12 double and equal-energy pulses (E _p=0.24 J/cm² each, with pulse duration t _p=430 fs, 800 nm laser wavelength) was varied between 0 and 14 ps and a decrease of the damaged area, crater depth size and periodicity of the induced subwavelength ripples (by 3–4 %) was observed with increasing pulse delay. The proposed underlying mechanism is based on the combination of carrier excitation and energy thermalization and capillary wave solidification and aims to provide an alternative explanation of the control of ripple periodicity by temporal pulse tailoring. This work demonstrates the potential of pulse shaping technology to improve ultra-fast laser-assisted micro/nanoprocessing.     

Physical mechanism of silicon ablation with long nanosecond laser pulses at 1064 nm through time-resolved observation

Nanosecond (ns) laser ablation can provide a competitive solution for silicon micromachining in many applications. However, most of the previous studies focus on ns lasers at visible or ultraviolet (UV) wavelengths. The research is very limited for ns lasers at infrared (e.g., 1064 nm) wavelengths (which often have the advantage of much lower cost per unit average output power), and the research is even less if the ns laser also has a long pulse duration on the order of ∼100 ns. In this paper, time-resolved observation using an ICCD (intensified charge-coupled device) camera has been performed to understand the physical mechanism of silicon ablation by 200-ns and 1064-nm laser pulses. This kind of work has been rarely reported in the literature. The research shows that for the studied conditions, material removal in laser silicon ablation is realized through surface vaporization followed by liquid ejection that occurs at a delay time of around 200-300 ns. The propagation speed is on the order of ∼1000 m/s for laser-induced plasma (ionized vapor) front, while it is on the order of ∼100 m/s or smaller for the front of ejected liquid. It has also been found that the liquid ejection is very unlikely due to phase explosion, and its exact underlying physical mechanism requires further investigations.     

Circular ripple formation on the silicon wafer surface after interaction with linearly polarized femtosecond laser pulses in air and water environments

Ultrashort laser pulse interaction with the surface of silicon wafer in air and water environments is investigated. Ti:sapphire laser with 40 femtosecond laser pulses at 790 nm and 10 Hz repetition rate was used. The ablation threshold of the silicon surface in the air was determined to be about 0.28 J cm^?2. The surface morphology was studied by using scanning electron microscope images. The size of the regular ripples formed in the air environment is a little smaller than the laser wavelength. Due to the nonlinear interaction and self-focusing before the target, the ripples size reduced to nearly a half of the laser wavelength in the water. Moreover, the spikes’ structure formation and their diameter in air and water were studied. Two regimes for spike formation in water are proposed that can explain the anomalous decrease of the spikes’ diameter in higher fluence. During the interaction of single linearly polarized femtosecond laser pulse with the surface, an irregular ripple formation that called circular ripple is observed. This structure which is a result of radiation pressure implies to the surface by the end of the pulse. A new physical model for interpretation of the circular ripples formation based on the ponderomotive force of an ultrashort pulse laser is proposed which can predict the size of the circular ripples. The calculated results are in accordance with our experimental findings.     

The evolution of a microstructure on Si by a femtosecond laser

Femtosecond laser micromaching silicon is investigated by a Ti:sapphire laser (800 nm, 1 kHz) with a pulse duration of 130 fs. It is investigated that the result is affected by fluences at a different number of pulse. In the experiment, we have observed the periodic surface structure such as microholes, and found that the direction of the array holes was parallel to the laser polarization direction. This result has potential application in the fabrication of two-dimensional photonic crystals. We have determined the optimal conditions for producing these surface structures. At the same time, when we change the fluence and the pulse number, the ripples and the columns emerge. If the pulse number remains unchanged, we also investigated the evolution of a crater at a different pulse fluence. The microholes will become regular cracks on the outer irradiation area, and the ripples will gradually disappear. But, the column structures will appear. At a higher fluence, ripples only appear when the number of pulses (N) is less than 200 and a column structure exists at the center region when N is above 600.     

The interaction and the surface crack of single-crystal silicon induced by a millisecond laser

When the silicon material is irradiated by laser, it absorbs the laser energy leading to the temperature rise and the thermal stress. The damage effect includes melting, vaporation and thermal stress damage. Once the thermal stress exceeds the stress strength the crack will initiate. The silicon surface cracks induced by a millisecond laser are investigated. The experimental results show that three types of cracks are generated including cleavage crack, radial crack and circumferential crack. The cleavage crack is located within the laser spot. The radial crack and circumferential crack are located outside the laser spot. A two-dimensional spatial axisymmetric model of silicon irradiated by a 1064 nm millisecond laser is established. To assess what stresses generate and explain the generation mechanism of the different cracks, the thermal stress fields during laser irradiation and the cooling process are obtained using finite element method. The radial stress and hoop stress within the laser spot are tensile stress after the laser irradiation. The temperature in the center is the highest but the thermal stress in the center is not always highest during the laser irradiation. The cleavage cracks are induced by the tensile stress after the laser irradiation. The radial crack and the circumferential crack are generated during the laser irradiation.