How to optimize ultrashort pulse laser interaction with glass surfaces in cutting regimes?

Accepted Manuscript Title: How to optimize ultrashort pulse laser interaction with glass surfaces in cutting regimes? Author: Nadezhda M. Bulgakova Vladimir P. Zhukov Adam Collins Danijela Rostohar Thibault J.Y. Derrien Tom´asˇ Mocek PII: DOI: Reference:

S0169-4332(14)02868-2 http://dx.doi.org/doi:10.1016/j.apsusc.2014.12.142 APSUSC 29377

To appear in:

APSUSC

Received date: Revised date: Accepted date:

31-7-2014 10-12-2014 20-12-2014

Please cite this article as: N.M. Bulgakova, V.P. Zhukov, A. Collins, D. Rostohar, T.J.Y. Derrien, T. Mocek, How to optimize ultrashort pulse laser interaction with glass surfaces in cutting regimes?, Applied Surface Science (2014), http://dx.doi.org/10.1016/j.apsusc.2014.12.142 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

How to optimize ultrashort pulse laser interaction with glass surfaces in cutting regimes? Nadezhda M. Bulgakova,1,2 Vladimir P. Zhukov,3,4 Adam Collins,5 Danijela Rostohar,1 Thibault J.Y. Derrien,1 and Tomáš Mocek1 1

HiLASE Centre, Institute of Physics ASCR, Za Radnicí 828, 25241 Dolní Břežany, Czech 2

Institute of Thermophysics SB RAS, 1 Lavrentyev ave., Novosibirsk, 630090, Russia

Institute of Computational Technologies SB RAS, 6 Lavrentyev Ave., 630090 Novosibirsk,

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Republic

Russia

Novosibirsk State Technical University, 20 Karl Marx ave., 630073, Novosibirsk, Russia NCLA, NUI Galway, Galway, Ireland

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Abstract. The interaction of short and ultrashort pulse laser radiation with glass materials is addressed. Particular attention is paid to regimes which are important in industrial

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applications such as laser cutting, drilling, functionalization of material surfaces, etc. Different factors influencing the ablation efficiency and quality are summarized and their

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importance is illustrated experimentally. The effects of ambient gas ionization in front of the irradiated target are also analyzed. A possibility to enhance laser coupling with transparent

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1. Introduction

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solids by bi-wavelength irradiation is discussed.

Pulsed laser ablation of solids is a complex phenomenon. The laser-matter interaction is influenced by various factors, which include laser wavelength, fluence and pulse duration, type of the ambient gas and its pressure, and surface roughness [1-3]. For multipulse irradiation regimes, the phenomenon becomes even more intricate, being complicated with various accumulation effects (heat, strain, defects [2,4-6]), generation of surface micro- and nanostructures (ripples, cones, columns, ridges, groves [7-14]). Formation of a deep crater whose walls also considerably changes laser energy absorption dynamics, due to the angled walls, and may prevent effective removal of ablation products [15]. The degree of manifestation of the listed factors strongly depends on the irradiation conditions and material properties. The variety of lasers, diversity of materials, and the desired laser application (drilling, cutting, surface structuring, scribing of thin film solar cells, etc. [5,16-19]) means

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laser micromachining is a complex task. Although cutting and drilling of transparent materials has been used in industry for decades, with the introduction of new lasers and the design of novel materials (e.g. new glasses for optical elements and touch-screen displays) new problems on laser processing quality have arisen. In this paper, we review the factors which influence micromachining of materials with short and ultrashort (nano-, pico-, and femtosecond) laser pulses. An attempt is made to

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identify and explain the origin of the problems encountered when cutting and drilling glass materials, including novel ultrathin glasses used in touch-screen technology. Although a

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viewpoint exists that short and ultrashort laser cutting is convenient only for producing delicate details such rounded corners and other curved shapes. However, due to the high cost

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of this technique in comparison with traditional cutting methods (diamond etching and continuous wave laser processing) [17], the possibilities of the technique remain insufficiently

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studied. Our study aims to revisit this issue.

The paper is organized as follows. In Section 2, we discuss the main factors affecting efficiency and quality of laser processing of different materials, focusing on glass materials

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and addressing similarities and dissimilarities with semiconductors and metals. To illustrate the main features and problems of glass processing, the results of comparative experiments on

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cutting and drilling of ultrathin borosilicate glass by 10 ps and 500 fs laser pulses are presented. Issues which require further investigations are highlighted. In Section 3, the effects of ambient gas on laser processing of surfaces are discussed, a topic which is still

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insufficiently studied. Section 4 addresses new possibilities to improve laser processing efficiency using bi-chromatic irradiation. In Conclusion, a summary of the presented analysis

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is given.

2. Main factors influencing pulsed laser processing of materials Laser interaction with transparent materials is a complicated phenomenon, which involves numerous interconnected physical processes occurring simultaneously and/or sequentially. To gain insight into this phenomenon, different influencing factors and irradiation parameters have to be thoroughly analyzed with assessment of their impact on the processing efficiency and quality. This section summarizes our accumulated experience on pulsed laser interaction with glass materials in an attempt to identify bottlenecks and outline possible solutions. The discussion is built based on the literature review and the results of extensive experimental studies of cutting and drilling of ultrathin borosilicate glass (Scott AF32) with the thickness of 50 and 110 µm by pico- and femtosecond laser pulses.

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The experiments were performed with the linearly polarized Gaussian laser beams, provided by Trumpf TruMicro 5050 laser (FWHM pulse duration of 10 ps, laser wavelengths of 1030 nm and 515 nm) and Amplitude Systems’ s-Pulse amplifier (FWHM pulse duration of 500 fs, laser wavelength of 1030 nm). Glass samples were mounted on a high precision XYZ stage (Aerotech). Laser beam was focused on the sample surface with a f-theta lens (focal length of 100 mm) of a scanner system (intelliSCAN 10 from Scanlab)

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as shown schematically in Fig. 1a. The laser power was varied using a laser attenuator which consisted of a motorized half-wave plate and a linear polarizer. Cutting of samples

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in open air was performed by scanning laser pulses at different fluences over the sample. To characterize the laser treated ultrathin glass samples, a special cross-sectioning

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technique was used, described in detail in [20]. It should be mentioned that, due to cleaving of samples, the method can introduce some additional cracks. At the same time due to

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brittle behavior of glasses, cracks cannot be avoided even during very slow polishing, a step in other sample preparation techniques. The results of glass cutting and drilling are presented in Figs. 1-5 and will be commented below in Section 2.4 devoted to multi-pulse

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irradiation regimes. To consider different factors that impact on material processing, we

2.1. Laser wavelength

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start from some basics of laser excitation of materials.

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Each class of materials has a wavelength- and material-specific optical response, which make them extremely sensitive to the choice of laser irradiation parameters. For band-

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gap materials, laser wavelength λ is a key parameter which determines the degree of nonlinearity N of laser energy absorption, N =

where Eg is the band gap and ħω is the

photon energy. The smaller the band gap and the larger the photon energy is, the more effective is excitation of electrons from the valence to the conduction band [21] that can result in decreasing of the ablation threshold (though accounting for transient change of optical response of materials). As soon as seed electrons are produced, they start to effectively absorb laser radiation via inverse bremsstrahlung process and exponentially create secondary electrons in collisions with material matrix [22]. This process leads to formation of a highly absorbing and reflecting skin layer (surface layer metallization, also called a mirror effect [23]) This highly reflecting layer reduces the efficiency of laser energy absorption and, as a consequence, the ablation efficiency.

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For metals, the effect of laser wavelength is determined by intrinsic optical properties n and k (refractive index and extinction coefficient respectively), governing respectively light reflection and absorption coefficients R = ((n – 1)2 + k2)/((n + 1)2 + k2) and α = 4πk/λ. However, for ultrashort laser pulses when free electrons are swiftly heated to high temperatures, the complex dielectric function ε of metals may considerably change [24,25], leading to strong variation of reflection and absorption of laser light (

= n + ik). A number

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of models for description of dielectric function variation vs. electron temperature have been proposed for the heating levels below, above, and close to the Fermi temperature, still giving

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rather contradicting results. Up to now, this aspect of ultrashort laser excitation of metals is debated and calls for further studies.

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Hence, for ultrashort laser pulses, there is no a straightforward method to estimate light absorption not only for dielectric materials where the free electron density is swiftly

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changed upon irradiation, thus bringing the laser-affected zone to a metallic state, but also for metal targets. It is not surprising that the control of the micromachining process is very complicated. Although the Drude model is widely used to describe transient optical response

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of band-gap materials, the model involves several parameters, such as effective mass of free electrons and their collision frequencies [26,27], with the latter depending on the transient

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electron density and energy. Additionally, reliable quantitative description of cumulative effects has started to be developed [28,29]. In this regard, rigorous models of laser energy 2.2. Laser fluence

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absorption are of high demand.

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At first glance, it can be expected that the higher the laser fluence is, the higher is the ablation rate per pulse and larger is the cutting/drilling efficiency. However, there are no direct relations between laser fluence applied and efficiency of laser processing. This is conditioned by several factors. As discussed in the previous paragraphs, the higher laser fluence may lead to formation of highly reflecting skin layers in band-gap materials and swift change of optical properties of metals. As will be shown below, increased fluence may lead to stress formation and consequent cracking of the processed material, thick layer of melt with its splashing behind the machining area, ejection and redeposition of debris. Besides, air plasma formation above the target and the plasma of the ablation plume can considerably intervene the ablation process for different pulse durations up to very short laser pulses [13,30-36]. Interestingly, the air breakdown above the irradiated surface can also partially screen the material surface from laser radiation, thus resulting in smaller ablation efficiency,

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and, on the contrary, facilitate the ablation process via radiative and conductive heat transfer to the target, depending on the irradiation conditions [33,34,37,38]. The importance of the laser plasma effects is still not completely realized despite of the fact that etching of different surfaces with discharge plasma is a widely studied phenomenon due to its extensive usage in plasma-chemical technologies [39]. More details on ambient plasma effects will be given in the Section 3.

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2.3. Pulse duration

Pulse duration τL is one of the key parameters of ultrashort laser processing of

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materials (see [40] and references therein). It is known that the laser damage threshold for the most materials is almost independent of the pulse duration in subpicosecond irradiation

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regimes up to few-cycle laser pulses [10,41]. However, for wide-band-gap dielectrics which are the focus of the present paper, the shorter pulses result in cleaner craters with reduced

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mechanical damage/cracking around the crater area, culminating in perfect craters obtained at 5 fs laser pulses [41]. This effect can be directly connected with the observation, in the case of fused silica irradiation, that at τL ≥ 10 ps both damage and white light emission thresholds

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from the laser affected zone are quite similar and follow the same dependence of ~τL1/2 [42,43]. It can be stated that under such conditions the threshold is associated with laser-

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induced breakdown of the material that leads to generation of hot electron plasma. However, at short laser pulses generation of white light requires much higher laser pulse energy as

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compared to damage threshold, yielding in dependence of ~τL-1. This effect highlights contribution of inverse bremsstrahlung with associated avalanche ionization to the damage

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threshold, depending on pulse duration. Once free electrons gain enough energy for production of secondary electrons, through considerable number of collisions with other electrons as well with lattice ions and atoms, inverse bremsstrahlung can lead to development of avalanche ionization [40,44]. The shorter the laser pulse is, the smaller number of collisions a free electron experiences during the laser pulse, thus remaining relatively cold and being unable to collisionally ionize lattice ions. On the other hand, the pulse shortening at the same laser fluence results in higher peak intensity and, hence, in much higher probability of photoionization. Comparing the data of [42] and [43], one can state that at τL < 1 ps the damage threshold is determined mostly by glass bond scissoring in photoionozation events while produced electrons stay relatively cold and cannot create a luminous cloud. However, at higher beam energies above the damage threshold, free electrons can absorb more photons

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from a denser photon flux and free-electron plasma becomes luminous [43]. Particularly, this may attributed to enhanced multiphoton absorption by the conduction electrons at increased laser intensities [45,46], the effect whose importance still requires studies. Laser pulse shortening down to few-cycle laser pulses culminates in producing of extremely smooth and clean craters of glass surface [41]. The absence of cracks around the produced crater indicates reduced stresses caused by laser energy coupling that could be highly favorable for advanced

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technologies requiring precise and efficient cutting techniques of glass materials. Relatively cheap methodology of laser pulse compression down to few-cycle pulses [47] can become a

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cost-effective solution for the use of femtosecond lasers (expecting a future price reduction) in the industrial sector of ultrafine material processing. At present it becomes clear that, for

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technologies where ultraprecise accuracy of material processing is required, picosecond laser pulses are not appropriate though they give much higher speed of cutting and drilling as

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compared to femtosecond laser systems [48].

2.4. Multipulse irradiation aspects: debris, strain, and phase separation problem Industrial use of pulsed lasers for material processing implies multiple irradiation of

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the same area on the surface or the same volume in the bulk at high repetition rates. The most explored effects of multipulse laser ablation are decreasing the ablation threshold due

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accumulation of defects [6,49-51], diminishing surface reflectivity as a result of the formation of surface micro- and nanostructures (including formation of black silicon and colorized metal surfaces) [7-12,18,52] and multiple reflection inside laser cuts [53], and accumulation of

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debris [32,54,55]. The debris accumulated on the crater walls and charged nanoparticles suspended in air inside the crater/trench represent one of the major problems of obtaining

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deep, high-aspect-ratio structures. They promote air breakdown by the next laser pulses and lead to strong light scattering and screening of the crater bottom, thus leading to pulse-bypulse and pass-by-pass decrease of the ablation rate [15,32,55]. At a certain depth, the structure deepening can completely stop as it is observed for different materials [53,56] while the crater bottom part may become branched and broadened [56]. Additionally, abundant deposition deposition of debris on the material surface beyond irradiation area [57] requires post-irradiation surface treatment. Figures 1-5 illustrate influences of different factors on the quality and efficiency of laser cutting and drilling of borosilicate glass in open air. Figures 1,b-d show SEM images of glass cuts (110 µm thick glass samples) produced by 10-ps 1030-nm laser pulses at 200 kHz repetition rate. In these experiments, the laser fluence was varied while keeping scanning velocity constant (0.38 m/s). Irradiation spot diameter was 35 μm (1/e2). With the fluence of

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10.8 J/cm2, 50 laser passes were not enough to cut glass through and the sample was cleaved after processing (Fig. 1b). At such fluence, the quality of cut is relatively high though the cutting process is slow. With increasing laser fluence (Figs. 1,c-d), a smaller number of beam passes is required to cut the sample through without necessity of cleaving (30 passes at 16.1 J/cm2 and only 10 passes at 19.3 J/cm2). However, the cut becomes coarse, indicating extensive melting of glass with the signs of unstable flow of molten material frozen in final

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cut structures. Hence, minimizing the laser fluence and maximizing the number of passes allows to improve the quality of cutting though, additionally to slower cutting, the energy

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balance estimations show a higher consumption of laser energy (approximately 3 times larger to produce cut in Fig. 1b compared to Fig. 1d).

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At femtosecond pulse duration, the cutting mechanism differs from that for picosecond laser pulses as can be seen in Fig. 2,a-c. The demonstrated tranches on 110-µm thick glass

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were produced by 500 fs laser pulses of 1030 nm wavelength with constant scanning velocity of 0.2 m/s (repetition rate of 10 kHz). All trenches were obtained with 40 beam passes with applying a cross-sectioning technique for sample characterization [20]. Two trenches were

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formed on opposite surfaces of the sample with perpendicular orientation as shown schematically in Fig. 2d. Cleaving along one trench allowed to characterize its sidewall (seen

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at the bottom of images) and the profile of one trench (at the top of images). At relatively low laser fluences (12.1 J/cm2 and 12.9 J/cm2) the trenches are almost identical and of good quality. Increasing the fluence to 15 J/cm2 does not lead to crater deepening but results in

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severe cracking of glass (Fig. 2c). At the same time, signs of melting are not observed. Such difference compared to picosecond cutting regimes (Fig. 1) can be explained by low

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contribution of bremsstrahlung absorption and subsequent avalanche ionization which are dominating mechanisms at picosecond laser excitation [22,40,42-44] and result in higher heating of the material. Another important parameter is the repetition rate, which is smaller for the used femtosecond laser system and, hence, implies heat accumulation. A simple analysis of the heat accumulation effect can be performed on the basis of the heat flow equation. The characteristic time of heat propagation by the distance r can be estimated as theat ~r2(ρc/λ) where ρ, c, and λ are the material density, heat capacity, and thermal conductivity respectively. For glass materials, it can be estimated that a heat wave typically propagates by ~1 μm during 1 μs. Complete dissipation of heat from the focal volume of micrometer size takes times up to 10 – 100 microseconds depending on the heating level and the size of the heat-affected zone. Such estimate enables controlling heat accumulation via pulse repetition rate and scanning speed (the latter for high repetition rate lasers). Thus, our picosecond laser

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system provides heat accumulation upon cutting at relatively low scanning velocity while femtosecond laser irradiation ensures cooling of the laser-affected region between pulses. Returning to Fig. 2c, it can be stated that, with increasing laser fluence, higher temperature (and hence pressure) gradients are formed, resulting in high localized stress released in material cracking. Surprising is that, at higher fluence, trench depth is not increasing. This bottleneck in the cutting process calls for further studies. The reasons can be

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in a more developed mirror-like effect [23] and/or change in the laser-affected material structure (e.g., accumulation of inhomogeneities on the way of the laser beam passing through

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the sample which scatter laser light of subsequent pulses).

An essential problem of laser processing of different materials is accumulation of

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debris as illustrated in Fig. 3. The holes shown in Fig.3,a-b were drilled in 50-μm thick glass by ps laser (515 nm wavelength, 50 pulses at 16 J/cm2 per pulse, repetition rate of 50 kHz)

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using trepanning method with irradiation spot overlap of ~90%. The trenches on 110-μm thick glass (Fig. 3,c-d) were produced by 500-fs laser pulses at 10.6 J/cm2 (repetition rate of 10 kHz, 30 laser passes at scanning speed of 0.2 m/s). Images (b), (c) and (d) were obtained by

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the FEI Phenom Pro Scanning Electron Microscope while the image (a) was made using the Hitachi S-4700 Scanning Electron Microscope. Images (a), (c) and (d) were done directly

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after the laser processing. Note that at the image (a) the focus of the SEM is inside the hole to show the number and size of debris. The image (b) was obtained after cleaning the sample in an ultrasound bath for 30 minutes. The remained debris on the surface around the hole were

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only possible to be removed by additional cleaning in acetone. From Fig. 3 it is evident that debris accumulation represents essentially the same

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problem for both pico- and femtosecond laser processing. The processing regimes were optimized to eliminate material cracking while debris can be partially removed only by additional cleaning procedure. Ultrasound bath cleaned interior of the hole (Fig. 3a) while the polluted surface area adjacent to the hole entrance is most probably the result of laser-induced air/ablation plasma etching (see Section 3) and hence can only be cleaned by chemical methods (Fig. 3b). The trenches obtained with 500-fs lasers are similarly subjected to external plasma etching (Fig. 3c) and debris accumulation inside the trench (Fig. 3, c and d). Heavy pollution of the interior of the laser-produced holes and trenches is related to the main mechanism of ultrashort laser ablation which is phase explosion [58,59]. As a result of swift heating of material to a highly superheated state, catastrophic growth of vapor bubbles causes the rupture of matter to a mixture of vapor and droplets that is one of the routes of effective laser-based production of nanoparticles [60]. However, for industrial cutting and drilling,

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nanoparticle formation is a highly undesired effect. The problem of eliminating or minimizing nanoparticle formation for the regimes of pico- and femtosecond laser processing of glass materials calls further extensive studies. We can speculate that, with reduced pulse duration down to few-cycle pulses, material ablation occurs mainly via atomic/ionic species ejection, thus eliminating the problem of debris [41]. Such regimes are still poorly explored while the few-cycle laser technique remains of limited use due to complexity of management of such

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pulses which can be technically resolved in the future. It should be mentioned that using of few-cycle pulses for material processing can meet other difficulties related to propagation of

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the beam, its distortion, air ionization, etc. On the other hand, at pulse durations shorter or comparable with free electron collision time, the mirror-like effect [23] will not develop. This

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enable better energy coupling to material, the topic which deserves further investigations. Two other problems of material processing quality, which have partially been touched

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above, are accumulation of heat and material strain. The first one results in material melting with expelling the melt from the processed structure and leading to uneven walls of the final holes and trenches (Fig. 1) while the strain accumulation unavoidably leads to material failure

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in the form of cracks (Fig. 2c) or porosity (see discussion below). Figure 4 demonstrates how sensitive glass cutting quality is to changes in irradiation conditions. Increasing the scanning

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velocity from 25 mm/s to 50 mm/s, which ensures 3% decrease of irradiation spot overlap (from 98% to 95%), results in much better cutting quality due to smaller material heating and varying irradiation load.

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reduction in melt thickness. Thus, the effect of heat accumulation can be controlled by Preventing the features of mechanical damage of the material caused by laser-induced

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strain accumulation is a more difficult task. Currently, strain accumulation is insufficiently understood and mechanical material damage in the form of cracks is poorly controlled. Purely mechanical mechanism of laser ablation of thin films is favorable for solar cell scribing [19,61] while is undesired for cutting and drilling technologies. Typical features of strain accumulation effects were demonstrated in Fig. 3c in the form of cracks and are shown in Fig. 5 in the form of both crack and porosity formation. Trench profiles shown in Fig. 5 were produced by picosecond (Fig. 5a) and femtosecond (Fig. 5b) laser cutting of glass (see Figure caption for details). It is seen that in both cases laser cutting is accompanied by crack formation with some features of porosity (grey areas at the trench edges where porous structure can be recognized). Note that cutting with picosecond laser resulting in formation of a thicker porous layer on the trench walls (Fig. 5a).

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The laser-induced porosity is one of the principal defects of high-power CW laser welding [62]. It is largely attributed to hot plasma formation at the limit of laser-supported detonation when local pressure on the molten metal pool may exceed 1 MPa level. At short and ultrashort laser pulses, the observed porosity (Fig. 5) is most probably of a common nature with meteoritic impact induced porosity [63] or ballistic impact damage of armor targets [64] resulted from brecciation and rapid uneven mass transport at high pressure loads.

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To provide crack-free laser processing, laser irradiation should be applied gently, avoiding conditions of material failure that would highly limit processing speed. A compromise is

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necessary between the desired processing quality and material removal rate. Again, one may expect that using extremely short powerful laser pulses down to few-cycle regime can be

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solution for a crack-free laser processing technology. At such extreme regimes, cold material ablation at reduced recoil pressure can be foreseen without accumulation of heat and 2.5. Notes on specific properties of materials

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associated strain.

It must be underlined that in multipulse irradiation regime each subsequent pulse

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arrives to a slightly modified matter as compared to the previous one. Thus, a gradual accumulation of the various intrinsic defect states in transparent crystals and glasses with

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larger excitation cross sections makes excitation easier for the subsequent pulses. On the other hand, the complex dielectric function within the irradiated region is also varying due to defect generation [6], material density change (compaction of rarefaction), and stress accumulation

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[4], thus affecting laser pulse coupling through the modified volume. As mentioned above, increasing surface roughness may lead to both higher absorptivity and larger scattering.

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Recently the effect of spatial separation of glass species was demonstrated [65] which

also should play an important role in laser-material coupling in multi-pulse irradiation regimes because of varying optical properties. In multicomponent materials, change in local composition within the irradiation zone can occur both due to diffusion of metallic species [65] and different volatilities of the constituents [66]. In such cases, the optical properties of irradiated material can be described within the concept of the Lorentz–Lorenz effective medium [66]. One more effect must be mentioned which is known for multicomponent glass materials but whose importance is not yet recognized for the regimes of laser micromachining. When laser light heats a multicomponent material, some atomic species may precipitate in the form of clusters (e.g., Na clusters in soda-lime glass [67]). Laser-induced formation of nanoparticles in the glass matrix may dramatically change optical response of the

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material towards considerable scattering of the laser light to large angles. This effect is poorly explored as well as the effect of abnormal “water-like” behavior of a number of materials among which silicon draws an utmost attention. Cyclic melting and solidification inside the processed crater/trench with expansion upon solidification adds most probably a complexity for the silicon drilling problem [68]. As seen from above analysis, in spite of several decades of exploiting lasers in

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material processing technologies and tremendous efforts of scientific community, many complicated problems still exist in the field which require further sophisticated investigations.

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One of such problems which is still poorly studied is the role of air ionization above the

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surface of laser-processed sample.

3. Effects of the ambient air on ultrashort material processing

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It is widely accepted that, due to rapid energy delivery to materials by ultrashort laser pulses in the regimes of surface irradiation, the laser – plume interaction is completely avoided and heat-affected zone is minimized. However, in the majority of technological

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applications material processing by ultrashort laser pulses is carried out in an ambient environment (air, inert or reactive gases, liquid media). Under such conditions, laser-induced we discuss its consequences.

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ionization of the gas or liquid surrounding an irradiated sample is almost unavoidable. Here It should be underlined that, even if ambient gas breakdown is not achieved,

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nevertheless substantial losses can be expected due to media ionization. It is important to underline here that, at subpicosecond irradiation regimes, collisional multiplication of

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electrons in gas media (avalanche) cannot be developed [33]. Typical cross sections for electron scattering by molecules of atmospheric gases (О2, N2, Ar) are of the order of 10−15 cm2. The electron velocities with the energies of several electron volts are of the order of ve ~ 106 m/s. Correspondingly, characteristic time between collisions of electrons with gas molecules is several picoseconds, much larger than the laser pulse duration. Furthermore, for gaining the energy exceeding ionization potential of ambient molecules via inverse bremsstrahlung process, an electron needs many collisions (up to ten collisions for 1.55 eV photons of fundamental harmonics of a Ti-sapphire laser). Hence, at near-infrared laser pulses of duration up to ~10-20 ps, only photoionization, more specifically, multiphoton or tunneling processes, depending on peak beam intensity reached in the beam propagation area, may cause ambient gas ionization. The time for avalanche development may be decreasing with increasing laser wavelength, accounting however for reducing bremsstrahlung absorption

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cross sections with increasing photon energy. The process is even more complicated accounting for dependence of absorption cross sections on the electron energy [69]. However, photoionization losses along the beam way toward the target can be substantial even at relatively low laser fluences and a high ionization degree in the ambient gas can be reached at atmospheric pressure. Thus, for 40 cm focusing lens as in [37], a 65-fs laser pulse with 3 J/cm2 fluence in the beam focus can lead to > 10% ionization of argon in a

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half-millimeter long region in front of a target with 100% ionization in the immediate proximity of the metal surface where incoming and reflected light interfere [38]. In air under

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the same conditions this effect can be even more pronounced due to the presence of molecular species with lower ionization potential and multistep ionization processes through the

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Rydberg states [70,71]. Hence, it is evident that on the way to the surface through the ambient gas, the laser beam can be considerably depleted due to photoionization of the gas while a

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photo-ionized gas region can be heated to a high temperature (note that electron recombination in air is a relatively fast process with a characteristic time of several nanoseconds under atmospheric conditions [72]). It must be emphasized that the effect of

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beam self-interference can be also important for transparent dielectric materials in view of swift formation of a “mirror-like” surface layer [23].

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The local laser-induced heating of the gas must result in a number of important consequences. It unavoidably leads to expansion of the heated gas with generation of compression (shock) waves. Such waves when propagating and contacting the surface of the

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target may lead to modification of the surface [57,73,74]. In particular, surface modification around the crater observed in Fig. 3b can be attributed to such shock-induced modification.

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As shown in [33,37,38], the discussed local heating of the ambient gas can even manifest itself as a “fictive” target absorptivity that is an enhanced target heating above the natural absorptivity level resulted from conductive and radiative heat exchange between the hot gas and the target [33,75,76].

Other consequences may be important for multi-pulse irradiation regime. Subsequent

laser pulses applied to the same irradiation spot (or having overlap with the previous spots in processing with scanning laser beams) couples with already modified surface as widely discussed in the literature. Such pulses also propagate through a “modified” ambient atmosphere above the target. The laser-induced dynamics of the ambient gas is a slowly relaxing process, due to gas inertia, with long-lasting vortex motions and hot/warm regions of reduced gas density up to millisecond time scale. Here we present the results of numerical

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simulations of laser-induced dynamics of ambient gas in order to get insight into levels of laser-induced gas expansion in the front of the processed surfaces and relaxation times. The model consists of two steps which were first developed separately and here we show the results of their combined application. At the first step, a simplified geometrical modeling is performed that describes the focused laser beam propagation towards the target, its partial absorption and reflection from the target, and gas ionization due to multiphoton

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mechanism. The details of this part of the model are given in [38]. By applying the twotemperature model, we also follow how the heat in the target is evolving. Material ablation is

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not taken into account at this stage in view of high complexity of our “simplified” approach that is a subject of further development. However, this model is very useful to demonstrate

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that, even without ablation, photoionization of the ambient gas solely can strongly influence laser micro-processing of materials. After the laser beam leaves the computational domain,

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the laser energy absorbed by the gas is partially radiated and converted locally into thermal energy of the gas through the processes of photo- and three-particle recombination that can be calculated by equations (22) and (23) of [38].

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Once the temperature distribution in the ambient gas is obtained, the second, hydrodynamic part of the model can be applied whose details are described in [77]. It consists

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of the Navier-Stokes equations for the ambient gas which naturally transfer into the heat flow equation inside the bulk at zero matter velocities. The temperature and the heat flow λ∂T/∂z (λ is the thermal conduction coefficient of each media) are continuous across the solid – gas

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interface that represents the boundary conditions on the solid surface together with the no-slip condition for the gas flow. The ambient gas computational domain is large enough to assume spot.

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that the gas is not disturbed at the outer gas boundaries which are remote from the irradiation The results of simulations for a platinum target irradiated by 65-fs, 3 J/cm2 laser pulse

in argon under normal atmosphere parameters for the experimental conditions of [37] are presented in Fig. 6. We underline once more that this is a model problem which can also be applied for dielectric materials accounting formation of a metalized surface layer. The argon density distributions (Fig. 6a and 6b) for two time moments, 25 ns and 2 μs, show formation and propagation of the two shock waves. The density is normalized by the initial argon density. One shock originates from the region near the irradiation spot (spot radius is 50 μm) where, according to simulations, 100% single ionization of argon atoms is reached. This strong shock wave with gas compression up to 4 times propagates half-spherically, leaving behind a gas region with reduced density. An interesting feature can be seen in the immediate

Page 13 of 35

contact of the shock wave with the target surface, ambient gas compression up to 8 times (Fig. 6a). This is conditioned by the heat exchange between the hot shock-compressed gas and the cold non-irradiated target area. Namely in this annular region on the surface the main conductive heat exchange takes place between the gas and the target that lead to enhanced “abnormal” absorptivity [37,75,76]. It should be mentioned that the radiative heating of the target from the gas plasma can take place [34] that calls for further studies at ultrashort

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irradiation regimes.

The second shock wave whose features can be recognized in images of work [74] has

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a quasi-cylindrical form and propagates from the laser beam axis. This wave is much weaker and propagates with smaller velocity. However, in view of shock wave geometries, the half-

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spherical shock wave dissipates faster in amplitude (compare Figs. 6a and 6b). The gas inside the shock wave structure remains at reduced density quite a long time up to millisecond time

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scale till the convective vortical flow initiated by the laser beam cools down the laser-affected region. Even at time scale of several dozens of millisecond the temperature of the gas is still high, even exceeding 1000 K (Fig. 6b). The convective gas flow is evident from the spatial

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distribution of the gas velocity component perpendicular to the target (Fig. 6d). Negative velocity corresponds to the direction away from the sample surface. Alternating velocity sign

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at the immediate proximity to the target indicates a vortical gas flow which may facilitate redeposition of ablation products on and around the irradiation spot. The demonstrated dynamics of the laser-induced motion of the ambient gas is still

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poorly studied but can have important consequences on the overall micromachining process. Reducing the quality of the processed surface can occur via facilitated debris redeposition and

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etching the surface by strongly compressed and ionized gas in the shock wave. It can also considerably influence laser light coupling to the material [34] that depends on competitive processes of beam energy depleting before it reaches the sample surface and heat supply to the target from the laser-heated gas. The balance between these processes and their impact on laser ablation rate and quality is to be studied. As most laser technologies assume material processing in open air, detailed knowledge on laser-induced flow dynamics is necessary for optimizing technological processes and promoting powerful lasers with short and ultrashort pulse durations for cost-effective use in industries.

4. What can be achieved in bi-wavelength irradiation regimes? Traditionally, monochromatic laser beams are used for material processing, modifications of structural, optical, mechanical, conductive, and thermal properties of

Page 14 of 35

different materials (both on the surface and inside the bulk), and for designing new nanostructured material systems. Although a considerable progress has been achieved in studies of chemical reactions induced by bichromatic laser waves [78], only little is known about material behavior upon irradiation by two- or three-color laser beams. The possibilities of using laser wave mixing for the purposes of material processing and designing have not been explored. Presently, attempts to theoretically study material behavior under

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multiwavelength laser excitation are lacking. So far there are only a small number of separate studies which indicate dramatic effects of bichromatic laser irradiation on surface

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modification. Among them, the following most important studies can be listed. Jia et al. [79] have shown that a small fraction of the second harmonics (SH) mixing to the fundamental one

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is a determining factor that can control the direction and improve the quality of laser-induced periodic structures on semiconductor surfaces (LIPSS). Enhanced efficiency of metal

an

nanoparticle production was demonstrated with two-color two-laser irradiation as compared to monochromatic light [80]. High-quality high-speed selected removal of insulating layers was achieved with a combination of 260 nm and 780 nm laser pulses [81]. 70% enhancement

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of the laser ablation yield of silicon by adding a small amount of the SH to fundamental ablating harmonics was shown by Zoppel et al. [82]. A tremendous role of a laser prepulse

ed

wavelength was found for improvement of LIBS signal [83]. Sugioka et al. [84] have demonstrated improvement of microfabrication of optical structures in hard materials with multiwavelength processing. Enhanced efficiency of drilling with dual laser frequencies has

pt

been found for both metals (steel [85]) and soft materials (PMMA, [86]). Asada et al. [87] have shown that the control over femtosecond laser microprocessing can considerably be

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improved by adding a pulse CO2 laser radiation which itself can produce material heating only after several seconds of application. Dual-wavelength can improve quality of thin films produced by pulsed laser deposition techniques [88]. A number of other studies can be cited including [89] which is probably a first attempt of bichromatic laser-matter interaction. Although the cited works have demonstrated a big potential of wave mixing for laser

processing of materials, they are rather empirical and are based on the trial-and-error procedure. On the other hand, several studies of laser damage threshold with dual laser harmonics demonstrate that energetically the laser energy absorption can be inefficient as compared to single wavelength irradiation that however cannot be evident at the first glance. Figure 7a shows the results of work [90] on laser damage of KH2PO4 crystal by simultaneous action of 1st and 3rd harmonics of Nd:YAG laser (pulse durations are 6.5 and 5.5 ns respectively, see [90] for more details). In the figure the results for the damage density of

Page 15 of 35

0.8 dam./mm2 from this work have been replotted to another scale for better demonstrating energetic inefficiency of dual wavelength application near the ablation threshold. Red dotted line connects the thresholds for fundamental and third harmonics, dividing the regimes to two regions, I and II. The experimental points in the region I below this line would mean a more efficient laser-surface coupling. If the experimental points fall to the region II above the red dotted line, this means that more energy expenses are needed for reaching damage as

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compared to energetically favorable regimes I. Green dashed line leads eye along the experimental data. At relatively low laser fluences, the damage threshold follows the red

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dotted line while, at higher fluences, the experimental points shift to the region II, culminating at point 1 where laser energy expenses needed to damage the glass surface are noticeably

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higher than the single damage threshold at fundamental harmonics.

The same situation was recently demonstrated for femtosecond laser pulses in

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experiments on ultraviolet multiphoton initiation of fused silica damage by the 1st and 3rd harmonics of Ti:sapphire laser (800 nm) [91]. It was found that the threshold fluence for the damage with the 3rd harmonics was considerably reduced by adding the 1st harmonics.

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However, similarly to ns pulses [90], the fused silica damage requires more laser energy from two combined wavelengths as compared to single wavelength irradiation regime as presented

ed

in Fig. 7b (the data for zero time delay between pulses have been redrawn from [91] to better see the energy inefficiency of such regimes, similar to Fig. 7a). The situation can be different at laser fluences well exceeding the ablation threshold as

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can be understood from literature review presented above. We made an attempt to estimate the laser energy absorption and heating of fused silica by dual laser pulses for the regimes of

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work [91]. For simulations, the model whose details can be found in [92] was used with adding two-photon ionization to the rate and energy balance equations. Two-photon ionization rate for 3rd harmonics was taken from [93] as 1.5×10-11 cm/W. To account for optical response of fused silica at 3rd laser harmonics, dielectric function was also calculated for UV laser pulse of the Gaussian temporal shape. A first series of simulations have been carried for the total laser energy of 4 J/cm2, varying the contributions from NIR and UV pulses. It was found that, at FNIR/FUV = 1.5:2.5, the surface layer of the sample was heated to more than 4500 K while the maximum temperature values at only NIR and UV pulses were respectively ~3200 K and ~3800 K. This result and findings reported in [79-89] lead to the conclusion that bichromatic laser light can be an effective tool for achieving highly efficient material processing with better control of energy deposition to materials. Much more studies are required to verify and experimentally support this conclusion and to find the optimum

Page 16 of 35

regimes for material processing with bichromatic laser beams (as combinations of wavelengths, fluence ratio, and time delays between pulses of different wavelength) which are to be material dependent. However, existing evidences already indicate that combining the laser pulses with different wavelengths can become a core technology for laser processing industries.

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5. Conclusion

In this paper, the current limits encountered for processing of glass materials in the

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context of cutting and drilling using short and ultrashort laser pulses have been reviewed. As previously understood by the scientific community, femtosecond lasers allow good and clean

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processing but are too expensive and slow for industry, whereas picosecond lasers allow a fast but lower quality material processing. The current knowledge regarding effects of laser

an

parameters such as laser wavelength, pulse duration and laser energy has been reviewed. Several insufficiently considered but important problems for large scale processing such as debris accumulation, necessity to use toxic substances (to reach industrial standards) after

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laser processing have been stressed, and the high sensitivity to relatively small changes of laser parameter such as fluence, scanning velocity, and laser repetition rate have been

ed

experimentally demonstrated. Additionally, new theoretical results demonstrating the importance of laser-induced dynamics of the ambient atmosphere as a result of ionization of the gas in front of the target were presented and analyzed.

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As a consequence, it becomes clear that optimizing laser parameters for glass processing with ultrashort-pulse lasers aiming to industrial applications cannot be obtained

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without further theoretical investigations and more subtle ways to control energy deposition to glass materials. Very recently, quantitative studies on the description of cumulative effects occurring in the time delay between successive laser pulses started to emerge. More efforts will hopefully allow reliable predictions on the total amount of laser energy absorbed by glass samples under different irradiation conditions that will lead to a grater control over laser cutting processes on large surface areas. Comprehensive studies must consider the complex nature of material reaction to laser action (melting, cracking, material softening or hardening, ablation, surface structuring, etc.). Expecting reduction in laser prices in non-far future, it can be predicted that novel laser systems such as few-cycle pulse lasers and application of new regimes for material irradiation such as combination two or even three beams with different wavelength simultaneously and/or with time delay between pulses will lead to breakthroughs in laser

Page 17 of 35

processing technologies. New advanced laser systems such as developed in HiLASE [94] draw the horizon towards large scale material processing with centimeter-sized irradiation spots and combined laser beams with different wavelengths. Preliminary theoretical investigations have revealed that combination of several wavelengths allow to control the size of the laser-affected region in transparent solids, thus opening the route for new and more

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subtle processing of dielectric and semiconductor materials for industry.

Acknowledgement

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This research was supported by the European Regional Development Fund, the European Social Fund and the state budget of the Czech Republic (project HiLASE: CZ.1.05/2.1.00/01.0027,

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project DPSSLasers: CZ.1.07/2.3.00/20.0143). Partial support of the Russian Foundation for Basic Research is also acknowledged (RFBR project No. 12-01-00510). Experimental work presented in

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this paper was conducted under the framework of the Irish Government Programme for Research in Third Level Institutions Cycle 5, National Development Plan 2007–2013 with the assistance of the European Regional Development Fund. Laser-Connect is a Marie Curie Industry-Academia

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Partnership and Pathways (IAPP) Project funded under EU FP7.

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79. T.Q. Jia, H.X. Chen, M. Huang, F.L. Zhao, J.R. Qiu, R.X. Li, Z.Z. Xu, X.K. Xe, J. Zhang, H. Kuroda, Formation of nanogratings on the surface of a ZnSe crystal irradiated by femtosecond laser pulses, Phys. Rev. B 72 (2005) 125429.

80. M. Sakamoto, T. Tachikawa, M. Fujitsuka, T. Majama, Two-color two-laser fabrication of gold nanoparticles in a PVA film, Chem. Phys. Lett. 420 (2006) 90-94. 81. M. Kumata, S. Tsujikawa, T. Simiyoshi, H. Sekita, Dual wavelength femtosecond material processing, CLEO/QELS 2007 1-5 (2007) 1207-1208. 82. Zoppel, R. Merz, J. Zehetner, G.A. Reider, Enhancement of laser ablation yield by two color excitation, Appl. Phys. A 81 (2005) 847-850.

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83. P.K. Diwakar, S.S. Harilal, J.R. Freeman, A. Hassanein, Role of laser pre-pulse wavelength and inter-pulse delay on signal enhancement in collinear double-pulse laser-induced breakdown spectroscopy, Spectrochem. Acta B 87 (2013) 65-73. 84. K. Sugioka, T. Akane, K. Obata, K. Toyoda, K. Midorikawa, Multiwavelength excitation processing using F2 and KrF excimer lasers for precision microfabrication of hard materials, Appl. Surf. Sci. 197-198 (2002) 814.

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85. W. Zhao, W. Wang, X, Mei, G. Jiang, B. Liu, Investigations of morphological features of picosecond dual-wavelength laser ablation of stainless steel, Opt. Laser Tech. 58

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86. B. Tan, K. Venkatkrishnan, N.R. Sivakumar, Laser drilling of thick material using

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femtosecond pulse with a focus of dual-frequency beam, Opt. Laser Tech. 35 (2003) 199-202.

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87. T. Asada, T. Tamaki, M. Nakazumi, E. Ohmura, K. Itoh, Laser-induced structural modification inside glass using a femtosecond laser and a CO2 laser, J. Laser Micro/Nanoeng. 9(2) (2014) 174-179.

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88. S. Witanachchi, K. Ahmed, P. Sakthivel, P. Mukherjee, Dual-laser ablation for particulate-free film growth, App. Phys. Lett. 66 (1995) 1469-1471. 143 (1988) 599-601.

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89. D. Pan, J. Chen, Etching enhancement of SiO2 in a two-laser field, Chem. Phys. Lett. 90. S. Reyné, G. Duchateau, J.Y. Natoli, L. Lamaignère, Competition between ultraviolet

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and infrared nanosecond laser pulses during the optical breakdown of KH2PO4 crystals, Appl. Phys. B 109 (2012) 695-706.

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Figure captions Fig. 1. (a) Schematics of the experimental setup. (b)-(d) SEM images of 110-µm thick glass samples after cutting with 10-ps 1030-nm laser pulses at 200 kHz repetition rate. Scanning velocity was 0.38 m/s. Laser fluence (pulse energy is given in brackets) was varied keeping all other parameters of irradiation unchanged: (b) 10.8 J/cm2 (52.0 µJ), (c) 16.1 J/cm2 (77.5

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µJ), (d) 19.3 J/cm2 (92.8 µJ). Required number of scanning passes to cut the sample were (b) 50, (c) 30, (d) 10. In the case (b), the sample was cleaved after laser processing.

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Fig. 2. SEM images of cross sections of tranches produced on 110 µm-thick glass sample by 1030 nm, 500 fs laser pulses at 10 kHz repetition rate. Irradiation spot size is 59.7 µm. Laser

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fluence (respective energy per pulse is given in brackets) was varied: (a) 12.1 J/cm² (170 µJ) (b), 12.9 J/cm² (180 µJ), (c) 15.0 J/cm2 (210 µJ). The trenches were obtained at 40 passes

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over sample surface with constant scanning velocity of 0.2 m/s. (d) Schematics of the crosssectioning technique for sample characterization (see text).

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Fig. 3. SEM micrographs of debris accumulated inside and outside the craters (a,b) and tranches (c,d) upon laser drilling and cutting. Holes were produced in 50-μm thick glass by 50 laser pulses using trepanning method with irradiation spot overlap of ~90% (515 nm

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wavelength, 10 ps pulse duration, fluence per pulse of 16 J/cm2, repetition rate of 50 kHz). The tranches were scribed on 110-μm thick glass using 500 fs laser pulses of 1030 nm

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wavelength at fluence of 10.6 J/cm2 (repetition rate of 10 kHz, 30 laser passes with scanning speed of 0.2 m/s). See text for further details.

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Fig. 4. SEM micrographs of cuts on 50-µm thick borosilicate glass produced by 10 passes of 10-ps laser pulses at 515 nm wavelength. Pulse energy of 9.74 J/cm2 (62.0 µJ energy per pulse), spot diameter of 40.2 µm, repetition rate of 50 kHz. Scanning velocities were 25mm/s (a) and 50 mm/s (b).

Fig. 5. SEM images of the laser-produced trenches demonstrating the strain effects on laser processing quality. (a) Tranches produced by 10 passes of picosecond laser (10 ps pulse duration) at fluence per pulse of 25.8 J/cm2. Pulse repetition rate of 50 kHz, scanning velocity of 0.13 m/s. (b) Trenches produced by 30 passes of femtosecond laser (500 fs pulse duration) at fluence per pulse of 17.1 J/cm2. Pulse repetition rate of 10 kHz, scanning velocity of 0.2 m/s. Wavelength was 1030 nm in both cases. For sample characterization, the crosssectioning technique was used (Fig. 2d), so two features are seen, trench cross section (top) and trench wall (bottom).

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Fig. 6. The results of modeling laser-induced dynamics of ambient gas (argon) above the femtosecond-laser irradiated metallic (Pt) target. Irradiation conditions: laser fluence of 3 J/cm2 at 800 nm wavelength, pulse duration of 65 fs. (a) and (b) Gas density distributions at 25 ns and 2 μs respectively after the laser pulse action. Target surface is located at z = 0. An intricate shock wave structure is seen with a quasi-cylindrical compressive wave propagating radially outward the laser beam axis (r = 0) and a stronger half-spherical wave moving away

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from the irradiation spot on the target. (c) The temperature distribution in argon (given as an increase of the temperature above the room value) nearby the irradiated sample at 18 μs after

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the laser pulse action. It is seen that the ambient gas remains relatively hot quite long after the laser pulse termination. (d) The distribution of the gas velocity component normal to the

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target at t = 2 μs after the laser pulse action.

Fig. 7. The damage thresholds at combining fundamental and third laser harmonics adapted

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from [90,91]. (a) Laser damage of KH2PO4 crystal at the damage density of 0.8 dam./mm2 produced by simultaneous action of 1st and 3rd harmonics of Nd:YAG laser (pulse durations are 6.5 and 5.5 ns respectively) [90]. (b) The results of experiments on ultraviolet multiphoton

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initiation of fused silica damage by a Ti:sapphire laser (800 nm) [91]. Near-infrared fundamental light considerably reduces the damage threshold fluence of the 3rd harmonics

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(pulse durations are 60 and 70 fs for 1st and 3rd harmonics respectively). The points are taken for zero delay between pulses. However, similarly to ns pulses [90], the fused silica damage requires more laser energy from two combined wavelengths as compared to single

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wavelength irradiation regime as presented in (b) (the experimental data are redrawn from [91] to better see the energy inefficiency of such regimes, similar to (a)). Red dotted line

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connects the thresholds for fundamental and third harmonics, dividing the regimes to two regions, I and II. The experimental points fall to the region II below this line would that indicates inefficiency of dual wavelength for inducing material breakdown. Green dashed line leads eye along the experimental data. Note experimental points marked “1” where laser energy expenses needed to damage the glass surface are noticeably higher than the single damage threshold at fundamental harmonics.

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Highlights: - The factors influencing laser micromachining of transparent materials are analyzed - Important role of ambient gas in laser processing is shown by numerical simulations - The large potential of bi-wavelength laser processing is demonstrated

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