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Abstract
Filamentary propagation and laser-induced damage have been demonstrated and investigated in fused silica excited by nanosecond deep-ultraviolet laser pulses. Long-range modification channels, up to 10 mm because of the filamentation and defect absorption in the bulk of the solid transparent materials, are observed. Here we investigate ways to control these laser-induced modifications by varying the laser pulse energy and number of exposures.The effects of objective lens focal length and working distance on the filament and modification are also discussed. Furthermore, the laser-induced fluorescence spectra indicate that dense absorbing defects in the damaged regions. Moreover, the mitigation of the induced damage through thermal effects are discussed.
© 2017 Optical Society of America
1. Introduction
One of the most active research fields in optoelectronics is the fabrication of embedded photonics devices in the bulk of transparent solid dielectrics. The ability to fabricate microstructures is crucial for the development of new materials and multifunctional microsystems and devices. By focusing laser pulses inside the bulk of transparent materials, the structure modification initiated by complicated nonlinear processes can be achieved [1]. The induced permanent structure modification in the samples, such as a large increase in refractive index in a region of the samples damaged by femtosecond laser pulses, can be applied to fabricate photonics structures, waveguide in a variety of glasses [2–4], gratings [5], and three-dimensional integrated optical devices inside optical materials [6, 7]. However, these applications rely on the comprehension of the physical mechanism and theoretical model of the laser-matter interactions. Furthermore, an increasing number of related technological applications such as laser machining and laser damage have stimulated basic research intended to develop the fundamental understanding in physical regime associated with laser-induced modification of materials and sampled by laser damage in dielectrics [8, 9].
Fused silica is one of the key optical materials used for excimer lasers because of its high optical transparency in the deep-ultraviolet (DUV) region, excellent optical homogeneity, and high durability when exposed to lasers. However, the transparency is degraded by preexisting and laser-induced defects, which in turn can lead to the excessive absorption of the propagating laser light, which generates localized high-energy depositions by exciting electrons from the valence band to the conduction band [10–12].
Laser material processing strongly depends on the pulse duration. For femtosecond pulses, the multi-photon ionization and avalanche have been proposed as the key processes for laser-induced damage inside the materials [13]. In contrast, for nanosecond pulses, thermal effects play a main role in the bulk damage [14, 15]. The heating of electrons through avalanche processes and the transfer of their energy to the lattice take place, resulting in the melting or boiling of the material, causing modifications such as tracks and cracks. The precise physical mechanisms and processes involved in the modifications induced by laser pulses have still only been partially studied and understood.
Laser pulse propagation phenomena are of great importance for laser-medium interactions in the bulk of transparent media. Bulk modification is a complicated process that includes light absorption, light ionization, self-focusing, and diffraction physical phenomena [16]. When a laser pulse with an input power larger than the critical power for self-focusing propagates in a Kerr medium such as fused silica, filaments appear due to the dynamics competition between Kerr self-focusing and defocusing induced by ionization and nonlinear absorption. Here , and denote the incident laser wavelength in vacuum, the linear and nonlinear refraction index of fused silica, respectively. In the filament’s core high intensities are reached and a plasma string is generated along its path. Filaments can propagate over distances that extend up to hundreds of times of the Rayleigh length [17–19]. Filaments and filamentation-induced bulk modifications are widely studied with ultra-short IR and UV laser pulses [20–22]. And the previous study of damages growth mainly focused on the radial direction [23–25]. However, the filamentation and induced modifications in fused silica focused by nanosecond DUV laser pulses have rarely been reported.
In this study, filamentation and induced permanent bulk modifications in fused silica with nanosecond DUV lasers are experimentally demonstrated. In a previous study we demonstrated the important link between the light absorption of laser-induced defects and the permanent damage structure [10]. We now focus our interest on the relation between the damage tracks and filamentation in the bulk of a transparent medium. In addition, the impacts of the objective lens focal length, working distance, laser pulse energy and total number of irradiated pulses on the structures and lengths of the filaments inducing bulk modification in fused silica are investigated in detail.
2 Experimental procedure
The filamentation and induced modification of high-purity fused silica are investigated with ArF excimer lasers with a center wavelength of 193.4 nm. The fused silica (FS8650, Corning) used in our setup is rectangular (6 mm × 30 mm × 42 mm) with optically polished surfaces.
The experiments are performed with a broadband 193 nm ArF excimer laser (EX100, GAMLaser Inc., pulse width τ = 20 ns, maximum pulse energy Emax = 7 mJ,maximum repetition rate of 500 Hz). The ArF excimer laser beam is focused into the fused silica sample with a microscope objective lens.An attenuator in front of the objective lens tunes the energy of the input laser pulses. An aperture in front of the lens is used to confine the laser beam from a 5 mm × 2 mm rectangle to a 2 mm diameter circle.
The sample is fixed onto a translation stage that allows the control of the laser pulse incident spot position. The optical images of the change in laser induced refractive index are observed by a digital microscope (VHX5000 KEYENCE).
By properly translating the sample, the structural modifications induced by laser filamentation characterized with a permanent refraction index change are studied in detail. Furthermore, the ways to control the modification are discussed. Long-range filamentary permanent structural modifications excited by nanosecond DUV laser pulses are reported in this paper.
3 Results and discussion
The laser-induced modification in fused silica is experimentally studied. Microscopic images of damaged channels in fused silica excited by different pulse energy laser beams and focal lengths are illustrated. As the laser pulse energy is larger than the damage threshold, permanent laser-induced damage in the bulk of the sample is produced along the laser propagation direction. With longer laser pulses, the laser induced damages characterized by a line shape is accompanied with squamous radially extending cracks. The long-range laser-induced damage track is observed to extend up to 10 mm.
By continuously tuning the input pulse energy irradiating the fused silica sample, we can determine that the critical power for self-focusing is about 0.03 MW for the given fused silica at 193 nm. The threshold is much lower in fused silica because of the irradiation with a short DUV laser wavelength [26]. The permanent traces indicate a refractive index modulation, which may be generated by the filamentation of the incoming laser beam at about a peak power of 0.5 MW. The long-range damaged channel can be explained as follows.
If the incident laser pulse power exceeds the critical power for self-focusing, a filamentary propagation of the beam inside the bulk of the fused silica should be observed. The balance between the self-focusing and defocusing effect of the weak plasma generated through multi-photon excitation could be reached at a low peak power in fused silica irradiated by the ArF laser pulses. Because of the dynamic interaction of the complicated nonlinear optics effects, the transmission of the laser pulses can be confined to a long-range channel in the form of filamentation [27–29].The time and spectrally resolved holographic technique demonstrated that the laser pulse nonlinear propagation inside the sample was accompanied by electron plasma strings [30]. Moreover, our experiment illustrates that the permanent material damaged channel is modified only along the string.
For tight focusing conditions, avalanche ionization plays a dominant role in the generation of free electrons. The damage threshold usually corresponds to a conduction electron density for which the energy density of the electrons is equal to the lattice binding energy. The strong plasma formation associated with the optical breakdown, either by the longer pulse duration or higher energy, can lead to scattering damages.
In fact, the laser-induced damage channel coincides with the track of the laser pulse filament in the fused silica. Therefore, the channel excited by the nanosecond excimer laser pulses exhibit thermal modifications that are confined to the determined track by optical filamentary propagation beams in the bulk of the fused silica. The diameter of the micro-explosions can be as large as 200 μm. The micro-explosions produced by the nanosecond ArF laser pulses differ dramatically from the damaged regions produced by infrared femtosecond laser pulses [31]. With the longer laser pulses, the resulting structures are much larger, irregularly shaped, and display radially extending cracks. The laser-induced defects can lead to excessive absorption of the propagating laser light, which generates localized high energy deposition. Rapid material heating and the associated thermal expansion result in stress fields, generating shock waves, which may cause cracks or micro-explosions [32–34]. Furthermore, the densification of the fused silica is a natural consequence of micro-explosions [35, 36]. As a result, the refractive index homogeneity of the fused silica is degraded by the laser-induced damage sites.
3.1 Impact of pulse energy on modifications
The impact of laser pulse energy on the induced modification of the fused silica is studied. As shown in Fig. 1, the microscope images of the induced bulk modification of the fused silica excited by different incident pulse energies are displayed. The ArF excimer laser beam is focused onto the fused silica sample with a 0.08 NA (3 × ) microscope objective lens (f = 33.6 mm in fused silica). The observation is linked to the nonlinear propagation of the intense laser pulses in the transparent medium in the form of a filament. As the input pulse energy increases from 0.13 to 1.25 mJ, the length of the damaged channel expands from 3.7 to 10.3 mm, and diameter of the channel increases from 91 to 200 μm. It should be noted that the diameter of the damaged channel is much larger than the theoretical focal spot size (5 μm) because of the cracking and explosion effects.
Fig. 1 Impacts of pulse energy (a, 0.13 mJ; b, 0.50 mJ; c, 1.25 mJ) on the filamentation induced damage channels with 2000 pulses.
This suggests that the channel length and diameter of the induced damages depend on the pulse energy. One can clearly observe that the channel length and diameter of the spatially confined filamentation-induced damages increase with the incident pulse energy.
The sensitivity of the damage length to the pulse energy can be interpreted in the framework of self-focus theories. It is known that the length of a filament, which can be approximated as the separation between the self-focus and the geometrical focus, depends on both the pulse peak power and external focusing condition [21]. As the incidence pulse energy increases, the position of the collapse moves from the linear focus position of the lens to the objective lens.
This can be explained by the fact that the higher energy laser pulse could excite a filament with a longer range in the medium. The modification and cracking of the fused silica are more severe with a higher pulse energy. Consequently, the radius of the filamentation induced damage is larger, and the cracking of the channel is more distinct when the incident pulse energy increases. The length of the damaged channel expands toward the laser source direction until the laser intensity is not sufficient to create small permanent damages. In fact, the length of the damage track is mainly controlled by the filamentation length.
It should be noted here that the damaged shape at both the top and tail of the track are different from the main center region of the channel. On one hand, the modification degree at the top of the damaged channel which locates closer to lens is lighter. Compared with the main center damaged region, the top region is characterized by a narrower modification width. Furthermore, the cracks that accompany the main center region are not yet excited at the top region. By observing the microscopic structure at the top of the channel, we can find that the modification channels contain dense spots that arranged in a line along the propagating laser beam direction. On the other hand, solid isolated spots instead of dense continuous dots at the center track are excited at the tail of the induced-damaged channel which locates farther from lens. Moreover, the resulting structures comprise irregularly shaped and dotted spots with cracks. The dotted spot damage can be explained by focusing-defocusing recurrent theory.
For nanosecond laser pulses, the filament preservation over a long distance is primarily due to the combined interplay of the self-focusing, multiphoton absorption (MPA) and avalanche ionization. A pulse with a peak power above Pcr should undergo collapse. However, when the beam becomes sufficiently intense, MPA attenuates the core of the beam and a plasma is generated near the collapse location, which defocuses the beam core. Once defocused, the beam may still have a power above the critical power and undergo another focusing-defocusing cycle. The repetition of this scenario sustains the long range, self-guided propagation in the form of a filament. The interplay between self-focusing, MPA and ionization is therefore a highly dynamic process, with recurrent, aperiodic strings or spots of ionization surging whenever the beam starts collapsing [37]. The recurrence of these cycles is illustrated more clearly as the peak power of the input pulse increases.
3.2 Effect of pulse number on modifications
A detail study on the length of the modification channels has been realized through the variation of the accumulated number of irradiation laser pulses. The number of laser pulses with the same repetition rate focused on the samples is controlled by a signal generator. By properly translating the sample, a group of damaged channels excited by different numbers of exposure laser pulses is demonstrated with the same discharge voltage, repetition rate and focusing lens (0.08 NA, 3 × ).
The microscope images and length of the damaged channels irradiated with different pulse numbers are detected by the microscope. The microscopic images of LID excited by 100 pulses in Fig. 2. It can be found that the crack of the damaged channel already exist at the beginning of the exposure. With pulse energy at 0.58mJ, the core diameter of the LID channel is about 20 μm, and the crack diameter can be expanded upto 200 μm. A reasonable explanation is that the damage channel induced by filament track is enlarged by cracks or micro-explosions resulted by thermal effects. In addition, the relation between the length of the damaged channels and number the irradiation laser pulses (100, 200, 300, 400, 500, 600, 700, 800, 900, 1000, 1500, 2000, 3000, 4000 and 5000) are plotted in Fig. 3.
Fig. 3 Length of the damaged channel as a function of accumulated exposure pulse number. The length of the damage channel increase and saturate with increasing irradiation laser pulse number
As discussed above, the diameters of the damaged channels are almost identical because of the stable incident pulse energy. However, the channel length is sensitive to the total number of incident laser pulses. As the number of incident laser pulses increase, the induced damage channel grows toward the laser source direction. This can be explained by an accumulation effect of small permanent refractive index changes that build up progressively in the region before the nonlinear focus where the intensities are lower [38]. The length of the damaged channel expands rapidly at the start of the increasing pulse number. However, since the filament length is controlled by the incident pulse energy [39], the length of the channel grows slowly as the number of irradiating pulses is increased. Eventually, the length of the filamentation induced damaged channel gets closer to saturation when the number of laser pulse reaches a certain value.
3.3 Focal length impact on LID
The fused silica damage focused by different focal length objective lenses are investigated. The focal length of two UV focusing lenses are 11.2 mm (0.25 NA, 10 × ) and 7.3 mm (0.32 NA, 15 × ) in the fused silica, respectively. The microscope images of the fused silica LID are shown in Fig. 4. The results indicate that the damage length is sensitive to the focal length. Although the two channels are characterized by similar diameters, the channel focused by the longer focal length lens exhibits a longer track length. Namely, the damaged channel length can be controlled by the focal length of the objective lens.
Fig. 4 Effects of focal length on the filamentation induced damages irradiated by 4000 pulses (0.35 mJ / 10 Hz).
It is observed that the damage length decreases as the focal length of the objective lens gets short. The is because the beam size focused by the shorter focal length lens is characterized by a larger beam divergence gradient. This implies that the weak geometrical convergence of the laser beam is characterized by the energy density being sufficiently high to ignite damages that can be sustained at longer distances. Thus, the filament, and tail structure of the damage tracks could be extended backward until the energy reservoir of the transmission beam decays to a value below the damage threshold. Conversely, no laser damage could be excited without filamentation inside the sample.
On the other hand, it could be qualitatively explained by the Rayleigh length of the Gaussian beam. The laws of Gaussian optics say that the width of a Gaussian beam with a flat spatial phase increases by a factor of when the beam propagates over a typical length, called the Rayleigh length. This length is defined as
Where is the beam waist before the lens, is the refractive index of the medium, and is the wavelength in vacuum. The laser beam is characterized by a long Rayleigh length focused by long focal length. In addition, the laser beam can be trapped at a long range with a high pulse energy density. As a result, the long focal length objective lens shows a longer damaged channel length.The damaged level is lighter at both ends of the channel. This phenomenon is similar to the typical filamentation where the center region shows a stronger intensity than that at the top and tail regions. A reasonable explanation for this is that the laser-induced defect density is closely related to the peak laser intensity of the filament core, which is inherently limited by the intensity clamping effect during laser filamentation [40]. Therefore, the defect density of two end regions are lower than that of the main center region, and the modification degree is affected by thermal effects.
However, the top and tail structure of the damaged channel characterized with different patterns when the sample focused by three focal length lenses. With relative short focal length lens, the top channel shows shorter length. No obvious weak top structure of the damaged channel is shown focused by the shortest facal length lens. In contrast, the tail channel illustrates longer length as the sample focused by shorter focal length lens. A reasonable explanation is that the focal length of the lens impacts the divergence of the energy reservoir for the transmitting beam. And the filamentation processes lead to damage only when sufficient energy is deposited at that intensity to lead to modification of the material [41].
3.4 Mitigation of LID
The length of the damaged channel grows toward the laser source as the number of exposure pulses increases. The homogeneity of the saturated growing damaged channel irradiated by the extra laser pulses is observed. The experimental results indicate that the refractive index homogeneity locates at the top of the damaged channel is mitigated after the continuous irradiation laser pulses, as shown in Fig. 4.
The mitigation of the damaged sample can be explained as follows. The density of the laser-induced defects increases rapidly within the damaged zone focused by the excimer laser. The accumulated heat transformed from light absorption caused by the laser-induced defects can be excited instantaneously to high temperatures in the localized damaged channel. A material re-melting procedure followed by rapid high heating improves the refractive index homogeneity of the damaged site.
In fact, the incident laser beam is strongly scattered by the damaged microstructures. The transmission of the following laser pulses is limited by the damaged sample. The following laser pulse cannot transmission along the damaged channel. Consequently, the mitigation of the damage is limited to the top end site of the damaged channel.
3.5 Effect of lens working distance on LID length
Due to the spatial distribution of the laser energy density of the focused beam, the working distance between the sample and objective lens impacts the focal point position and nonlinear optical transmission of the laser beam. As a result, the formation of the laser filament and damaged channel could be modulated. The length of the filament induced damaged channel with different working distances is investigated with a 0.25 NA (10 × ) objective lens, as shown in Fig. 5. The value 0 represents the laser beam being focused on the incident surface of the sample. The negative and positive working distance implies that the laser beam is focused inside and outside the sample, respectively.
Fig. 5 Working distance impact on the length of the filamentation as well as the damaged channel. The length of the damaged channel is shorten sharply as the focal point change from inside to outside of the sample.
The depicted curves indicate that the length of the channel could maintain a high value as the working distance changed from −10 mm to 0, where the position of the objective lens focus is inside the transparent solid media. However, the length of the structure decreases dramatically when the laser beam focuses on the surface or outside the sample.
For the close focus condition, the laser beam is focused inside the fused silica. There is enough space for the filament to excite and interact with the transparent optical material. And the induced-damaged channel shows a long length. The length slightly decreases as the working distance increases from 0 to 10 mm. The reason is that the laser pulse energy gets a little weaker due to the absorption of oxygen in the air for longer working distances [42].
However, the length of the laser-induced damage channel length drops sharply as the working distance changes to the position where the focus of the lens is close to the rim of the incident sample surface. The microscope image illustrates that the damage is mainly spread vertically at the rim of the incident surface of the sample. The damaged structure is similar to the surface damages that were excited by other wavelengths nanosecond laser pulses [23]. Experimental results show that the damaged channel cannot be excited if the focal point of the lens located on the surface or outside of the sample. It confirms that the dominant process for the structure modification in fused silica is the filamentation formed before the focal point for the tightly focused beam.
3.6 Laser-induced fluorescence
Optical defects in optical materials can cause optical absorption and thermal effects [43, 44]. The basic point defects of fused silica, which are induced by laser irradiation, include the oxygen-deficiency center (ODC), non-bridging oxygen hole center (NBOHC), and E0 center [45]. Laser-induced defects in fused silica can be created by ArF excimer laser pulses through two photon absorption processes [10, 44]. The laser-modified fused silica exhibits intense broad laser-induced fluorescence (LIF) bands due to NBOHCs (at 650 nm) and ODCs (at 281 and 478 nm). These spectra are captured by imaging the emission from the irradiated volume of the glass into a fiber coupled spectrometer (Maya 2000, Oceanoptics Inc.). As shown in Fig. 6, pristine and damaged region LIF spectra are recorded before and after bulk damages experimentation without tuning the sample position, respectively. Both LIF spectra are excited by laser pulses without objective lens. It shows that the LIF bands of the damaged region are much stronger than that of the pristine regions in fused silica.
Fig. 6 LIF spectra both from the damaged region and pristine region of the fused silica. The stronger LIF intensity from the damaged region indicates a higher density of defects.
The stronger LIF intensity from the damaged region indicates a higher laser-induced defect density at the filament channel in the fused silica. The filament accompanied by a strong plasma string is responsible for the increase in laser-induced defects. The defects can lead to the excessive absorption of the propagating laser light, which generates localized high energy depositions. Rapid material heating and the associated thermal expansion results in stress fields, generating shock waves, which may cause cracks or micro-explosions [17, 31]. As a result, the damage track is significantly rougher than the filament string. Therefore, the LIF spectra further support our conclusion that filamentation is responsible for the laser-induced damage channels in the fused silica irradiated by nanosecond DUV laser pulses.
4 Conclusions
We have demonstrated and studied filamentation induced modification in the bulk of fused silica by nanosecond DUV excimer laser pulses. The microscope images reveal that the location of the filament coincides with that of the permanent refractive index change channel. The length and diameter of the modification channel are sensitive to the irradiation laser pulse energy and focal length of the lens. The length of the damaged channel grows toward the laser sources as the number of exposure pulses increases. The LIF spectra of the damaged region indicate that the physical process of the laser-induced damages is evoked mainly through the absorption and thermal effects of the induced defects excited by filaments. The focal length and working distance provide methods for controlling the modification length. Thermal melting is responsible for mitigation of the refractive index homogenization at the top end site of the damaged channel. To further improve the whole homogeneity of the damaged channel, additional experiments will be performed using CO2 laser proceeding. These results maybe give some indications for develop the physical mechanism of the laser-induced damaged in solid fused silica.
Funding
National Natural Science Foundation (No. 61308024 and 61405202); National Science and Technology Major Project (No. 2013ZX02202003); National Key Research and Development Program (NO.2016YFB0402201); K. C. Wong Education Foundation; Program of Shanghai Technology Research Leader (No. 17XD1424800); Shanghai Sailing Program (No. 17YF1421200); Key technologies Research and Development Program of Jiangsu (No. BE2014001, BE2016005-4); Natural Science Foundation of Shanghai (No.16ZR1440100, 16ZR1440200).
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