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Abstract
Fused silica irradiated with 6.8 ns 355 nm laser pulses is studied by micro-Raman scattering spectroscopy. Results show that, for laser fluences above the laser-induced breakdown threshold (Fth ~3.9 J/cm2), irradiation results in the formation of four laser-induced defect-related Raman bands centered at 1363, 1557, 1605.9 and 2330 cm−1. Bands centered on 1363, 1557 and 2330 cm−1 are attributed to Si = O, interstitial O2 and Si-H bond. However, defects giving rise to a broad band at 1605.9 cm−1 are unknown. Based on these results, we discuss physical processes occurring during the laser-induced fused silica breakdown, leading to the formation of Si-H bond and interstitial O2 and the fracture of fused silica.
© 2018 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Fused silica (amorphous silicon dioxide, a-SiO2) is widely used in high power laser system because of its excellent optical, thermal and mechanical properties. It is an increasingly important material in modern high tech industries, a prominent example being bulk material as transparency optics for large high power output systems such as National Ignition Facility (NIF) [1], Laser Megajoule (LMJ) [2], and the SG series laser facility [3]. The growth of laser induced damage on the surface of fused silica plays a major role in determining the operation fluence and optics lifetime in high power laser system.
Previous studies have shown that laser-damaged regions, typically initiated at the surface of a-SiO2, grow rapidly with repetitive pulses. Such damage growth is associated with two aspects: (1) The laser field intensities is above the laser-induced damage threshold (LIDT) at microscopic cracks produced by previous pulses; (2) there is light absorption at laser-induced optical defects. Hence, elucidating the nature of light absorbing of the optical defects in laser-modified a-SiO2 is important for controlling the evolution of laser damage and for designing an effective damage mitigation procedure.
It has previously been shown that tightly focused 355 nm ns laser pulse irradiation of a-SiO2 to fluences above the LIDT can create structural changes leading to modify the Raman spectroscopy bands centered on 435, 491, 603, 798, and 1065.9 cm−1 [4–16]. The 435 cm−1 bond is assigned to a Si–O–Si bending mode, the signals at 798 and 1065.9 cm−1 to Si–O–Si symmetric and asymmetric stretching modes, respectively, and two bonds at 491 and 603 cm−1 to four- and three-fold Si–O rings. The 2330 cm−1 band is attributed to Si-H stretching bond [17,18].
In this letter, we report on a combination of optical microscope and micro-Raman scattering spectroscopy study of a-SiO2 modified by 6.8 ns 355 nm laser pulses under a wide range of irradiation conditions. We also discuss the physical processes responsible for the formation of Si-H stretching bond and interstitial O2 during the catastrophic laser-induced material breakdown.
2. Experimental details
2.1 Sample manufacturing
To test their consistency and repeatability, UV-grade Heraeus Suprasil S312 high-quality optical fused silica samples were polished by a conventional pitch-polishing process with CeO2, divided into five block samples with a dimension of 20 mm × 20 mm × 10 mm (marked as A, B, C, D, and E), and irradiated by a 355 nm, 6.8 ns UV pulsed laser. The laser radiation was focused on the surface of a sample to form a spot with a size of 0.22 mm in diameter. Sample A was used as the blank sample, and Sample B was irradiated by a 50-shot pulsed laser with fluences at 4.2 J/cm2 and 4.5 J/cm2. The laser emission frequencies employed for four sites were 1, 2, 5, and 10 Hz. Sample C was irradiated by the 1 Hz laser with a fluence of 4.5 J/cm2, and the numbers of shots were 1, 5, 10, 20, 50, 80, and 100. Sample D and E were irradiated by 1 shot, 1 Hz laser, and different irradiation fluences from 1 to 45 J/cm2.
2.2 Experimental test of Raman spectroscopy
UV laser-excited Raman spectroscopy was used to characterize the angle change of the Si-O-Si bridging bond angle. The 325 nm laser with power of 200 mW was used to excite Raman scattering and the counting time was 120 s. Raman light was collected in the back-scattering geometry. Raman spectra in the range 50–2500 cm−1 was measured for each specimen and the spectra resolution was 1.25 cm−1.
3. Results
3.1 Raman spectra of high energy single shot laser irradiation
When the fused silica sample is irradiated by a shot laser with the fluence larger than 10 J/cm2, the damaged fused silica samples were observed by scanning electron microscopy as shown in the picture. The size of the damage pit is between several microns and tens of micrometers as shown in Fig. 1.
Fig. 1 Scanning electron microscopy of the surface morphology of fused silica after fused silica irradiated by a shot laser with the fluence larger than 10 J/cm2.
The Raman spectrum of fused silica irradiated by a 6.8 ns 355 nm laser beam with a fluence of 15 J/cm2 can be seen in Fig. 2. The micro crystal appears on the exit surface of the fused silica element, and the Si-O bond breaks and the Si-H bond is formed. The Raman spectra of the undamaged fused silica and damaged fused silica are shown in Fig. 2 as curves a) virgin region and (b) damage region, respectively, in which all the well-known characteristic bonds of silica appeared. The 431.4 cm−1 band is assigned to a Si–O–Si bending mode, the signals at 797 and 1063.5 cm−1 to Si–O–Si symmetric and asymmetric stretching modes, respectively, and two bonds at 487 and 602 cm−1 to four- and three-fold Si–O rings.
Fig. 2 Optical image of a-SiO2 irradiated by a single high energy laser (10 J/cm2) at the front surface.
Compared with undamaged fused silica, the Raman scattering spectrum of the damaged fused silica is obviously different. One interesting feature in the Raman spectra of the damaged fused silica, curved (b) in Fig. 3, is that there are three stretching bonds around 1557, 1604.8 and 2330 cm−1. The 1557 and 2330 cm−1 stretching bond is due to the stretching vibration of free O2 in its ground state and the Si–H stretching mode, respectively. However, defects giving rise to a broad bond at 1604.8 cm−1 are unknown. In the controllable experiments we have found that damage procedure has a significant effect on the Raman spectra of the fused silica.
Fig. 3 Typical Stokes Raman spectra of the selective regions is also shown for comparison. (a) virgin region; (b) damage region.
3.2 Raman spectra of low energy multiple shots laser irradiation
When irradiated by 50 shots laser with the flux less than 10 J/cm2, the damaged fused silica samples were observed by scanning electron microscopy as shown in the picture. The sizes of the damage pits are all more than a few hundred microns as shown in Fig. 4.
When irradiated by 50 shots laser with the fluence less than 10 J/cm2, the sizes of the damage pits are all more than a few hundred microns. A damage pit is irradiated by 50 shots 6.8 ns 355 nm laser beam with an radiation fluence of 4.5 J/cm2 above the LIDT(3.9 J/cm2) at the front surface at different regions: (a) virgin region, (b) the damage in the surrounding area, and (c) at the center of the melting damage in Fig. 5.
Fig. 5 Image of a-SiO2 irradiated with 50 pulses 6.8 ns 355 nm laser beam to a fluence above the LIDT at the front surface corresponding to Raman measurements. (a) virgin region; (b) fracture region; (c) melt region.
It can be seen in the Raman spectra that the structure of the melting damage area more perfect after recrystallization in fused silica melt, the Raman peak of 1605.9 cm−1, we do not know if it represents what form of vibration, accompanying the formation of Si = O bond. The Raman spectra of a-SiO2 in fracture region and melt region are shown in Fig. 6 as curves (b) and (c), respectively, in which all the well-known characteristic bonds of silica appeared. Compared with the undamaged a-SiO2, one interesting feature in the Raman spectra of the damaged fused silica, curved (b) and (c) in Fig. 6, is that there are two stretching bonds around 1363, and 1557 cm−1. The Raman scattering spectrum of a-SiO2 in melt region is obviously different from that in fracture region. i.e. a more peak of 1363 cm−1 and a less peak of 1557 cm−1. The 1363 cm−1 stretching bond is due to the Si = O stretching mode, instead of the carbon signal [19]. The 1557 cm−1 stretching bond is due to the stretching vibration of free O2 in its ground state, respectively. The results indicate that owing to rapid material heating and the shock wave propagation as during laser-induced breakdown, Si-O bond break lead to the formation of Si-H bond and interstitial O2 observed in fracture region. A rapid re-solidification of the damage core accompanying oxygen loss, Si = O bond can be formed in melt region. However, the 1605.9 cm−1 bond maybe attributed to the unknown defect [20,21].
Fig. 6 Typical Stokes Raman spectra of a-SiO2 irradiated with 50 pulses 6.8 ns 355 nm laser beam to a fluence above the LIDT at the front surface on the right. A Raman spectrum of the selective regions is also shown for comparison. (a) virgin region; (b) fracture region; (c) melt region.
3.3 Raman spectra in different positions
Optical images of the ablated spot different regions are shown in Fig. 7(a). The condition of the irradiated laser is 50 pulses 6.8 ns 355 nm laser beam with a fluence of 5 J/cm2. The Raman spectra of a-SiO2 in fracture region and melt region are shown in Fig. 7(b), respectively. The higher the degree of crystallization near the center, the higher the peak value corresponding to the 1605.9 cm−1 Raman peak.
Fig. 7 Typical Stokes Raman spectra of a-SiO2 irradiated with 50 pulses 6.8 ns 355 nm laser beam to a fluence above the LIDT at the front surface. A Raman spectrum of the selective regions is also shown for comparison. (a) Optical images of the ablated spot different regions; (b) Raman spectra in fracture and melt region.
4. Discussion
In order to understand the mechanisms of the laser induced material fracture and the formation of optical defects, we briefly discuss the physical processes occurring in a-SiO2 during the laser-induced breakdown. In ideal a-SiO2, there is no intrinsic linear absorption of the incident sub-bond-gap laser light. Nonlinear or defect-assisted absorption processes are, hence, responsible for the coupling of the laser beam with a-SiO2, resulting in the formation of a dense plasma. If the rate of energy deposition by the laser pulse is higher than the energy dissipation rate, ultrafast material melting and subsequent re-solidification can occur in the laser-damaged core. In fact, a recent experimental study has suggested temperatures of ~5000 K at ~10 ns after the laser pulse in a-SiO2 exposed to nanosecond-high-power laser irradiation. Rapid material heating and associated thermal expansion result in large stress fields, generating a shock wave. Such ultrahigh pressure and shock waves can cause phase transformations, plastic deformation, and the formation and propagation of cracks in the surrounding (cold) material.
Based on the above discussion, the formation of Si-H bond and interstitial O2 observed in Fig. 6 and Si = O bond observed in Fig. 4 can be attributed to Si-O bond break, typically accompanying rapid material heating and the shock wave propagation as during laser-induced breakdown. In this process, non-bridging oxygen hole center (NBOHC) can be formed in fracture regions exposed to plastic deformation and cracking, leading to the formation of dangling bonds. Furthermore, material fracture revealed by optical microscope studies see Fig. 7, in which the strong Raman spectroscopy signal can be attributed to Si-H bond and interstitial O2 in fracture regions. A rapid re-solidification of the damage core accompanying oxygen loss, Si = O bond can be formed in melt region. However, the fact that both NBOHC and oxygen deficient center (ODC) can form in a-SiO2 under a variety of ionizing radiation conditions, such as various particle and electromagnetic radiation suggests that electronic relaxation (radiolytic) processes could also contribute to the formation of NBOHC and ODC defects during the laser induced damage.
5. Conclusion
In conclusion, we have studied a-SiO2 damaged by high-intensity nanosecond laser pulses with different parameters using optical microscope and micro-Raman scattering spectroscopy. Our results have shown that, for all the laser irradiation conditions studied, laser-damaged a-SiO2 exhibits intense broad Raman spectroscopy bands due to interstitial O2 at 1557 cm−1 and Si = O at 1363 cm−1, and some other laser-induced defects at 1605.9 cm−1, whose structure is presently unknown. These results may have important implications for control and mitigation of laser-induced damage in a-SiO2. The Raman peaks corresponding to 1605.9 cm−1 should be due to the crystallization of fused silica. The reason is that there is no 1605.9 cm−1 in the Raman spectra of the undamaged fused silica sample, and there is a weak 1609 cm−1 Raman peak in the fracture region, however, the strong 1605.9 cm−1 Raman peak only occurs in the melting region. It means that the fused silica structure is more perfect and closer to the crystal in the melting region.
Funding
National Natural Science Foundation of China under Grant (NNFC) (51402173); Fundamental Research Funds for the central universities (FRF-TP-15-099A1).
Acknowledgments
We acknowledge Wenyong Cheng and Lisong Zhang for their help during the samples preparation.
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