In this study, porous silica films, which have particle accumulation microstructure, were prepared using the sol–gel method. For comparison, compact silica films were deposited using the electron-beam-heating method. These films were then irradiated using nanosecond-pulsed laser beams with wavelengths of 1064 and 532 nm. Laser-induced damage thresholds were recorded and the film microstructures, as well as damage photographs, were observed using scanning electron microscopy. The experimental results show that different kinds of stripes formed on the surface of the silica films with particle accumulation structure. A kind of subwavelength periodic straight stripe was observed in the case of the 1064 nm wavelength, whereas another kind of annular stripe around the small damage pits was observed in the case of the 532 nm wavelength.
© 2012 OSA
Among the physical, chemical, and mechanical nanofabrication techniques, laser-assisted nanofabrication techniques are efficient and environmentally friendly . In laser ablation, the remarkable formation of laser-induced periodic surface structures occurs in various materials under diverse irradiation conditions . A previous study on laser ablation has shown that laser irradiation can facilitate the formation of periodic structures on a material surface . Before the advent of the femtosecond laser, a long-pulse or continuous laser was used to obtain stripes with large periods. In a normal-incident situation, the intervals of these stripes, which are called classic stripes, are generally close to the laser wavelength . On the other hand, ultra-short pulsed laser-induced novel stripes  have spatial periods that are considerably less compared with those of laser wavelength. These stripes are strictly perpendicular to the direction of laser polarization, and their structures are similar with those of the grating structures, that is, a steep edge exists between the slot and the ridge. This kind of subwavelength periodic stripe has opened a new avenue for laser micro–nano production. Consequently, subwavelength periodic stripes have become an important topic in laser studies and related fields.
Subwavelength periodic stripes mainly appear in femtosecond and picosecond laser irradiation experiments . In these experiments, the damage is mainly caused by an electronic Coulomb explosion upon the femtosecond pulsed-laser irradiation . However, the damage is mainly caused by thermal ablation upon irradiation by a nanosecond pulsed-laser. In generally, a nanosecond pulsed-laser is seldom used for etching subwavelength gratings because of eruptions produced during fusing .
In our previous work, subwavelength periodic stripes were etched on the surface of silicon oxide (SiO2) films by using nanosecond laser pulses . A silicon oxide film with nanoporous microstructure was deposited on the surface of a B270 glass substrate by using the sol-gel method. Subwavelength periodic stripes were then etched on the coated glass surface by using a nanosecond pulsed-laser. The laser was a neodymium: yttrium-aluminum-garnet (Nd: YAG) type with wavelength of 1064 nm and pulse width of 12 ns. Moreover, Theppakuttai and Chen  observed submicron ripples on the surface of a borosilicate glass substrate when irradiated with a nanosecond Nd: YAG laser (10 ns, 1064 nm) by using silica nano-spheres.
Numerous theories, such as those on scattering waves, organization, secondary harmonics, and laser interaction with plasma, have been proposed [10–13]. Huang et al.  systematically investigated the theory of laser interaction with plasma. Surface-plasma-interference-formed standing wave was shown to determine the initial spatial period of the subwavelength stripes. The main etching mechanism was a nanoscale Coulomb explosion in the grooves of the stripes. The orientation of the subwavelength stripes depends on the cavity mode and on the surface plasma transverse mode (TM). Upon analysis of the subwavelength stripe formation mechanism, the laser–plasma interaction model was deemed significantly successful. These theoretical models were developed based on experimental results corresponding to femtosecond pulsed-laser-etching subwavelength periodic stripes. However, the formation mechanism of nanosecond pulsed-laser etching subwavelength periodic stripes is unclear.
In the current study, different microstructures of silica films and different wavelengths of laser pulses were used in the experiments. Moreover, the microstructures, microparticle sizes, laser damage characteristics, and damage thresholds of the SiO2 films were examined. As an emphasis of this paper, the etching process was analyzed.
2. Experimental details
2.1. Preparation of SiO2 film
(1) Porous SiO2 films
Reagents were combined based on a 1.0: 2.0: R: 37 ratio of tetraethylorthosilicate (TEOS), H2O, NH3, and C2H5OH (R = 0.1, 0.2, 0.4, 0.6, and 0.8). Then, the solution was completely sealed and fully mixed at 50 °C. The solution was then filtered at low pressure by using a filtration membrane with pore radius of 0.8 μm. The pulling method was used to prepare coatings at 20 °C to 25 °C and 60% relative humidity. The cleaned substrate (25 mm × 75 mm × 2 mm B270 glass) was dipped in the solution for 60 s before being smoothly, uniformly, and vertically lifted at a speed of 1.5 mm/s. Heat treatment was performed in a furnace, as follows: from room temperature to 100 °C at approximately 2 °C/min to 5 °C/min and maintained for 30 min under 100 °C; from 100 °C to 250 °C at approximately 2 °C/min to 5 °C/min and maintained for 1 h at 250 °C; and finally, from 250 °C to 450 °C at approximately 2 °C/min to 5 °C/min and maintained for 0.5 h at 450 °C. Subsequently, the furnace was closed and the sample was allowed to cool to room temperature. Before annealing, a beaker filled with ammonia was placed in the muffle furnace to allow the film to be heated under ammonia atmosphere. The film only covered half of the substrate surface (approximately 25 mm × 40 mm), and the final film thickness of approximately 200 nm was detected using a step profiler.
(2) Compact SiO2 films
SiO2 films were prepared on a B270 glass substrate (25 mm × 75 mm × 2 mm) through electron beam heating in a physical evaporation deposition system. SiO2 (>99.99% purity) was used as the coating material. The deposition chamber had a base pressure of approximately 5.0 × 10−3 Pa and a working pressure of approximately 1.2 × 10−2 Pa. The deposition beam current was 180 mA. The film thickness was monitored according to the deposition time (approximately 90 s) until a final value of about 200 nm was obtained.
2.2. Laser-etching of subwavelength periodic stripes
The schematic diagram of the experimental setup for laser etching is shown in Ref . Nd: YAG laser system was operated at the TEM00 mode. The laser pulse width was 12 ns, and the laser wavelengths were 1064 and 532 nm. The laser beam was focused on the target plane with 1 mm diameter spot (1/e2) by a non-spherical lens with 250 mm focal length. The incident angle of the irradiating laser beam was 0°. During etching, only one laser pulse was imposed on the etching point on the specimen surface. The laser pulse energy was adjusted using an attenuator that consists of a half-wavelength plate and a polarizer. The direction of the polarization of the irradiation light was determined using the polarizer and was independent of the laser pulse energy. A marking line parallel to the direction of polarization of the irradiation light was made on the specimen prior to etching. This marking line help determine the relationship between the polarization direction of the irradiation light and the orientation of the formed subwavelength ripples. A Nomarski microscope was used to determine the damage of the radiation sites at 100 × magnification.
2.3 Damage characteristics record
A total of 100 points were etched for each specimen. Every 10 points were exposed to laser irradiations with the same energy density, and the portion of the damaged points under this energy density was recorded. The damage threshold of a specimen was decided by fitting the curve of damage portions under different laser energy densities. The fitting line crossed the horizontal axis at the energy density of the damage threshold. In addition, scanning electron microscopy (SEM) was used to observe film microstructure and damage morphology.
3. Experimental results
3.1. Film microstructure
Figure 1 shows the SEM images of the SiO2 films, where the SiO2 particle accumulation structure can be observed in the porous silica films. The cracks in the films were caused by the preparation process, which affected the integrity of the etched stripes. This particle accumulation structure was not observed in Fig. 1(f), which shows the SEM image of the compact silica film.
Figure 2 shows the particle radii of the porous SiO2 films prepared from sols containing different quantities of ammonia. The radii of SiO2 particles were 5, 7.5, 12.5, 20, and 30 nm when the values of NH3/TEOS were 0.1, 0.2, 0.4, 0.6, and 0.8, respectively.
The rates of hydrolysis of TEOS and of nucleation increased with increasing ammonia concentration. The rate of polymerization of SiO2 was relatively slower than the hydrolysis reaction because of steric effects. Polymerization was multi-dimensionally conducted after the completion of hydrolysis. A kind of short-chain cross-linked structure, which is the early-stage polymerized product, was continuously strengthened, and the crosslink was eventually converted into a spherical particle . Ammonia is the main factor that influences particle morphology. The number of nucleus increased with increasing ammonia concentration under other constant conditions. Ammonia also promotes the growth of the nucleus, thereby gradually increasing the sizes of SiO2 particles.
3.2. Laser damage threshold and characteristics
(1) Damage threshold
Figure 3 shows the relationship between the laser damage threshold of the sol-gel film and R. Laser damage thresholds were fitted according to damage probabilities under different irradiation energy densities. R was converted to a suitable particle radius r. The film samples exhibited high laser damage threshold when R = 0.4 (r = 12.5 nm). For the SiO2 film prepared via electron beam evaporation, the damage threshold was approximately 6.0 J/cm2 at a laser wavelength of 1064 nm. This value was lower compared with those of the sol–gel films because of the high purity of the sol–gel films. Moreover, the sol–gel films had a loose microstructure that can ease the local thermal stress caused by laser irradiation. The thermal conductivity of the sol–gel films was less than that of the compact film, which is unsafe for the film in resisting laser-induced damage. However, the nanosecond-pulsed laser-induced damage process is commonly believed to be thermal stress dominated, and a loose microstructure can help improve the laser damage threshold [16, 17].
(2) Damage characteristics
Figure 4 shows the SEM images of the damage spots. The damage spots of the two kinds of films had completely different characteristics. The sol–gel film damage spot was smaller, had stripes, and did not exhibit effects of eruption. At a wavelength of 532 nm, few small damage pits in the damage spot were arbitrarily and discretely distributed. Concentric ring stripes around some damage pits were present. These findings were not observed at the 1064 nm wavelength. Nevertheless, parallel-oriented stripes were seen. For the electron beam evaporation film, the damage spot size was large, traces of material melting were obvious, and the damage spot caused by material eruption was deeper. No stripe in the damage spot was observed.
3.3. Ripple microstructure
(1) SEM images
Figure 5 shows the periodic stripes in the sol–gel film damage spot as they emerge. Figure 5(a) shows the stripes that were etched using a laser pulse with wavelength of 532 nm. These stripes appeared around some micro-damage pits. The spatial periods of the stripes in different damage spots were in the range of 0.5 λ to 1.5 λ, where λ is the etching laser wavelength. The slot and the ridge exhibited smooth transitions. Overall, the height distributions of the annular stripes had similar sine-curve characteristics. Figure 5(b) shows the stripes that were etched using a laser pulse with wavelength of 1064 nm. These stripes mainly appeared at the center of the damage spots. The stripe spatial period was approximately 0.5 λ. These periodic stripes had the same spatial period, which was significantly less than the laser wavelength. Moreover, the periodic stripes exhibited similar orientations, that is, they were perpendicular to the direction of polarization of the laser irradiation. Similarity to nanograting structures was observed, with steep edge between the slot and the ridge . Compared to classic stripes, the periodic stripes exhibited basic characteristics that appear in the subwavelength periodic stripes that were etched using a femtosecond laser pulse. These subwavelength periodic stripes were observed only in the damage pit. Neither the slot nor the ridge preserved the original particle accumulation structure, indicating that the stripe structure was formed after the fusion of the materials.
As Ref . showed, a higher energy density of the laser pulse results in better integrity in the stripe and larger duty cycle of stripe structure. However, excess energy density leads to material eruption and stripe damage.
(2) Ripple period
As Ref . showed, a wide range of energy densities of laser pulses is suitable for forming subwavelength periodic stripes. The stripe spatial period is related to the energy density of the laser pulse. A higher energy density results in a larger stripe spatial period. However, the relationship between the stripe spatial period and the size of silica particles in the sol–gel film is not evident.
4. Discussions of ripple formation mechanism
In the case of nanosecond-pulsed laser irradiation, damage is mainly induced by the strong absorption of impurities or defects in a material. Heat action is the main distinction between nanosecond and femtosecond pulsed-laser damages . The local area of the material may reach a sufficiently high pressure and then erupt. This eruption damages probably formed periodic stripes.
A plasma flash is generally observed in nanosecond laser-induced damage processes. Huang et al.  proposed a core part of the subwavelength-stripe-etching mechanism that does not repel the case of nanosecond laser radiation when some unfavorable factors of the damage process, such as local area absorption and high-temperature material eruption, can be overcome. In the current study, sol–gel porous SiO2 films with particle accumulation structure were used in the experiments. The sol–gel method can significantly improve film purity and reduce damage probability. Moreover, the molten material can expand and penetrate to some extent when the porous films melt or gasify. This process can reduce the pressure in the molten material and delay or even prevent material eruption , thereby maintaining the probably formed periodic stripes.
Considering that the core part of the subwavelength-stripe-etching mechanism does not repel the case of nanosecond laser radiation, the general process of nanosecond-laser-etching subwavelength stripes can be proposed accordingly. When nanosecond-pulsed laser irradiates on a dielectric film, the ionization actions, particularly the avalanche ionization, cause high ionization degree in the film material, produce many free electrons, and cause the medium to exhibit metallic characteristics . Then, with the help of the particle silica structure, high-frequency waves become excited because of the scattering, leading to the formation of a surface plasma wave with TM wave feature. This surface plasma wave is coupled with the laser TM wave [22, 23]. A Coulomb explosion of the surface plasma etches out grooves that are oriented along the vertical direction of laser polarization. In TM-oriented grooves, the coupled TM wave is continuously strengthened, further etching the channel. In this procedure, the higher-frequency light should be excited via the scattering of the particle silica because the visible or infrared light cannot directly induce the formation of a surface plasma wave.
Plasma etching is the core of the nanosecond-pulsed laser-etching mechanism. The subwavelength ripples that appeared in the experiments were caused by two key factors, namely, the porous microstructure that maintained the formed period structure and the particle silica microstructure that assisted the formation of the surface plasma wave.
The annular stripes formed by the laser pulse with wavelength of 532 nm were different from those formed by laser pulse with wavelength of 1064 nm because laser radiation with shorter wavelength is more sensitive to film defects . The structural and atomic defects in the sol–gel films were the main absorption defects. Structural defects cause locally enhanced laser field distribution that results in local enhanced absorption. However, locally enhanced laser field distribution will not cause different damage characteristics for laser irradiations with different wavelengths. Atomic defects cause the band gap to be narrow. Suppose E1064 is the photon energy of the laser radiation with wavelength of 1064 nm and E532 is the photon energy of the laser radiation with wavelength of 532 nm. If the band gap energy is greater than E1064 and lower than E532, then the laser radiation with wavelength of 532 nm will be more sensitive to atomic defects than that with wavelength of 1064 nm, and the film damage process will be defect absorption dominated. Figure 6 shows the damage caused by a laser pulse with energy density near the damage threshold. As can be seen in Fig. 6, subwavelength periodic ripples with approximately 250 nm space periods formed in the case of the 532 nm wavelength. However, these ripples were destroyed by a defect-initialized damage pit. Hence, if the laser energy density is high, then the periodic ripples are destroyed, as shown in Fig. 5(a).
When a laser beam with short wavelength (532 nm in this study) irradiates on films, the photo-ionization rate in the defective area of the films is high during the early stage of laser irradiation. This situation causes a fierce avalanche ionization, and accordingly, high electronic density , fast rate of temperature rise, and early material melting. Moreover, the temperature of the melted material in the defective area further increases and the pressure rises. As the liquid material erupts, a shockwave travels through the surrounding area and a stress field distribution is created. A damage pit surrounded by annular stripes appears after eruption because of the stress wave .
The other theory that a porous structure can alter the laser field distribution is not reasonable for interpreting the formation of periodic ripples. If the stripe structure is formed by the laser field redistribution caused by a particle silica structure, then the stripe space period will change according to particle size. However, the experimental results conflict with such theory. In addition, silica particles have two-dimension periodic arrangement, whereas periodic ripples have one-dimension arrangement. The formation of an enhanced laser field around the particles cannot explain the formation of periodic ripples. Furthermore, the probable self-organization behavior  is not clear and concrete and remains a black box.
In this study, the sol–gel method was used to process optical glass surfaces. Nanoporous-structured SiO2 films formed on the surface of the optical glass. Nd: YAG laser beams with wavelengths of 1064 and 532 nm and pulse widths of 12 ns were used to etch subwavelength periodic stripes on the coatings. Based on the experimental results, the following conclusions were drawn: first, porous silica films can preserve the probably formed ripple structure because of its stress relief action; and second, laser pulses with wavelengths of 1064 and 532 nm can etch subwavelength periodic straight ripples, but those with wavelengths of 532 nm are more sensitive to defects. The formed parallel ripples were destroyed by defect-initialized damage pits, resulting in the formation of another kind of annular stripe around the damage pits. The surface-plasma-wave-etching theory is reasonable for interpreting the formation process of subwavelength periodic straighter ripples. Moreover, stress wave is believed to cause annular stripes.
The authors gratefully acknowledge the financial support of the National Science Foundation of China (Grant 10974150 and 10804090), the Open Research Fund of Key Laboratory of Material for High Power Lasers,— Chinese Academy of Sciences, the China Postdoctoral Science Foundation (Grant no. 2012M511691), as well as the Self-determined and Innovative Research Funds of WUT (Grant no. 2012-IV-014).
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