Open Access
Add to BibSonomy
Abstract
It has been reported that periodic nanostructures with a period size of 200–330 nm can be formed on silicon suboxide (SiOx, x ≈ 1) with 800-nm, 100-fs laser pulses at a fluence much smaller than that needed for nanostructuring on glasses such as fused silica and borosilicate glass. We demonstrated that a homogeneous SiO2 nanostructure with a period of ∼240 nm can be produced using a two-step ablation process and heat treatment in air at 1000°C for 144 hours. Optical microscopic images of the nanostructured surface illuminated by non-polarized visible light show that the transmittance increases as the reflectivity decreases.
© 2022 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
Glasses such as fused silica, synthetic quartz, and borosilicate glass are widely used as materials for transmissive optical elements such as windows, lenses, and polarizers because of their high transparency in the visible region, high chemical stability, high hardness, and low cost. Many researchers have focused on improving the anti-reflective properties of glass surfaces because surface reflection causes energy loss. This can be achieved by coating the glass with multilayers [1–4] and by forming sub-wavelength structures on the surface [4–6]. The former can be produced on a large scale using deposition processes, while the optical properties strongly depend on the wavelength, the polarization direction, and the incident angle of the light. For the latter method, the optical properties are hardly affected by these factors. Nanoimprinting is popularly used to produce sub-wavelength structures [7], while this requires the fabrication of molds. In addition, the mold size restricts the area of the optical elements. Therefore, the direct fabrication of nanostructures on glass is expected to save on the cost and time required to produce large-area optical devices.
Recently, it was found that nanostructures with a period of d = 100–200 nm can be directly formed inside or on fused silica by the superposition of multiple shots of tightly focused femtosecond (fs) laser pulses a few micrometers in diameter with a fluence of a few to 10 J/cm2, by using a high-numerical-aperture (NA) lens such as a microscope objective [8–10]. However, the need for focal spots of a few micrometers and the short focusing depth required for this nanostructuring restrict the possible applications. Current limitations include slow processing times and short working distances between the target and the focusing lens [11]. In recent work aiming to overcome these limitations, nanostructures have been successfully formed with d = 200–330 nm on a silicon suboxide (SiOx, x ≈ 1) [12–15] surface irradiated in air with 800-nm, 100-fs laser pulses at ∼700 mJ/cm2 with a spot size 120 µm in diameter, using a low NA lens [16]. These results showed that the formation of a thin layer of high-density electrons and excitation of surface plasmon polaritons (SPPs) induced with intense fs laser pulses are responsible for the formation of the nanostructure on the silicon suboxide (SiOx, x ≈ 1) film. Because this material can be easily oxidized to SiO2 by thermal treatment at ∼1000°C in air [15], a nanostructured SiO2 surface is expected to be obtained. SiOx (x ≈ 1) has the thermal parameters such as specific heat capacity, thermal diffusivity, melting point similar to those of SiO2, while the complex refractive index is quite different [15].
In this paper, we report that homogeneous SiO2 nanostructures with a single spatial frequency can be formed using a two-step ablation method and heat treatment, and that the transmittance of nanostructured surfaces with a period of d ∼ 240 or 320 nm increases as the reflectivity decreases.
2. Experimental
As a target, we used a silicon suboxide (SiOx, x ≈ 1) film deposited on a fused silica substrate by thermal evaporation (Leybold UNIVEX 350, Cologne, Germany) [15]. The film thickness was set to be 1.4 µm to reduce interaction of the laser beam with the nearby interface between the film and substrate. A homogeneous nanostructure with a single spatial frequency was formed on the SiOx surface using a two-step ablation process with fs laser pulses [17]. The optical setup for the ablation process was almost the same as that used in previous experiments [17–20]. Briefly, a linearly polarized laser pulse with wavelength of λ ∼ 800 nm, pulse width of 100 fs, and repetition rate of 10 Hz output from a chirped-pulse amplification Ti:sapphire laser system was split into two beams (Beams 1 and 2) by a half mirror. The beams were focused by 300-mm focal-length lenses and then spatially and temporally superimposed at the target surface. The horizontally polarized Beams 1 and 2 were normally and obliquely incident on the target, respectively. The incident angle θ was set to θ = 60° to form an interference fringe with a period of Λ = λ/sinθ ∼ 924 nm. In the optical line of Beam 2, we set a retroreflector on an x stage to align the delay time between Beams 1 and 2. The focused spot diameter of the beams measured with a CCD camera was ∼135 µm at 1/e2 intensity. The energy of each beam was adjusted by a pair of half-wavelength plates and polarizers placed in each optical path. In the first step, a fringe pattern was formed on the target through single pulse ablation with the interference fringes produced by spatially and temporally overlapping the two beams, each having a fluence of F1. In the second step, superimposed multiple shots of only beam 1 on the fringe pattern with N pulses at fluence F2 produced the nanostructure.
After laser irradiation, the target was annealed at 1000°C in air using a tube furnace. To observe the modification of the SiO2, we measured the transmission spectrum of the target at normal incidence using white light of λ = 200–1000 nm from a laser-driven light source (EQ-99, Energetiq Technology, Inc., USA) and a fiber spectrometer (HR4000CG-UV-NIR, Ocean Insight, USA). The surface morphology of the target was observed with a scanning electron microscope (SEM, JSM-6510, JEOL, Japan) and a scanning probe microscope (SPM, SPM-9700, Shimadzu Corporation, Japan). In the SEM observation, the target was coated with a platinum layer ∼6 nm in thickness to improve the conductivity of the surface in order to obtain a clear image. A two-dimensional Fourier transformation (using the image processing software, SPIP, Image Metrology, Denmark) was performed on the SEM image in a 5 × 5 µm2 region to acquire the spatial frequency distribution along the polarization direction and to evaluate the period size d.
The reflectivity and transmittance of the target surfaces were evaluated based on the color brightness of photographs taken with an optical digital microscope (VH-Z500R & VH-5500, Keyence, Japan). Figures 1(a) and 1(b) show schematic illustrations of the optical configuration for the measurement procedures of the reflectivity and transmittance, respectively. As shown in Fig. 1(a), non-polarized white light from a halogen lamp was reflected with a half mirror installed in the lens barrel of the optical microscope to irradiate the target surface placed on a black plate at normal incidence. As shown in Fig. 1(b), the white light from the lamp was incident vertically from below the target placed on a glass plate. The microscopic images of the surfaces illuminated with the light were acquired with a CCD camera. The spectrum of the light from the halogen lamp was measured with a spectrometer. As shown in Fig. 1(c), the light has a broad spectrum in the visible region of λ = 350–800 nm. To evaluate the reflectivity and transmittance of the target, the exposure of the camera was adjusted with the following procedure. Firstly, we took a photograph of a black plate to determine the zero-point reflectivity and transmittance. Next, we took photographs of a silver mirror and the glass plate to set the exposure, so as not to saturate the color brightness in the image data of the surface. Here, the reflectivity R and the transmittance T were determined to be R = 97.1% and T = 100%. Finally, we confirmed that the measured values of R = 7.8% and T = 92.1% of a synthetic quartz plate (2 mm in thickness) placed on the black plate or the glass plate were in good agreement with those calculated from the refractive index of the synthetic quartz by using the Fresnel equation. Here, the values of R and T for the transparent materials include the reflection at two surfaces of the material.
Fig. 1. Schematic drawing of measurement procedures for the reflectivity (a) and the transmittance of the nanostructured SiO2 surface (b). (c) The spectrum of the light from the halogen lamp.
3. Results and discussion
In a preliminary experiment, we observed the nanostructures formed on SiOx targets irradiated with single fs laser pulses (Beam 1) for comparison with the homogeneous nanostructures formed by the two-step ablation process. Figure 2(a) shows the SEM and SPM images of the surface irradiated with 100 pulses of Beam 1 at a fluence of 410 mJ/cm2, together with the spatial frequency spectrum. The images clearly show that line-like structures extending in the direction perpendicular to the polarization direction are formed on the target and have a broad spatial-frequency distribution, corresponding to non-uniform periods in a range of d = 230–920 nm having a peak at d ∼ 359 nm. The average depth of the nanostructure was ∼150 nm.
Fig. 2. SEM (upper) and SPM (middle) images of SiOx surfaces irradiated with fs laser pulses, and the corresponding spatial frequency spectra (bottom). The surface structures were formed with (a) single beam fs laser irradiation of 100 pulses at a fluence of 410 mJ/cm2 at normal incidence and the two-step ablation process for (b) N = 0, (c) N = 10, and (d) N = 20 at F1 = 420 mJ/cm2, F2 = 310 mJ/cm2. E denotes the direction of the polarization. The number in the frequency spectrum denotes the period of the structure in nm. The red triangles represent the position of the grooves formed in the first step.
Using the two-step ablation process, a homogeneous nanostructure was formed on the SiOx surface. In the first step, we produced a fringe pattern on the surface irradiated with one fs pulse (Beams 1 and 2) each having a fluence of F1 = 420 mJ/cm2, as shown in Fig. 2(b). The pattern exhibits an average depth of ∼30 nm and frequency peak at Λ = 923 nm with its harmonic components. In the second step, the fringe pattern was divided into finer fringes with N superimposed single fs pulses (Beam 1) at F2 = 310 mJ/cm2. Figures 2(c) and 2(d) show the results for N = 10 and 20, respectively. The red triangles denote the position of grooves formed by the first step. After N = 10, the surface image in Fig. 2(c) represents parallel dark lines of ablation traces on the ridge near the initial groove, while no strong ablation takes place in the groove. This preferential ablation on the ridge is due to the near-field enhancement at the high-curvature regions along the ridge edge. Increasing N to 20, the surface structure becomes a homogeneous nanostructure with a uniform period of d = 244 nm ∼ Λ/4, as also shown by the isolated peak in the spectrum shown in Fig. 2(d). The average depth was measured to be ∼170 nm.
The previous paper [16] has reported that the nanostructure is formed on a SiOx film by plasmonic near-fields generated in the interface between SiOx and the surface layer having high-density electrons exited by the fs pulse. Calculations using this physical model as reported in the previous paper [16] reproduce the period d of nanostructure observed in our present work well. In addition, the period d of the homogeneous nanostructure formed by the two-step ablation process is in a good agreement with Λ/q, where q is an integer. The result shows that a standing wave mode of SPPs could be excited on SiOx, as shown in the previous paper reported about GaN, stainless steel, and TiN [17–20].We also observed a nanostructure with a uniform period of d = 479 nm ∼ Λ/2 formed on SiOx irradiated for F1 = 420 mJ/cm2, F2 = 470 mJ/cm2, and N = 10, and one with d = 323 nm ∼ Λ/3 for F1 = 420 mJ/cm2, F2 = 440 mJ/cm2, and N = 10. The large period d formed with the high fluence F2 is as same as that of homogeneous nanostructures formed on GaN [17,19], stainless steel [18,20], and TiN [18]. These results suggest that the wavelength of SPPs increases by increasing the thickness of the high-density electron layer generated in the SiOx surface with rising fluence F2 of the fs laser pulse, as shown in the previous work using a diamond-like carbon film [21,22].
Figure 3 shows the transmission spectra of the targets after annealing, compared with that before annealing. The initial SiOx target has an optical absorption edge at λ = 400 nm, while its position shifts to λ = 350 and λ = 300 nm as the annealing time increases to 48 and 96 hours, respectively. After annealing for 144 hours, the transmittance T reaches T ∼ 92% in a range of λ = 210–1000. The result shows that SiOx is modified to SiO2 [15]. Figure 4 shows the SEM images of the nanostructured surfaces after annealing in air at 1000°C for 144 hours, together with the spatial frequency spectra. As shown in this figure, the spatial frequency spectra of the nanostructure were not changed by annealing.
Fig. 3. Transmission spectra of SiOx films on fused silica before (black line) and after annealing in air at 1000°C. The green, blue, and red lines denote annealing times of 48, 96, and 144 hours, respectively.
Fig. 4. SEM (upper) images of SiOx surfaces irradiated with fs laser pulses and annealed in air at 1000°C for 144 hours, and the corresponding spatial frequency spectra (bottom). The surface structures were formed with (a) F1 = 420 mJ/cm2, F2 = 470 mJ/cm2, N = 10, (b) F1 = 420 mJ/cm2, F2 = 440 mJ/cm2, N = 10, and (c) F1 = 420 mJ/cm2, F2 = 310 mJ/cm2, N = 20. E denotes the direction of the polarization. The number in the frequency spectrum denotes the period of the structure in nm.
Figure 5 shows the reflectivity and transmittance of targets having homogeneous nanostructures formed using the two-step ablation process and annealed in air at 1000°C for 144 hours, plotted as a function of the period size d. The insets represent examples of microscopic images in the vicinity of the ablation traces. Because the homogeneous nanostructures were formed in an area of ∼6 µm in radius from the focal spot center, the reflectivity and transmittance were evaluated from the average brightness of these areas. As shown in Fig. 5, the reflectivity is almost constant at R ∼ 4%, and does not depend on d. For the target with d = 480 nm, the transmittance of T ∼ 91.7% is lower than that of the non-irradiated target. Decreasing d to 320 nm and 240 nm, the transmittance of the targets increases to T ∼ 92.4% and T ∼ 92.8%. Here, the reflectivity and transmittance of the target having an inhomogeneous nanostructure formed by single-beam fs laser pulses, as shown in Fig. 2(a), were R ∼ 2.9% and T ∼ 91.4%. These results clearly show that the reflectivity of a surface having a homogeneous nanostructure smaller than the wavelength decreases as the transmittance increases, and that the two-step ablation process is effective for inducing anti-reflection properties.
Fig. 5. (a) Reflectivity and (b) transmittance of target having homogeneous nanostructures plotted as a function of the periodicity. The insets depict examples of microscopic images in the vicinity of the ablation traces. The dashed lines represent the reflectivity and transmittance of the non-irradiated target.
We next discuss the anti-reflection properties of a surface having a homogeneous line-like nanostructure formed by fs laser pulses. Assuming that one surface has no reflection, the reflectivity and transmittance will be measured as R ∼ 4% and T ∼ 96% in this experiment, respectively, because the camera image is acquired from the light reflected at the opposite surface. As shown in Fig. 5, for the surface having a homogeneous nanostructure with d = 240 nm, this is indeed the case for the reflectivity: R decreases to ∼4%, while T only increases to ∼93%. Three possible reasons for this phenomenon are discussed below.
Firstly, the line-like nanostructure was smaller than the wavelength of light along the direction parallel to the periodicity, and the illumination light was non-polarized. The anti-reflection effect, therefore, should be less than that for a nanostructure with a non-directional periodicity such as a dot-like nanostructure [4]. Moreover, our nanostructured target has low-spatial-frequency surface roughness in the direction perpendicular to the periodicity. The scattering of the illuminating light induced by the roughness would lead to a decrease in R and T.
Second, the spatial frequency spectrum was generated by transformation of the SEM image of the nanostructured surface in a 5 × 5 µm2 region, as shown in Figs. 2(d) and Fig. 4(c), showing that the surface has a narrow spectrum with a peak at d = 240 nm. On the other hand, surface roughness with a size of a few hundred nanometers from debris on the surface can be also seen in the SEM and SPM images of Fig. 2(d) and the SEM image of Fig. 4(c). This roughness would induce scattering of the illuminating light to prevent an increase in T. This can be seen from the spot-like change in the transmittance near the center of the ablation trace, as seen in the inset of Fig. 5(b).
Third, the depth of the nanostructures is not large and the cross-sectional shape is not needle-like, which are typical features of ideal anti-reflective structures [4]. The cross-sectional shape of the periodic nanostructures formed by superimposed shots of fs laser pulses strongly depends on various parameters such as the material properties, laser fluence, and the number of laser pulses [23]. In particular, it has been reported that the depth of nanostructures depends mainly on the pulse number [17]. Optimizing the pulse number is expected to enable the formation of deep nanostructures and further increase the transmittance of the surface.
4. Conclusion
We have investigated the reflectance and transmission of SiO2 surfaces having homogeneous nanostructures formed by fs laser pulses. Using a two-step ablation process and low-NA focusing lens, low-fluence fs laser pulses produced a nanostructure with a uniform period of 240 nm on a SiOx film, which has a large nonlinear optical absorption coefficient compared to other glass materials. Heat treatment in air at 1000°C oxidized the target to form SiO2. This took 144 hours for a 1.4 µm thick SiOx film. Thinner films will be oxidized much faster. The results of reflectivity and transmittance measurements showed that the formation of homogeneous nanostructures smaller than the wavelength decreases the reflectivity as it increases the transmittance, and that the two-step ablation process is effective for creating glass surfaces with anti-reflection properties.
Funding
Grant-in-Aid for Scientific Research (B) (18H01894); The Izumi Science and Technology Foundation (2021-J-024); the Joint Usage/Research Program on Zero-Emission Energy Research, Institute of Advanced Energy, Kyoto University (ZE30C-0, ZE31C-01); Deutsche Forschungsgemeinschaft (IH 17/27-1).
Acknowledgments
The authors would like to thank Y. Oki for performing the preliminary experiment and K. Sugioka and A. Narazaki for their advice on the physical process of femtosecond laser processing of glass.
Disclosures
The authors declare no conflicts of interest. The funders had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript; or in the decision to publish the results.
Data Availability
Data underlying the results presented in this paper are not publicly available at this time but may be obtained from the authors upon reasonable request.
References
1. W. H. Southwell, “Coating design using very thin high- and low-index layers,” Appl. Opt. 24(4), 457 (1985). [CrossRef]
2. D. Poitras and J. A. Dobrowolski, “Toward perfect antireflection coatings. 2. Theory,” Appl. Opt. 43(6), 1286–1295 (2004). [CrossRef]
3. J. Q. Xi, M. F. Schubert, J. K. Kim, E. F. Schubert, M. Chen, S. Y. Lin, W. Liu, and J. A. Smart, “Optical thin-film materials with low refractive index for broadband elimination of Fresnel reflection,” Nat. Photonics 1(3), 176–179 (2007). [CrossRef]
4. S. Chattopadhyay, Y. F. Huang, Y. J. Jen, A. Ganguly, K. H. Chen, and L. C. Chen, “Anti-reflecting and photonic nanostructures,” Mater. Sci. Eng. R Reports 69(1-3), 1–35 (2010). [CrossRef]
5. P. B. Clapham and M. C. Hutley, “Reduction of lens reflexion by the “moth eye” principle,” Nature 244, 281–282 (1973). [CrossRef]
6. S. J. Wilson and M. C. Hutley, “The optical properties of ‘moth eye’ antireflection surfaces,” Opt. Acta (Lond) 29(7), 993–1009 (1982). [CrossRef]
7. S. Y. Chou, P. R. Krauss, and P. J. Renstrom, “Imprint lithography with 25-nanometer resolution,” Science 272(5258), 85–87 (1996). [CrossRef]
8. Y. Shimotsuma, P. G. Kazansky, J. Qiu, and K. Hirao, “Self-organized nanogratings in glass irradiated by ultrashort light pulses,” Phys. Rev. Lett. 91(24), 247405 (2003). [CrossRef]
9. V. R. Bhardwaj, E. Simova, P. P. Rajeev, C. Hnatovsky, R. S. Taylor, D. M. Rayner, and P. B. Corkum, “Optically produced arrays of planar nanostructures inside fused silica,” Phys. Rev. Lett. 96(5), 057404 (2006). [CrossRef]
10. R. Wagner, J. Gottmann, A. Horn, and E. W. Kreutz, “Subwavelength ripple formation induced by tightly focused femtosecond laser radiation,” Appl. Surf. Sci. 252(24), 8576–8579 (2006). [CrossRef]
11. E. Kannatey-Asibu Jr., “Principles of Laser Materials Processing,” (John Wiley & Sons, 2008), pp. 409–430.
12. H. R. Philipp, “Optical properties of non-crystalline Si, SiO, SiOx and SiO2,” J. Phys. Chem. Solids 32(8), 1935–1945 (1971). [CrossRef]
13. S. Hernández, P. Pellegrino, A. Martínez, Y. Lebour, B. Garrido, R. Spano, M. Cazzanelli, N. Daldosso, L. Pavesi, E. Jordana, and J. M. Fedeli, “Linear and nonlinear optical properties of Si nanocrystals in SiO2 deposited by plasma-enhanced chemical-vapor deposition,” J. Appl. Phys. 103(6), 064309 (2008). [CrossRef]
14. S. Minissale, S. Yerci, and L. Dal Negro, “Nonlinear optical properties of low temperature annealed silicon-rich oxide and silicon-rich nitride materials for silicon photonics,” Appl. Phys. Lett. 100(2), 021109 (2012). [CrossRef]
15. T. Fricke-Begemann, J. Meinertz, R. Weichenhain-Schriever, and J. Ihlemann, “Silicon suboxide (SiOx): laser processing and applications,” Appl. Phys. A 117(1), 13–18 (2014). [CrossRef]
16. T. Takaya, G. Miyaji, I. Takahashi, L. J. Richter, and J. Ihlemann, “Fabrication of periodic nanostructures on silicon suboxide films with plasmonic near-field ablation induced by low-fluence femtosecond laser pulses,” Nanomaterials 10(8), 1495 (2020). [CrossRef]
17. K. Miyazaki and G. Miyaji, “Nanograting formation through surface plasmon fields induced by femtosecond laser pulses,” J. Appl. Phys. 114(15), (2013).
18. K. Miyazaki, G. Miyaji, and T. Inoue, “Nanograting formation on metals in air with interfering femtosecond laser pulses,” Appl. Phys. Lett. 107(7), 071103 (2015). [CrossRef]
19. G. Miyaji and K. Miyazaki, “Fabrication of 50-nm period gratings on GaN in air through plasmonic near-field ablation induced by ultraviolet femtosecond laser pulses,” Opt. Express 24(5), 4648 (2016). [CrossRef]
20. Y. Tamamura and G. Miyaji, “Structural coloration of a stainless steel surface with homogeneous nanograting formed by femtosecond laser ablation,” Opt. Mater. Express 9(7), 2902 (2019). [CrossRef]
21. G. Miyaji and K. Miyazaki, “Origin of periodicity in nanostructuring on thin film surfaces ablated with femtosecond laser pulses,” Opt. Express 16(20), 16265–16271 (2008). [CrossRef]
22. Y. Iida, S. Nikaido, and G. Miyaji, “Sub-100-nm periodic nanostructure formation induced by short-range surface plasmon polaritons excited with few-cycle laser pulses,” J. Appl. Phys. 130(18), 183102 (2021). [CrossRef]
23. K. Miyazaki and G. Miyaji, “Mechanism and control of periodic surface nanostructure formation with femtosecond laser pulses,” Appl. Phys. A 114(1), 177–185 (2014). [CrossRef]
