Irradiation inside some transparent materials such as fused silica can induce nanograting structures at the focal area. Here, we investigate experimentally how the nanograting formation can be influenced by tuning the ionization property of the transparent material, which is achieved by irradiation inside a porous glass immersed in water doped with NaCl at variable concentrations. Our results show that the doping of NaCl not only reduces the threshold fluence of optical breakdown, but also leads to nanograting structures with shorter periods. These effects may be attributed to the enhanced photoionization in water doped with NaCl.
©2013 Optical Society of America
Extreme nonlinear interaction of focused femtosecond laser pulses with transparent materials has led to interesting phenomena at both micrometer and nanometer scales, including refractive index modification, formation of nanoripples, nanogratings, nanovoids, nanocrystallizations, etc [1–9]. The micro- and nanostructures have found many applications ranging from information processing  to chemical and biological sensing [11, 12]. Among these discoveries, formation of nanogratings inside transparent materials by femtosecond laser irradiation has attracted broad attention because of its sub-diffraction-limit nature and its potential for optical, polarization sensitive and fluidic applications [13, 14]. Generally speaking, the nanogratings formed by femtosecond laser irradiation emerge as a succession of variable density layers oriented perpendicular to the laser polarization. The period of the nanograting is typically around 200~300 nm, whereas the width of a single perpendicular layer can be merely a few tens of nanometers. Traditionally, the nanogratings formed in fused silica are revealed by scanning electronic microscopy (SEM) examination after etching with hydrofluoric acid. Until now, although great effort has been made on investigating the mechanism behind this phenomenon, a clear picture is still lacking [15, 16].
Recently, it has been found that nanogratings can also be formed in a porous glass immersed in water by irradiation with femtosecond laser pulses of carefully selected pulse energies . This discovery has enabled formation of three-dimensional (3D) nanofluidic channels for DNA analysis . For the mechanism exploration, such porous material also provides unique opportunities to continuously tune the material properties by filling different types of solutions into the nanopores. For example, optical properties could be tuned by filling liquids with different nonlinear optical coefficients. In this work, we place our focus on tuning the ionization property of the porous glass by doping the immersion water with NaCl at different concentrations. Although there is no report on quantitative investigation of strong field ionization in water doped with NaCl as a function of concentration of NaCl, it has been known that the Na(H2O)n clusters formed in water have much lower ionization potentials than that of H2O molecules, leading to reduction of average ionization potential in water doped with NaCl . The results we obtained not only suggest a practical way for promoting the fabrication efficiency due to the reduction of the ablation threshold in water by doping of NaCl, but also reveal interesting dependence of the characteristic parameters of nanograting (e.g., grating period, etc) on the concentration of NaCl. Equally important, the discoveries raise new questions that could shed new light on the mechanism behind the nanograting formation, as presented in the discussion section.
Details of experimental setup can be found elsewhere . Briefly speaking, in our experiment, home-made high-silicate porous glass samples were used as the substrates, which were produced by removing the borate phase from phase-separated alkali-borosilicate glass in hot acid solution. The composition of the porous glass is approximately 95.5SiO2-4B2O3-0.5Na2O (wt.%). The pores with a mean size of ~10 nm are homogeneously distributed in the glass and occupy ~40% in volume of the glass. This special property uniquely allows for tuning the physical and chemical properties inside glass by filling the pores with different kinds of liquid. In this work, since we intend to investigate the influence of ionization property in the glass on the nanograting formation process, water solutions doped with NaCl at different concentrations are chosen as the immersion liquid.
To induce the nanograting structure, a high-repetition regeneratively amplified Ti:Sapphire laser (Coherent, Inc., RegA 9000) with a pulse duration of ~100 fs, a central wavelength of 800 nm and a repetition ate of 250 kHz was used. The Gaussian laser beam with an initial 8.8 mm diameter was trimmed to 1~3 mm-dia. for ensuring a high beam quality. The femtosecond laser beam was then tightly focused into the porous glass with a microscopy objective (N.A. = 1.10), as shown in Fig. 1. As mentioned above, to adjust the properties of the substrate, the porous glass was immersed in pure water as well as water doped with NaCl at different concentrations.
The laser power was controlled by using a combination of a polarizer, a waveplate, and a set of neutral density filters. A half-wave plate was used to change the polarization direction of the linearly polarized femtosecond laser. The samples were translated by a computer-controlled XYZ stage with a resolution of 1 μm and a speed of 30 μm /sec. In all the experiments, the laser pulses were focused ~180 μm below the surface of samples. As the estimated focal spot diameter is ~0.9 µm for the water immersion objective lens we chose, the exposure time in each focal spot can be estimated to be ~1.5 ms, and the number of pulses applied per focal spot is ~370.
To characterize the morphology of the embedded nanograting structures, the samples were cleaved along the plane perpendicular to the writing direction to access the cross section areas of the laser-modified zones. The revealed nanograting structures were directly characterized by SEM (Zeiss Auriga 40). Since hollow cracks were directly formed in the glass, which constitutes the nanogratings, it is not necessary to use chemical etching before the SEM observations.
Figure 2 shows the SEM images of a series of single nanocracks formed inside the porous glass immersed in water doped with NaCl at different concentrations. The polarization of the laser light is horizontal, while the nanocracks are aligned vertically, i. e., perpendicular to the polarization direction of laser beam. It is shown in Fig. 2(a) that the threshold power for obtaining a single nanocrack of a width ~32 nm was 75 mW when the porous glass was immersed in purified water. However, when the porous glass substrate was immersed in NaCl solution at 0.01 M concentration of NaCl, the threshold power for obtaining a single nanocrack of a width of ~36 nm was reduced to 63 mW, as shown in Fig. 2(b). Further increasing the concentration of NaCl to 0.06 M and 0.1 M in the NaCl solution leads to even lower threshold powers of 60 mW and 52 mW for obtaining a single nanocrack, as shown in Figs. 2(c) and 2(d), respectively. From the results in Figs. 2 (a)-2(d), we conclude that the threshold power for inducing optical breakdown in the porous glass immersed in NaCl solution decreases with increasing of the concentration of NaCl. The same trend can also be seen in Fig. 2(e), in which the dependence of the threshold fluence of single-crack formation on the concentration of NaCl is plotted. It is noteworthy that the same trend has also been observed when examining the dependence of the threshold fluence of optical breakdown in plain NaCl solution on the concentration of NaCl, as shown in Fig. 2(f). To determine the optical breakdown threshold, we focus the femtosecond laser beam with the same objective lens into NaCl solution (the parameters of the laser and objective lens can be found in Sec. 2). By gradually increasing the power of femtosecond laser, a weak but visible plasma spot appears on the CCD camera, which indicates the onset of optical breakdown. The consistency between the results in Figs. 2(e) and 2(f) indicates that the damage behavior in porous glass can be associated with the NaCl solutions.
One may notice that with an increasing concentration of NaCl solution, its refractive index will increase. Such effect can influence the focusing conditions, and therefore, may contribute to the reduction of the optical breakdown threshold as well. However, as we can see from Figs. 2(a) and 2(b), adding a small amount of NaCl at a concentration of only 0.01 M into the purified water can lead to a sudden reduction of the optical breakdown threshold by ~20%. The refractive index increase is small in such a case, suggesting that the major cause of the decreasing in breakdown threshold is the lowering of ionization potential in the NaCl solution.
It should be mentioned that the single nanocracks formed in Fig. 2 all have a similar transverse width of ~30 nm, which does not vary significantly with the laser peak power or the concentration of NaCl in the solution. It has also been observed that (see, e. g., Fig. 3 below) even for the nanogratings consisting of an array of nanocracks, which can be produced by increasing the peak power of femtosecond laser pulses, the width of each single nanocrack does not change much although the nanocracks are distributed in different areas within the focal spot (corresponding to different laser peak intensities). The mechanism for such phenomenon deserves further investigation, but it could be more or less associated with the thermo-mechanical (elasto-plastic) properties of the material .
Next, in Fig. 3, we show different morphologies of nanogratings formed inside the glass by irradiation with femtosecond laser pulses at a fixed average power of 75 mW for different concentrations of NaCl in water. Figure 3(a) shows that a single nanocrack with a width of ~32 nm has been obtained inside the glass immersed in purified water at this power. However, as shown in Fig. 3(b), a nanograting composed of five nanocracks was created when replacing the purified water with a NaCl solution at a 0.01M doping concentration. The width of the whole grating (i.e., the horizontal distance between the first and last vertical lines in the nanograting, as indicated in Fig. 3) is measured to be ~1.05 μm in the horizontal direction, and the period of the grating is ~240 nm. Figure 3(c) shows that when the concentration of NaCl further increases to 0.6 M, a nanograting of seven self-organized nanocracks with an overall width of ~1.13 µm and a period of ~182 nm was formed at the same laser power of 75 mW.
Figure 4 quantitatively shows how the period of nanograting evolves with the increasing laser power and increasing concentration of NaCl in water. Generally speaking, at a fixed concentration of NaCl, the period of nanogratings decreases with increasing power, which is consistent with the results reported previously [21, 22]. On the other hand, we find that at a fixed output power of femtosecond laser, the period of nanogratings decreases with increasing concentration of NaCl in water. The comparison of these observations seems to suggest that a stronger ionization, which could be realized either by increasing the laser peak intensity or by reducing the ionization threshold, would lead to smaller period of the nanograting structures. This is consistent with the previous observation . In our opinion, the decreasing period with enhancement of photoionization may suggest that an above critical density has been reached, resulting in a semiopaque plasma. Below in Sec. 4, we will try to provide a tentative understanding on these trends.
Until now, the mechanism behind formation of nanograting structures inside transparent materials by femtosecond laser irradiation is still an open question and under hot debate. Previously, several models have been proposed which suggest that the interaction of laser and generated bulk electron plasma, accompanied by multi-pulse irradiation induced self-organization effect play important roles [4, 15]. Recently, it is also suggested that exciton mediated self-organization could cause formation of nanogratings in glass [24, 25]. To date, none of these models have been widely accepted. To facilitate the discussion on the mechanism behind nanograting formation, we list the major findings as follows:
1. We have observed formation of single nanocracks in porous glass by reducing the peak intensity of femtosecond laser pulses . For the volume nanograting formed inside glass or crystals, such threshold phenomenon, to our knowledge, has not been reported previously. It should be mentioned that formation of single ripple on the surface of silica glass has been observed, suggesting that there exists some connection between the formation mechanisms of surface nanoripples and volume nanogratings .
2. We have observed that despite the fact that the porous glass immersed in water is microscopically inhomogeneous due to the random locations of the pores as well as the relatively large pore size (dia. ~10 nm), self-organized periodic nanogratings can still be formed with high reproducibility by femtosecond laser irradiation.
3. We have observed that the threshold intensity for formation of single nanocracks decreases with the increasing concentration of NaCl in water.
4. We have observed that at a fixed laser intensity, the number of nanocracks in the nanogratings increases with increasing concentration of NaCl in water, whereas the period of the nanograting decreases in such a case.
5. We have observed that by continuously scanning the laser beam in a horizontal direction, the nanocracks can be smoothly connected into a through nanochannel inside the glass. The cross sectional shape of the nanochannel is very uniform along the nanochannel .
These facts actually offer a lot of useful information for gaining deeper insights on nanograting formation in glass. First of all, the first observation on the threshold effect provides a clear evidence that this technique is able to concentrate light to an extremely small region in comparison with the writing wavelength (i.e., less than 1/20 of the writing wavelength as we have shown in  and in this experiment). There are, however, two possible mechanisms either of which could allow for formation of such single nanocrack. In the first mechanism, the formation of single nanocrack could originate from a highly localized field enhancement, without the need of excitation of a plasma wave spanning the entire focal spot area. The second mechanism is fundamentally different from the first one, which suggests that single nanocrack could also be formed with the excitation of a broad plasma wave spanning the entire focal spot. Due to a threshold effect, only in the central region of the focal spot, the electron density can be sufficiently high for inducing ablation of material. Identification of the mechanism requires new experimental schemes which should enable direct observation of the transit plasma excitation instead of the final nanostructures.
Secondly, the combination of the second and the third observations indicate that in our experiment, the ionization must originate from the immersion liquid but not from the glass, as tuning of ionization threshold is achieved by changing the concentration of NaCl in water. As a result, it is the free electrons generated from the liquid that induce the subsequent ionization and ablation in the porous glass. However, as we can see in Fig. 3, periodically self-organized nanogratings span the entire focal spot area. Therefore, the following questions are raised: how the plasma waves excited in the liquid communicate with the plasma waves excited in glass, and why periodic gratings could still be formed with the nanopores randomly distributed in glass? Understanding these questions will probably provide important insight for uncovering the mechanism behind the nanograting formation.
Thirdly, with the fourth observation listed above, we may safely conclude that the formation of nanograting also depends on the density of electrons in the excitation region, as higher electron density could be achieved with either higher peak intensity or lower ionization potential, which in turn causes formation of the gratings of smaller period .
Lastly, the fifth observation is a clear indication of the role of “seeding effect” in the formation of the nanograting, i. e., the structures formed earlier will participate the later process which leads to the formation of new structures. Thus, during the scan of the laser beam, the consecutive single nanocrack can be precisely aligned along a straight line to form the nanofluidic channel, despite the facts that there are instabilities in the laser pointing directions, the translation of XYZ stage, and so on, which could lead to deviation of the focal spot from the predesigned writing tracks.
Although we have not completely clarified the mechanism, we stress here that the porous glass indeed provides us a unique route for investigating the mechanism behind nanograting formation, as one can modify the intrinsic properties of the material with much flexibility.
To conclude, we have carried out a systematic investigation on the formation of nanograting structures in porous glass immersed in NaCl solutions of various concentrations. In a general sense, the nanopores could enhance local fields, leading to enhanced ionization as well as strongly localized plasmon excitation in the beginning. However, it is now still unclear how these effects have influenced the formation of nanogratings in the porous glass. The doping of NaCl allows us to continuously tune the ionization potential in the porous glass, providing an additional control knob for studying the mechanism underlying nanograting formation in glass. The threshold intensity for inducing ablation in the porous glass can be efficiently reduced at high concentrations of NaCl in the immersion solution. From the application point of view, this finding will be of benefit to high-throughput, cost-effective fabrication of micro and nanofluidics in glass. Although in the current experiment, only NaCl-doped water solution is used for altering the intrinsic ionization property in the porous glass, it will be straightforward to extend this strategy to other types of immersion liquids for tuning either mechanical or optical properties in the porous material.
Fadhil A. Umran and Yang Liao contributed equally to this work. The authors would like to thank Rüdiger Grunwald, Stefan Nolte, and Peter Kazansky for their helpful discussion. This research is financially supported by National Basic Research Program of China (2011CB808100), and National Natural Science Foundation of China (60921004, 61108015, and 61275205).
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