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We present a controllable fabrication of nanogratings and nanosquares on the surface of ZnO crystal in water based on femtosecond laser-induced periodic surface structures (LIPSS). The formation of nanogrooves depends on both laser fluence and writing speed. A single groove with width less than 40 nm and double grooves with distance of 150 nm have been produced by manipulating 800 nm femtosecond laser fluence. Nanogratings with period of 150 nm, 300 nm and 1000 nm, and nanosquares with dimensions of 150 × 150 nm2 were fabricated by using this direct femtosecond laser writing technique.
© 2014 Optical Society of America
1. Introduction
For a long time, nano-scientists are dreaming to realize arbitrary nanostructures with feature size of sub-100 nm. Direct femtosecond laser writing (DFLW) provides an extremely versatile approach for achieving this goal. In the past decade, nanostructures with a resolution of sub-100 nm have already been realized by two-photon polymerization [1–5]. However, this technology is limited to resin materials.
Periodic nanoripples on semiconductors, dielectrics and metals induced by femtosecond laser pulses have attracted much attention [6–17]. Taylor et al. reported their experimental efforts towards developing photonic and biophotonic applications of femtosecond laser-induced self-organized planar nanocracks inside fused silica glass [18]. They fabricated one, two, three and multiple nanoslots with width of 20 nm by controlling laser pulse energy near the threshold for nanocrack formation. By reducing femtosecond laser fluence to slightly higher than the threshold value, Liao et al. achieved a single 37 nm wide hollow channel in glass by DFLW method [19], and after, three dimensional integration of nanofluidic channels was fabricated inside glass [20]. Buividas et al. also reported the fabrication of single 50-70 nm wide groove by laser ablation via a controlled ripple formation on the surface of sapphire [12].
Because of the great potential applications in new type optoelectronic devices, wide band gap semiconductors have attracted increasing attention. Because of the high efficiency of blue luminescence, ZnO has been investigated intensely for searching novel techniques to fabricate nanostructures that exhibit excellent optical properties [21–24].
In this paper, based on the experiments of 800 nm femtosecond laser-induced periodic surface structures (LIPSS) on semiconductors, we study the fabrication of single groove with width less than 40 nm and double grooves with distance of 150 nm on the surface of ZnO crystal in pure water. Gratings with periods of 150 nm, 300 nm and 1000 nm and periodic nanosquares with dimensions of 150 × 150 nm2 and 150 × 250 nm2 are further achieved.
2. Experimental
The schematic diagram of experimental setup is shown in Fig. 1. A Ti:sapphire amplifier (Legend Elite, Coherent), operated at a central wavelength of 800 nm, pulse duration of 50 fs and repetition rate of 1 kHz, was used as the light source. A combination of a half wave plate and a Glan prism was used to control the polarization direction and the laser intensity. The laser beam was spatially cleaned by a pin hole with diameter of 100 μm. Hereafter, a near perfect Gaussian laser beam was entered into the microscope. A water immersion objective with numerical aperture (NA) of 1.2 was used to focus the beam. The input aperture of the objective is 5 mm, and it is over covered by the laser beam with a diameter of 10 mm. A CCD was used to monitoring the process of laser fabrication. There are two types of definition of focus diameter of high NA objective. One is defined at the 1/e2 intensity level according to the formula of dfoc = 1.22λ/NA [12]. Another is defined as the full width at half maximum (FWHM) of intensity profile of point spread function, and the diameter is dfoc = 0.61λ/NA, here λ is laser wavelength in air [2, 25]. In this paper, the focus diameter was estimated to be of 400 nm according to the second definition, and laser fluence is calculated with it.
Fig. 1 Schematic diagram of the experimental set-up. HWP: Half wave plate, GP: Glan prism, L1 and L2: Lens with focus length of 400 mm, PH: Pin hole with diameter of 100 μm.
The sample is a commercial ZnO crystal with dimensions of 10 × 10 × 1 mm3 (MTI Corporation, Hefei, China). The two surfaces were both optically polished with roughness less than 10 nm. The sample was dipped in a water cell and mounted on a 3D pizeo translation stage controlled by a computer. The laser beam was normally focused on the sample surface. After radiation, the sample was immersed in ethanol and cleaned for 10 min with an ultrasonic cleaner. The surface nanostructures in ablation area were observed by scanning electron microscope (SEM).
3. Results and discussion
3.1 Fabrication of single nanogroove
Figure 2 shows the SEM images of the surface nanostructures radiated by 800 nm femtosecond laser. The exposure time is 75 ms, which means 75 pulses overlapped at each ablation spot. The structures in Figs. 2(a)-2(c) were fabricated by using water immersion objective and Figs. 2(d)-2(f) were prepared in air atmosphere. Nanocracks with orientation perpendicular to the direction of laser polarization are formed in the ablation spot, which is a common phenomenon in laser-induced surface ripples [26].
Fig. 2 (a)-(c): SEM images of nanocracks fabricated by using water immersion objective. The laser fluences are (a) 2.74 J/cm2, (b) 2.64 J/cm2, and (c) 2.54 J/cm2, respectively. (d)-(f): SEM images of nanocracks fabricated by using dry objective with laser fluences of (d) 1.52 J/cm2, (e) 1.44 J/cm2, and (f) 1.36 J/cm2, respectively. The pulse number is 75 in all pictures. The arrow in (a) indicates the laser polarization (E).
Three nanocracks appeared in the ablation spot in Fig. 2(a) when the laser fluence is of 2.74 J/cm2. The distance between two nanocracks is about 155 nm, and the crack width is of 41 nm. At lower laser fluence of 2.64 J/cm2, the distance of double nanocracks is of 150 nm, and the crack width is 35 nm, as shown in Fig. 2(b). In Figs. 2(a) and 2(b), the sample surface is obviously melted. By decreasing the laser fluence to 2.54 J/cm2, a single 32 nm wide nanocrack with less melt matter is found in the ablation spot in Fig. 2(c). If the laser fluence is less than 2.50 J/cm2, there is no nanocrack observed. This means we can fabricate one, two, and three nanocracks in a controllable way. Huang et al. reported ultrafast ablation on ZnO by femtosecond laser pulses. The feature sizes of laser-induced periodic ripples decreased spontaneously with the pulse increasing and the crater extending, and eventually approached 10-nm scale. The narrow single nanocrack was proposed to be caused by ultrafast, nonthermal ablation for extraordinary electrostatic field enhancement [27].
We have also studied the formation of nanocracks radiated by laser pulses with different number N, such as 100, 25, 15, 10, and 7. If the pulse number N<10, there is no nanocrack appeared in the ablation spot no matter how much the laser fluence is.
The formation mechanisms of femtosecond laser-induced periodic surface structures (LIPSS) on semiconductors, metals and dielectrics have been studied intensely in the last ten years [6–17]. Although a clear formation mechanism is still lacking, it is believed that surface plasmon polariton (SPP) plays an important role. The period of low spatial frequency LIPSS (LSFL) is approximately equal to the SPP wavelength. With further radiation of laser pulses, high spatial frequency LIPSS (HSFL) formed via splitting of LSFL [10,27]. Makin and Vorobyev et al. also reported similar experimental results in metal surface [28,29]. We studied the formation process of HSFL, and found that low periodic ripples with a distance of 330 nm split into two ripples with a distance of 170 nm. We have researched the formation of nanoripples on stainless steel in air and in water [10,30]. The results showed that the interaction of local SPP with the incident laser induces the localization of laser on the protuberance, which leads to the splitting of ripples. In one word, we propose that the HSFL on ZnO surface is caused by the splitting of LSFL. However, the physical mechanism of the formation of LIPSS is very complex, and needs further studies.
The sample was immersed in water in the experiment because the water can carry away most of the ablated material and spare heat generated during laser ablation [31]. We have also researched the formation of nanocracks on ZnO by using dry objective (NA = 0.9). Figures 2(d)-2(f) are SEM pictures of the nanocracks formed in air atmosphere at different laser fluences. By comparing Figs. 2(a)-2(c) with Figs. 2(d)-2(f), we find that the nanocracks formed in water are smoother and more uniform.
According to Rayleigh formula dz = 2λ/NA2, the FWHM of Rayleigh length of 800 nm laser is only of 1100 nm. In order to focus the laser beam on the sample surface accurately, we changed the sample position along Z-axis and the laser fluence. When the laser fluence is larger than 2.60 J/cm2, the ablated lines can be observed by optical microscope in real time. We fabricated many lines and moved the sample simultaneously along Z-axis at an interval of 200 nm. Comparing the CCD and SEM images, we find that the position error of laser focus can be controlled in a range of 600 nm. Figure 3 shows that the single nanogroove is very good if the laser focus vary in the range of –300- + 300 nm. Here, the minus “–” means the sample surface is in front of the center of laser focus, and the plus “+”, behind the laser focus.
Fig. 3 SEM images of the nanogrooves fabricated when the sample surfaces are at (a) –500 nm, (b) –300 nm, (c) + 300 nm, and (d) + 500 nm, respectively. The laser fluence is of 2.52 J/cm2, and writing speed keeps at 10 μm/s.
Figure 4 shows nanogrooves fabricated at different writing speeds, where the laser fluence keeps at 2.56 J/cm2. When the writing speed is 2 μm/s, the nanogrooves are broken and irregular, as can been seen in Fig. 4(a). This is because the water is ionized intensely at the slow translation speed of 2 μm/s. Bubbles and turbulence are formed in water, which leads to scattering and defocus of the incident laser. When the speed increases to 6 or 8 μm/s, nanogrooves become continuous but also irregular. Single regular nanogroove can be achieved at the writing speed of 10 or 12 μm/s. However, when the writing speed is higher than 13 μm/s, the nanogroove becomes discontinuous. These results indicate that the writing speed is a critical parameter for the formation of single nanogroove at appropriate laser fluence.
Fig. 4 SEM images of nanogrooves induced by laser with a fluence of 2.56 J/cm2. The writing speeds are (a) 2 μm/s, (b) 6 μm/s, (c) 8 μm/s, (d) 10 μm/s, (e) 12 μm/s, and (f) 13 μm/s, respectively. The laser polarization (E) and writing direction (S) are shown in (a).
Figure 5 presents the formation of nanogrooves changing with laser fluences and writing speeds. When laser fluence is higher than 2.70 J/cm2, or writing speed is lower than 8 μm/s, there are three or more grooves formed in the ablation area. Single or double regular grooves are formed easily at proper laser fluence and writing speed, as shown in the blue and red rectangles. Here, the uncertainty of laser fluence is 2%.
Fig. 5 A parameter space for the formation of nanogrooves: laser fluence (J/cm2) vs. writing speed (μm/s).
A combination of a Glan prism and a power meter was used to measure the uncertainty of laser polarization in front of the objective. Because the laser is linearly polarized, laser power changes when rotating the Glan prism. By measuring the maximum value Imax and minimum value Imin, we got the uncertainty of laser polarization is less than 5% according to the equation P = (Imax-Imin)/(Imax + Imin). The nanogrooves shown in Figs. 3 and 4 indicate that the laser with the polarization uncertainty of 5% is done well on the fabrication of regular nanogrooves.
Figure 6 presents SEM images of the nanocracks fabricated on the surface of ZnO crystal radiated by laser with different polarization directions. The laser was linearly polarized in the two cases. Both the writing direction (S) and the polarization (E) are shown in each panel. It can be seen that the orientations of the nanocracks are always perpendicular to the laser polarization direction.
Fig. 6 SEM images of nanocracks induced by 800 nm femtosecond laser. The angles between the writing direction (S) and the laser polarization (E) are (a) 45°, and (b) 0°. The laser fluences are (a) 3.0 J/cm2, and (b) 3.2 J/cm2, and the writing speeds are both at 14 μm/s.
3.2 Fabrication of nanogratings
Figure 7 presents nanogratings fabricated by DFLW method with the writing speed fixed at 10 μm/s. The electronic shutter was closed during the acceleration and deceleration processes of the pizeo stage so that the sample was only radiated by laser when the stage moved at a constant velocity. When the laser fluence was 2.64 J/cm2, double uniform nanogrooves with a distance of 150 nm were formed, as shown in Fig. 7(a). By decreasing the laser fluence to 2.54 J/cm2, a single nanogroove with width less than 40 nm was formed, as shown in Fig. 7(b). The groove edges were sharp and clean, and almost no ablated material deposited in the surrounding region. Single- and double-line gratings with a period of 1 μm were fabricated by DFLW method, as shown in Figs. 7(c) and 7(d). If laser fluence was higher than 2.72 J/cm2, three-line gratings were fabricated, but the nanogrooves were irregular, and the edges were covered with melted materials.
Fig. 7 (a) SEM images of double-line nanogroove induced by laser with a fluence of 2.64 J/cm2, and (b) single nanogroove for laser fluence of 2.54 J/cm2, (c) double-line grating for laser fluence of 2.61 J/cm2, and (d) grating for laser fluence of 2.52 J/cm2. The laser polarization (E) and writing direction (S) are shown in (a).
Realizing large area, uniform nanostructures by DFLW method is very important for future applications. It is easy to obtain a single nanogroove with length of several tens micrometers. However, the external random vibration affects the experimental results during laser direct writing, and leads to distortions in the nanogrooves. In order to fabricate uniform nanostructures with large area, we will improve the experimental setup in two aspects: one is isolating the external vibration, and another is equipping with an auto-focusing system with resolution less than 100 nm.
Figures 8(a) and 8(b) show two regular nanogratings fabricated by DFLW method with periods of 300 nm and 150 nm. The nanogratings were fabricated by raster scanning from left to right. Only one nanogroove was carved in each scanning, and the writing speeds were both at 10 μm/s. The laser fluences were (a) 2.47 J/cm2, and (b) 2.42 J/cm2, which are slightly lower than the value of fabricating single nanogroove. This is because the period is smaller than the FWHM of laser focus, and the preceding laser writing creates defects and causes the ablation threshold decreasing. The width of single groove was less than 40 nm - only 1/20 of the laser wavelength. The distance between two adjacent grooves in Fig. 8(b) was only 150 nm - breaking the diffraction limit successfully. Table 1 presents the fabrication conditions of the nanogrooves and nanogratings shown in Figs. 7 and 8.
Fig. 8 SEM images of nanogratings for the laser fluence of (a) 2.47 J/cm2, and (b) 2.42 J/cm2, respectively.
Table 1. Fabrication Conditions of Nanogrooves and Nanogratings in Figs. 7 and 8.
In order to measure nanogroove depth, we prepared a sample with dimensions of 1 × 1 × 5 mm3. A nanograting with a period of 200 nm was fabricated at the sample edge with a laser fluence of 2.43 J/cm2 and writing speed of 10 μm/s. The SEM image of the cross section of nanograting is shown in Fig. 9. The depths of the nanogrooves are between 200 nm and 300 nm. The proper laser fluence and writing speed to achieve single regular nanogroove are in a small range, as shown in Fig. 5. Therefore, the nanogroove depth is limited to 150-320 nm.
Fig. 9 SEM image of the cross section of nanogratings for the laser fluence of 2.43 J/cm2 and writing speed of 10 μm/s.
3.3 Fabrication of nanosquares
We fabricated nanosquares by using two-step DFLW method. First, a nanograting with a period of 250 nm was fabricated along the X-axis with a laser fluence of 2.45 J/cm2 and a writing speed of 10 μm/s. And then the laser polarization direction was rotated by 90°. The second nanograting with a period of 150 nm was fabricated on the nanograting along the Y-axis with a laser fluence of 2.20 J/cm2 and a writing speed of 10 μm/s. Figure 10 presents the SEM image of the nanorectangles with dimensions of 150 × 250 nm2. There are many ablated particles and debris deposited in the nanogrooves, which scatter greatly the laser during the second writing, and induce broken and tortuous nanogrooves. The nanorectangles are very irregular. Periodic nanosquares with dimensions of 150 × 150 nm2 are also fabricated, which is much worse. In order to improve the quality of nanosquares, the sample was immersed in ethanol and cleaned for 10 min with an ultrasonic cleaner after the fabrication of nanogratings. In this case, the nanosquares become much cleaner and more regular, as shown in Fig. 11.
Fig. 10 SEM images of nanorectangles with dimensions of 150 × 250 nm2. The scale bar is 300 nm. The writing directions “X” and “Y” are shown on the right side.
Fig. 11 SEM images of nanosquares for the laser fluences at the second writing along the Y direction are of 2.23 J/cm2 (a), and 2.16 J/cm2 (b). The scale bars are 300 nm in (a) and (b). The writing directions “X” and “Y” are shown on the right side.
The laser fluence during the second laser writing affects the square quality greatly. Figure 11(a) shows that if the laser fluence is too high, the squares are ablated greatly, and some debris was deposited on the surface. The size and shape of the squares are different from each other. Figure 11(b) presents the SEM image of nanosquares fabricated with lower laser fluence. The nanosquares are uniform. However, some of the nanogrooves are broken, and some nanosquares are not split thoroughly. After try many times, we found it is better for the laser fluence at the second writing less than the first one by 0.19-0.23 J/cm2, and the writing speed keeps in the range of 9-11 μm/s. The laser focus is adjusted by the same way as the discussion in Fig. 3.
By controlling the laser fluence, focus, and writing speed carefully, 150 nm periodic nanosquares were fabricated, as shown in Fig. 12(a). Figure 12(b) shows the SEM image of nanorectangles with dimensions of 150 × 250 nm2. Table 2 presents the fabrication conditions of the nanosquares shown in Figs. 10, 11 and 12.
Fig. 12 (a) SEM images of nanosquares with dimensions of 150 × 150 nm2, and (b) nanorectangles with dimensions of 150 × 250 nm2. The scale bars are 300 nm in (a) and (b). The writing directions “X” and “Y” are shown on the right side.
Table 2. Fabrication Conditions of Nanosquares in Figs. 10, 11, and 12.
4. Conclusions
In conclusion, we have demonstrated the formation of nanogrooves on the surface of ZnO crystal in water based on femtosecond laser-induced periodic surface nanostructures. By controlling the laser fluence slightly higher than the threshold value, and the writing speed at 10 μm/s, single regular nanogroove and double nanogrooves were fabricated by DFLW method. Moreover, nanogratings with the periods of 150 nm, 300 nm and 1000 nm, and nanosquares with dimensions of 150 × 150 nm2 were fabricated by using this technique, and the width of each nanogroove was less than 40 nm. The 150 nm periodic nanogratings and the 150 nm periodic nanosquares are both far beyond the diffraction limit. We prospect this DFLW method can be used in fabricating functional super-resolution nanostructures on semiconductors, dielectrics and metals with potential applications in nanophotonics, plasmonics, nanofluidics, biosensor, and optoelectronic devices.
Acknowledgments
This work is supported by the National Natural Science Foundation of China (11474097, 11274116, 113740995, 11104178, 51132004), National Special Science Research Program of China (2011CB808105), the Program of Introducing Talents of Discipline to Universities (B12024), and the Open Fund of the State Key Laboratory of High Field Laser Physics (Shanghai Institute of Optics and Fine Mechanics).
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