An approach for fabricating large area uniform nanostructures by direct femtosecond (fs) laser ablation is presented. By the simple scanning technique with appropriate irradiation conditions, arbitrary size of uniform, complanate nano-grating, nano-particle, and nano-square structures can be produced on wide bandgap materials as well as graphite. The feature sizes of the formed nanostructures, which can be tuned in a wide range by varying the irradiation wavelength, is about 200 nm with 800 nm fs laser irradiation. The physical properties of the nano-structured surfaces are changed greatly, especially the optical property, which is demonstrated by the extraordinary enhancement of light transmission of the treated area. This technique is efficient, universal, and environmentally friendly, which exhibits great potential for applications in photoelectron devices.
©2008 Optical Society of America
Over the past 20 years, nanofabrication techniques have been attracting abundant research interests for the novel electronic, optical, and mechanical properties of various nanostructures exhibiting broad potential applications in new hi-tech products. In a mass of physical, chemical, and mechanical techniques for nanofabrication, the laser-assisted methods have shown to be efficient and environmentally friendly, and aroused researchers’ greatest attention . For example, the pulsed-laser deposition for the preparation of nanostructures is a well-known and mature technique for nanofacbrication [1, 2]. Nowadays, the development and wider applications of femtosecond (fs) laser provided with the advantages of high peak fluence and minimal thermal effect , have opened new routes for nano-processing, such as all kinds of laser direct-write techniques [4-8] by virtue of multi-photon absorption that can break through the diffraction limit and machining the inside of transparent materials.
In laser ablation, a remarkable phenomenon — the formation of laser-induced periodic surface structures , has been observed extensively in various materials with multifarious irradiation conditions. In particular, the interesting phenomenon that fs laser irradiation can induce ripple structures with periodicities significantly shorter than laser wavelength on a variety of materials, has again aroused great interest for the origin and applications of such subwavelength structures [10-19]. This novel ripple structure, whose periodicity can be as small as 1/11 of the laser wavelength , and morphological characteristic resembles the grating fabricated by advanced nanofabrication technologies, is greatly different from the classic ripples that always exhibit a larger periodicity and smooth profile. Using circularly polarized light, sub-wavelength particle structures analogously can be formed on material surface with similar irradiation condition [14, 21]. Compared to other physical and chemical methods for preparation of nanostructures, this technique — direct induced nanostructures on material surface by ultrafast laser ablation is quite simple and efficient, which can open new roads for material nanofabrication. However, due to some technical limitations, this approach has not been widely applied for material processing till now. Firstly, for the Gaussian profile of laser intensity in space, which always induces bowl-like ablation crater, it is hard to obtain uniform, complanate large area nanostructures on material surface. Secondly, with the scanning technique, the pre-formed nanostructures may be polluted by the redeposited materials induced by laser plume. Thirdly, laser ablation treatment always changes the crystal structure of material and induced thick amorphous layer, which will alter the physical properties of material and is harmful to the excellent performance of devices. These problems hinder the further promotion of this technique for applications in nanofabrication.
In this paper, we demonstrate that arbitrary size of uniform, complanate nano-grating and nano-particle structures can be produced on wide bandgap materials, even the opaque material — graphite, by fs laser irradiation with a simple scanning technique under appropriate processing conditions. The surface morphologies of the large area nano-structures are controlled by the laser fluence and polarization, the scanning speed and interval, and the focussing condition. In addition, the feature sizes of the formed nanostructures can be tuned in a wide range by varying the irradiation wavelength. The physical properties of the nano-structured surfaces are changed greatly, especially the optical property, which is demonstrated by the extraordinary enhancement of light transmission of the treated area. This nanofabrication technique is simple, efficient, universal, and environmentally friendly, which exhibits a broad application prospects for nano-processing of material surface.
2.1 Experiment setups
A polished ZnO crystal sample (KMT, surface roughness < 5 Å), a polished ZnSe crystal sample (KMT, surface roughness < 5 Å), and a highly-oriented pyrolytic graphite sample (SPI Supplies, Grade SPI-II) were used in this study. A regenerative Ti:Sapphire amplifier system (Spectra Physics Hurricane) with a central wavelength of 800 nm, pulse duration of 125 fs, and repetition rate of 1 kHz is used in the experiments for fabricating large area nanostructures. The wavelength dependence experiments are implemented by the optical parametric generators (OPG, EKSPLA, PG411/511) pumped by a mode locked Nd:YAG laser (EKSPLA, PL2143B) with pulse duration 30 picosecond (ps) and repetition rate 10 Hz. A variable neutral density filter was utilized to adjust the irradiation fluence, and the linearly polarized Gaussian laser beam was focused on the sample surface by a convex lens. The sample was placed in a three-dimension micro-positioning stage with the surface perpendicular to the propagation direction of laser beam. The laser processing is carried out by translating the sample in the plane parallel to sample surface, and focusing is achieved by translating the sample in the perpendicular direction. The surface morphologies of the formed nanostructures were analyzed with scanning electron microscopy (SEM, JEOL JSM-6380 and Quanta 400F), and analyzed by laser micro-Raman spectrometer (MRS, Renishaw inVia, excited at 514.5 nm).
2.2 Demonstration of processing techniques
2.2.1 Obtain complanate ablated crater
In order to make the area ablated by focused fs laser pulse flat and uniform, we used three techniques. Above all, the transverse mode of laser should be homogenized. A simple method was employed to achieve this objective (Fig. 1(a)). The laser beam was first focused by a convex lens with a short focal length (125 mm), and then diverged by the confocal convex lens with a long focal length (250 mm) to get the parallel light beam with a large diameter. Then the large Gaussian light beam was controlled by an aperture to get light beam with small diameter, which was provided with an almost flat-top intensity distribution — a symmetrical transverse mode. Note that although the incident beam is almost flat-top, the intensity distribution in the focus is not flat-top. In other words, via this method we are able to obtain a highly axisymmetric intensity distribution in the focus, but it is still hard to make the ablation crater be complanate for such a radial intensity distribution. Nevertheless, in virtue of the advantages of the fs laser — the well-defined ablation threshold and “non-thermal” ablation mechanism , we can achieve this objective in a simple way as shown in Fig. 1(b). When the pulse peak fluence is near ablation threshold, only a small area of the focus will be ablated for the “non-thermal” characteristic of fs laser ablation. Thus, in the ablated area the light intensity is almost even — amounting to the ablation threshold, and as a result, the ablated area is complanate, which is clearly demonstrated by crater II in Fig. 1(b). In addition, under such an irradiation condition, nanostructures can appear with fewer pulse number than the case of high fluence, as the morphologies of the crater centers irradiated by different fluences shown in Fig. 1(b). Note that if the pulse number is very large, the crater profile will likewise turn to a bowl-like. Hence, in order to ensure the complanation of the treated area, the pulse number in each location should be moderate, i.e., the laser irradiation time should be appropriate. Another technique for reinforcing the processing complanation is the use of convex lens with long focal length (300 mm), i.e. low numerical aperture (NA). Thus, the focus has a larger size, i.e., a wider complanate area can be obtained by a single scan, which will greatly improve the efficiency of processing.
2.3 Reduce thermal effects
How to reduce the influence of the redeposited materials induced by laser plume and the amorphous layer induced by thermal effects of laser is an important issue for obtaining a clean surface with the intrinsic crystal structure. Actually, these negative factors can be reduced greatly by ensuring the laser fluence near the ablation threshold, which can bring the “non-thermal” ultrafast mechanism, for instant the Coulomb explosion (CE) mechanism [3,20], as the dominant ablation mechanism for nanostructure formation. In this way, the plume will have a high spurting speed, and not redeposit on the pre-formed area. In addition, with the CE mechanism, the thermal effects can be enormously reduced (see crater II in Fig. 1(b)). On the other hand, the condition of laser fluence near ablation threshold is consistent with the above condition for the realization of complanate processing area. Contrarily, the fluence far higher than ablation threshold will induce abundant melted (see crater I in Fig. 1(b)) and redeposited structures, and hinder the nanostructure formation.
2.4 Scanning technique
Based on the above techniques, we can produce an arbitrary size of complanate nanostructure by a simple scanning technique as shown in Fig. 1(c). By properly adjusting the interval (D) of two adjacent scanning lines, we can bring the whole processing area to be highly uniform and complanate. As described above, laser fluence is the decisive condition for the roughness on the ablated region (in other words “local roughness”, which can be seen from the different morphologies of the scanning lines in case I and II of Fig. 1(c)), whereas the appropriate interval of scanning lines is the prerequisite for obtaining a complanate ablated surface in a large scale, which is demonstrated by the distinct morphologies of the ablated areas in case II and III of Fig. 1(c)). In practice, we make the adjacent scanning lines overlapping each other with a majority of the ablated region (generally, the scanning interval is smaller than half of the line width of a single scanning), so that the whole ablated area is more complanate. Because when two adjacent scanning lines overlap each other with a majority of the ablated region, the nano-grating (or other structures) produced by the latter scanning can “inherit” the order of the nano-grating produced by the former scanning. Thus, a regular pattern is formed in the whole ablated area, where the respective scanning lines can not be distinguished. But if the overlapping of two adjacent scanning lines is too much, an “over-hatch” effect may appear: the “hatch” time for the nano-grating formation becomes too long, i.e., pulse number at given position is too large, which is unfavorable for the evenness of the ablated area. In addition, the movement speed of the stage is another key parameter for the nanostructure morphology. In general, if the stage translates too fast, the irradiation time for each position will be not long enough to incubate the nanostructure; but if the speed is too slow, the “over-hatch” effect will occur — the complanate nanostructure will be destroyed, and the surface will be covered by the bumpiness structures for the positive feedback mechanism of cupped structures. In a word, the interval of two adjacent scanning lines and the scanning speed should be moderate in order to obtain a complanate ablated surface (make the roughness of the whole ablated area small).
3. Results and discussion
3.1 Large area uniform nanostructures fabricated by the method
3.1.1 Nano-grating structures on wide bandgap materials
Figures 2(a) and 2(b) shows SEM images of ZnO and ZnSe irradiated by the 800 nm fs laser of linear polarization with the following processing conditions: irradiation fluence slightly higher than respective ablation thresholds, scanning speed of 1 mm/s, and scanning interval of 8 µm. With these processing parameters, the 200×200 µm2 area can be produced in 46 sec., demonstrating the high efficiency of this nanofabrication technique. In the whole ablated areas, the uniform, complanate nano-gratings are formed with few redeposited and amorphous materials, which demonstrate the neglectable thermal effects. The nano-gratings have periodicities about 200 nm, and their orientations are perpendicular to the laser polarization, which is a remarkable characteristic for such an ultrafast laser induced nanostructure. In addition, our results with vertical and horizontal scanning manners demonstrate that the scanning direction has little influence on the morphologies of the formed nanostructures.
Above we have shown the nano-grating structures fabricated by linearly polarized laser. To our surprise, by the irradiation of elliptically polarized laser with major to minor axis rate 2, we can obtain a highly uniform and complanate nano-grating as shown in Fig. 2(c). It seems that the use of elliptically polarized laser can reduce the strong feedback of polarization dependence and make the treated area more even. The phenomenon is further confirmed by the circular polarization experiments as described following. Our experiments also demonstrate that, when the major to minor axis rate of elliptically polarized laser approaches 1, i.e., the elliptical polarization becomes circular polarization, the formed nanostructure will change from nano-grating to nano-particle, which is agree with the intuition.
3.1.2 Nano-particle structures on wide bandgap materials
The surface morphologies of ZnO and ZnSe treated by circularly polarized laser with similar processing conditions as the linearly polarized case are shown in Figs. 2(d) and 2(e) respectively. The area of Fig. 2(d) locates at the verge of the treated area, so the untreated surface can be seen in the right side of the figure. In general, the whole processed areas are covered by the uniform nano-particles, and the sizes of the nano-particles for ZnO and ZnSe are 210 and 160 nm respectively. Obviously, the nano-particle structures induced by circularly polarized light, which do not exhibit gurgitation in a large scale, is extraordinarily complanate. In detail, the nano-particles are closed packed in a self-organization manner to achieve the largest duty cycle on a planate surface. Our results under different irradiation conditions demonstrate the complanation is easier to obtain with circular polarization than linear polarization. This phenomenon should be due to axisymmetric distribution of electric field vector of circularly polarizated laser, which do not induce the orientation ablation of polarization sensitiveness that is prominent in linearly polarized laser ablation.
3.1.3 Nanostructures on graphite
It is general accepted that the laser induced nanostructures are apt to appear in wide bandgap material ablated by fs laser. Nowadays, our ablation experiments on highly-oriented pyrolytic graphite show that it is also easy to fabricate nanostructures on graphite by short pulse laser irradiation . Here we show that with the scanning technique, large scale nanostructures can also be fabricated on graphite surface (see Fig. 2(f)). Interestingly, the nano-gratings on graphite have multi-periodicities about 200 nm and 120 nm as shown in area I and II of Fig. 2(f).
3.1.4 Nano-square structures on wide bandgap materials
Other than the nano-grating and nano-particle structures, we further fabricated the novel nano-square structure with certain special ablation techniques, such as two beam alternate ablation technique  and perpendicular overlapped processing technique. The two beam alternate ablation technique in fixed position can produce uniform nano-square structure in a crater. Here we further combine such a technique into the scanning technique for the fabrication of large area nano-square structure (see Fig. 3(a)). Although the nano-square here is not as uniform as in the fixed position case , it is complanate in a large scale and beneficial to application. Interestingly, there are some black holes on the surface, which are due to the nano-squares breaking off at these locations.
Otherwise, nano-square can be obtained by the perpendicular overlapped processing technique in a simpler manner: first fabricate a large area nano-grating structure, then rotate laser polarization or sample with an angle of 90 degrees, and repeat the processing on the preformed nano-grating area. In this way, the two orthogonal overlapped nano-gratings will form the nano-square. Note that choice of the conditions for the second processing should be cautious, for the perpendicular nano-grating is likely to completely cover the pre-formed nano-grating if the ablation time is too long. The nano-square fabricated by this technique (see Fig. 3(b)) exhibits different morphology from that by the two beam alternate ablation. During the second processing, we have observed a scattering effect due to the fact that the surface has been nanostructured by the first processing, which can arise the scattering of incident light. But the effect has not seriously influenced the formation of the second perpendicular nano-grating. We consider this phenomenon is as a result of the strong polarization dependence of the grating direction. It means that the linearly polarized laser only efficiently induces nano-grating with corresponding direction, and the pre-formed orthogonal nano-grating acts little to incident laser for the polarization selection. In fact, the phenomenon is similar to that in the case of two beam alternate ablation , where two beams with orthogonal polarizations can induce orthogonal nano-gratings in a manner of mutual non-interference.
3.2 The influence of processing parameters on nanostructure morphologies
3.2.1 Laser fluence and scanning interval
The influence of laser fluence as well as scanning interval on the morphologies of the nanostructures is shown in Fig. 4. It can be seen that when the irradiation fluence is suitable (slightly higher than ablation threshold), the ablated surface is approximately a plane and the roughness is small, which should be better than 100 nm (see area I of Fig. 4(a)); when the fluence is somewhat higher than the ablation threshold, the planeness of the treated area gets worse (see area II of Fig. 4(a)); further, when the fluence is far higher than the ablation threshold, the redeposited materials occur, the ablated surface becomes bumpy, and the roughness is large, which may be at the micron scale (see area III of Fig. 4(a)). When the scanning speed is fixed, the scanning interval will determine the pulse number imposed in a given position. Comparing area II with area IV in Fig. 4(a), we can see that the morphology of nano-grating fabricated by small interval has no significant difference from the large interval case (although in detail, the morphology in the case of small interval slightly deteriorates due to the “over-hatch” effect as described above). These results imply the laser fluence plays the key role for the morphology and quality of the formed nanostructure when the scanning interval is in an appropriate range. In addition, at threshold fluence, the nano-grating is modulated by the classic ripple (see area I of Fig. 4(b)), which can not be observed in the high fluence case (see area II of Fig. 4(b)). These results also illustrate that along with the increase of laser fluence, the size of the nanostructure increases.
3.2.2 Laser wavelength
The above results demonstrate that the technique — large scale nanostructures directly fabricated by scanning fs laser ablation, is universal for wide bandgap materials as well as graphite. Here we further show that the size of the formed nanostructure can be tuned by varying the incidence laser wavelength. The linear relationship of the laser wavelength with the periodicity of the nano-grating on ZnO (Fig. 5) illustrates that the size of the formed nano-aperture can be controlled in a wide range by a convenient way — varying the laser wavelength, which is easy to realize benefited from the popularization of the optical parametric amplifier (OPA) that can output ultra-short laser pulse in a wide spectrum range.
3.3 The structural and optical characteristics of formed nanostructures
3.3.1 Structural characteristics
In order to evaluate the thermal effects on material surface induced by laser processing, we use the micro-Raman spectroscopy (MRS) to study the morphological and structural characteristics of the formed nanostructures. The MRS of the treated areas (Fig. 2(d) and Fig. 4(b) II) and untreated area are demonstrated in Fig. 6. The sharpest peak at 438 and 99 cm-1 can be assigned to the high and low frequency branch of E2 mode of non-polar optical phonons , which are the strongest modes in wurtzite crystal structure. In the treated areas (spectra b and c), the peaks at 207, 332, 545, and 986 cm-1 attributed to the second order Raman spectrum arising from zone-boundary phonons (multi-phonon processes) and the peak at 573 cm-1 assigned to the A1 LO modes , are more distinct than those in intrinsic surface (spectrum a). In our opinion, this phenomenon should be due to the formation of the nanograting on ZnO, which can active the modes that is forbidden in the configuration of normal incidence of excitation light onto the ZnO (0001) surface, as the case of spectrum a. Although the amorphous structures may also contribute to those peaks, the strong, narrow E2(H) peak indicates that the nano-grating area exhibits mainly the intrinsic crystal structure of bulk ZnO, rather than the thick amorphous structures induced by strong thermal effect that always occurs in long duration pulse laser ablation.
3.3.2 Optical characteristic.
Taking into account the nanoscale of the formed structures, we can expect a strong change of the optical and electric properties of material treated with the techniques. From Fig. 7 — the CCD images of nano-grating and nano-particle structures with a area of 200×200 µm2 taken by a optical microscope with backside illumination of white light, we can see that the treated areas are quite bright, which implies a strong scattering and high transmission efficiency of light in the area. As is well known, the incline incident light has a high reflection ratio on a smooth surface of high refractive index material. Our results demonstrate that with the smooth surface processed by fs laser, the formed nanostructure can suppress the reflection of incline incidence light and extraordinarily enhance the total transmission of incidence light. In addition, interestingly, we can see that the nano-grating areas and the nano-particle areas exhibit different colors under white light illumination: pale yellow and light purple respectively. This phenomenon should be owing to the different morphologies and aperture sizes of the two type nanostructures. It means that the spectrum of the transmitted light can be adjusted via changing the size and morphology of the formed nano-apertures. On the other hand, the phenomenon of strong light transmission enhancement in the treated area implies that such nano-structured surfaces are antireflective, i.e., strongly light-absorbing for opaque fabricated by the technique can lead to broad potential applications in the new photoelectric devices, where higher photoelectric conversion and light transmission efficiency are always challenges, for examples the surface roughness of device for the improvement of luminescence efficiency of LED ; nano-structured the photodiode and solar cell for higher absorption efficiency of light in the visible spectrum range .
Our results show that by fs laser ablation with the simple scanning technique and appropriate irradiation conditions, arbitrary size of nano-grating, nano-particle, and nano-square structures can be fabricated on material surface. The formed nanostructures, which are provided with excellent morphology and chemical characteristics — few redeposited components, thin thermal induced amorphous layer, and dominant intrinsic crystal structure, are uniform and complanate, and exhibit novel and unique photoelectric properties. This technique is simple, efficient, universal, and environmentally friendly, which might attract tremendous interest for the applications in the fields of nano-photoelectron and nano-mechanics.
The authors are grateful to Y. F. Liu and X. R. Zeng for their supports in the experiments. This work has been supported by grants from the Shanghai Institute of Optics and Fine Mechanics (SIOM), Chinese Academy of Sciences (CAS).
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