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We report on the formation of nanoscale tungsten spikes generated on subwavelength periodic ripples which built up by single beam 800 nm femtosecond laser pulses. The nanospikes have a diameter ranging from 10 to 100 nm and are up to 250 nm in length. The nanospikes orientate from the ridges toward the valleys of the ripple structures independent of the polarization of the light. The heterogeneous nucleation of the liquid phase at the irradiated surface and the inhomogeneous surface roughness are considered as the mechanism of this nanospike formation.
©2007 Optical Society of America
1. Introduction
Fabrication of nanostructures by laser technology has attracted great attention since nanotechnology revealed its great potential. Nanostructures, from zero- to three-dimensional structures such as quantum dots, nanowires, and nanogratings, etc., have been achieved in metal, dielectrics, semiconductors, and polymers by using laser technology [1–10]. Among them, femtosecond laser fabrication of nanostructures has especially become the focus of intensive research due to the ultra-high intensity and ultra-fast properties of femtosecond lasers [3–10]. Several approaches have been used to fabricate nanoscale structures by femtosecond lasers, including: (a) taking advantage of the well-defined modification threshold of materials to make only the central part of the beam able to modify the material, the diffraction limit can be overcome by choosing the peak laser fluence slightly above the threshold value [4, 5], (b) using femtosecond pulses in combination with a scanning near-field optical microscope to directly ablate materials to achieve nanostructuring [6], (c) applying single beam femtosecond laser pulses interference with excited plasma waves to induce nanoscale periodic structures [3], (d) using femtosecond pulses to induce chemical reactions for the production of nanocomposites [7], (e) using femtosecond laser direct ablation to induce nanoparticles or nanowires [8, 9].
It is well-known that ripple-like periodic structures can be generated by a single laser beam on the surface of semiconductor [11], metal [12], and dielectric [13] materials due to the interference of incident pulses with light scattered from defects at the surface [14–17]. Very recently, we found that ripples with spatial period from subwavelength to half-wavelength can be observed by using linearly and circularly polarized femtosecond light with variation of laser fluence, number of pulses, and incident angle [18], which is in contradiction to the conventional observations [11–17].
In this paper, we report on the formation of nanoscale tungsten spikes grown on subwavelength periodic ripples induced by single beam, 800 nm femtosecond laser pulses. The orientation of the nanospikes is from the ridges towards the valleys of the ripple structures independent from the polarization-type of the light (linear or circular). The nanospikes have a diameter ranging from 10 to 100 nm and are up to 250 nm in length. Our observation provides a new approach to the fabrication of nanostructures which is different from the approaches (a) to (e) in the above classification.
2. Experimental
A commercial multi-pass amplified Ti:Sapphire mode-locked laser system (Femtopower Compact PRO, Femtolasers Produktions GmbH) with 800 nm wavelength, 33 fs pulse duration, ~1 mJ average energy, and 1 kHz repetition rate was used as the irradiation source. After passing through a variable neutral density filter and a mechanical shutter, the beam was guided into a microscope and was focused normally on the sample surface by a 10× objective (NA=0.30). The sample could be moved with the help of a computer controlled 3D-XYZ stage. The number of laser pulses applied to the sample was controlled by using a frequency control shutter system (FPC, Femtolasers Produktions GmbH). The laser polarization was adjusted by inserting half-wave or quarter-wave plates in the beam path. All experiments were performed in air under atmospheric pressure. The morphology of femtosecond laser-induced surface modifications is inspected by using an optical microscope and an ultra-high resolution field emission scanning electron microscope (FE-SEM, Hitachi S-4800). Tungsten foils with thickness of 50 µm were used in this study. Before and after laser irradiation, the samples are cleaned by acetone to remove ambient dust and most of the plume deposited on the sample surface.
3. Results and discussion
Figures 1(a)–(d) show the subwavelength ripple structures induced by linearly and circularly polarized light beams with normal incidence under a fluence of 4.2 J/cm2 with 100 shots were applied for all polarization states. Ripple-like periodic structures with periods of approximately 550 nm have been observed. The period is in contrast with previous findings in the literature which found the period of ripples to be approximately equal to the laser wavelength at normal incidence of the light [11–17]. Furthermore, the orientation of ripples is aligned perpendicularly to the direction of polarization for linearly polarized light, while +45° for left circularly polarized light and -45° for right circularly polarized light with respect to the incident plane.
Fig. 1. SEM images of subwavelength ripple structures induced by different polarization and their 2D Fourier transformation. These structures are generated with 100 shots irradiation under a fluence of 4.2 J/cm2 at (a) vertical, (b) horizontal, (c) left circular, and (d) right circular polarization, respectively. The arrows indicate the direction of polarization. The corresponding 2D Fourier transformations are shown in (e)–(h).
In order to investigate the induced ripple structures in more detail, the images in Figs. 1(a)–(d) were subjected to a two-dimensional Fourier transformation, as shown in Figs. 1(e)–(h). One can see that the central areas reflect large spatial frequencies, such as large-scale corruption and, since the Fourier images are symmetrical to their center, four points in each figure correspond to the frequencies of the ripples in the original SEM images. Note that the four points lie on a vertical line [Fig. 1(e)], a horizontal line [Fig. 1(f)], and diagonal lines [Figs. 1(g) and 1(h)] through the image centers, because the image intensities in the spatial domain change the most when traversing them vertically, horizontally, and diagonally, respectively. If the surface morphology is observed with higher magnification, some interesting nanostructures on the subwavelength ripples can be observed, as shown in Fig. 2. A great number of nanospikes with smooth sidewalls are observed that have formed on the subwavelength ripples induced by different polarized light beams. If careful observation is made of the apparent random growth direction, it is found that most of the nanospikes root on the ridge of the ripples and grow towards the valley of the ripples. Explicitly, the formation of the nanospikes is not dependent on the ripple formation, be it by linearly or circularly polarized light. Most of the nanospikes grow with caps like mushrooms and a few of them without caps, like needles. The nanospikes have a diameter ranging from 10 to 100 nm (at the neck of the mushroom-like spike) and are up to 250 nm in length.
Fig. 2. SEM images of nanospikes grown on ripples induced by (a, e) vertical, (b, f) horizontal, (c, g) left circular, and (d, h) right circular polarization. Each row has the same magnification.
When the SEM accelerating voltage is increased (15 kV instead of 10 kV used for Figs. 1 and 2) a double-layer structure can be observed in the nanospikes (Fig. 3). Here we only give cases of the structures induced by vertically and left circularly polarized light, as the structures induced by horizontally and right circularly polarized light are similar.
Fig. 3. SEM images of double-layer nanospikes induced under a fluence of 7 J/cm2 with 30 shots irradiation at (a, b) vertical and (c, d) left circular polarization, respectively, which are detected at a higher accelerating voltage of 15 kV.
Further investigation was made of the evolution of the formation of nanospikes by applying varied fluences and number of pulses. We applied 1 to 100 shots with fluences in the range from 3 to 12 J/cm2. The SEM analysis indicates that the formation of nanospikes depends on both the laser energy and the number of shots. With a single shot, no nanospikes were observed within the applied fluence range. With a minimum fluence of 6 J/cm2 and 10 shots, the formation of nanospikes starts with morphologies similar to those presented in Fig. 2. It is worth mentioning that some interesting surface structures were observed when a different number of shots with a fluence of 3 J/cm2 was applied, In Figure 4(a) the structural changes caused by 10 shots are shown. It can be observed that a new class of nanoscale ripples with periods ranging from 30 to 100 nm with orientation parallel to the direction of polarization. This type of nanoripples is similar to type c structures defined in [17], i.e. periodic structures with a spacing of λ/cosθ and orientation parallel to the polarization [17], where λ is laser wavelength and θ is incident angle, while the period of nanoripples in our case is much less than the wavelength of the laser. We suggest that the inhomogeneous surface roughness is the origin of the nanoripples. When the laser irradiates the sample surface, surface plasmon waves are generated, whose density would not be uniform on the nanoscale area due to this inhomogeneous surface roughness. The nonuniform plasmon waves will interfere with incident light, causing a localized melting or ablation area with a much smaller size than the laser wavelength. Figure 4(b) shows some nanospheres with diameters ranging from 20 to 120 nm on the spot when the sample was irradiated with 40 shots at 3 J/cm2 pulse energy. Figure 4(c) shows that the nanospikes occur when the sample is irradiated with 100 shots. Compared to the nanospikes shown in Fig. 2, their dimension is smaller. For comparison, we as well show an image of sample surface before laser irradiation in Fig. 4(d).
Fig. 4. Nanoripples (a), nanospheres (b) and nanospikes (c) induced by femtosecond laser with number of shots 10 (a), 40 (b) and 100 (c) under fluence 3 J/cm2. (d) Surface without femtosecond laser irradiation.
Based on the evolution of the nanospike formation, several points can be concluded: 1) The nanospike formation has a threshold fluence for any number of shots no matter if ripples occur or not, 2) The dimension of nanospikes changes slightly with variation of fluence and number of shots, 3) The orientation of nanospikes is random without ripple formation. When ripples are formed, the nanospikes are mostly perpendicular to the ripples.
Studies of the ultrafast laser induced solid-liquid phase transitions in metals [19, 20], performed by time-resolved pump-probe techniques, suggest that the melting of the surface layer can take from several to hundreds of picoseconds. At such a time scale, the heterogeneous nucleation of the liquid phase at the irradiated surface and propagation of the melting front can penetrate into the bulk of the crystal. On the other hand, the original surface roughness will increase the optical absorption, causing melting on localized nanoscale sites, which will subsequently cool and freeze to form nanospikes. With the increase of the number of pulses, the formation of nanospikes will further enhance laser absorption to excite surface plasmons that will interfere with the incident light. The interference patterns will be inscribed on the surface to form the subwavelength ripple structures. Without subwavelength ripple formation, the orientation of the nanospikes is random. With subwavelength ripple formation, the orientation of the nanospikes is from the ridge toward the valley of the ripple. This is because a periodic modulated intensity field is formed due to the incident light interfering with the excited plasmons. Consequently, the temperature at the surface heated by high intensity parts of the intensity field is higher than that of surface heated by low intensity parts of the intensity field, causing a large temperature gradient from the ridge to the valley of the ripple. Thus the orientation of nanospikes is mainly from the bridge to the valley of the ripple, following this temperature gradient. The exact physical mechanism of the formation of nanospikes is still under investigation.
4. Conclusions
In summary, we have observed the formation of tungsten nanospikes growing on periodic ripples induced by single beam 800 nm femtosecond laser pulses. The formation of nanospikes depends both on laser fluence and the number of shots. The heterogeneous nucleation of the liquid phase at the irradiated surface and the inhomogeneous surface roughness are considered as the mechanism of this nanospike formation. Other nanostructures, nanoripples and nanospheres were observed on surfaces without the ripple-like structure. With this method of femtosecond laser material processing a new way for structuring material surfaces at the nanometer scale is demonstrated, which may find applications in modifying field emission [21], improving directivity of thermal light sources [22], and enhancing surface catalytic activities [23].
Acknowledgments
Q. Z. Zhao acknowledges fellowship support from the Alexander von Humboldt Foundation.
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