This paper reports the ultrafast imaging on the formation of periodic surface ripples induced by a single 800 nm, 50 fs laser pulse. The evolution process is observed on a Si surface with a prefabricated nanogroove. The ripples emerge very quickly, only 3 ps after the laser pulse with a fluence of 0.18 J/cm2 irradiating on the surface, and last for several hundreds of picoseconds. The ultrafast dynamics of laser-matter interaction, such as free carrier excitation, carrier and lattice heating, surface plasmon polariton (SPP) excitation, etc, are studied theoretically. The theoretical and experimental results support that the periodic ripples are caused by the periodic energy deposition due to SPP excitation. The emerge time could identify the surface melting causing the formation of periodic ripples, and exclude the other thermal effects, for example, hydrodynamics.
© 2018 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
Laser-induced periodic surface structures (LIPSS) is a universal phenomenon observed in semiconductors, dielectrics and metals [1–13]. According to their periods, the ripples can be separated into high spatial frequency LIPSS (HSFL, Λ < 0.5λ) and low spatial frequency LIPSS (LSFL, 0.5λ < Λ < λ), here λ is the laser wavelength . Several different models have been supposed to explain the formation mechanisms of LSFL, such as surface plasmon polariton (SPP) [15, 16], scattering light of rough surface  and capillary instability in the surface molten layer . Huang et al. attributed the formation of LSFL to the SPP excitation . Das et al. researched the variation of the ripple periods on ZnO surface with laser fluence, and the experimental results agreed well with the SPP model . However, the formation mechanism of LSFL is mainly studied by the combination of theoretical simulation and experimental results of the final periodic structures, and the mechanism of laser-induced periodic ripples is still an open problem.
Femtosecond LIPSS develops very quickly and becomes a powerful and flexible technique for the fabrication of functional surfaces, such as structural color, wettability, and surface-enhanced Raman spectroscopy [20–22]. Study on the ultrafast dynamics occurring in the formation of ripples is useful to control the size, depth and uniformity of ripples, and is crucial in achieving the designing functions .
A complete physical scenario of the LSFL formation with ps resolution is necessary for further understanding the formation mechanism. In order to study the dynamics, many experiments were performed to image the formation process [24–28]. Murphy and associates found that the onset of LSFL began at 50 ps, in the timescale of material ejection, by a non-collinear pump-probe imaging technique . Garcia-Lechuga et al. used a moving-spot multiple-pulse irradiation approach, which was able to resolve spatially and temporally the birth and growth of individual ripples on silicon surface . They demonstrated that the formation process was initiated from free carrier generation, leading to non-thermal melting, liquid phase overheating and rapid solidification into the amorphous phase. Jia et al. employed an ultrafast time-resolved imaging technique to study LIPSS formation on silicon surface. They observed the onset of periodic ripples formation at tens of picoseconds .
The formation of LSFL is related to the surface melting. In order to achieve the variation of the surface temperature, two-temperature model (TTM) is used to simulate the lattice and carrier temperature. Yang et al. studied the melting and evaporation on Si surface according to the evolution of lattice temperature calculated by the TTM, which explained well the erasure of the LIPSS in the center of the ablation region . Wang et al. performed theoretical study of LIPSS on noble metals by using the TTM model, and found that the electron-phonon coupling strength played the dominant role in the formation of LIPSS .
In this paper, the formation dynamics of surface ripples after irradiation by single femtosecond pulse is studied using pump-probe imaging technique. The evolution process of surface ripples is visualized clearly on Si surface around a prefabricated nanogroove . The formation of LSFL after irradiation by a single femtosecond laser pulse, which excluding the irregular and random surface defects created by the multiple pulses, avoids the disturbance of these structures to the observation of LSFL formation. The ripples appear on Si surface at a delay time of 3 ps irradiated by a 50 fs laser pulse with a fluence of 0.18 J/cm2. The timescale could identify the surface melting is the mainly reason for the LSFL formation. In that timescale, many other effects have not happened yet, for example, hydrodynamics . The ripple position keeps unmoved during the evolution processes. The theoretical prediction of the melting time is close to the experiment result according to the two temperature model and the Drude model (TTM-Drude). The LSFL period is calculated by the SPP theory and coincides with the experiment result. The experiment and theoretical results demonstrate that the formation of LSFL is caused by periodic energy deposition due to the SPP excitation.
Figure 1(a) shows the experimental setup of ultrafast pump-probe imaging, which is similar with that shown in . A commercial Ti: sapphire regenerative amplifier laser system (Legend Elite, Coherent) delivers laser pulses at a center wavelength of 800 nm with a pulse duration of 50 fs, a maximum pulse energy of 3.5 mJ and a repetition rate of 1 kHz. The laser beam is separated into two beams by a beam splitter. One laser beam is used as pump beam to induce surface structures on Si crystal. The probe beam is a white-light pulse produced by focusing the other 800 nm laser beam into a water cell of 10 mm thick, and served as the lighting source. The probe pulse goes through a delay line with a resolution of 1 μm, corresponding a temporal resolution of 6.6 fs. The irradiation and imaging are performed with a microscope (Nikon, 80i), in which the pump and probe beams are collinearly focused by an objective lens (100 × , NA = 0.9) on Si surface at normal incidence. The optical spectrum of the white-light pulse is in the range of 450-570 nm. The reflected optical micrograph (OM) image of sample surface is observed by a CCD camera with a spatial resolution of 300 nm.
In order to clearly observing the micro/nanostructures induced by laser pulse, the sample surface had to be moved to the object plane. However, the object plane was very close to the focus plane. The focus diameter is very small, only of 2.6 μm, which is too small to fabricate enough ripples. Therefore, f = −150 mm concave lens is placed in front of the objective lens to diverge the pump laser. The intensity distribution of laser field at the object plane is measured by a CCD camera via the emission light from a ZnSe crystal surface. Figure 1(b) shows that the Gaussian laser becomes a flat-roofed distribution. The laser focus is enlarged to 28 μm in diameter with several rings coming from the diffraction effect of the concave lens. The emission intensity fluctuates mainly in the range of 0.7-1.0 in the inner region for radius r < 12 μm, as shown in Fig. 1(c). The blue emission from ZnSe crystal is excited by a two-photon absorption of 800 nm light. Therefore, the intensity of pump laser field changes in the range of 0.84-1.0 with an average value of 0.87. The laser pulse energy Ep is measured by an energy sensor (PE9-C, OPHIR) with a radius of sensing area of 5 mm. It is put at 3 mm below the objective lens so as to ensure the laser spot radius less than 3 mm. Laser fluence F is calculated by F = Ep / S, where S is the area of laser focus at the object plane.
In this paper, the intensity of the probe pulse is less than 10−4 J/cm2, which is very weak and does not disturb the measurement.
The duration of white-light pulse is a very important parameter because it determines the temporal resolution in the experiments. However, it is very difficult for us to measure directly the pulse duration. Therefore, the pulse duration is estimated by the optical dispersion in the water cell and objective lens. Figure 1(d) shows that the optical spectrum of white-light pulse is in the range of 400-1000 nm, which is narrowed in the range of 450-570 nm by placing a short-wave pass filter with cutoff edge at 550 nm in behind of the water cell. The duration of white-light pulse is estimated by the difference of optical path between the 450 nm and 570 nm light. The refractive index of water is 1.337 for 450 nm, and 1.333 for 570 nm light. The duration of white-light pulse is elongated by 130 fs as passing through the water cell with a length of 10 mm. The objective lens contains mainly a battery of silica lenses, and the effective length is about 20 mm. The refractive index of fused silica is 1.4656 for 450 nm, and 1.4591 for 570 nm light. The white-light pulse width increases by 430 fs as passing through the objective lens. In total, the duration of white-light pulse extends to 0.6 ps as it arrives at the sample surface.
The sample is a commercial available undoped Si-wafer (100) (MTI-group, China) with a thickness of 0.5 mm. The surface is optically polished with a roughness <1 nm. The nanogroove served as surface defects is produced by laser direct writing with a 800 nm, 1 kHz, 50 fs laser focused by a water immersion lens (100 × , NA = 1.2). Uniform nanogroove of 400 nm wide is fabricated for laser fluence F = 0.54 J/cm2 and a scanning velocity of 60 µm/s, as shown in Fig. 2(a). Figures 2(b) and 2(c) shows the image of nanogroove measured by atomic force microscope (AFM). The depths vary in the range of 44 nm to 54 nm with an average value of 46 nm.
The Si wafer is moved by a two-axis translation stage with 1 μm accuracy, which ensure that each pump pulse radiates at a fresh nanogroove. After laser irradiation, the periodic surface ripples are observed by scanning electron microscope (SEM, JEOL JSM-5600) and atomic force microscope (AFM, Nanonavi E-Sweep).
3. Results and discussion
3.1 Zero point
Femtosecond time-resolved microscopy has been used to analyze the structural transformation dynamics (melting, ablation, etc.) induced by intense fs laser pulses in semiconductors [32 – 35]. Pump laser pulse excited the surface layer and initiated the melting and ablation. The excited spot was illuminated with a probe pulse, and observed with an optical microscope. The insets in Fig. 3 present three typical images at the delay time of - 0.5 ps, 0.3 ps and 0.8 ps. The normalized reflectivity is 1.0, 1.09, and 1.20, respectively. The reflectivity increases quickly due to the high density plasma excited by the pump pulse . The excitation laser fluence is 1.7 J/cm2, much larger than the damage threshold of 0.17 J/cm2. In this experiment, the normalized reflectivity is set as the average value of the normalized intensity of 5 images within the delay time of −0.6ps — −1.8 ps. Similar with Ref. 34, the error of zero point determined by this method is less than ± 0.5 ps.
3.2 Ultrafast dynamics
After irradiation by a single pump pulse at fluence of 0.18 J/cm2 with polarization perpendicular to the nanogroove, LSFL with a period of 680 ± 15 nm can be observed and its direction is parallel to the nanogroove, as shown in Fig. 4(a). The annular structures are due to the diffraction effect caused by the f = −150 mm lens. By irradiating the Si surface with a single pump laser pulse, we can distinguish the moment when LIPSS begin to appear and avoid the complications due to irradiation by multiple laser pulses. If irradiating the sample with multiple laser pulses, surface defects, such as debris, crater edges, and periodic structures formed after each laser pulse irradiation. It is difficult to discern separate mechanisms that maybe contribute to the LIPSS formation.
The SEM image in Fig. 4(b) is nearly same with that observed by the optical microscope, which confirms that the experimental system in Fig. 1(a) can be well used in the ultrafast imaging of the formation of LSFL. Figure 4(c) shows the average values of the height profiles measured by AFM in the two rectangles perpendicular to the nanogroove as shown in Fig. 4(b). The height fluctuation is less than 0.6 nm without a given spatial periodicity. This indicates that the surface ripples are observed because of the periodic variations in surface reflectivity rather than height. The reflectivity of the diffraction ring and LSFL regions increases due to the formation of amorphous Si after surface melting [25, 36]. Because both the refractive index and extinction coefficient of the amorphous Si (n = 4.755, k = 0.522) are much higher than those of the crystal Si (n = 4.087, k = 0.041) for the illumination light with the center wavelength of 550 nm, the reflectivity of amorphous Si is larger than that of Si crystal .
The evolution process of laser-induced ripples primarily occurred in the timescale of 0-600 ps. Figure 5 shows the time-resolved OM images of LSFL at different delay times after a single pump pulse irradiation. When the pump and probe pulses irradiate at Si surface simultaneously, only the prefabricated nanogroove is observed in Fig. 5(a). To our surprise, the periodic ripples emerge very quickly. At the delay time of 3.0 ± 0.5 ps, the ripples are visualized beside the nanogroove as shown in Fig. 5(d), and several diffraction rings can be observed due to the reflectivity changes by the formation of surface plasma layer. As time elapsing, the ripples become clearer and brighter due to the reflectivity enhancement of further melting surface.
Figure 6 shows the respective intensity profiles of CCD pixels along the white lines in Figs. 5(c)-5(h). The relative positions of these curves are same on each laser spot. The intensity curves for different delay times are normalized and the curves “2 ps”, “3 ps”, “3.25 ps”, “3.5 ps”, “3.75 ps” and “4 ps” are manually shifted upwards for convenient comparison. The magnitude contrasts of crests and valleys become larger with longer delay time, and decreases gradually away from the nanogroove. Besides, the positions of crests and valleys keep unmoved during the evolution of LSFL formation.
Figure 5 show the evidence that the onset of surface ripples appears at 3 ps after the arrival of pump pulse. It is necessary to emphasize that the time domain is close to the moment of carrier-lattice temperature equilibrium via electron-phonon scattering during ultrafast laser ablation in semiconductors . Rethfeld et al. also considered the typical timescale for lattice heating due to electron-phonon collisions was in a few picoseconds . Shank et al. demonstrated that the non-thermal melting caused by high density plasma excited by intense fs laser occurred in less than 1 ps . Moreover, other thermal and structural effects, such as ablation, material removal, are usually happened at several tens of picosecond after pump pulse irradiation, and can’t participate in the initial process of LSFL formation . Consequently, the formation of LIPSS originates from local surface melting, resulted from the deposition of laser energy at a specific spatial frequency.
High density plasma is excited on Si surface via one-photon and two-photon absorption of femtosecond laser pulse. Oscillations of free carriers at the surface can sustain SPP. The laser-SPP interaction results in the spatial modulated laser field and the free carrier generation. Subsequently, carrier-phonon interaction will transfer the carrier energy to the lattice locally, leading to periodic melting and grating-like LIPSS structures.
3.3 Carrier excitation and two temperature model
When Si crystal is irradiated by fs laser pulse, electrons are excited in a few femtoseconds through single-photon and two-photon absorption, and thermalized owing to strong electron-electron scattering in a several tens of femtoseconds. Because of the thermalization, the electron temperature increases rapidly while the lattice remains cool. The TTM were widely used to describe the non-equilibrium system of hot electrons and cold lattice in Si after irradiation by femtosecond laser pulse [28, 40]. The electron energy transfers to the lattice through electron-phonon scattering, which takes typically several to tens of picoseconds. Moreover, very high density of free carriers (> 1021 cm−3) are generated on Si surface, and the optical properties change from semiconductor to metallic-like material. Therefore, the Drude model was used to modify the TTM . In order to further demonstrating the evolution of the lattice temperature after femtosecond pulse hitting the Si surface, we perform theoretical calculations of the carrier excitation, carrier and lattice temperatures by using the TTM-Drude model .
The evolution of carrier density Ne, electron Te and lattice temperature Tl are calculated by solving the Boltzmann’s transport equations, where both single-photon and two-photon absorption are considered in the theoretical model .
A finite difference method is used to numerically solve the TTM-Drude equations. The parameters are listed in Table 1 [40–43]. Besides, the initial carrier density of silicon is set as 1018 cm−3. Refractive index and extinction coefficient of bulk silicon for 800 nm light are n = 3.68 and k = 0.005, respectively. The Von Neumann boundary conditions are adopted ignoring heat losses at the front and back surfaces. In the theoretical calculation, laser pulse is with a FWHM of 50 fs and fluence in the range of 0.04 - 0.5 J/cm2.
Figure 7(a) shows the carrier density at Si surface irradiated by 50 fs laser pulse with laser fluence of 0.18 J/cm2. One can notice that high density carriers are excited from the valence band to the conduction band, and reach to a peak value of 6.6 × 1021 cm−3 in 0.08 ps due to one-photon and two-photon absorption, as shown as the last two terms in Eq. (1). A rapid decrease of the carrier density is mainly owing to Auger recombination process, where the free carriers are captured by ionized donors and lose their energy nonradiatively .
Figure 7(b) presents the maximum values of carrier density for different laser fluences. The carrier density increases quickly when laser fluence changes in the range of 0.04 < F < 0.14 J/cm2, and it approaches to saturation as F > 0.22 J/cm2. This is because the light absorption and carrier density increases with the laser fluences, but the Auger recombination rate increases with the cubic of carrier density, as shown in Eq. (1).
Figure 7(c) shows the evolution of carrier and lattice temperature for the laser fluence of 0.18 J/cm2. It is found that the carrier temperature is drastically increased to 3.6 × 104 K due to the heat of laser pulse. Subsequently, the highly energetic carriers heats the lattice through electron-phonon scattering. The melting temperature of silicon is about 1687 K. It can be seen that about 3 ps after irradiation the lattice temperature exceeds the melting point. More importantly, the time scale of thermal melting is very close to the birth of LSFL as shown in Fig. 5. The result further demonstrates the LSFL are formed due to the surface melting caused by the periodic energy deposition. The equilibrium temperature is slightly larger than the melting temperature (Tmelt = 1687 K), but less than the vaporization temperature (Tvapor = 2700 K), as shown in Fig. 7(d). This means the pump laser fluence F = 0.18 J/cm2 is slightly larger than the single pulse melting threshold, but less than the ablation threshold, which excludes the intensive ablation affecting the formation of LSFL, such as evaporation and phase explosion [23, 44]. The melting time, namely, the duration for Tl increases to the melting temperature, becomes shorter for higher laser fluence. It is less than 0.7 ps as laser fluence F > 0.45 J/cm2, which explains well the experimental results shown in Fig. 3 that the reflectivity increases by 20% within 1 ps after laser pulse irradiating on the silicon surface.
3.4 Surface plasmon polarization and periodic ripples
Periodic ripples on metal and semiconductor induced by femtosecond laser pulses have been studied intensely, and are attributed to the laser-induced SPP . The dispersion curves of surface plasmon and light in air are described by Figure 8 shows that for equal frequency, the wavenumber is always larger than that of photon in air, which means SPP cannot be excited directly by a normal incident light. Many references reported that if defects are prepared on the sample surface in advance, SPP can be efficiently excited by femtosecond laser pulse because the surface structure provides the extra wave vector . The interaction between the incident laser field and SPP causes a periodic spatial modulation of the laser field, and further induces the formation of periodic surface ripples . For a normal incident laser, SPP and periodic ripples are both perpendicular to the direction of laser polarization, and the ripples period is equal to the SPP wavelength . In this paper, the extra wavevector component comes from the prefabricated nanogroove. Figure 4 shows that the ripple period is of 680 nm induced by 800 nm laser. The additional wave vector provided by the nanogroove is estimated to be of 2500 cm−1.
High density carriers are excited and oscillate along the surface when femtosecond laser pulse incidents on Si surface . According to the Drude model, the dielectric constant changes simultaneously. The real part of dielectric constant is described as Table 1. According to Eq. (5), the real part of the dielectric constant is calculated as a function of the carrier density and shown in Fig. 9(a). The maximum carrier density is of for laser fluence of 0.18 J/cm2, and the real part of dielectric constant is −3.57, less than −1.0. Therefore, the Si surface changes from a semiconductor to metallic properties, and can support SPP excitation.
At normal incident of femtosecond laser, the LSFL period Λ is equal to the SPP wavelength and can be written asEqs. (5) and (6), and the numerical results shown in Fig. 7(b), we can calculate the dependence of LSFL period Λ on the laser fluence. Figure 9(b) shows that the LSFL period is 679 nm for laser fluence of 0.18 J/cm2, agreeing well the experiment results. We also studied the formation of LSFL on the surface of GaP crystal by femtosecond pulses at different laser fluences, and found the ripple periods became larger with the laser fluence increasing, as expected as the SPP model . We conduct experiments to study the formation of periodic ripples induced by fs pulse with different laser fluence. The ripple periods are of 680 nm, 710 nm and 730 nm for the laser fluence of 0.18 J/cm2, 0.29 J/cm2 and 0.45 J/cm2, respectively. These experimental results accord well with the SPP model. Therefore, laser-induced SPP is responsible for the formation of LSFL.
3.4 Polarization effect
We rotate the nanogroove by 90° so that the laser polarization direction is parallel to the nanogroove. Figure 10 shows the time-resolved OM images of surface structures at different delay times after irradiation by a single pump pulse at the laser fluence of 0.18 J/cm2. The laser spot and diffraction rings are similar to that for perpendicular laser polarization as shown in Fig. 5. Murphy et al. reported laser-induced periodic ripples resulting from single-pulse irradiation of Au microstructures on Si substrate. Faint ripples were observed as fs laser polarization is parallel to the edge of Au mesa . However, Fig. 10 shows that no surface ripples appear on Si surface in the whole evolution process. This is caused by the different optical properties of groove in Si surface and the Au microstructures.
Depending on the irradiation conditions, SPP excitation and light diffraction originating from the nanogroove contribute to the spatial distribution of light energy . The SPP excitation is a polarization dependent process. When the laser polarization is perpendicular to the direction of nanogroove, the SPP excitation increases the local intensity at the sample surface. As the laser polarization direction is parallel to the nanogroove, SPP can’t be efficiently launched. According to the pump-probe results, clear and regular ripples are observed under the condition of laser polarization direction perpendicular to the nanogroove rather than that of parallel polarization, which indicates that the SPP excitation dominates the origin of periodic ripples.
In summary, we conduct a collinear pump-probe imaging experiment, and study the ultrafast dynamics of the LSFL formation induced by a single 800 nm, 50 fs laser pulse on Si surface with a prefabricated nanogroove. The onset of surface ripples is observed in 3 ps after the arrival of pump pulse with a fluence of 0.18 J/cm2, and the ripple positions keep stable during the evolution of LSFL formation. The well-defined ripples are visualized when the laser polarization direction is perpendicular to nanogroove, but no ripples are observed with the laser polarization direction parallel to nanogroove.
The excitation of free carriers, the evolution of carrier and lattice temperature are studied by the TTM-Drude model. The theoretical results indicate that the melting time is about 3 ps for laser fluence of 0.18 J/cm2, and it becomes shorter for higher laser fluence, which are very close to the experiment results. These indicate that the surface ripples are due to the surface melting, and exclude other thermal dynamics, such as ablation, hydrodynamics, etc. The LSFL period is predicted according to the SPP model and coincide with the experiment result. The results demonstrate that the LSFL are caused by the surface melting, and the formation of LSFL is due to the periodic energy deposition caused by the SPP excitation.
National Natural Science Foundation of China (Grant NO. 11474097, 11374099, 11274116); Open Fund of the State Key Laboratory of High Field Laser Physics.
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