Focused on silicon surface in water, superimposed multiple shots of linearly polarized 800-nm, 100-fs, 10-Hz laser pulses at lower fluence than the single-pulse ablation threshold are shown to produce two kinds of periodic nanostructures with almost constant periods of 150 nm and 400 nm. Surface plasmon polaritons excited in the surface layer illustrates well the formation of nanostructures and its dynamic properties observed. Pump and probe measurements of the ultrafast change in surface reflectivity during the interaction have demonstrated that the multiple low-fluence fs pulses are crucial to the nanostructuring through the accumulation of non-thermal bonding structure change and the subsequent nanoscale ablation.
© 2012 OSA
During the last decade, intense femtosecond (fs) laser pulses have been demonstrated to produce periodic nanostructures on the surface of solids such as dielectrics [1–3], semiconductors [4–6] and metals [7,8], and also inside transparent materials [9,10], where the structure size observed is typically 1/10 – 1/5 of the laser wavelength λ. The periodic nanostructure formation observed for various kinds of solid materials suggests a new field of nanoscale, ultrafast light-matter interaction physics and its potential routes to laser nano-processing beyond the diffraction limit.
Formation of periodic surface structure on the order of λ has long been known as a universal phenomenon, so-called ripple induced in laser ablation. The ripple structure is mostly understood to arise from the interference between the incident light and the surface electromagnetic wave associated with scattered light or surface plasmons . Recent studies have shown that the interference process including the surface plasmons can produce sub-wavelength ripples as small as ~λ/2 [12,13].
Since the traditional ripple formation mechanism can never account for the origin of nanoscale periodicity much smaller than λ, as well as the characteristic property of nanostructuring observed, various mechanisms have been proposed so far, such as the self-organization of surface instability [1,14], second-harmonic generation [4,15], refractive index change [6,16], and nanoplasma formation . Recently we have shown for diamond-like carbon (DLC) film that fs laser pulses induce near-field strong enough to initiate nanoscale ablation on the dielectric surface even if the fluence is smaller than the single-pulse ablation threshold . Based on the results obtained, we have proposed that the origin of nanoscale periodicity can be attributed to the excitation of surface plasmon polaritons (SPPs) in the glassy carbon layer formed on the DLC surface , where the model calculation reproduces well the observed nanosize of periodicity.
Despite the intensive studies, the fundamental process of periodic nanostructure formation is still insufficiently understood, and there have been few pictures valid for different target materials. The diverse discussion on the physical mechanism is due most likely to the fact that the nanostructuring strongly depends on fs-laser parameters, target materials, surface conditions, and ambient materials of the target. For further understanding the detail of nanostructuring mechanism, we have focused our attention on the experimental conditions that have been used extensively for the formation of nanostructures, i.e., superimposed multiple shots of fs laser pulses at lower fluence than the single-pulse ablation threshold.
In this paper we report the study of the periodic nanostructure formation mechanism for crystalline silicon (Si) surface irradiated in water under the conditions of interest. The experimental results obtained represent nanostructuring at almost constant periods of 150 nm and 400 nm and its characteristic properties. It is shown that the excitation of SPPs in the Si surface layer illustrates well the nanostructuring process and the structure sizes observed. Measurements of temporal change in surface reflectivity have demonstrated that the multiple shots of low-fluence fs laser pulses are crucial to the formation of a new layer on the target surface and the subsequent periodic nanoscale ablation.
Polished p-type crystalline Si (100) substrate of 380 μm in thickness was set in a small cell filled with distilled water, as nanostructuring of Si is known to depend on the ambient materials [5,19]. Through the 2-mm thick water layer and a thin quartz window of the cell, the Si surface was irradiated with linearly polarized, 800 nm, 100-fs laser pulses from a Ti:sapphire laser system operated at a repetition rate of 10 Hz. In the experiment for nanostructuring, the laser beam having a well-defined Gaussian spatial profile was focused at normal incidence with a 1000-mm focal-length lens to the focal spot size w0 ~200 μm in 1/e2 radius. For accurate measurements of the spatial beam profile and w0, a CCD camera was used to image the focused beam and a 100-μm-diameter tungsten wire as a reference length on the focal point. The peak fluence on the target was estimated by the relation F = 2Epulse/πw02 with the measured laser pulse energy Epulse and w0.
Throughout the experiments, the fluence on the target surface was F = 100 – 200 mJ/cm2, corresponding to Epulse = 64 – 128 μJ. This fluence was smaller than the ablation threshold Fth = 400 – 500 mJ/cm2 and the melting or modification threshold Fm = 260 – 350 mJ/cm2 of crystalline Si for a single fs laser pulse at λ ~800 nm in air [20,21]. At the low fluence, more than a hundred shots of fs laser pulses were necessary for the onset of ablation.
The morphological change of Si surface was observed with a scanning electron microscope (SEM). With the two-dimensional Fourier transform we analyzed the SEM images to see the distribution of spatial periodicity in the surface structure.
3. Results and discussion
3.1 Formation of periodic nanostructures
Figure 1 shows a pair of the SEM image and its frequency spectrum observed as a function of the superimposed shot number of fs pulses N at F = 120 mJ/cm2. The image represents the line-like patterns extended to the direction perpendicular to the laser polarization vector, and the spectrum provides the distribution of the pattern period d measured along the laser polarization. The periodicity along the polarization direction is a spontaneous nature resulting from the enhanced near-field due to the excitation of SPPs discussed below.
In Fig. 1, the periodic structure starts to be formed in the central part of the focal spot at N = 400, where the spectrum represents a broad peak at d ~400 nm. With increasing N to 500, high frequency components grow up to form double peaks in the spectrum. With a further increase in N to 800 – 2000, the fine nanostructure with d ~150 nm rapidly develops to cover the whole area, as seen by the isolated sharp peak in the spectrum, while the coarse structure with d ~400 nm is greatly suppressed. This morphological change suggests that the fine structure is efficiently produced in succession to the coarse structure, as if the coarse structure would be the source of the fine structure. It is noted that the periods of d ~150 nm and d ~400 nm are almost constant with an increase in N.
In the experiment, the fine nanostructure formation at d ~150 nm was observed only for a range of F = 120 – 140 mJ/cm2. The Si surface was never ablated at F < 120 mJ/cm2, and only the coarse nanostructure was formed at F > 140 mJ/cm2. Figure 2 shows the morphological change observed with N = 1000 at different values of F. At the lowest fluence, the multiple shots produce the fine structure with d ~150 nm, while the higher fluence leads to a relative increase in the density at d ~400 nm or produces only the coarse structure.
Figure 3 plots the isolated peak position of d in the spectrum observed as a function of N at four different values of F. The periodic structure is mostly formed with two sizes of d ~150 nm and d ~400 nm. Note that the fine structure formation is always preceded by the coarse structure as N increases, while an increase in F increases the additional number of pulses ΔN required for the fine structure formation. The fine nanostructure is no longer produced at the highest value of F in Fig. 3. The characteristic behavior of nanostructuring is illustrated well with the excitation of SPPs in the surface layer, as discussed below.
3.2 Nanostructuring through the excitation of SPPs
Despite the experimental condition of F < Fth and F < Fm, the superimposed shots of fs laser pulses are shown to induce the ablation to form periodic nanostructures. One of the major roles of multiple pulses in nanostructuring is to accumulate structural change in the surface layer that leads to a decrease in the ablation threshold [20–23]. For crystalline Si (c-Si), such multiple shots of low-fluence fs laser pulses can produce an amorphous Si (a-Si) layer of a few tens of nanometers in thickness [20,23]. Since the absorption coefficient of a-Si is by an order of magnitude larger than that of c-Si , the bonding structure change to form the a-Si layer increases the laser energy density absorbed in the uppermost layer of the target. This could lead to the initiation of random nanoscale ablation as N increases. In fact we observed such random ablation traces, prior to the formation of periodic structure, as for DLC .
Thus the Si target to be nanostructured with multiple shots of fs laser pulses can be modeled by a surface consisting of the a-Si and c-Si layers, as shown in the inset of Fig. 4 , where the media a and c denote water and c-Si substrate with the relative dielectric constants εa = εwater and εc = εc-Si, respectively, and the medium b represents the a-Si layer formed on c-Si. The incident fs laser pulse predominantly produces a high density of free electrons Ne in the a-Si layer, as mentioned above. In a pump pulse duration, the increase in Ne induces an ultrafast change in the relative dielectric constant εb = εa-Si. Then, the SPPs can transiently be excited via the coherent coupling of the incident laser pulse with the randomly corrugated surface, where the a-Si layer including Ne works as a thin metal layer between water and the c-Si substrate for the excitation of SPPs .
The SPPs can be excited at two interfaces A (water/layer b) and B (layer b/c-Si) when the familiar dispersion relation24], where kspp is the plasmon wave number, k0 is the wave number of the incident light in vacuum, εa,c is εa or εc, and εb′ is the relative dielectric constant of the layer b, including the effect of Ne. Using the Drude model, εb′ in the laser field is written as εb′ = εb – [ωp2/(ω2 + iω/τ)], where ω is the incident light frequency in vacuum, τ is the Drude damping time of free electrons, and ωp = [e2Ne/(ε0m*m)]1/2 is the plasma frequency with the dielectric constant of vacuum ε0, the electron charge e and mass m, and the optical effective mass of carriers m* . For the normal incidence, near-field should periodically be enhanced along the laser polarization direction at a period of the half SPP wavelength, λspp/2 = (1/2)2π/(Re [kspp]), due to the spatial standing wave. In the calculation of kspp for λ = 800 nm, we used εc-Si = 13.5 + i0.0384, εa-Si = 14.9 + i0.627 and εwater = 1.76  with m* = 0.2 and τ = 1 fs .
Figure 4 shows the period Λ = λspp/2 calculated as a function of Ne for the interfaces A and B, together with the calculated skin depth in the layer b. The condition of εb′ < 0 has to be satisfied for the excitation of SPPs , which corresponds to the shaded region of Ne > 5.5 × 1021 cm–3 in Fig. 4. In a different experiment, we have confirmed that the ablation for nanostructuring certainly takes place at Ne in this region, as described below. The calculated periods are Λ ~300 nm for the interface A and Λ = 100 – 200 nm for the interface B, being in good agreement with the observed coarse and fine periods of d ~400 nm and ~150 nm, respectively. In more detail, however, the calculated value of d at the interface A is smaller than the observed one in the whole range of Ne. This is most likely due to the simplified modeling of the target, where we have disregarded the thin SiO2 layer that is usually present on the initial uppermost surface of c-Si substrate . We have found that the calculation including a partial contribution of SiO2 to εb′ leads to Λ = 350 – 450 nm, while the definite estimation of its contribution is difficult.
The result shown in Fig. 4 demonstrates that the skin depth δ in the layer b rapidly decreases down to δ ~35 nm as Ne increases to Ne ~5.5 × 1021 cm–3 for εb′ < 0. The calculated critical value of δ coincides with the a-Si layer thickness (38 – 42 nm) observed so far for c-Si surface [20,23]. This indicates that the energy absorption and resulting structural change predominantly takes place in the layer b, as expected.
Referring to the calculated results, we can illustrate the characteristic behavior of ΔN in the fine and coarse structure formations shown in Fig. 3. With increasing N of fs laser pulses, the a-Si layer would be created on the target prior to the onset of ablation for the coarse structure formation at the interface A. In the present experiment, the a-Si layer thickness should be less than δ ~35 nm so that the fine nanostructure is formed through the excitation of SPPs at the interface B, as mentioned above. When F is increased, the initial multiple shots of fs laser pulses create a thicker a-Si layer before ablation. When δ exceeds the critical value, the excitation of SPPs at the interface B is hardly induced until the a-Si layer thickness is reduced to δ < ~35 nm through the ablation. Thus a higher value of F increases ΔN for the fine nanostructure formation, as seen in Figs. 3(a) – (c). An additional increase in F increases excess energy deposited into the surface layer and resulting thermal effects [5,27] to restrict the fine structure formation.
3.3 Generation of a high electron density for nanoscale ablation
To confirm the nanostructuring mechanism discussed above, we measured the reflectivity of Si in water to estimate Ne on the surface during the interaction. A pump-probe technique was used in this experiment, of which optical configuration is shown in Fig. 5(a) . The p-polarized pump beam was obliquely incident at the angle of 12þ, focused with 500-mm focal length lens to the spot size w0 = 115 μm, while the orthogonally polarized probe pulse was incident at normal with a time delay Δt to cover the whole pumped area. The reflected probe beam was recorded for every shot of the pump pulse with a CCD camera, and its reflectivity R(Δt) at Δt was measured for the central pump beam area of 40-μm in diameter.
Figure 5(b) shows the temporal change in the reflectivity η = R(Δt)/R0 for a single pump pulse at F = 120 mJ/cm2, normalized to the initial value R0 = 22.1% of the non-irradiated c-Si surface in water, where the target was translated shot by shot so that each pump pulse hits a fresh surface area. The pump pulse produces a high density of free electrons in the surface to induce the rapid decrease of η to ~0.8 at Δt ~0.5 ps. After the end of interaction, η slowly decays due primarily to the energy transfer from free electrons to the lattice . Using the Drude expression, we can estimate Ne ~1.0 × 1021 cm–3 for the minimum of η at Δt ~0.5 ps.
Keeping the time delay at Δt = 0.5 ps, we measured η as a function of the superimposed shot number N of the pump pulses at the same fluence, and the result is shown in Fig. 5(c). With increasing N, the initial value of η ~0.8 is almost constant up to N ~600, while no morphological change on the target surface was observed. With a further increase in N, η rapidly grows up to η ~1.2 at N ~700, followed by a continuous decrease down to η ~0.2 or less. We observed that the ablation starts to take place on the surface at N = 600 – 700, and the fine periodic nanostructure with d ~150 nm is formed at N ~1000. These results demonstrate that the abrupt enhancement of η at N ~700 is certainly due to the generation of a high electron density sufficient to induce the onset of ablation, being preceded with the accumulated structural change from c-Si to a-Si in the region of N < ~700. We estimated Ne = (0.6 – 1.0) × 1022 cm–3 for the peak value η ~1.2, assuming a thin a-Si layer. This value of Ne reconciles well with the condition for the excitation of SPPs in Fig. 4.
3.4 Formation of periodic structures in air
The same experiment of nanostructuring was made for c-Si in air with multiple shots of low-fluence fs laser pulses. The results have shown that only a ripple-like structure with d ~600 nm could be produced at the higher ablation threshold by about 30% than in water. This suggests that the formation of fine nanostructures is limited in air by the thermal effect [5,29], as the fine nanostructure is formed in water at the lower fluence of F = 120 – 140 mJ/cm2. In air the excess energy must be deposited into the a-Si layer and would destroy the fine energy distribution at the interface B. We have confirmed such thermal effects in the pump-probe measurement of η. The results will be reported in detail elsewhere. The fine nanostructure might be created on Si even in air , if the thermal effect can effectively be suppressed.
The excitation of SPPs in the surface layer can be the fundamental mechanism of periodic nanostructure formation on Si surface irradiated in water with superimposed multiple shots of fs laser pulses at lower fluence than the single-pulse ablation threshold. This mechanism is shown to illustrate well the observed nanostructuring properties, as well as the period sizes in surface structures. The pump-probe measurements of surface reflectivity have demonstrated that the multiple low-fluence fs pulses accumulate structural change in the surface layer to induce an abrupt increase in Ne and subsequent nanoscale ablation. It is also shown that the non-thermal process should be crucial to produce such a fine periodic energy distribution in the surface layer. The nanostructuring mechanism should be valid for different materials.
This work is partially supported by the Grant-in-Aid for Scientific Research 23360034, 22110506, and 24686011.
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