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In this study, we aimed to create the nano-micro hierarchical surface structures on Ni and analyze their optical second harmonic generation (SHG) response. The hierarchical surface structures were found to significantly modify the optical nonlinearity of the metal surface. The macroscopic symmetry of the surface’s shape influenced SHG, and the excitation of surface plasmon (SP) enhanced SHG. On the other hand, the nanostructures on the micro-cubes had an additional effect on the generated SHG. The mechanism of anisotropic SHG enhancement by the nano-micro structures has been investigated.
© 2016 Optical Society of America
1. Introduction
Nanotechnology has developed rapidly in recent year with the marked improvement in analysis techniques for characterizing nanomaterials. Nanomaterials are characterized by several parameters. Among them, symmetry parameter is important as it influences the dielectric property through the electric wave function in their consisting atoms and bondings. In particular, second-order nonlinear optical phenomena are forbidden for centrosymmetric structured materials [1,2]. Hence, the second-order nonlinear optical process should be sensitive to the symmetry of nanostructured shape.
Optical second harmonic generation (SHG) is a coherent nonlinear optical process and its efficiency depends not only upon the electronic properties but also upon the symmetry of the geometrical structure of the medium [3]. In general, SHG does not generate in centrosymmetric bulk media [1–3]. SHG processes are used in a wide range of applications, such as developing-devices for optical processing [4], nonlinear imaging [5], and phase-sensitive amplification [6]. Since nonlinear processes through photon-photon interactions are intrinsically weak, studies on enhancement of nonlinear efficiency are crucial [1–3,7].
So far, enhancement of SHG responses by surface plasmons (SPs) has been studied extensively [8–14]. SPs are coherent electron oscillations localized on a metal surface [9]. Since SHG light intensity is proportional to the squared incident light intensity, therefore the SP-induced electric field enhancement at the surface yields significantly enhanced SHG emission [8]. This is evident as few studies have focused on studying SHG enhanced by the localized SPs on Ni nanostructures [15].
Recently, Vorobyev et al. created a type of hierarchical surface structure which consisted of parallel micro-grooves covered with a large number of nanostructures, known as nanostructure-covered laser-induced periodic surface structures (NC-LIPSSs) [16]. They found that NC-LIPSSs could significantly change the optical properties of the metal surfaces [17]. Both micro-scale periodic grooves and nanostructures on NC-LIPSSs can affect SPs. Therefore, NC-LIPSSs are likely to exhibit unusual SP-induced nonlinear optical effects, and this is indicated from their recent study on photoelectron emission from NC-LIPSSs on platinum (Pt) [18]. Essentially, nanostructures on LIPSSs change the dispersion relation of the SP modes [19]. With the excitation of SPs, they observed an enhanced multi-photon photoelectron emission from the NC-LIPSSs.
Despite the extensive research on the SP-assisted nonlinear processes in pure metals or composite materials [11,20,21], a systematic study on the effects of micro-nano hierarchical structures on nonlinear optical processes is currently lacking. As mentioned above, both nano- and micro-structures are important for generating SPs. Accordingly, a systematic study of a combined effect of nano-micro hierarchical structures is necessary for its potential applications of hierarchical structures in future nonlinear optical devices. The NC-micro-cubes (NC-MCs), NC-LIPSSs, is a novel hierarchical surface structure which consisted of micro-cubes covered with a large number of nanostructures. One interesting aspect of the structure is that SPs are excited by the dominant features of the different scales of the surface structures. The micro-cubes on the NC-MCs induce localized SPs; however, the smaller structures on the hierarchical structure can excite additional SPs: nano-structures on NC-MCs can generate localized SPs. By performing a systematic investigation of SPs induced by different individual structures, the interactions of SPs induced by different individual structures, and the final combined SPs due to the hierarchical structures, we expect that we can control the SHG efficiency in nano/micro structures. In this work, we create the NC-MCs on Ni through a laser-ablation method and observe their SHG signal. By comparing the signal from NC-MCs with the signal from nanostructures (NSs) and micro-cubes (MCs) structures, we characterize the relation between nano and/or micro structure and SHG.
2. Experimental
Our experimental setup employed an amplified Ti:Sapphire femtosecond (fs) laser system. The high-energy fs laser system generated 60-fs pulses with a central wavelength of 800 nm; the average power of the laser was 1.2 W at 1 kHz repetition rate.
We first produced a Ni sample covered with NC-MCs. We established a new setup using the high-energy fs laser system, as provided in a previous study [22]. An experimental setup for surface nano-/micro-structuring on a Ni is shown in Fig. 1(a). The laser beam from the light source was divided into two beams by using a beam splitter. In an optical path, a half-wave-plate was used to rotate the laser polarization by 90°, and a neutral density filter was used to adjust the laser power irradiated on the sample. The laser fluence was set to 0.137 J/cm2. The two beams were controlled using a chopper. The number of pulses per pulse burst was determined to be 1. If the chopper frequency is set at 500 Hz, the time interval when the chopper slot is open for a specific beam is 0.001 s; therefore, the two beams alternately passed through the chopper pulse by pulse. Then, by focusing and scanning in zigzag the beams onto a Ni sample mounted on the XY-translational stage, we succeeded in fabricating Ni NC-MCs samples with a large area. This method is newly named two-beam zigzag alternate ablation (TBZAA).
Fig. 1 Nano-micro-structuring of Ni surface. (a) Experimental setup for surface nano/micro-structuring on a Ni. The laser polarization is indicated with a double arrow in the plot. α is the angle between the two laser polarization directions. (b) An optical microscope image of the created sample of Ni. (c) SEM image of NC-MCs on Ni showing micro-cubes. (d) Zoomed view, showing smaller features of (c).
Using this method, two-dimensional arrays of the NC-MCs sample with an overall area of more than 1 cm2 was fabricated, and a part of them was observed by an optical microscope (OM), as seen in Fig. 1(b). After this laser-treatment, the surface morphologies of NC-MCs were characterized by using a scanning electron microscope (SEM). Figures 1(b) and 1(c) show the SEM image of the laser-treated micro-/nano- hierarchical structures on Ni. Figure 1(d) shows that micro-cubes are covered with nanostructures. The average measured period Λ of the micro-cubes on Ni was 600 nm, and the measured depth h of the micro-structures was ~100 nm.
Once the sample was fabricated using a high energy fs laser system, the SHG measurements of the Ni sample were performed using the same laser system. Figure 2(a) shows the configuration of the SHG intensity measurements of the Ni sample. The s-polarized light at ~800 nm was focused onto the Ni NC-MCs sample at the incidence angle of 45°, and the laser fluence was ~3.09 mJ/cm2. Here, the direction for s-polarization corresponds to the y-direction, and the rotation angle φ was defined as the angle between the incident plane and the groove direction. When φ is set at 0°, the incident light travels in the x-direction as shown in Fig. 2(a). In the experiment with the Ni sample, we studied the dependence of SHG on the laser polarization. The generated SHG reflection was passed through a blue filter (BG39) and a band-pass filter (FB400-10) to filter the fundamental frequency, and subsequently, a polarizer was used to select the s-polarization. To account for the hyper-Rayleigh scattering (HRS) [23], the reflected SHG light was collected through a focusing lens. The reflected SHG light signal was detected by a photomultiplier tube (PMT). In order to get high signal-to-noise ratio of the SHG intensity, the SHG light intensity pulse was accumulated for ~7 mins for each SHG data point.
Fig. 2 SHG intensity measurement. (a) Optical configuration of the reflected SHG intensity measurement for the Ni NC-MCs sample at an incidence angle of 45°. The sample rotation angle φ is defined as the angle between the incident plane and the groove direction. The laser polarization is indicated with a double arrow in the scheme. (b) Power dependence of the SHG intensity for the NC-MCs plotted on a log-log scale. The slope value for the fits to these data is ~two, confirming the second-order nature of the emitting light.
3. Results and discussions
We first defined the relationship between the reflected SHG light I(2ω) and the nonlinear susceptibility χ(2) of the metallic sample, as done in a previous study [24]. The s-polarized SHG intensity |Es (2ω)|2 { = Is(2ω)} generated using the χ(2)YYY element is given as:
where χijk is the nonlinear susceptibility with i, j, and k representing x, y, and z directions. The coordinate system is oriented so that the X and Y coordinates are in the plane and the Z coordinate is in the direction normal to the substrate surface. F(2ω) and F(ω) represent the Fresnel factors at the SHG and fundamental frequency, containing the contribution by the plasmonic excitation. The local electric field Eloc has the relation as Eloc = F(ω)E(ω). Consequently, Eq. (1) shows that the s-polarized SHG intensity |Es (2ω)|2 depends on the local electric field Eloc of Y-direction component.For the SHG measurement, we first confirmed the quadratic dependence on the fundamental light intensity as shown in Fig. 2(b). The conversion efficiency from fundamental to SHG light photons for Ni sample was ~10−13 at maxima, for s-in/s-out polarization configuration.
We first analyzed the χ(2) of the NC-MCs surface of Ni. The χ(2) is sensitive for the symmetry of the shape [25,26]. Since the shape of Ni micro-cubes has C4v symmetry with four mirror planes, an independent nonlinear susceptibility element χ(2)YYY should be permitted. Assuming that the contribution of χ(2)YYY is effective for s-in/s-out polarization configuration, the SHG intensity should be zero at φ = 0°, 45°, and 90° in ranging from φ = 0° to 90°, as shown in Fig. 3(a). In conclusion, the χ(2)YYY depending on a sin(4φ) function dominates the SHG intensity from the NC-MCs on Ni. Thus, the symmetry for the Ni NC-MCs was fitted by a sin2(4φ).
Fig. 3 SHG enhancement. (a) The contribution of χ(2) for the Ni sample, at φ = 0°, 22.5°, 45°, 67.5°, and φ = 90°. Red dotted lines indicate the mirror plane. (b) Electric field distributions for the Ni NC-MCs calculated for the configurations at an incidence angle of 45° and φ = 0°, 22.5°, 45°, 67.5°, and φ = 90° by using the Finite Difference Time Domain (FDTD) method, and Enhancement Factors (EFs) curve for the sample rotation angle φ. The effective refractive index due to the nanostructures atop the micro-grooves, η = 1.333 for the Ni sample, was used in this calculation. |E| is the electric field magnitude. The color scale bar indicates the electric field strength and the values show the local electric field magnitude. Double arrows indicate the electric field vector E of an incident beam. Blue dotted curve represents EFs consisting of the |E|2 values calculated by using FDTD method. (c) Simulated SHG intensity curves for the sample rotation angle φ. The red curve is the theoretical intensity curve, described as {|χ(2)||E||E|}2.
NC-MCs in this study was created using a normal incident beam. In this case, the periodicity Λ of the surface grating formed by interference between the incident laser light and the excited SP wave is given by
with the grooves formed normal to the electric field direction [17]. Here, λ is the incident light wavelength, andis the effective refractive index. For the smooth Ni samples, η was calculated as 1.041 for the Ni (ε1 = −13.0 and ε2 = 21.7) [27]. By using Eq. (2), the periodicity of the grooves was found to be 769 nm for the Ni sample. However, the observed periodicity was about 600 nm for the Ni sample, as observed in Fig. 1(d). By using this observed periodicity value, we found that η = 1.333 for the Ni sample. This effective refractive index might arise from the NSs atop the microstructures. Using this effective refractive index value, we next investigated the electric field distribution at an incidence angle of 45° on a periodic cube surface on the Ni sample.The local enhancement of an electric field by metallic nanostructures can enhance the SHG response [28]. To find the origin of the enhanced SHG, we calculated the local electric-field Eloc intensity on the sample by using the Finite Difference Time Domain (FDTD) method. The electric field distribution from φ = 0° to 90° is shown in Fig. 3(b). The color scale bar indicates the electric field magnitude |E|. Sharper metal tips can produce strong local electromagnetic fields, which are called lightning-rod effect [2]. Recently, studies have been reported lightning rod effect induced at the tips at the Ni nanostructure for s-in/s-out polarization configuration [15]. Thus, lightning rod effect should be induced at the edge parts of the Ni sub-micro-cube structures. In Fig. 3(b), the enhancement of the local electric field Eloc at the edge parts is clearly seen, and this enhancement is led by the lightning rod effect. The value in Fig. 3(b) indicates the maximal electric field magnitude |Eloc|max in the simulation. At φ = 45°, the value of |Eloc|max was higher, and at φ = 0° and 90°, the values of |Eloc|max were lower. This relation is represented as the blue dotted curve in Fig. 3(b). Since the s-polarized SHG intensity is given by Eq. (1), the expected intensity curve is represented using the red curve in Fig. 3(c).
Next, we observed the azimuthal angle dependence of the SHG from NC-MCs at an incidence angle of 45°, as shown in Fig. 2(a). The sample rotation angle φ was defined as the angle between the incident plane and the sub-micro-groove direction (x or y-direction). Figure 4 illustrates the SHG intensity in the s-in/s-out polarization configuration for the Ni sample, as a function of the sample rotation angle φ from the Ni NC-MCs. The simulated curve in Fig. 3(c) is patterned in Fig. 4. Here, note that the Ni sample with μm thickness for z-direction is not appropriate for the measurement in p-polarization configuration. In fact, the SHG pattern in the s-in/s-out polarization configuration exhibited eight dim minima at φ = 0°, 45°, 90°, 135°, 180°, 225°, 270°, and 315°. Thus, the SHG intensity pattern for the s-in/s-out polarization combination strongly depends on the rotation angle φ, and it was clear that the intensity is sensitive to not only the symmetry parameter but also enhancement factor (EF) of electric field. Namely, when the structure give the largest near-field enhancement, the smallest SHG signal is provided both in the sin2(4φ) model and in the SHG data due to the symmetry-selection properties of SHG. On the other hand, the nano-micro hierarchical structure consisting of structures with different sizes may induce cascaded plasmon field enhancement [29]. If the nanostructures are in contact with the corners of micro-cube structures, a large local field enhancement will be obtained by cascading field enhancement effect originating from the “lightning rod” effect. Then, the E-field concentration might boost the finite SHG intensity up at azimuthal angle φ without mirror symmetry.
Fig. 4 Angular SHG intensity. The SHG intensity pattern of the NC-MCs as a function of the sample rotation angle φ. The data points are connected by lines to guide the eye. The solid curve shows the simulation curve calculated in Fig. 3(c). The dotted show the isotropic intensity pattern formed by averaged intensity at φ = 0°, 45°, 90°, 135°, 180°, 225°, 270°, and φ = 315°.
Here, we comment on the contribution of the quadrupolar effect [30] in our SHG signals. The SHG intensity for s-in/s-out polarization from a smooth Ni bulk sample was at the noise level as we found out in a separate experiment. Thus, it implies that the electronic quadruple effect from the Ni bulk sample can be disregarded. On the contrary, the SHG from Ni NC-MCs with nm thickness generally includes both dipole and electronic quadruple effects. Dipolar SHG originating from the third-rank tensor is sensitive to the asymmetry of the structure, while quadrupolar SHG originating from the fourth-rank tensor is insensitive to it. In Fig. 4, the angular SHG patterns observed from Ni NC-MCs shows sensitive dependence on the asymmetry of the microstructures. Therefore, it is suggested that the quadrupolar effect is not dominant.
However, Fig. 4 also reveals a non-zero SHG intensity at φ = 0°, 45°, 90°, 135°, 180°, 225°, 270°, and 315°, and the contribution from ISHG with the NC-MCs on the Ni does not completely reproduce our observed SHG intensity pattern, as seen in Fig. 4. This fact indicates that SHG in s-in/s-out polarization cannot be solely explained by the symmetry and SPs on the micro-cube structured surface.
Thus, we considered the nano-structural effects on the SHG intensity pattern, as observed in Fig. 4. Extensive NSs from Fig. 1(c) was observed atop the MCs. It is known that localized SPs can be generated from NSs [31,32], and the localized SPs, if they exist, may generate an isotropic SHG enhancement in s-in/s-out polarization configurations. To clarify this, we additionally produced NSs on Ni using the TBZAA technique and measured their SHG as a function of rotation angle φ. Figure 5(a) shows the NSs induced on Ni by performing ablation at a laser fluence of 0.050 J/cm2 using the setup as seen in Fig. 5(b). Most of structures were the nanodots with a diameter in the range of 5-80 nm, as seen in Fig. 5(c). In the SHG measurement, we observed isotropic SHG enhancement from NSs on Ni in s-in/s-out polarization configuration, as shown in Fig. 5(d). Thus, we can say that the non-zero SHG components around φ = 0°, 45°, 90°, 135°, 180°, 225°, 270°, and 315° in Fig. 4 arise from SHG enhancement due to the localized SPs on the NSs. We found that the SHG in Fig. 4 is formed by the excitation of SPs from both MCs and NSs.
Fig. 5 Nanostructural effects on the SHG intensity pattern. (a) The SEM image of the surface of Ni NS sample. (b) Configuration of the NSs in the SHG intensity measurement. (c) The expanding image of (a). (d) The SHG intensity pattern of the NSs as a function of the sample rotation angle φ. The data points are connected by lines to guide the eye. The dotted circle show the isotropic intensity pattern formed by averaged intensity.
In order to control the plasmonic enhancement effect of SHG, we tried to remove nano-structural effect on the Ni sample. As shown in Fig. 1(c), there are many ablated nanodots on micro-cubes, leading to isotropic SHG as seen in Fig. 4. The sample was immersed in isopropanol and distilled water and cleaned for ~10 min with an ultrasonic cleaner, after the creation of NC-MCs to remove the NSs on micro-cubes. By doing so, the micro-cubes become much cleaner. The SEM images of the surface of the obtained MCs sample are shown in Fig. 6(a). We next measured their SHG signal, as shown in Fig. 6(b), using the MCs sample as shown in Fig. 6(a) and 6(c). Figure 6(d) shows the SHG intensity pattern in s-in/s-out polarization configuration for MCs on the Ni sample. The pattern clearly exhibited eight lobes at minima with φ = 0°, 45°, 90°, 135°, 180°, 225°, 270°, and φ = 315°, and the minimal intense [“b”seen in Fig. 6(d)] was relatively ~34.7% lower than that for the NC-MCs [“a”seen in Fig. 4]. Together it indicates that NSs could not be completely removed, but the isotropic SHG emission from the NSs was suppressed with decreasing amount of NSs.
Fig. 6 Effect of NSs on SHG emission. (a) The SEM image of the surface of Ni MCs sample. (c) The expanding image of (a). (b) Configuration of the MCs in the SHG intensity measurement. (c) The SHG intensity pattern of the MCs as a function of the sample rotation angle φ. The data points are connected by lines to guide the eye. The solid curve show the simulation curve calculated using η = 1.041 in Fig. 3(c). The dotted show the isotropic intensity pattern formed by averaged intensity at φ = 0°, 45°, 90°, 135°, 180°, 225°, 270°, and φ = 315°.
However, in fact, most of the nanostructures were removed, as seen in Fig. 6(c). Although most of the nanostructures were removed, it is not quite clear why the minimum intensities in the pattern do not show the large decrease. Thus, we considered the dephasing of the light field, induced by the NSs on the Ni micro-structures seen in Fig. 1(c) and Fig. 6(c). In general, SH radiation collected from nanometer-scale areas is strongly dephased [33]. Stockman et al. have discussed the dephasing effect by the roughness of metal surfaces on the optical processes upon them [34]. Based on their theorem, we considered the propagating delay of fundamental light by the roughness in nanoscale.
The origin of the dephasing is the delocalization of the linear local field [34]. The actual delocalization of the nanoscale optical fields induced in the Ni surface causes their concentration areas (i.e. hot spots) to overlap for different propagations. Since these propagating modes have randomly different frequencies, this leads to the random phase shifts and results in the dephasing [34]. More materially, a timing of the penetration of the propagating light is off when the NSs exists on a surface, resulting in the dephasing. The dephasing by the propagation delay may cause noise in the intensity data. Therefore, we concluded that the SHG pattern in Fig. 6(d) may contain noise due to the dephasing at rotation angle φ under forbidden conditions.
4. Conclusions
We analyzed the SHG intensity from the constituted NC-MCs hierarchical surface structures on a metal. We observed an unique anisotropic SHG enhancement causing symmetry of shapes and excitation of SPs. The SHG patterns are attributed to the localized/localized SPs on nano-/micro- surface structures. The contribution of micro-structure to SHG was found to be larger than that of nanostructures. Furthermore, the separation of nano/micro structure related to the decomposition of the SHG intensity patterns. These interesting physical phenomena can find possible applications in novel symmetry-sensitive plasmon optical devices.
Acknowledgment
I would like to thank K. Misawa in Tokyo University of Agriculture and Technology and G. Mizutani in Japan Advanced Institute of Science and Technology for thier valuable advices.
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