ABSTRACT
In this review, the authors study their creation of nano/micro-hierarchical structures on Ag and Ni substrates by femtosecond laser treatment and their investigation of their optical second-harmonic generation (SHG) signal intensities by SHG spectroscopy. The authors obtained the nanostructure-covered microgroove and microcube structures. These hierarchical surface structures were found to modify significantly the optical nonlinearity of the metal surfaces. The macroscopic symmetry of the surface’s shapes influenced SHG, and the excitation of surface plasmons enhanced SHG. On the other hand, the nanostructures on the microstructures had an additional effect on the generated SHG. For the microcube structures, the additional SHG emission was suppressed by removing a large amount of nanostructures. Left SHG was discussed by the effect of the propagating delay.
1. Introduction
Nonlinear optical phenomena in nanostructured materials have been challenging our perceptions of nonlinear optical processes that have been explored since the invention of lasers [Citation1]. The symmetry parameter is important because the symmetry of nanostructures should have an influence on their dielectric property through the electric wave function in their constituent atoms and bondings. In particular, second-order nonlinear optical phenomena are forbidden for centrosymmetric structured materials [Citation2,Citation3]. Hence, the second-order nonlinear optical process should be sensitive to the symmetry of nanostructured shape [Citation4].
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 [Citation2,Citation5,Citation6]. In general, SHG does not generate in centrosymmetric bulk media [Citation2]. Recently, advances of nanofabrication techniques [Citation7,Citation8] have enabled control of the sizes, shapes, and alignment of nanostructures of a wide range of materials, and their SHG responses have been investigated. For example, the reports for SHG responses from the GaAs nanoneedles grown by the chemical vapor deposition method [Citation9], the nanocups deposited Au by electron-beam evaporation on to silica nanoparticles [Citation10], the ZnO nanorods grown on glass substrates by means of the low-temperature chemical bath method [Citation11], the Pt nanowires created by the physical vapor deposition method [Citation12], and the Au nanoholes fabricated by the focused ion beam method [Citation13] attracted most researchers. These SHG processes are used in a wide range of applications, such as developing devices for optical processing [Citation14], nonlinear imaging [Citation4], and phase-sensitive amplification [Citation15]. Because nonlinear processes through photon–photon interactions are intrinsically weak, studies on enhancement of nonlinear efficiency are crucial [Citation16].
So far, enhancement of SHG responses by surface plasmons (SPs) has been studied extensively [Citation17–Citation19]. Surface plasmons are coherent electron oscillations localized on a metal surface [Citation18,Citation20]. Because SHG light intensity is proportional to the incident light intensity squared, the SP-induced electric field enhancement at the surface yields significantly enhanced SHG emission. As representative works, Quail and Simon [Citation21] and Pipino et al. [Citation22] have focused on studying SHG enhanced by propagating SPs on metal nanograting. They found that the diffracted SHG was enhanced at a specific incident angle. Also, Baida et al. [Citation23] and Hubert et al. [Citation24] have focused on studying SHG enhanced by localized SPs on metal nanostructures. They found that the SHG was enhanced at a specific incident wavelength. However, the good development of nonlinear optical devices requires detailed understanding and modeling of the nonlinear optical response of nanoscale metallic systems.
Recently, the femtosecond (fs) laser has been shown to be an effective tool for producing surface structures on the nanoscale and microscale [Citation25,Citation26]. For example, a number of nano/micro-hierarchical patterns were created by fs laser surface structuring, such as nanostructure-covered laser-induced periodic surface structures (NC-LIPSSs) [Citation26]. The uniqueness of the configuration is that they contain a hierarchy of nanostructures superimposed on microstructures. Two layers of these hierarchical patterns are seen; random nanostructures at tens to hundreds of nanometers are superimposed on microscale periodic grooves. The periodicity of microgrooves for NC-LIPSSs is on the order of the generating laser wavelength (~800 nm).
So far, we have produced NC-LIPSSs on many kinds of metal surfaces. We found that NC-LIPSSs could significantly change the optical properties of the metal surfaces [Citation26,Citation27]. Both microscale periodic grooves and nanostructures on NC-LIPSSs can affect SPs; the grooves can couple optical pulse energy to propagating SPs, while the nanostructure can couple incident photons to localized SPs. Therefore, NC-LIPSSs are likely to exhibit unusual SP-induced nonlinear optical effects, and this can be seen from one of our recent studies on photoelectron emission from NC-LIPSSs on Pt [Citation27]. Essentially, nanostructures on LIPSSs change the dispersion relation of the SP modes [Citation28]. With the excitation of SPs, we observed multiphoton nonlinear photoelectron emission from the NC-LIPSSs. Following this work, we set out to study the nonlinear optical effects of the metallic NC-LIPSSs.
Despite the extensive research on the SP-assisted nonlinear processes in pure metals, hybrid, or composite materials [Citation22,Citation29–Citation33], a systematic study on the effects of micro/nano-hierarchical structures on nonlinear optical processes is currently lacking. As mentioned above, both nanostructures and microstructures are important for generating SPs. Accordingly, a systematic study of the combined effect of nano/micro-hierarchical structures is necessary for its potential applications of hierarchical structures in future nonlinear optical devices. The nanostructure-covered micro cubes (NC-MCs) is a novel hierarchical surface structure which consists of micro cubes covered by a large number of nanostructures. Throughout this study, we shall relate to both NC-LIPSSs and NC-MCs. One interesting aspect of the structure is that SPs are excited by the dominant features of the different scales of the surface structures. The major difference between the two types of structure is that different types of SP are excited by the dominant features on the different types of surface structure. For example, the microgrooves on NC-LIPSSs favor the excitation of propagating SPs (Figure 1(a)). However, the smaller structures on each type of hierarchical structure can excite additional SPs: nanostructures on NC-LIPSSs can generate localized SPs (Figure 1(b)). The microcubes on the NC-MCs induce lightning rod effect (LR) (Figure 1(c)); however, the smaller structures on the hierarchical structure can excite additional SPs: nanostructures on NC-MCs can generate localized SPs (Figure 1(b)). 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 (such as PSP/LSP or LR/LSP as illustrated in Figure 1(d) and 1(e)) due to the hierarchical structures, we expect that we can control the SHG efficiency in nano/microstructures. In this work, we create the NC-LIPSSs and NC-MCs on Ag and Ni through laser ablation methods and investigate their SHG responses from the viewpoint of ‘symmetry’ and ‘SP’ [Citation34,Citation35]. By comparing the signals from NC-LIPSSs and NC-MCs with the signal from nanostructures and microstructures, we characterize the relation between nanostructures and/or microstructures and SHG.
2. Materials and methods
Our experimental setup employed two amplified Ti:Sapphire fs laser systems, a high-energy system, and a high-repetition-rate 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. The high-repetition-rate laser system generated 65-fs pulses at a central wavelength of 810 nm; the average power of the laser was 400 mW at 273 kHz repetition rate.
2.1. Laser treatment of the Ag surface
As described in [Citation26], we first produced a Ag sample covered with NC-LIPSSs using the high-energy fs laser system. An experimental setup for surface nano/microstructuring on Ag is shown in . The laser polarization was oriented to the ground using a polarizer, and a neutral density (ND) filter was used to adjust the laser power irradiated on the sample. The laser fluence was set at ~0.150 J/cm2. Then, by focusing the beam on to a Ag sample mounted on a stage, we obtained the Ag NC-LIPSS sample.
2.2. Laser treatment of the Ni surface
We next 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 [Citation36]. An experimental setup for surface nano/microstructuring on Ni is shown in ). 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 (HWP) was used to rotate the laser polarization by 90°, and a ND 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 one. 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 the beams on to a Ni sample mounted on the stage, we succeeded in fabricating the Ni NC-MC sample.
shows the optical configuration for the sample preparation. Using this method, two-dimensional arrays of the NC-LIPSS and NC-MC sample were fabricated, and part of them was observed by an optical microscope, as seen in ,). After these laser treatments, the surface morphologies of NC-LIPSSs and NC-MCs were characterized by using a scanning electron microscope (SEM). ,) shows the SEM image of the laser-treated micro/nano-hierarchical structures on Ag and Ni, respectively. ) shows that microgrooves and microcubes are covered with nanostructures. The average measured periods Λs of the microgrooves and microcubes on Ag and Ni were 672 and 600 nm, respectively. The measured depth h of both microstructures was ~100 nm.
Before the SHG measurements of the Ag sample, the hierarchical structures were coated with a thin SiO layer (5.3 nm thick) by physical vapor deposition in a UHV chamber. The protective SiO layer serves two purposes: (1) to protect the surface from atmospheric gases, especially oxidation; and (2) to avoid any damage to the surface [Citation5]. This coating did not affect the conclusions drawn from our study because the SiO layer was very thin and thus was transparent to both 800 and 400 nm wavelength light on a metal–SiO interface.
2.3. SHG measurement for the Ag and Ni samples
Once the sample was fabricated using a high-energy fs laser system, the SHG measurements of the Ag and Ni samples were performed using the high-repetition and high-energy laser system, respectively. shows the configuration of the SHG intensity measurements of the Ag and Ni samples. The p- and s-polarized light at ~800 nm were passed through an orange filter to cut the SHG photons from optics and were focused on to the Ag NC-LIPSS and Ni NC-MC samples at an incidence angle of 45° through a lens. The laser fluences were set at ~25.5 and ~30.9 mJ/cm2. Here, the direction for p-polarization corresponds to the inner production between the x- and z-directions. The direction for s-polarization corresponds to the y-direction. 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 ). In the experiment with the Ag sample, we studied the dependence of SHG on the incident beam angle and the laser polarization. For this, the sample was mounted on two rotation stages. In the experiment with the Ni sample, we studied the dependence of SHG on the laser polarization. For this, the sample was mounted on just one rotation stage. 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 p- and s-polarization for Ag and Ni samples, respectively. To account for the hyper-Rayleigh scattering (HRS) [Citation37,Citation38], the reflected SHG light was collected through a focusing lens with a large numerical aperture. Here, HRS is incoherent second-harmonic (SH) scattering from small feature structures based on Mie theory; up to now, HRS signals from spherical and non-spherical lacking centrosymmetric structures have been observed [Citation39,Citation40]. Hence, the consideration of HRS is important, and the experimental setup has been referred to in some papers [Citation38,Citation41]. Following polarization selection, the reflected SHG light signals were 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 min for each SHG data point.
3. Results and discussion
3.1. SHG signal from Ag samples
We first defined the relationship between the reflected SHG light I(2ω) and the nonlinear susceptibility χ(2) of the metallic groove sample, as done in a previous study [Citation17]. The p-polarized SHG intensity |Ep(2ω)|2 {= Ip(2ω)} generated using the χ(2)XXZ element is given as:
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. L(2ω) and L(ω) represent the local field factors at the SHG and fundamental frequency containing the contribution by the plasmonic excitation. The local electric field Eloc has the relation Eloc = L(ω)E(ω). Consequently, Equation (1) shows that the s-polarized SHG intensity |Ep(2ω)|2 depends on the local electric field Eloc of the X- and Z-direction component.
3.1.1. Power dependence of the SHG from Ag NC-LIPSSs
shows the excitation power dependence of the SHG signals measured on Ag NC-LIPSSs. The data show that the SHG signal has a nearly quadratic dependence on the excitation intensity (slope: 1.81), which indicates a characteristic of the second-order nonlinear processes. The conversion efficiency from fundamental to SHG light photons for the Ag sample was ~10–12 at maxima for p-in/p-out polarization configuration.
3.1.2. Incident angle dependence of the SHG from Ag NC-LIPSSs
We next measured the SHG intensity from a Ag sample as a function of the incident angle θ. shows the angle-resolved SHG intensity from the Ag NC-LIPSSs in p-in/p-out polarization configuration. When the laser polarization was parallel to the microgrooves (the E//groove configuration), the SHG intensity did not vary noticeably with the incident angle θ. However, when the laser polarization was normal to the microgrooves (the E⊥groove configuration), SPs could be excited using the laser beam, and the SHG signal was significantly enhanced between θ = 40° and 55°. We believe that this is owing to the enhancement of electric field E by SP excitation in the propagating mode. At an incident angle of 45°, the SHG intensity for the E⊥groove configuration was ~fourfold stronger than that for the E//groove configuration. We also observed that the NC-LIPSSs on the Ag relax the phase-matching conditions for SP excitation owing to the mismatch of groove period Λ or broadened wave vector k on NC-LIPSSs [Citation27] and significantly broaden the range of angular-coupling, leading to an enhanced SHG light emission over a broad range of incident angles. We also observed SHG for other incident angles. We believe this is due to the nanostructures covering the microstructures. From these data, we say that the SHG from nanostructures does not dominate at incident angles.
3.1.3. Sample rotation angle dependence of the SHG from Ag NC-LIPSSs
To investigate the origin of the anisotropic enhancement of the generated SHG, we analyzed the nonlinear optical susceptibility χ(2) of the NC-LIPSS surface of Ag. As indicated in Appendix 1, because the shape of Ag microgrooves had (quasi-)C2v symmetry, five independent nonlinear susceptibility elements, χ(2)XXZ, χ(2)ZXX, χ(2)ZZZ, χ(2)YYZ, and χ(2)ZYY, are permitted [Citation2,Citation6]. Here, the subscripts X, Y, and Z indicate the laboratory coordinates. Symmetry breaking in the Z-direction does not exist, which disobeys the fact that there is a symmetry breaking in the Z-direction when the non-zero nonlinear susceptibility element has an odd number of Z in the suffixes. From this, the contribution of χ(2)ZZZ should be zero. In addition, symmetry breaking in the Y-direction does not exist. From this, the contributions of χ(2)XXZ and χ(2)ZXX are much stronger than the contributions of χ(2)YYZ and χ(2)ZYY. We also studied an s-in/p-out polarization configuration, and we did not find any SHG in this configuration (not shown here). This allowed us to rule out χ(2)ZXX; thus, the contribution is determined by χ(2)XXZ. It should lead to a pattern with two lobes at φ = 0° and 180° in p-in/p-out polarization. From the above, we estimate that the χ(2)XXZ forming a cos(φ) function strongly dominates on the SHG intensity from the NC-LIPSSs on Ag. Thus, the |χ(2)|2 curve was fitted by a cos2(φ) function.
NC-LIPSSs in this study were 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 [Citation26,Citation27,Citation42]. Here, λ is the incident light wavelength, and ε' and εd are the dielectric constants of the real part and of the surrounding dielectric material, respectively. When the laser ablation is performed in air, εd = 1. For the smooth Ag sample, periodicity of the grooves Λ was calculated as ε1 = −29.7 at λ = 800 nm for the Ag [Citation43]. By using Equation (2), Λ was found to be 786 nm for the Ag sample. However, the observed periodicity was about 672 nm for the Ag sample (). By using the observed periodicity value, we found that ε'Ag = 2.4 for the Ag sample. We believe that this effective dielectric constant arises from the nanostructures on top of the microstructures. Using the effective dielectric constant, we next investigated the electric field distribution at θ = 45° on a periodic groove surface on the Ag sample.
As mentioned above, the local enhancement of an electric field by metallic nanostructures can enhance the SHG response [Citation13]. We should notice that the SP enhancement of SHG is very sensitive to the linear dielectric ε(1), leading to the enhancement of the incident electric field E to fourth order, as explained in Appendix 2. To find the origin of the SP-enhanced SHG, we calculated the local electric field Eloc intensity on the NC-LIPSSs by using the finite difference time domain (FDTD) method. Here we used commercial software (Lumerical Solutions, Canada, https://www.lumerical.com/tcad-products/fdtd/). The electric field distribution for the x-direction at θ = 45° is shown in . Here, the red and blue color scales in ) indicate the distributions of positive and negative change, respectively, for electric field E. A propagating surface wave on a continuous metal–dielectric interface is a SP polariton. SP polaritons were clearly observed, implying propagating surface waves on the continuous metal–dielectric interface. The value in ) indicates the maximal electric field magnitude |E|max calculated in the simulations. At φ = 0° and 180° (E⊥groove), the values of |E|max were higher, and at φ = 90° and 270° (E//groove), the |E|max value was lower. This relationship provides the field enhancement curve as a function φ.
By using the χ induced by the structure’s symmetry and the local field enhancement induced by SP, the expected intensity curve is represented as the black curve in ). The analysis method based on the relation between χ and the near field has been reported elsewhere [Citation17,Citation44]. The expected SHG curve is described as ranging from φ = 0 to φ = 180, so the whole simulation pattern for the SHG forms two lobes consisting of the modified cos2(φ) function.
In fact, we measured the SHG as a function of the sample rotation angle φ from the NC-LIPSSs at θ = 45° using the setup shown in . The sample rotation angle φ was defined as the angle between the incident plane and the sub-microgroove direction (y-direction), or between the laser polarization and the k vector of the microgrooves. ) shows the SHG intensity for the Ag sample as a function of φ from the Ag NC-LIPSSs. Here, the SHG pattern in the p-in/p-out polarization configuration exhibited maxima at φ = 0° and 180° (E⊥groove). We should notice that the experimental data fit relatively well to the simulated curve shown in ). The SHG intensity pattern for the p-in/p-out polarization combination depends strongly on the rotation angle φ, and it is clear that the intensity is sensitive not only to the symmetry of the sub-microgroove structure but also to local field enhancements.
However, the contribution from ISHG with the NC-LIPSSs on the Ag does not completely reproduce our observed SHG intensity pattern, as seen in ). (Although SH radiation collected from nanometer-scale areas is strongly depolarized [Citation45] and dephased [Citation46], we did not consider these influences in the Ag sample.) This fact indicates that SHG in p-in/p-out polarization cannot be explained solely by propagating SPs on the microgrooves’ surface.
Thus, we considered the nanostructural effects on the SHG intensity pattern. Extensive nanostructures from ) were observed on top of the LIPSSs. It is known that localized SPs can be generated from nanostructures [Citation24,Citation47,Citation48], and the localized SPs, if they exist, may generate an isotropic SHG enhancement in the p-in/p-out polarization configuration. Here, we believe that the non-zero SHG components around φ = 90° and 270° in ) arise from SHG enhancement due to the localized SPs. In addition, we suggest that the finite SHG on the Ag at other angles, as seen in ), is due to localized SP polariton excitation in nanostructures. In , the difference between the generated SHG at φ = 0° and 90o is very small at most angles. This is because the localized SPs in both conditions with incident electric field normal and parallel to the groove direction are excited. We believe that the SHG in ) is formed by the propagating and localized SPs from microgrooves and nanostructures.
In order to control the plasmonic enhancement effect of SHG, we tried to remove the nanostructural effect on the Ag sample. As shown in ) and ), there are many ablated nanodots, leading to isotropic SHG, by using setup of ), as seen in ). The sample was immersed in isopropanol and distilled water and cleaned for ~10 min with an ultrasonic cleaner after the creation of NC-LIPSSs to remove the nanostructures (NSs) on microgrooves. shows the SEM image of the obtained sample. A small amount of NS was removed; however, because the difference in surface quality before and after cleaning was very small, a smoother microgroove structure was not obtained. One reason, that SHG intensity at φ = 0° and 90° did not show the difference before and after cleaning as shown in ), is plausible. Thus, to control the SHG intensity on the Ag sample, a more appropriate sample was required. In the next section, we shall select the Ni substrate as an appropriate medium.
3.2. SHG signal from Ni samples
As in the metallic groove sample, we first defined the relationship between the reflected SHG light I(2ω) and the nonlinear susceptibility χ(2) of the metallic cube sample, as done in a previous study [Citation17]. The s-polarized SHG intensity |Es(2ω)|2 {= Is(2ω)} generated using the χ(2)YYY element is given as:
The local electric field Eloc has the relation Eloc = L(ω)E(ω). Consequently, Equation (3) shows that the s-polarized SHG intensity |Es(2ω)|2 depends on the local electric field Eloc of the Y-direction component.
3.2.1. Power dependence of the SHG from Ni NC-MCs
shows the excitation power dependence of the SHG signals measured on Ni NC-MCs. The data show that the SHG signal has a nearly quadratic dependence on the excitation intensity (slope: 2.10), which indicates a characteristic of the second-order nonlinear processes. The conversion efficiency from fundamental to SHG light photons for the Ni sample was ~10–13 at the maxima for the s-in/s-out polarization configuration. The conversion efficiency of the Ni sample was somewhat weaker than that of the Ag sample.
3.2.2. Sample rotation angle dependence of the SHG from Ni NC-MCs
To investigate the origin of the anisotropic enhancement of the SHG, we analyzed the χ(2) of the NC-MC surface of Ni. The χ(2) is sensitive to the symmetry of the shape [Citation5,Citation6]. As indicated in Appendix 1, if we assume that the Ni microcubes (system: tetragon) have C4v symmetry with four mirror planes, three nonlinear susceptibility elements, χ(2)ZZZ, χ(2)ZXX = χ(2)ZYY, χ(2)XZX = χ(2)YZY = χ(2)XXZ = χ(2)YYZ, should be permitted. On the other hand, the χ(2) element for the s-in/s-out polarization configuration should not appear. However, by considering mirror symmetry with respect to the φ change, the χ(2)*YYY panel is assumed to be newly induced, as illustrated in . The candidate for this induced panel is that the symmetry of the microscopic shape indicates Cs symmetry lower than C4v symmetry. If so, 10 independent nonlinear susceptibility elements should be permitted and thus real χ(2)YYY for the s-in/s-out polarization configuration should be accepted [Citation2,Citation5]. The other candidate is that the χ(2)YYY panel can be induced under C4v symmetry in a specific configuration [Citation17], or the shape of the microcubes has broken symmetry at the air–metal interface [Citation24]. However, the consideration of the E field of the incident light being converted from s- to p-polarization by a polarization scrambling effect [Citation6,Citation45] is plausible as one of the candidates.
Thus, we assume that the E field of the incident light being converted from s- to p-polarization is due to the polarization scrambling effect originating from the surface nano-roughness, namely the conversion of the s-polarized incident field of frequency ω by the roughness ζ(x, y), as illustrated in . Ogata et al. [Citation6] have discussed the general effect of the roughness of the metallic nanowire’s surfaces on the optical process upon them. Using their formalism, we consider the conversion of s-polarized light into p-polarized light due to the roughness of the nanowire structure used here. We consider the conversion of the s-polarized incident field of frequency x by the roughness ζ(x, y) of the metal cube structure. Here, ζ(x, y) represents the deviation of the height of the metal surface of the microcube from that of the ideal. We consider the case of φ = 0° so that the wave vector component of the incident light parallel to the substrate is in direction x. The electric field of the s-polarized incident light is in direction y. The electric field, EωNF, near the microcube is generally expanded with respect to ζ(x, y) as:
Here, C(i)jk is a coefficient describing the rate of the conversion of the j-polarized electric field into the k-polarized one, and and are unit vectors in the X- and Y- directions, respectively. Averaging Equation (4) over the area on the substrate of a wavelength size, the second term vanishes and Equation (4) becomes:
since ζ(x, y). Hence, the third component of the averaged electric field is
Here, we suggest that the contribution of χ(2)YYZ was primarily responsible for the enhancement of the SHG intensity, as expected in ). This nonlinear susceptibility element gives a nonlinear polarization:
Substituting the third component of Equation (6) for EωZ in Equation (7), we obtain
Equation (8) indicates that the roughness on the microcube generates SHG in the s-in/s-out polarization configuration at φ = 0 . If EωNF,Z depends weakly on the direction of the incident electric field, the SHG intensity generated by Equation (8) in s-in/s-out should be as follows:
Here, χ(2)YYZ is non-zero under C4v symmetry. Therefore, Equation (9) is large when the incident electric field is in direction y. In this configuration, the pattern exhibits eight lobes at the minima with φ = 0°, 45°, 90°, 135°, 180°, 225°, 270°, and 315°, as seen in Equation (9). Hence, if we consider the polarization scrambling effect, the formation of a SHG pattern with eight lobes can be explained. Namely, if we assume that the contribution of χ(2)YYZ is effective for the s-in/s-out polarization configuration in this work, the SHG intensity should be zero at φ = 0°, 45°, and 90° ranging from φ = 0° to φ = 90°, as shown in ). In conclusion, the χ(2)YYZ 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 sin2(4φ).
The NC-MCs in this study were 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 Equation (2), as well as in the Ag case. For the smooth Ni samples, the periodicity of the grooves Λ was calculated as εNi' = −13.0 for the Ni [Citation43]. By using Equation (2), Λ was found to be 769 nm for the Ni sample. However, the observed Λ was about 600 nm for the Ni sample, as observed in ). By using this observed Λ value, we found that εNi' = −4.00 for the Ni sample. This effective dielectric constant might arise from the NSs on top of the microstructures [Citation27]. Using this effective dielectric constant, we next investigated the electric field distribution at an incidence angle of 45° on a periodic cube surface on the Ni sample.
Likewise, to find the origin of the enhanced SHG, we next calculated the local electric field Eloc intensity on the sample using the FDTD method. The electric field distribution from φ = 0° to φ = 90° is shown in ). The color scale bar indicates the electric field magnitude |E|. Sharper metal tips can produce strong local electromagnetic fields, called the lightning rod effect. Recently, studies have reported the lightning rod effect induced at the tips at the Ni nanostructure for the s-in/s-out polarization configuration [Citation49]. Thus, the lightning rod effect should be induced at the edge parts of the Ni sub-microcube structures. In ), 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 ) 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 relationship provides the field enhancement curve as a function φ.
Using the χ induced by the structure’s symmetry and the local field enhancement induced by SP, the expected intensity curve is represented as the black curve in ). The expected SHG curve is described as ranging from φ = 0° to φ = 90° so that the whole simulation pattern for the SHG forms eight lobes consisting of the modified sin2(4φ) function.
We observed the azimuthal angular dependence of the SHG from NC-MCs at an incidence angle of 45°, as shown in . Here, 45° incidence is appropriate for defining nonlinear polarization with third-rank tensors. For example, when θ = 0°, the component for the z-direction does not exist; therefore, the nonlinear polarization should be defined using x- and y-directions only, as such small amounts of information do not play a role in χ analysis. When 0°<θ < 45° and 45°<θ < 90°, the nonlinear polarization should be defined by modifying the information for the beam-spot size and E field/wave vector(s) on a case-by-case basis; thus, χ analysis may be complicated. The design of a 45° incidence with a large volume of information and an easier understanding is valid for the experiments. The sample rotation angle φ was defined as the angle between the incident plane and the sub-microgroove direction (x- or y-direction). ) 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 ) is patterned in ). 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 not only to the symmetry parameter but also to the enhancement factor (EF) of the electric field. Namely, when the structure gives 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 [Citation50]. If the nanostructures are in contact with the corners of microcube structures as seen in , a large local field enhancement will be obtained by the cascading field enhancement effect originating from the ‘lightning rod’ effect. Then, the E field concentration might boost the finite SHG intensity at the azimuthal angle φ without mirror symmetry.
) also reveals, however, 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 ). This fact indicates that SHG in s-in/s-out polarization cannot be explained solely by the symmetry and SPs on the microcube-structured surface.
Here, we merely comment on the contribution of the quadrupolar effect [Citation51] in our SHG signals. Then, what is the quadrupolar effect? We should note that the nonlinear polarization PNL includes a dipole and a quadrupole. The nonlinear polarization PNL including the quadrupole is written as:
Here, superscripts D and Q indicate the electric dipole and quadrupole, and χijk and χijkl indicate third- and fourth-rank tensor, respectively. The fourth-rank tensor shows a finite value even for centrosymmetric systems. In general, the bulk crystal in which the fluctuation of the electric field is smoother than the wavelength can be disregarded; meanwhile, the thin film or part with a large contribution to the surface and interface cannot be disregarded. In this work, the SHG intensity for s-in/s-out polarization from a smooth Ni bulk sample was at the noise level, as we found in a separate experiment. This implies that the electronic quadruple effect from the Ni bulk sample can be disregarded. On the contrary, the SHG from Ni NC-MCs with nanometer 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 ), 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 for s-in/s-out polarization is not dominant, namely χYYYYEY∇YEY ≒ 0. (In this work, since the magnetic field is not applied, the magnetically induced nonlinear optical effect [Citation17,Citation52] is disregarded.)
Thus, we considered the nanostructural effects on the SHG intensity pattern as observed in ). Extensive NSs from ) were observed on top of the MCs. It is known that localized SPs can be generated from NSs [Citation47], 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 technique shown in ) and measured their SHG as a function of rotation angle φ using the setup shown in . Figure 16(a) shows the NSs induced on Ni by performing ablation at a laser fluence of 50.0 mJ/cm2 using the setup illustrated in ). Most of the structures were nanodots with a diameter in the range of 5–80 nm, as seen in ). In the SHG measurement, we observed isotropic SHG enhancement from NSs on Ni in the s-in/s-out polarization configuration, as shown in ). Thus, we can say that the non-zero SHG components around φ = 0°, 45°, 90°, 135°, 180°, 225°, 270°, and 315° in ) arise from SHG enhancement due to the localized SPs on the NSs. We found that the SHG in ) is formed by the excitation of SPs from both MCs and NSs.
To control the plasmonic enhancement effect of SHG, we tried to remove the nanostructural effect on the Ni sample. As shown in ), there are many ablated nanodots on microcubes, leading to isotropic SHG, as seen in ). 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 microcubes. By doing so, the microcubes become much cleaner. The SEM images of the surface of the obtained MC sample are shown in Figure 17(a,b). We next measured their SHG signal, as shown in ). ) 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 the minima with φ = 0°, 45°, 90°, 135°, 180°, 225°, 270°, and 315°, and the minimal intensity (‘b’ in )) was approximately ~34.7% lower than that for the NC-MCs (‘a’ in )). Together, these findings indicate that NSs could not be completely removed, but the isotropic SHG emission from the NSs was suppressed with decreasing amounts of NSs. We expect that the nanostructure-covered microstructures make the polarization scrambling effect small with a decreasing amount of nanostructures. If so, the SHG pattern with eight lobes for the MC structure seems to be small and the total SHG pattern seems to be large and circular. However, the measured SHG seen in ) is formed by the large eight lobes and a small circle. This means ‘the polarization scrambling effect relates to nanostructure better than the localized SP effect’. The incomplete removal of NSs makes the behavior of SHG complicated.
Incidentally, to remove NSs perfectly, laser irradiation was also tested on the Ni surface at normal incidence in water [Citation42] using the experimental setup shown in ). However, as seen in ,), we could neither remove the NSs nor fabricate the cube structures. Unremoved NS is due to the dramatic change of the dielectric for the metal–water interface by performing the laser ablation in water. According to the SEM image, the periodicity of broken cubes was 300 nm. Considering this, if we calculate the dielectric for the metal–water interface using εNi' = −4.00 in Equation (7), it is determined to be εd' = 2.56. This fact shows a large quantitative change from εd' = 1.00 to εd' = 2.56. On the other hand, regarding why the cube sample cannot be created, plasma generation in water is a plausible explanation. In any case, the sample obtained by laser ablation in water is not appropriate for our SHG experiment, and thus is excluded from our work.
Returning to the main topic. How many NSs could be removed by the cleaning method? According to quantitative analysis, the existence probability of NSs on NC-MCs was found to be 11.71% of the whole surface seen in ). On the other hand, the existence probability of NSs on MCs was found to be 2.65% of the whole surface seen in ). Namely, we can thus say that 9.06% of the NSs on the whole surface was removed.
In fact, however, most of the nanostructures were removed, as seen in ). Although most of the nanostructures were removed, it is not entirely clear why the minimum intensities in the pattern do not show a large reduction. Thus, we considered the dephasing of the light field induced by the NSs on the Ni microstructures seen in ). In general, SH radiation collected from nanometer-scale areas is strongly dephased [Citation53]. Stockman [Citation46] has discussed the dephasing effect in terms of the roughness of metal surfaces on the optical processes upon them. Based on their theorem, we considered the propagating delay of fundamental light by the roughness on a nanoscale.
The origin of the dephasing is the delocalization of the linear local field [Citation46]. 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 random phase shifts and results in dephasing [Citation46]. More materially, the timing of the penetration of the propagating light is off when the NSs exist on a surface, resulting in dephasing. Namely, when the incident wave comes to a MC structure with NSs as illustrated in Figure 18(a), the hit-part of the wave to material is slightly different from an ideal MC structure, as illustrated in ), due to the existence of NSs. As a result, a random phase shift in the incident wave, called dephasing by propagation delay, occurs. The dephasing may cause noise in the intensity data. Therefore, we concluded that the SHG pattern in ) may contain noise due to the dephasing at rotation angle φ under forbidden conditions.
4. Conclusions
We have systematically analyzed the SHG intensity from the constituted NC-LIPSSs and NC-MC hierarchical surface structures on metals. We observed unique anisotropic SHG enhancements causing symmetry of shapes and excitation of SPs. The SHG patterns are attributed to the localized/propagating and localized/localized SPs on nano/micro-surface structures on Ag and Ni, respectively. The contribution of microstructure to SHG was found to be larger than that of nanostructure. Furthermore, the separation of nano/microstructure related to the decomposition of the SHG intensity patterns. Left SHG was explained by the dephasing effect of propagating delay. These interesting physical phenomena can find possible applications in novel symmetry-sensitive plasmon optical devices.
Acknowledgments
The authors would like to thank A. Vorobyev for his support in the ablation experiments and B. McIntyre for his help in the surface analyses. The work was supported by US Army Research Office and the Bill & Melinda Gates Foundation.
Disclosure statement
No potential conflict of interest was reported by the authors.
Additional information
Notes on contributors
Yoichi Ogata
Yoichi Ogata is an Enterprise Cooperation Researcher at the Center for Advanced Photonics at RIKEN. His research is in the area of nanotechnology and nonlinear optics. He received his PhD in Materials Science from the Japan Advanced Institute of Science and Technology. He worked for several years as a Project Assistant Professor at the Tokyo University of Agriculture and Technology in the Department of Applied Physics, Visiting Research Associate and then later on as Research Associate with the University of Rochester in the Institute of Optics. He brought to the table his passion for studying and teaching at a university level ‘Nonlinear Optics of Metallic Nano and/or Micro structures’ as it has been a primary focus of his career. Specifically, he has developed an expertise in second-harmonic generation spectroscopy and nanofabrication. He has had more than 10 papers published in international journals.
Chunlei Guo
Chunlei Guo is a Professor in The Institute of Optics and Department of Physics and Astronomy at the University of Rochester. His research is in the area of femtosecond lasermatter interactions. He received his PhD in Physics from the University of Connecticut and postdoctoral training at the Los Alamos National Laboratory. His work at Rochester has led to the discoveries of a range of highly functionalized materials, which may find a broad range of applications and have been featured extensively by the media, including four New York Times features. He is a Fellow of the American Physical Society and Optical Society of America. Currently, he serves as the Editor-in-Chief for one of the most widely used handbooks in lasers and optics, CRC Handbook of Laser Technology and Applications (2nd Edition).
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Appendices. Symmetry and plasmonic effect on nonlinear optics
Appendix 1. Nonlinear susceptibility elements χijk(2) permitted by symmetry of the medium
We shall describe χ(2) elements related to the symmetry of the medium based on the book written by Boyd [Citation2], case by case.
(i) χijk(2) elements under C2v symmetry
If we describe the symmetry of the metal groove surface using group theory, Cnv = C2v (n = 2). When some plane is rotated by 360/2 = π for the symmetry axis C, it is back in the original position.
Here, we perform the following operation to withdraw effective nonlinear susceptibilities under C2v symmetry. The nonlinear polarization PNL excited by the electric field E of the incident light is expressed as:
First, we convert third-rank tensors of ijk to coordinate the axis of xyz. When we do not consider the symmetry, PNL with x, y, and z components are given as the following equations:
The groove structure has two mirrors normal to the surface and the symmetry is broken in the z-direction. Taking this into account, the inversion operation for C2v symmetry can be performed twice with respect to x- and y-axes. If we perform the inversion operation for the x-axis as (Px(2), Py(2), Pz(2))⇒(-Px(2), Py(2), Pz(2)) and (Ex, Ey, Ez)⇒(-Ex, Ey, Ez) for Equations (A.2), (A.3), and (A.4), they are modified as Equations (A.5), (A.6), and (A.7):
When Equations (A.5), (A.6), and (A.7) given by the inversion operation have the same sign as Equations (A.2), (A.3), and (A.4) given by the identity operation, the same terms remain, as shown below:
If we next perform the inversion operation for the y-axis as (Px(2), Py(2), Pz(2))⇒(Px(2), -Py(2), Pz(2)) and (Ex, Ey, Ez)⇒(Ex, -Ey, Ez) for Equations (A.8), (A.9), and (A.10), they are modified as Equations (A.11), (A.12), and (A.13):
When Equations (A.11), (A.12), and (A.13) given by the inversion operation have the same sign as Equations (A.2), (A.3), and (A.4) given by the identity operation, the same terms remain as below:
If we consider the permutation (jk⇔kj) of Kleinman’s symmetry, non-zero independent nonlinear susceptibility elements χxxz(2) = χxzx(2), χyyz(2) = χyzy(2), χzxx(2), χzyy(2), χzzz(2) are permitted under C2v symmetry with the groove surface.
(ii) χijk(2) elements under C4v symmetry
If we describe the symmetry of a metal cube’s (tetragonal) surface using group theory, Cnv = C4v (n = 4). The cube structure has four mirrors normal to the surface and the symmetry is broken in the z-direction. Taking it into account, the inversion operation for C4v symmetry can be performed twice with respect to x- and y-axes and, furthermore, the permutation operation between x and y as (Px(2), Py(2), Pz(2))⇒(-Py(2), Px(2), Pz(2)),(Ex, Ey, Ez)⇒(-Ey, Ex, Ez).
Under C2v symmetry, five non-zero independent nonlinear susceptibility elements, χxxz(2) = χxzx(2), χyyz(2) = χyzy(2), χzxx(2), χzyy(2), χzzz(2), are permitted. Here, if the permutation operation between x and y is added, three independent nonlinear susceptibility elements, χzzz(2), χzxx(2) = χzyy(2), χxzx(2) = χyzy(2) = χxxz(2) = χyyz(2), are permitted.
Appendix 2.
Local field effect on nonlinear optics
Here, we perform the expansion of local field L based on the book written by Boyd [Citation2].
Since the polarization P has both linear and nonlinear contributions,
We represent the linear contribution as
Here, the local electric field Eloc is written as
Therefore, the linear polarization PL is expanded as
Here, let us derive the linear susceptibility χ(1). Generally, χ(1) is defined as
If we derive χ(1) using Equation (B.4),
Here, the linear dielectric constant ε(1) is defined as
If we use Equations (6) and (7), PL is solved as
We next consider the displacement vector . If the expression for PL is substituted into , which is derived as
Since the nonlinear source term PNLS is actually the nonlinear polarization PNL multiplied by the factor (ε(1)+2)/3, is simply rewritten as
Here, we consider the wave equation. In general, the optical wave equation is written as
We now use Equation (B.10) to eliminate from this equation. If we replace the magnetic permeability μ0 by1/ε0c2, we obtain the wave equation:
This equation shows that the local field effects are incorporated into the wave equation.
We finally define the relation between the second-order nonlinear polarization P(2ω) and the nonlinear susceptibility χ(2). The nonlinear polarization P(2ω) in the metal structures at the SH frequency is written as a function of the incident electric field E at ω as
where χijk is the third-rank tensor of nonlinear susceptibility, while i, j, and k represent any of the Cartesian coordinates X, Y, and Z. 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. Here, if we assume that the local electric field Eloc = L(ω)E(ω), where L(ω) represents that the local field factors at the fundamental frequency, the nonlinear polarization PNL can be written as follows:
By solving Equation (12), PNLS is given as
As mentioned above, the relation between PNLS and PNL is PNLS = (ε(1) + 2)/3 PNL. Thus, PNL is given as
Using Equations (B.3), (B.5), and (B.7), we relate local and macroscopic fields Eloc and E as
Therefore, using the relation L(ω) = (ε(1) + 2)/3, PNL is rewritten as
This equation shows that the nonlinear polarization PNL including L factors is dominated by the linear dielectric ε(1). Thus, we can say that SP excitation is very sensitive to the linear dielectric change, and SP excitation can lead to SHG enhancement from this equation.